Pulsatile
Reperfusion After Cardiac Improves Neurologic Outcome
Arrest
MARK P. ANSTADT, M.D., MICHAEL J. STONNINGTON, B.A., MARK TEDDER, M.D., BARBARA J. CRAIN, M.D., PH.D., MICHAEL F. BROTHERS, M.D., DAVID J. HILLEREN, M.D., RICHARD J. RAHIJA, D.V.M., PH.D., J. ALAN MENIUS, JR., M.H.S., and JAMES E. LOWE, M.D.
Cardiopulmonary bypass (CPB) using nonpulsatile flow (NPF) is advocated for refractory cardiac arrest. This study examined cerebral outcome after resuscitation with pulsatile flow (PF) versus NPF. Dogs arrested for 12.5 minute were reperfused with NPF (n = 11) using roller pump CPB or PF (n = 11) using mechanical biventricular cardiac massage. Pump flows were similar between groups; however early arterial pressures were greater during PF versus NPF, *p < 0.05. Circulatory support was weaned at 60 minutes' reperfusion. Neurologic recovery of survivors (n = 16) was significantly better after PF versus NPF, *p = 0.01. The presence of brain lesions on magnetic resonance images did not significantly differ between groups at 7 days. Brains then were removed and regions examined for ischemic changes. Loss of CAl pyramidal neurons was more severe after NPF versus PF, tp = 0.009. Ischemic changes were more frequent after NPF in the caudate nucleus (tP = 0.009) and watershed regions of the cerebral cortex (tp = 0.062), compared with PF. These results demonstrate that PF improves cerebral resuscitation when treating cardiac arrest with mechanical circulatory support (* = MANOVA with repeated measures, t = categorical data analysis).
C
EREBRAL ISCHEMIA RESULTS in the tragic loss of neurologic function if not adequately reversed in a timely fashion. This circumstance occurs in the setting of cardiac arrest, the most common cause of clinical death. Ironically therapies that improve cardiac resuscitation have raised concern about the increased po-
tential for successfully resuscitating severely brain-damaged patients. This dilemma seems inevitable in view of the brain's limited tolerance to ischemia. Brief periods of circulatory arrest (5-10 minutes) can irreversibly damage the brain. 1-5 In contrast the heart can tolerate significantly longer periods of ischemia and subsequently sustain life.6
From the Departments of Surgery, Radiology, and Pathology, Duke University Medical Center, Durham, North Carolina
Therefore preserving cerebral function has become a major challenge for investigators attempting to improve resuscitation from cardiac arrest. Recent attention has been directed toward mechanical circulatory support as an adjunctive treatment for refractory cardiac arrest. Previous experience found these devices effective for recovering cardiac function.7 New advances in cannulation techniques make institution of blood pumps possible during cardiac arrest.8" Although these support systems vary somewhat, most use a standard cardiopulmonary bypass (CPB) circuit with either roller or centrifugal pumps. Both of these devices provide resuscitative support with nonpulsatile flow. Certain characteristics of pulsatile flow, however, may be critical for effective resuscitation. Hydraulic forces generated by pulsatile flow are thought to be required for maintaining the patency of capillary beds.'2-14 In a similar manner, the cerebral microcirculation may depend on "critical opening pressures" during reperfusion. Although cardiac arrest represents the most common clinical condition causing global cerebral ischemia, prior investigations have not compared the effects of pulsatile and nonpulsatile flow on neurologic outcome. The purpose of this study, therefore, was to assess cerebral recovery after cardiac arrest and subsequent resuscitation using either pulsatile or nonpulsatile reperfusion.
Presented at the 11 1th Annual Meeting of the American Surgical Association, April 1 1-13, 1991, Boca Raton, Florida. Address reprint requests to James E. Lowe, MD, Duke University Medical Center, Box 3954, Durham NC 27710. Accepted for publication April 23, 1991.
478
Methods The study protocol was approved by the Animal Care and Use Committee at Duke University Medical Center and performed in accordance to the "Guide for the Care and Use ofLaboratory Animals" prepared by the National Research Council of the NIH (NIH Publication No. 86-
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PULSATILE FLOW FOR CARDIAC ARREST
479
23, Revised 1985). Twenty-two young (12-16-month-old) conditioned coon hounds (23 ± 1 kg) from the same breeding colony were chosen for this study. Only dogs in good physical health, tested negative for heart worms, and with no evidence of respiratory disease or infection were selected for experimentation. The experiment modeled cardiac arrest by electrically inducing ventricular fibrillation (VF). Mechanical circulatory support was employed for resuscitation after 12.5 minutes of VF. Animals were separated into two groups for either pulsatile or nonpulsatile circulatory support. Nonpulsatile flow (NPF) was generated from a standard cardiopulmonary bypass system (Fig. 1). The system used two venous return cannulae positioned in the superior and inferior vena cavae, a Cobe membrane oxygenator, and a calibrated roller pump that delivered nonpulsatile arterial return through a femoral cannula. Pulsatile flow (PF) was produced by a method termed direct mechanical ventricular actuation (DMVA) (Fig. 2). Direct mechanical ventricular actuation is achieved by a pneumatic
FIG. 2. Schematic of DMVA used to generate PF. Regulated by a pneumatic drive system, an assistor cup actuates the ventricles into systolic and diastolic configurations.
