Nitrous Oxide Impairs Functional Recovery of Stunned Myocardium in Barbiturate-Anesthetized, Acutely Instrumented Dogs Daniel Siker, MD, Paul s. Pagel, MD, Lorie R. Pelc, PhD, John P. Kampine, MD, PhD, William T. Schmeling, MD, PhD, and David c. Wadtier, MD, PhD Departments of Anesthesiology, Pharmacology, and Medicine, Medical College of Wisconsin; Zablocki Veterans Administration Medical Center; and Children's Hospital of Wisconsin, Milwaukee, Wisconsin
The purpose of this investigation was to characterize the effects of nitrous oxide or nitrogen (70%) on systemic and regional hemodynamics and myocardial tissue perfusion after a brief coronary artery occlusion (15 min) and reperfusion (3 h). Two groups of experiments (14 experiments total) were completed with 24 open-chest, barbiturate-anesthetized dogs. Coronary collateral blood flow was diverted from the ischemic zone during coronary artery occlusion to eliminate a source of variability in degree of ischemia produced by differences in degrees of collateral blood flow among animals. Seven of 16 dogs treated with nitrous oxide and 7 of 8 dogs treated with nitrogen survived coronary occlusion and reperfusion ( P < 0.05). Coronary artery occlusion produced paradoxical systolic bulging in the ischemic zone in both groups of experiments. After reperfusion, segment shortening gradually returned toward control levels but remained depressed from the preocclusion state after 3 h in the nitrogen-treated control group. Similar results were observed after
R
eperfusion after brief periods of coronary artery occlusion is associated with delayed recovery of contractile function despite return of blood flow to normal levels (1). This reversible postischemic myocardial dysfunction, termed "stunned' myocardium (2), may be prolonged but occurs without tissue necrosis (3,4). This phenomenon appears to play an important role in the natural history of ischemic heart disease because spontaneous reperfusion of ischemic myocardium after thrombosis or coronary vasospasm is known to occur frequently in This work was supported by U.S. Public Health Service Grants HL 36144 and HL 32911, Anesthesiology Research Training Grant GM 08377, and VA Medical Research Funds. Accepted for publication April 30, 1992. Address correspondence to Dr. Warltier, Medical College of Wisconsin, MFRC, Room A1000, 8701 West Watertown Plank Road, Milwaukee, WI 53226. 01992 by the International Anesthesia Research Society 0003-2999/92./$5.00
reperfusion in the nitrous oxide group; however, segment function in the ischemic region was significantly ( P < 0.05) depressed throughout the 3-h reperfusion period compared with the control group. Transmural coronary collateral blood flow during occlusion was not significantly different (P > 0.05) between groups, indicating that differences in recovery of contractile function observed between groups could not be attributed to differences in myocardial oxygen supply. In addition, the similarity in systemic hemodynamics between the nitrous oxide and control groups indirectly suggests that differences in recovery of function could not be attributed to differences in myocardial oxygen demand. The results indicate that 70% nitrous oxide produces greater mortality after coronary artery occlusion and reperfusion and reduces functional recovery of postischemic, reperfused myocardium compared with 70% nitrogen in open-chest, acutely instrumented dogs. (Anesth Analg 1992;75:53943)
patients with coronary artery disease. In addition, patients with acute myocardial ischemia often undergo coronary artery reperfusion through interventions, such as thrombolytic therapy, angioplasty, and emergency bypass surgery, in attempts to salvage myocardium at risk. The pathogenesis of stunned myocardium remains unclear despite extensive study (3). Although tissue reperfusion is necessary to preserve myocardial integrity, several ischemia-induced cellular abnormalities have been implicated in the subsequent development of reperfusion injury (1,3-7). Intracellular calcium overload (8), depletion of high-energy phosphates (9-ll), production of oxygen-derived free radicals (12,13), and uncouphg of myocyte excitation and contraction (14) have all been suggested as contributing etiologies for the contractile dysfunction observed in stunned myocardium. Pharmacologic Anesth Analg 1992;75:53948