FIG. 1. Schematic
diagram of cardiopulmonary bypass circuit used to
generate NPF. Bicaval cannulation return.
was
used to ensure maximal venous
device that alternately compresses and ditates the ventricular chambers and has been described in detail elsewhere.'5-20 The device can best be understood as a mechanical method of internal cardiac massage. Direct mechanical ventricular actuation application during VF can reliably generate pulsatile flow that is virtually identical to the normal physiologic state. Animal Preparation Dogs were fasted the evening before experimentation with access only to water. An intravenous (I.V.) line was placed for infusion of Lactated Ringer's (LR) solution on the morning of the study. Sodium pentobarbital (I.V.) was administered slowly until the corneal reflex was ablated; this reflex was subsequently monitored until completion of the operative procedure. The animals were intubated, given procaine penicillin G (600,000 units) and dihydrostreptomycin sulfate (750 mg) (I.M.) and then prepared for a lateral thoractomy using aseptic technique. An electronic thermistor probe was positioned in the rectum for measuring core temperature. Respiratory support
480
ANSTADT AND OTHERS
was begun with a pressure-regulated ventilator (InterMed Bear 1) before any operative procedures. All surgery was performed in a standard sterile fashion. Polyethylene catheters positioned in the femoral artery allowed monitoring of arterial blood pressures and blood gases. The opposite femoral artery was exposed for arterial cannulation in those animals selected to receive CPB. Animals receiving DMVA had a Swan-Ganz catheter advanced to the pulmonary artery for calculating blood flow during resuscitation. Lateral thoracotomies were then made through the right or left chest for CPB or DMVA, respectively. After confirming surgical hemostasis, heparin (100 U/kg, I.V.) was administered 5 minutes before cardiac arrest in both groups. Ventricular fibrillation was induced by low-voltage AC current. During cardiac arrest an arterial return cannula was advanced into the femoral artery of dogs receiving CPB. The right atrium was then exposed by pericardiotomy and two venous cannulas positioned with their tips in the proximal superior and inferior vena cavae. After 10 minutes of arrest, the pericardium was widely opened in both groups of animals. At 12 minutes of arrest, an appropriately sized assistor cup was positioned over the hearts of dogs receiving DMVA.
Resuscitation Protocol
Circulatory support was initiated after 12.5 minutes of VF arrest in both groups. Cardiopulmonary bypass flow rates were advanced to 100 ml/kg/min during the first 2 minutes of support. Flow rates were constantly adjusted thereafter, to match maximal venous return. Direct mechanical ventricular actuation compression rates were begun at 60 cycles/minute with actuating forces adjusted to the lowest degree allowing full compression and dilatation of the ventricles. Systolic durations remained at 50% throughout DMVA support. Cycle rates, as well as systolic and diastolic forces, were gradually increased during the first 2 minutes of support to 120/minute, + 125 mm Hg, and -1 10 mm Hg, respectively. Hearts were defibrillated after 15 minutes of reperfusion in both groups. Fibrillation refractory to three successive countershocks (10, 20, and 30 J) was treated by similar defibrillation attempts every 15 minutes. Total circulatory support was continued for a full hour in both groups. Arterial blood pressures, pump flows, serum electrolytes, and arterial blood gases were recorded at 5, 15, 30, 45, and 60 minutes of reperfusion. All animals were given sodium bicarbonate (2 mEq/kg) during the first 5 minutes of reperfusion. Thereafter sodium bicarbonate, potassium chloride, and calcium chloride were administered every 15 minutes as needed to correct arterial blood gases and serum electrolytes. Respiratory settings and membrane oxygenator gas flows were adjusted
Ann. Surg. * October 1991
to keep arterial P02 between 150 and 250 mm Hg and
PCO2 between 30 and 45 mm Hg. After 60 minutes of reperfusion, circulatory support was discontinued. In animals treated by CPB, the entire pump volume (1 L of LR prime) was returned to the circulation over 15 minutes. One unit of fresh whole blood was then given over 45 minutes, followed by LR at 100 mL/hour for 12 hours. Animals supported by DMVA received 1 L LR during support. Thereafter fluid administration was decreased to 100 mL/hour for 12 hours after
removal of the device. Hemostasis was obtained by electrocautery, followed by application of Thrombin spray (Thrombostat 20,000 units) to all surgical incisions. A size 20 French chest tube was placed and the chest was closed. Arterial pressures and blood gases were monitored for 1 hour after support. The catheters then were removed and the groin incisions closed. Ventilatory support was maintained for 3 hours after cessation of circulatory support. Animals then were placed on 50% oxygen and transferred to their cages. Four hours after resuscitation, 10 mL 50% dextrose and 25 mg flunixin (a nonsteroidal antiinflammatory agent) were administered I.V. Animals were extubated 6 hours after resuscitation. Oxygen supplementation was continued for the following 12 hours. Analgesia was insured by administering morphine sulfate (0.5 mg/kg I.V.) immediately after circulatory support with two subsequent doses of 0.25 mg/kg at 6 and 12 hours (after the neurologic assessments, below). Intravenous fluids were decreased to 50 mL/hour 16 hours after resuscitation and discontinued once animals began taking oral fluids. Animals were fed and watered by hand two times per day until daily requirements were taken spontaneously. Tube and hand feeding were continued if voluntary intake was inadequate. Assessment of Neurologic Function
Neurologic function was based on two scoring systems. One rated neurologic recovery during the 7-day postoperative period. The other scoring system measured persistent neurologic deficits at the end ofthe recovery period. Neurologic recovery scores were based on those used by previous investigators.8'21-23 Only the neurologic activities that could be objectively assessed were chosen for monitoring recovery. Scores were assigned to each of 12 categories: corneal reflex (present = 1, absent = 0); respiratory effort (present = 1, absent = 0); pupillary response (normal = 3, sluggish = 2, pinpoint = 1, fixed & dilated = 0); swallow (present = 1, absent = 0); visual perception (gross objects = 2, light only = 1, none = 0); pain response (present = 1, absent = 0); touch response (present = 1, absent = 0); voice response (present = 1, absent = 0); food intake (without assistance = 2, hand
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PULSATILE FLOW FOR CARDIAC ARREST
feeding = 1, none = 0); sternal recumbency (yes = 1, no = 0); standing (yes = 1, no = 1); and walking (without falling = 2, with falls = 1, unable = 0). Neurologic function was assessed at 3, 6, 12, 18, and every 24 hours thereafter by a veterinarian unaware of the animal's resuscitative therapy. A separate neurologic assessment determined persistent neurologic deficits after 7 days of recovery. Animals were assigned one point for unremitting visual impairments, gait defects, ambulatory incapacity, or indifference to food on day 7, as well as for any seizure activity during the entire recovery period. Each animal received a deficit score based on the presence or absence of these five abnormalities (range, 0[normal] to 5[severe deficit]). Magnetic Resonance Imaging After 7 days of recovery, animals were anesthetized with pentobarbital for magnetic resonance imaging (MRI). Brain imaging was performed in a 1.5-Tesla clinical imager (GE Signa) using a quadrature head coil. Coronal T2-weighted 5-mm-thick images were obtained at 7.5mm intervals. Sequence parameters for imaging included: a TR of 2800 msec, TE of 30 and 80 msec; number of excitations = 0.75; and a field view of 16 cm. Comparisons were made between the two echoes to separate normal areas of high signal intensity (i.e., cerebrospinal fluid) from brain lesions. All images were interpreted by a radiologist blinded to the animals' treatment protocol. Brain Histopathology The day after MRI assessments (8 days after resuscitation), animals were killed and their brains placed in 10% formalin for at least 2 weeks. The brains then were sectioned coronally and inspected for gross lesions. Regions most frequently injured on MRI images were examined histopathologically. Representative blocks of the hippocampus, caudate nucleus, cerebellum, and watershed areas ofthe frontal and parietal cortex were taken from each hemisphere, placed in 20% sucrose/10% formalin, and cut on a freezing microtome. Representative sections were mounted on gel-coated slides and stained for neurons with cresyl violet. Hippocampal damage was graded by scoring pyramidal cell loss within the CAl region. Those hippocampi with no identifiable injury were assigned a score of 1 (normal). Those with complete absence of CA pyramidal cells were assigned a score of 5 (severe). Those hippocampi that did not fall into these two extremes were categorized according to their overall relative appearances: scant pyramidal cell loss = 2, approximately 50% pyramidal cell loss = 3, and only scattered pyramidal cells remaining = 4. Scores from
the left and right hippocampus were averaged to provide a single score for each animal. The caudate nucleus was examined for evidence of ischemic injury as evidenced by microinfarction or diffuse neuronal loss. Abnormal findings were categorized into three groups: absent, unilateral, or bilateral. The frontal and parietal watershed regions of brains were examined for evidence of histologic damage (microinfarction or diffuse cell loss). Results then were categorized for the two groups: injury found in one or more watershed regions versus absence of ischemic changes in all examined regions. The cerebellum was examined for microinfarction or selective Purkinje cell loss. All brain regions of surviving animals were histologically evaluated. One tissue block was unavailable for each region. Therefore each region was examined in 15 rather than in 16 cases. Tissue examinations and gradings were performed by a neuropathologist who was blinded to the experimental groups.
Statistical Analysis Univariate analysis and between-group comparisons (PF vs. NPF) were performed using SAS statistical software (SAS Institute Inc., Cary, NC). Pre-arrest parameters known to affect neurologic outcome (temperature, pH, PCO2, P02, and anesthetic dosage) were compared between groups using a nonpaired Student's t test. Variables known to affect neurologic outcome during resuscitative support (blood flow, mean arterial pressure, pulse pressure, temperature, pH, PCO2, and P02) also were tested for between-group differences. Group (time by group effect) and within-group (within-group effect) effects were determined using multivariate analysis of variance with repeated measures over time (MANOVA). In addition overall between-group differences were tested using area under the curve analysis (group effect). Experimental measures of cerebral recovery then were assessed. Serial neurologic scores were compared between groups using MANOVA. All measures of outcome that were categorized (MRI lesions, neurologic deficit scores, histologic grading, and occurrence of regional ischemic damage) were assessed by comparing row mean scores using a chi-squared statistic. Comparisons were considered statistically significant at alpha levels less than 0.05. Results
Survival Sixteen animals (73%) survived for the full 1-week observation period. All deaths occurred during the initial 18 hours of recovery. There was no significant difference in
ANSTADT AND OTHERS
482
TABLE 1. Parameters Measured Immediately Before Cardiac Arrest (mean ± SEM)
Parameter Temperature (°C)* pH pCO2(mmHg)
P02(mmHg) Pentobarbital (mg/kg)
PF
NPF
p
35.9 ± 0.2 7.41 ± 0.02 31 ± 2 247 ± 19 31.2 ± 1.1
35.4 ± 0.2 7.43 ± 0.01 28 ± 1 208 ± 18 33.0 ± 1.7
0.09 0.50 0.41 0.18 0.30
* Temperatures tended to be 0.5 C lower in animals that received NPF versus PF. (PF versus NPF, nonpaired Student's t test). NPF, nonpulsatile flow; PF, pulsatile flow.
mortality after PF (2/1 1) versus NPF (4/1 1), p = 0.31. One animal (PF group) succumbed to respiratory failure because of underlying pulmonary disease. The remaining deaths were of undetermined origin. Necropsies found no evidence of pneumothorax or hemothorax, pulmonary edema, or internal bleeding. Only results from the 16 surviving animals were analyzed in this study.
Control and Resuscitation Conditions Prearrest variables that may affect neurologic outcome were kept similar in both groups (Table 1). Both groups of animals had mild hypothermia created by open thoracotomy. Core temperatures were slightly lower in animals before NPF compared with PF, but this difference was not statistically significant. Blood gas values exhibited little between-group disparity and mean sodium pentobarbital doses differed by less that 2 mg/kg. All animals were normotensive with a stable sinus rhythm before induction of ventricular fibrillation. Variables monitored during the period of resuscitative circulatory support are summarized in Table 2. Blood flow was calculated by thermodilution and a calibrated
Ann.