539
540
SIKER ET AL. NITROUS OXIDE AND STUNNED MYOCARDIUM
therapy has focused on modulation of these diverse pathophysiologic abnormalities and a variety of treatment modalities, including calcium channel blockers (15-18), vasodilators (15,19), P-adrenergic antagonists (20), specific bradycardic agents (21), and superoxide dismutase and catalase (22), have all been shown to enhance recovery. The effects of anesthetics on contractile function recovery in postischemic, reperfused myocardium have been examined only recently. Volatile anesthetics have been shown to improve functional recovery of stunned myocardium when these agents are administered in vivo (23) and in vitro (24) before and during, but not after (25), brief periods of myocardial ischemia. The effects of nitrous oxide on stunned myocardium have not been previously described. Use of this anesthetic gas in patients with ischemic heart disease remains controversial. Studies in experimental models of coronary artery constriction have suggested that nitrous oxide may contribute to development of myocardial ischemia (26-31). Most clinical investigations have been unable to confirm these findings in patients with coronary artery disease (32-38). Accordingly, systemic and coronary hemodynamics, regional contractile function, and intramyocardial blood flow, as evaluated by the radioactive microsphere technique, were recorded during brief coronary artery occlusion (15 min) and 3 h of reperfusion in control (70% nitrogen) and nitrous oxide (70%)groups. Because dogs have varying degrees of coronary collateral circulation, retrograde coronary flow was diverted during occlusion to eliminate the variability of endogenous collateral blood flow among animals. The present investigation was performed to characterize actions of nitrous oxide on recovery of function of stunned myocardium in open-chest, barbiturate-anesthetized dogs.
Methods Experimental Preparation All experimental procedures and protocols used in this investigation were reviewed and approved by the Animal Care Committee of the Medical College of Wisconsin. Furthermore, all conformed to the Guiding Principles in the Care and Use of Animals of the American Physiologic Society and were in accordance with the Guide for the Care and Use of Laboratory Animals (DHEW [DHHS] publication No. [NIH] 85-23, revised 1985). Twenty-four adult male and female mongrel dogs, weighing 25-30 kg, were fasted overnight and anesthetized with sodium pentobarbital (25 mgkg) and sodium barbital (200 mg/kg). After tracheal intubation, ventilation was controlled (Ohio Medical Prod-
ANESTH ANALG 1992;75:53948
ucts, Arc0 Inc., Madison, Wis.) with an oxygen (30%)nitrogen (70%) mixture (1.0 L/rnin). Respiratory rate and tidal volume were adjusted to maintain arterial blood gases within normal physiologic limits (pH 7.357.45; Pco, 30-35 mm Hg; Po, 85-100 mm Hg). Temperature was monitored and maintained at 37” & 1°C (mean ? SE) by a humidified circuit, warming light, and heating pad connected to a servomechanical controller. Dual-pressure transducer-tipped catheters (SPR277, Millar, Houston, Tex.) were placed into the left ventricle and ascending aorta through the left carotid artery for measurement of continuous left ventricular and arterial pressures, respectively. Peak positive dP/dt, an index of global left ventricular contractility, was obtained by electronic differentiation of the left ventricular pressure pulse. A triangular wave signal of known slope was used to calibrate the differentiator. The right femoral vein and left femoral artery were catheterized for fluid administration and for withdrawal of reference arterial blood samples, respectively. A thoracotomy was performed at the left fifth intercostal space. The lung was gently retracted, and the heart was suspended in a pericardial cradle. A catheter was placed in the left atrium through the atrial appendage for injection of radioactive microspheres. The right carotid artery was isolated and cannulated. A 1.0-1.5-cm segment of the left anterior descending coronary artery (LAD)was isolated distal to the first diagonal branch