Surg. October 1991 -
roller pump during PF and NPF, respectively. Flows were entirely dependent on the rate of maximal venous return in both groups. Calculated flows were not significantly different between groups. Pilot studies and prior investigations using radiolabeled microspheres under similar experimental conditions found roller pump values underestimate actual flow by approximately 25% compared with thermodilution. Therefore actual flows were more closely matched between groups than measured values suggest. Accordingly calculated flows were compared to illustrate venous return trends for each group. Despite similar flows PF initially generated greater mean arterial pressures (MAP) than did NPF. Resuscitative pressures significantly differed at 5 and 15 minutes of reperfusion. However MAP disparities diminished throughout reperfusion and did not significantly differ between groups after 15 minutes. Pulse pressures remained constant during PF and contributed to the relatively early elevations in MAP. Nonpulsatile flow exhibited small pulsations due to roller pump oscillations, which were periodically intensified by effective cardiac contractions. However PF pulse pressures consistently exceeded those during NPF by fivefold to tenfold. Arterial blood gases were similar between groups, with the exception of pH values. Reperfusion with NPF resulted in rapid normalization of arterial pH, whereas a more gradual return occurred during PF. Thus mean pH values were significantly lower during the initial 15 minutes of PF compared with NPF. Thereafter pH remained similar between groups. Between-group differences in P02 were inconsequential, as they remained between 100 and 250 mm Hg in both groups. Core temperatures also behaved differently between groups. Although mean values were identical at 5 minutes' reperfusion, core temperatures subsequently increased
TABLE 2. Reperfusion Parameters During 1 Hour of Resuscitative Support with Either PF or NPF Flow (mean ± SEM)
ReperfusionTime (min) Parameter Flow (ml/kg/min)*
Method
5
15
30
45
60
p
89 ± 11 148 ± 15 159 ± 20 161 ± 18 146 ± 15 0.059 110±4 116±7 108±5 102±2 100±3 ± MAP (mmHg) ± 125 6 109 3 101 ± 6 100 ± 7 99 ± 6 0.033 64 ± 7t 74 ± 8 69 ± 7t 79 ± 11 90 ± 11 Pulse pressure (mmHg) 75 ± 2 83 ± 8 79 ± 11 87 ± 8 79 ± 11 0018 8±3 9±3 14 ± 4 10 ± 2 18 ± 10 ± ± ± ± Temperature (C) ± 35.5 0.2 35.5 0.2 35.5 0.02 35.3 0.3 35.4 0.3 0.022 35.3 ± 0.1 36.0 ± O.lt 36.4 ± 0.3t 36.4 ± 0.2t 36.5 ± 0.2t 7.17 ± 0.02 pH 7.25 ± 0.02 7.36 ± 0.02 7.38 ± 0.01 7.39 ± 0.02 0.072 7.36 ± 0.04t 7.37 ± 0.04t 7.43 ± 0.02 7.38 ± 0.01 7.35 ± 0.02 ± ± pCO2(mmHg) ± 46 ± 3 2 41 36 2 34 1 36 ± 2 40 ± 4 34 ± 3 0.162 30 ± 2 32 ± 1 38±4 185±24 165±25 160±20 136± 19 P02 (mmHg) 123± 15 196 ± 19 223 ± 29 0.072 194 ± 26 188 ± 19 203 ± 23 * Flow was calculated by thermodilution (PF) and a calibrated roller (MANOVA). pump (NPF). PF, pulsatile flow; NPF, nonpulsatile flow; MAP, mean arterial prest p < 0.05 between groups for within group rate differences sure. PF NPF PF NPF PF NPF PF NPF PF NPF PF NPF PF NPF
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PULSATILE FLOW FOR CARDIAC ARREST
during NPF. This small increase in temperature was statistically significant during the latter 45 minutes of reperfusion. At completion of circulatory support, mean temperature was 1 C higher in the NPF group. Hemodilution was controlled by fluid administration. Animals in each group received equal volumes of crystalloid during resuscitation. Furthermore 1 unit fresh whole blood was given to animals after NPF to compensate for any hemolysis. Despite these factors mean hematocrit was somewhat decreased at 1 week after NPF (33.4 ± 1.8%) versus PF (39.4 ± 1.4%), p = 0.058.
(Severe) 5
A
4
A P-0.08
0
3
0
CO)
2
f
A
1
*
A
(Normal) 0
Neurologic Outcome Neurologic function returned significantly more rapidly during the first 24 hours in animals with PF, as evidenced by serial neurologic scores (Fig. 3). Although mean scores remained higher in the PF group than the NPF group, differences were not statistically different after 24 hours. This trend toward persistent neurologic discordance between the two groups was reflected in the neurologic deficit scores at 7 days (Fig. 4). Compared with four animals after NPF, only two animals treated by PF had an abnormal deficit score. Although the median deficit score was greater after NPF (1) than PF (0), the difference did not reach statistical significance. Furthermore there were no significant between-group differences with respect to individual neurologic abnormalities used in determining the neurologic deficit categories. Magnetic Resonance Imaging Magnetic resonance imaging detect ed brain le lesions on T2-weighted scans (Fig. 5). Le: present in 4 of 13 brains scanned. The absence lesions in each group (5/6 with PF vs. 4/7 with NPF), diid not significantly differ (Fig. 6). Lesions numbered fronn two to nine per
sionsswer lesionseinpreacht of
483
-000000 PF
AAA
NPF FIG. 4. Neurologic deficit scores after 7 days of recovery from cardiac arrest. Animals that received NFP showed a trend toward more severe
deficits than those that received PF. Each symbol indicates an individual
dog. (PF versus NPF, categorical analysis).
brain and occurred most commonly in the region of the caudate. Other areas damaged, in order of lesion incidence, included: the hippocampus, pons/midbrain junction, and cerebellum, with isolated lesions in the brainstem and frontal cerebral cortex. In contrast visual inspection of formalin-fixed coronal brain sections found no gross evidence of lesions. Only microscopic examination was able to identify areas of ischemic damage.
Brain Histopathology Hippocampal injury was found in 10 of 15 animals, three of eight with PF versus seven of seven NPF, as determined by grading CAl pyramidal neuronal loss (Fig. and right hippocampal injury scores were aver7). Left however grades were identical in 11 of 15 brains. aged; No areas of microinfarction were noted in any of the hippocampal sections. The extent of damage differed significantly between groups, with more severe injuries occurring in animals treated with NPF
(Fig. 8).
The occurrence of caudate injury also was significantly *_____
18 16
in
l ll l
14
O 12 10
0
0
SO8 0D6 z
4 2
-
PF
-
NPF
*p
Reperfusion After Cardiac Improves Neurologic Outcome
Arrest
MARK P. ANSTADT, M.D., MICHAEL J. STONNINGTON, B.A., MARK TEDDER, M.D., BARBARA J. CRAIN, M.D., PH.D., MICHAEL F. BROTHERS, M.D., DAVID J. HILLEREN, M.D., RICHARD J. RAHIJA, D.V.M., PH.D., J. ALAN MENIUS, JR., M.H.S., and JAMES E. LOWE, M.D.
Cardiopulmonary bypass (CPB) using nonpulsatile flow (NPF) is advocated for refractory cardiac arrest. This study examined cerebral outcome after resuscitation with pulsatile flow (PF) versus NPF. Dogs arrested for 12.5 minute were reperfused with NPF (n = 11) using roller pump CPB or PF (n = 11) using mechanical biventricular cardiac massage. Pump flows were similar between groups; however early arterial pressures were greater during PF versus NPF, *p < 0.05. Circulatory support was weaned at 60 minutes' reperfusion. Neurologic recovery of survivors (n = 16) was significantly better after PF versus NPF, *p = 0.01. The presence of brain lesions on magnetic resonance images did not significantly differ between groups at 7 days. Brains then were removed and regions examined for ischemic changes. Loss of CAl pyramidal neurons was more severe after NPF versus PF, tp = 0.009. Ischemic changes were more frequent after NPF in the caudate nucleus (tP = 0.009) and watershed regions of the cerebral cortex (tp = 0.062), compared with PF. These results demonstrate that PF improves cerebral resuscitation when treating cardiac arrest with mechanical circulatory support (* = MANOVA with repeated measures, t = categorical data analysis).
C
EREBRAL ISCHEMIA RESULTS in the tragic loss of neurologic function if not adequately reversed in a timely fashion. This circumstance occurs in the setting of cardiac arrest, the most common cause of clinical death. Ironically therapies that improve cardiac resuscitation have raised concern about the increased po-
tential for successfully resuscitating severely brain-damaged patients. This dilemma seems inevitable in view of the brain's limited tolerance to ischemia. Brief periods of circulatory arrest (5-10 minutes) can irreversibly damage the brain. 1-5 In contrast the heart can tolerate significantly longer periods of ischemia and subsequently sustain life.6
From the Departments of Surgery, Radiology, and Pathology, Duke University Medical Center, Durham, North Carolina
Therefore preserving cerebral function has become a major challenge for investigators attempting to improve resuscitation from cardiac arrest. Recent attention has been directed toward mechanical circulatory support as an adjunctive treatment for refractory cardiac arrest. Previous experience found these devices effective for recovering cardiac function.7 New advances in cannulation techniques make institution of blood pumps possible during cardiac arrest.8" Although these support systems vary somewhat, most use a standard cardiopulmonary bypass (CPB) circuit with either roller or centrifugal pumps. Both of these devices provide resuscitative support with nonpulsatile flow. Certain characteristics of pulsatile flow, however, may be critical for effective resuscitation. Hydraulic forces generated by pulsatile flow are thought to be required for maintaining the patency of capillary beds.'2-14 In a similar manner, the cerebral microcirculation may depend on "critical opening pressures" during reperfusion. Although cardiac arrest represents the most common clinical condition causing global cerebral ischemia, prior investigations have not compared the effects of pulsatile and nonpulsatile flow on neurologic outcome. The purpose of this study, therefore, was to assess cerebral recovery after cardiac arrest and subsequent resuscitation using either pulsatile or nonpulsatile reperfusion.
Presented at the 11 1th Annual Meeting of the American Surgical Association, April 1 1-13, 1991, Boca Raton, Florida. Address reprint requests to James E. Lowe, MD, Duke University Medical Center, Box 3954, Durham NC 27710. Accepted for publication April 23, 1991.
478
Methods The study protocol was approved by the Animal Care and Use Committee at Duke University Medical Center and performed in accordance to the "Guide for the Care and Use ofLaboratory Animals" prepared by the National Research Council of the NIH (NIH Publication No. 86-
VOL. 214.- NO. 4
PULSATILE FLOW FOR CARDIAC ARREST
479
23, Revised 1985). Twenty-two young (12-16-month-old) conditioned coon hounds (23 ± 1 kg) from the same breeding colony were chosen for this study. Only dogs in good physical health, tested negative for heart worms, and with no evidence of respiratory disease or infection were selected for experimentation. The experiment modeled cardiac arrest by electrically inducing ventricular fibrillation (VF). Mechanical circulatory support was employed for resuscitation after 12.5 minutes of VF. Animals were separated into two groups for either pulsatile or nonpulsatile circulatory support. Nonpulsatile flow (NPF) was generated from a standard cardiopulmonary bypass system (Fig. 1). The system used two venous return cannulae positioned in the superior and inferior vena cavae, a Cobe membrane oxygenator, and a calibrated roller pump that delivered nonpulsatile arterial return through a femoral cannula. Pulsatile flow (PF) was produced by a method termed direct mechanical ventricular actuation (DMVA) (Fig. 2). Direct mechanical ventricular actuation is achieved by a pneumatic
FIG. 2. Schematic of DMVA used to generate PF. Regulated by a pneumatic drive system, an assistor cup actuates the ventricles into systolic and diastolic configurations.