and cannulated with a large-bore, metal-tipped catheter. Heparin (300 U/kg) was administered intravenously. Perfusion to the distal LAD was reestablished within 2 min after cannulation via a silastic shunt from the right carotid artery. A cannulating electromagnetic flow probe (Statham SP7515, Gould Instruments, Cleveland, Ohio) for measurement of coronary blood flow was placed proximal to a side arm in the shunt with a three-way stopcock. The stopcock was suspended at the level of the heart and used for passive diversion of coronary collateral blood flow during coronary artery occlusion. A steel clamp was positioned around the silastic shunt immediately distal to the flow probe for production of coronary artery occlusion. The site of LAD cannulation was chosen so that approximately equal regions of the left ventricle were subjected to ischemia in all animals. The mass of the left ventricle subjected to occlusion and reperfusion was 27.7% t 1.0% (mean -+ SE) of the total left ventricular mass in all dogs. Myocardial contractile function (percent segment shortening [%SS]) was measured in the ischemic (LAD) and normal (left circumflex coronary artery [LCCA]) regions with ultrasonic segment-length transducers (5 MHz) implanted (10-15 mm apart and 7-9 mm deep) into the subendocardium of each region parallel to fiber orientation. Segment-length
ANESTH ANALG 1992;75:53948
signals were monitored and driven by an ultrasonic amplifier (Hartley, Houston, Tex.). End-systolic segment length (ESL) was determined at maximal negative dP/dt, and end-diastolic segment length (EDL) was determined at the onset of left ventricular isovolumetric contraction. The lengths were normalized according to the method described by Theroux et al. (39). Percent segment shortening was calculated using the following equation: %SS = (EDL - ESL) x 100/EDL. All hemodynamic data were continuously recorded on a polygraph (model 7, Grass Instrument Co., Quincy, Mass.). At the end of each experiment, the heart was electrically fibrillated, immediately removed, and fixed in 10% formaldehyde for 24-48 h. India ink (5 mL) was injected distally through the LAD cannula to darken the area of myocardium subjected to coronary occlusion and reperfusion. The left ventricle and septum was separated from the remainder of the heart, divided into stained and unstained regions, and weighed to determine the mass of the left ventricle subjected to ischemic insult.
Regional Tissue Perfusion Carbonized plastic microspheres (15 2 2 pm in diameter) (New England Nuclear, Boston, Mass.) labeled with 14'Ce, lo3Ru,51Cr,or 95Nbwere used to measure regional tissue perfusion (40). Immediately before injection, the sphere suspension was ultrasonicated (E/MC Corporation, model 450) for 15 min and agitated in a vortex mixer (Cole-Parmer, model 4722) for 5 min. The microsphere injection consisted of approximately 2-3 x lo6 spheres injected into the left atrial chamber as a bolus dose over 10 s and flushed in with 10 mL of warm (37°C) sterile saline solution. A few seconds before each microsphere injection, a timed collection of reference arterial blood from the femoral arterial catheter was initiated and maintained at a constant rate of 7 mL/min for 3 min (precalibrated Harvard infusiodwithdrawal pump, model 1941). At the conclusion of each experiment, transmural myocardial tissue samples were selected for mapping of regional myocardial blood flow. The samples were subdivided into subepicardial, midmyocardial, and subendocardial layers of approximately equal thickness. Samples were weighed, placed in scintillation vials, and the activity of each isotope was determined. The activity of each isotope in the reference blood samples was also determined. The radioactivity in the tissue and reference blood samples was analyzed with a gamma counter (Minaxi 5000, Packard, Downers Grove, Ill.). Myocardial blood flow (Q,) (mL.min-l.g-l) was calculated from the following equation: Q, = Q;C,/C,, where Q, is the rate of withdrawal of the reference blood flow sample (mL/
SIKER ET AL. NITROUS OXIDE AND STUNNED MYOCARDIUM
541
min), C, is the activity (counts per minute [cpm]) of the reference blood flow sample, and C, is the activity (cpm/g) of the tissue sample.