FIG. 1. Schematic
diagram of cardiopulmonary bypass circuit used to
generate NPF. Bicaval cannulation return.
was
used to ensure maximal venous
device that alternately compresses and ditates the ventricular chambers and has been described in detail elsewhere.'5-20 The device can best be understood as a mechanical method of internal cardiac massage. Direct mechanical ventricular actuation application during VF can reliably generate pulsatile flow that is virtually identical to the normal physiologic state. Animal Preparation Dogs were fasted the evening before experimentation with access only to water. An intravenous (I.V.) line was placed for infusion of Lactated Ringer's (LR) solution on the morning of the study. Sodium pentobarbital (I.V.) was administered slowly until the corneal reflex was ablated; this reflex was subsequently monitored until completion of the operative procedure. The animals were intubated, given procaine penicillin G (600,000 units) and dihydrostreptomycin sulfate (750 mg) (I.M.) and then prepared for a lateral thoractomy using aseptic technique. An electronic thermistor probe was positioned in the rectum for measuring core temperature. Respiratory support
480
ANSTADT AND OTHERS
was begun with a pressure-regulated ventilator (InterMed Bear 1) before any operative procedures. All surgery was performed in a standard sterile fashion. Polyethylene catheters positioned in the femoral artery allowed monitoring of arterial blood pressures and blood gases. The opposite femoral artery was exposed for arterial cannulation in those animals selected to receive CPB. Animals receiving DMVA had a Swan-Ganz catheter advanced to the pulmonary artery for calculating blood flow during resuscitation. Lateral thoracotomies were then made through the right or left chest for CPB or DMVA, respectively. After confirming surgical hemostasis, heparin (100 U/kg, I.V.) was administered 5 minutes before cardiac arrest in both groups. Ventricular fibrillation was induced by low-voltage AC current. During cardiac arrest an arterial return cannula was advanced into the femoral artery of dogs receiving CPB. The right atrium was then exposed by pericardiotomy and two venous cannulas positioned with their tips in the proximal superior and inferior vena cavae. After 10 minutes of arrest, the pericardium was widely opened in both groups of animals. At 12 minutes of arrest, an appropriately sized assistor cup was positioned over the hearts of dogs receiving DMVA.
Resuscitation Protocol
Circulatory support was initiated after 12.5 minutes of VF arrest in both groups. Cardiopulmonary bypass flow rates were advanced to 100 ml/kg/min during the first 2 minutes of support. Flow rates were constantly adjusted thereafter, to match maximal venous return. Direct mechanical ventricular actuation compression rates were begun at 60 cycles/minute with actuating forces adjusted to the lowest degree allowing full compression and dilatation of the ventricles. Systolic durations remained at 50% throughout DMVA support. Cycle rates, as well as systolic and diastolic forces, were gradually increased during the first 2 minutes of support to 120/minute, + 125 mm Hg, and -1 10 mm Hg, respectively. Hearts were defibrillated after 15 minutes of reperfusion in both groups. Fibrillation refractory to three successive countershocks (10, 20, and 30 J) was treated by similar defibrillation attempts every 15 minutes. Total circulatory support was continued for a full hour in both groups. Arterial blood pressures, pump flows, serum electrolytes, and arterial blood gases were recorded at 5, 15, 30, 45, and 60 minutes of reperfusion. All animals were given sodium bicarbonate (2 mEq/kg) during the first 5 minutes of reperfusion. Thereafter sodium bicarbonate, potassium chloride, and calcium chloride were administered every 15 minutes as needed to correct arterial blood gases and serum electrolytes. Respiratory settings and membrane oxygenator gas flows were adjusted
Ann. Surg. * October 1991
to keep arterial P02 between 150 and 250 mm Hg and
PCO2 between 30 and 45 mm Hg. After 60 minutes of reperfusion, circulatory support was discontinued. In animals treated by CPB, the entire pump volume (1 L of LR prime) was returned to the circulation over 15 minutes. One unit of fresh whole blood was then given over 45 minutes, followed by LR at 100 mL/hour for 12 hours. Animals supported by DMVA received 1 L LR during support. Thereafter fluid administration was decreased to 100 mL/hour for 12 hours after
removal of the device. Hemostasis was obtained by electrocautery, followed by application of Thrombin spray (Thrombostat 20,000 units) to all surgical incisions. A size 20 French chest tube was placed and the chest was closed. Arterial pressures and blood gases were monitored for 1 hour after support. The catheters then were removed and the groin incisions closed. Ventilatory support was maintained for 3 hours after cessation of circulatory support. Animals then were placed on 50% oxygen and transferred to their cages. Four hours after resuscitation, 10 mL 50% dextrose and 25 mg flunixin (a nonsteroidal antiinflammatory agent) were administered I.V. Animals were extubated 6 hours after resuscitation. Oxygen supplementation was continued for the following 12 hours. Analgesia was insured by administering morphine sulfate (0.5 mg/kg I.V.) immediately after circulatory support with two subsequent doses of 0.25 mg/kg at 6 and 12 hours (after the neurologic assessments, below). Intravenous fluids were decreased to 50 mL/hour 16 hours after resuscitation and discontinued once animals began taking oral fluids. Animals were fed and watered by hand two times per day until daily requirements were taken spontaneously. Tube and hand feeding were continued if voluntary intake was inadequate. Assessment of Neurologic Function
Neurologic function was based on two scoring systems. One rated neurologic recovery during the 7-day postoperative period. The other scoring system measured persistent neurologic deficits at the end ofthe recovery period. Neurologic recovery scores were based on those used by previous investigators.8'21-23 Only the neurologic activities that could be objectively assessed were chosen for monitoring recovery. Scores were assigned to each of 12 categories: corneal reflex (present = 1, absent = 0); respiratory effort (present = 1, absent = 0); pupillary response (normal = 3, sluggish = 2, pinpoint = 1, fixed & dilated = 0); swallow (present = 1, absent = 0); visual perception (gross objects = 2, light only = 1, none = 0); pain response (present = 1, absent = 0); touch response (present = 1, absent = 0); voice response (present = 1, absent = 0); food intake (without assistance = 2, hand
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feeding = 1, none = 0); sternal recumbency (yes = 1, no = 0); standing (yes = 1, no = 1); and walking (without falling = 2, with falls = 1, unable = 0). Neurologic function was assessed at 3, 6, 12, 18, and every 24 hours thereafter by a veterinarian unaware of the animal's resuscitative therapy. A separate neurologic assessment determined persistent neurologic deficits after 7 days of recovery. Animals were assigned one point for unremitting visual impairments, gait defects, ambulatory incapacity, or indifference to food on day 7, as well as for any seizure activity during the entire recovery period. Each animal received a deficit score based on the presence or absence of these five abnormalities (range, 0[normal] to 5[severe deficit]). Magnetic Resonance Imaging After 7 days of recovery, animals were anesthetized with pentobarbital for magnetic resonance imaging (MRI). Brain imaging was performed in a 1.5-Tesla clinical imager (GE Signa) using a quadrature head coil. Coronal T2-weighted 5-mm-thick images were obtained at 7.5mm intervals. Sequence parameters for imaging included: a TR of 2800 msec, TE of 30 and 80 msec; number of excitations = 0.75; and a field view of 16 cm. Comparisons were made between the two echoes to separate normal areas of high signal intensity (i.e., cerebrospinal fluid) from brain lesions. All images were interpreted by a radiologist blinded to the animals' treatment protocol. Brain Histopathology The day after MRI assessments (8 days after resuscitation), animals were killed and their brains placed in 10% formalin for at least 2 weeks. The brains then were sectioned coronally and inspected for gross lesions. Regions most frequently injured on MRI images were examined histopathologically. Representative blocks of the hippocampus, caudate nucleus, cerebellum, and watershed areas ofthe frontal and parietal cortex were taken from each hemisphere, placed in 20% sucrose/10% formalin, and cut on a freezing microtome. Representative sections were mounted on gel-coated slides and stained for neurons with cresyl violet. Hippocampal damage was graded by scoring pyramidal cell loss within the CAl region. Those hippocampi with no identifiable injury were assigned a score of 1 (normal). Those with complete absence of CA pyramidal cells were assigned a score of 5 (severe). Those hippocampi that did not fall into these two extremes were categorized according to their overall relative appearances: scant pyramidal cell loss = 2, approximately 50% pyramidal cell loss = 3, and only scattered pyramidal cells remaining = 4. Scores from
the left and right hippocampus were averaged to provide a single score for each animal. The caudate nucleus was examined for evidence of ischemic injury as evidenced by microinfarction or diffuse neuronal loss. Abnormal findings were categorized into three groups: absent, unilateral, or bilateral. The frontal and parietal watershed regions of brains were examined for evidence of histologic damage (microinfarction or diffuse cell loss). Results then were categorized for the two groups: injury found in one or more watershed regions versus absence of ischemic changes in all examined regions. The cerebellum was examined for microinfarction or selective Purkinje cell loss. All brain regions of surviving animals were histologically evaluated. One tissue block was unavailable for each region. Therefore each region was examined in 15 rather than in 16 cases. Tissue examinations and gradings were performed by a neuropathologist who was blinded to the experimental groups.
Statistical Analysis Univariate analysis and between-group comparisons (PF vs. NPF) were performed using SAS statistical software (SAS Institute Inc., Cary, NC). Pre-arrest parameters known to affect neurologic outcome (temperature, pH, PCO2, P02, and anesthetic dosage) were compared between groups using a nonpaired Student's t test. Variables known to affect neurologic outcome during resuscitative support (blood flow, mean arterial pressure, pulse pressure, temperature, pH, PCO2, and P02) also were tested for between-group differences. Group (time by group effect) and within-group (within-group effect) effects were determined using multivariate analysis of variance with repeated measures over time (MANOVA). In addition overall between-group differences were tested using area under the curve analysis (group effect). Experimental measures of cerebral recovery then were assessed. Serial neurologic scores were compared between groups using MANOVA. All measures of outcome that were categorized (MRI lesions, neurologic deficit scores, histologic grading, and occurrence of regional ischemic damage) were assessed by comparing row mean scores using a chi-squared statistic. Comparisons were considered statistically significant at alpha levels less than 0.05. Results
Survival Sixteen animals (73%) survived for the full 1-week observation period. All deaths occurred during the initial 18 hours of recovery. There was no significant difference in
ANSTADT AND OTHERS
482
TABLE 1. Parameters Measured Immediately Before Cardiac Arrest (mean ± SEM)
Parameter Temperature (°C)* pH pCO2(mmHg)
P02(mmHg) Pentobarbital (mg/kg)
PF
NPF
p
35.9 ± 0.2 7.41 ± 0.02 31 ± 2 247 ± 19 31.2 ± 1.1
35.4 ± 0.2 7.43 ± 0.01 28 ± 1 208 ± 18 33.0 ± 1.7
0.09 0.50 0.41 0.18 0.30
* Temperatures tended to be 0.5 C lower in animals that received NPF versus PF. (PF versus NPF, nonpaired Student's t test). NPF, nonpulsatile flow; PF, pulsatile flow.
mortality after PF (2/1 1) versus NPF (4/1 1), p = 0.31. One animal (PF group) succumbed to respiratory failure because of underlying pulmonary disease. The remaining deaths were of undetermined origin. Necropsies found no evidence of pneumothorax or hemothorax, pulmonary edema, or internal bleeding. Only results from the 16 surviving animals were analyzed in this study.
Control and Resuscitation Conditions Prearrest variables that may affect neurologic outcome were kept similar in both groups (Table 1). Both groups of animals had mild hypothermia created by open thoracotomy. Core temperatures were slightly lower in animals before NPF compared with PF, but this difference was not statistically significant. Blood gas values exhibited little between-group disparity and mean sodium pentobarbital doses differed by less that 2 mg/kg. All animals were normotensive with a stable sinus rhythm before induction of ventricular fibrillation. Variables monitored during the period of resuscitative circulatory support are summarized in Table 2. Blood flow was calculated by thermodilution and a calibrated
Ann.
Surg. October 1991 -
roller pump during PF and NPF, respectively. Flows were entirely dependent on the rate of maximal venous return in both groups. Calculated flows were not significantly different between groups. Pilot studies and prior investigations using radiolabeled microspheres under similar experimental conditions found roller pump values underestimate actual flow by approximately 25% compared with thermodilution. Therefore actual flows were more closely matched between groups than measured values suggest. Accordingly calculated flows were compared to illustrate venous return trends for each group. Despite similar flows PF initially generated greater mean arterial pressures (MAP) than did NPF. Resuscitative pressures significantly differed at 5 and 15 minutes of reperfusion. However MAP disparities diminished throughout reperfusion and did not significantly differ between groups after 15 minutes. Pulse pressures remained constant during PF and contributed to the relatively early elevations in MAP. Nonpulsatile flow exhibited small pulsations due to roller pump oscillations, which were periodically intensified by effective cardiac contractions. However PF pulse pressures consistently exceeded those during NPF by fivefold to tenfold. Arterial blood gases were similar between groups, with the exception of pH values. Reperfusion with NPF resulted in rapid normalization of arterial pH, whereas a more gradual return occurred during PF. Thus mean pH values were significantly lower during the initial 15 minutes of PF compared with NPF. Thereafter pH remained similar between groups. Between-group differences in P02 were inconsequential, as they remained between 100 and 250 mm Hg in both groups. Core temperatures also behaved differently between groups. Although mean values were identical at 5 minutes' reperfusion, core temperatures subsequently increased
TABLE 2. Reperfusion Parameters During 1 Hour of Resuscitative Support with Either PF or NPF Flow (mean ± SEM)
ReperfusionTime (min) Parameter Flow (ml/kg/min)*
Method
5
15
30
45
60
p
89 ± 11 148 ± 15 159 ± 20 161 ± 18 146 ± 15 0.059 110±4 116±7 108±5 102±2 100±3 ± MAP (mmHg) ± 125 6 109 3 101 ± 6 100 ± 7 99 ± 6 0.033 64 ± 7t 74 ± 8 69 ± 7t 79 ± 11 90 ± 11 Pulse pressure (mmHg) 75 ± 2 83 ± 8 79 ± 11 87 ± 8 79 ± 11 0018 8±3 9±3 14 ± 4 10 ± 2 18 ± 10 ± ± ± ± Temperature (C) ± 35.5 0.2 35.5 0.2 35.5 0.02 35.3 0.3 35.4 0.3 0.022 35.3 ± 0.1 36.0 ± O.lt 36.4 ± 0.3t 36.4 ± 0.2t 36.5 ± 0.2t 7.17 ± 0.02 pH 7.25 ± 0.02 7.36 ± 0.02 7.38 ± 0.01 7.39 ± 0.02 0.072 7.36 ± 0.04t 7.37 ± 0.04t 7.43 ± 0.02 7.38 ± 0.01 7.35 ± 0.02 ± ± pCO2(mmHg) ± 46 ± 3 2 41 36 2 34 1 36 ± 2 40 ± 4 34 ± 3 0.162 30 ± 2 32 ± 1 38±4 185±24 165±25 160±20 136± 19 P02 (mmHg) 123± 15 196 ± 19 223 ± 29 0.072 194 ± 26 188 ± 19 203 ± 23 * Flow was calculated by thermodilution (PF) and a calibrated roller (MANOVA). pump (NPF). PF, pulsatile flow; NPF, nonpulsatile flow; MAP, mean arterial prest p < 0.05 between groups for within group rate differences sure. PF NPF PF NPF PF NPF PF NPF PF NPF PF NPF PF NPF
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during NPF. This small increase in temperature was statistically significant during the latter 45 minutes of reperfusion. At completion of circulatory support, mean temperature was 1 C higher in the NPF group. Hemodilution was controlled by fluid administration. Animals in each group received equal volumes of crystalloid during resuscitation. Furthermore 1 unit fresh whole blood was given to animals after NPF to compensate for any hemolysis. Despite these factors mean hematocrit was somewhat decreased at 1 week after NPF (33.4 ± 1.8%) versus PF (39.4 ± 1.4%), p = 0.058.
(Severe) 5
A
4
A P-0.08
0
3
0
CO)
2
f
A
1
*
A
(Normal) 0
Neurologic Outcome Neurologic function returned significantly more rapidly during the first 24 hours in animals with PF, as evidenced by serial neurologic scores (Fig. 3). Although mean scores remained higher in the PF group than the NPF group, differences were not statistically different after 24 hours. This trend toward persistent neurologic discordance between the two groups was reflected in the neurologic deficit scores at 7 days (Fig. 4). Compared with four animals after NPF, only two animals treated by PF had an abnormal deficit score. Although the median deficit score was greater after NPF (1) than PF (0), the difference did not reach statistical significance. Furthermore there were no significant between-group differences with respect to individual neurologic abnormalities used in determining the neurologic deficit categories. Magnetic Resonance Imaging Magnetic resonance imaging detect ed brain le lesions on T2-weighted scans (Fig. 5). Le: present in 4 of 13 brains scanned. The absence lesions in each group (5/6 with PF vs. 4/7 with NPF), diid not significantly differ (Fig. 6). Lesions numbered fronn two to nine per
sionsswer lesionseinpreacht of
483
-000000 PF
AAA
NPF FIG. 4. Neurologic deficit scores after 7 days of recovery from cardiac arrest. Animals that received NFP showed a trend toward more severe
deficits than those that received PF. Each symbol indicates an individual
dog. (PF versus NPF, categorical analysis).
brain and occurred most commonly in the region of the caudate. Other areas damaged, in order of lesion incidence, included: the hippocampus, pons/midbrain junction, and cerebellum, with isolated lesions in the brainstem and frontal cerebral cortex. In contrast visual inspection of formalin-fixed coronal brain sections found no gross evidence of lesions. Only microscopic examination was able to identify areas of ischemic damage.
Brain Histopathology Hippocampal injury was found in 10 of 15 animals, three of eight with PF versus seven of seven NPF, as determined by grading CAl pyramidal neuronal loss (Fig. and right hippocampal injury scores were aver7). Left however grades were identical in 11 of 15 brains. aged; No areas of microinfarction were noted in any of the hippocampal sections. The extent of damage differed significantly between groups, with more severe injuries occurring in animals treated with NPF
(Fig. 8).
The occurrence of caudate injury also was significantly *_____
18 16
in
l ll l
14
O 12 10
0
0
SO8 0D6 z
4 2
-
PF
-
NPF
*p