Experimental Protocol Dogs were assigned to receive either nitrous oxide (70%)or nitrogen (70%) in combination with oxygen (30%)before and during LAD occlusion in a random fashion in separate experimental groups. After surgery was completed, the preparation was allowed to stabilize for 30 min during ventilation with room air enriched with oxygen (30%). Control hemodynamics and regional contractile function were then recorded, and a radioactive microsphere injection was completed. Hemodynamics and regional myocardial blood flow were again measured after a 30-min period of equilibration during administration of nitrous oxide (70%) or nitrogen (70%) in oxygen (30%). End-tidal gases were continuously measured by mass spectrometry (Advantage 2000, Marquette Electronics, St. Louis, Mo.). The LAD flow was then interrupted for 15 min by the mechanical occluder in both groups. During LAD occlusion, retrograde LAD coronary flow from the cannulas was allowed to drain by gravity to reduce collateral perfusion within the ischemic zone. After 12 min of LAD occlusion, hemodynamics and segment function were measured, and another set of radioactive microspheres was administered. The occlusion was removed after 15 min, and reperfusion was allowed to occur. All dogs (in both control [nitrogen] and nitrous oxide groups) received lidocaine (1 mg/kg IV) and procainamide (5 mg/kg IV) immediately before reperfusion. Nitrous oxide (70%) or nitrogen (70%) was continued throughout the occlusion and 15 min into the reperfusion period in separate groups. Nitrous oxide was then discontinued, and room air enriched with oxygen (30%)was reinstituted. Hemodynamics and regional myocardial function were recorded at 5, 15, 30, 60, 120, and 180 min during the reperfusion period. A final set of radioactive microspheres was administered at the completion of the 3-h reperfusion period. Thus, systemic hemodynamics, regional myocardial function, and myocardial tissue perfusion before and during acute coronary artery occlusion and during 3 h of reperfusion were studied in control (70% nitrogen) and nitrous oxide (70%)-exposed groups. Twentyfour dogs were used to provide seven experiments in each group.
Statistical Analysis Statistical analysis of data within and between groups before and during coronary artery occlusion and reperfusion was performed using analysis of variance
542
ANESTH ANALG 1992;75:53948
SIKER ET AL. NITROUS OXIDE AND STUNNED MYOCARDIUM
Table 1. Hemodynamics During Coronary Artery Occlusion and Reperfusion in Control Experiments in Dogs Preocclusion control
HR (beatdmin) M A P (mm Hg) LVSP (mm Hg) LVEDP (mm Hg) RPP (beats.min mm Hg x lo3) dP/dt (mm Hg/s) LAD MCBF (mL/min) LAD SS (%) LCCA SS (%)
Coronary occlusion
Reperfusion (min)
5
121 5 6 105 t 10 125 2 10 4tl 14.82 2.0
110% 5" 107 t 4" 8426" 82 t 6" 105 2 5" 99 2 6" 7 2 2" 7 2 1" 10.5 t 0.4 11.2 t 1.0"
2040 2 70 35 t 5 19.5 2 3.3 10.4 t 2.7
1660 2 57 t -3.3 t 1.5" 9.4 2 14.5 2 3.2 12.0 2 1690 2 110" O t o "
15
30
111 t 4"
114 5 89 5 7 107 2 7" 722 12.32 1.2"
87 2 6 107 2 5" 7 t 2" 11.6f 0.9"
*
60
120
180
114 2 5 92 11 116 +. 9 622 12.6 2 1.8
123 2 6 99 2 10 124 2 9 622 14.22 1.8
121 2 5 102 f 8 122 f 9 7f2 14.1 f 12.7
*
130" 1750 2 1W 1800 2 120" 1860 t 120 27 t 3 29 t 3 36 2 6 7" 2.6" 8.1 2 2.4 8.9 2 2.9" 10.1 2 2.7' 8.9 2 2.4 9.5 2 2.7 3.3 10.5 t 3 3
1840 2 29 2 9.8 2 8.4 2
140 1820 2 120 29 f 5 4 1.9" 7.9 t- 1.9" 9.3 2 2.3 1.9
HR, heart rate; MAP, mean arterial pressure; LVSP, left ventricular systolic pressure; LVEDP, left ventricular end-diastolic pressure; RPP, rate-pressure product; LAD, left anterior descending coronary artery, MCBF, mean coronary blood flow; SS, segment shortening; LCCA, left circumflex coronary artery Data are mean 3- SE ( n = 7). "Significantly (P < 0.05) different from preocclusion control values
(ANOVA) with repeated measures, followed by application of Bonferroni's modification of the t-test. Changes from control within and between groups were considered statistically significant when the probability (P) value was
The purpose of this investigation was to characterize the effects of nitrous oxide or nitrogen (70%) on systemic and regional hemodynamics and myocardial tissue perfusion after a brief coronary artery occlusion (15 min) and reperfusion (3 h). Two groups of experiments (14 experiments total) were completed with 24 open-chest, barbiturate-anesthetized dogs. Coronary collateral blood flow was diverted from the ischemic zone during coronary artery occlusion to eliminate a source of variability in degree of ischemia produced by differences in degrees of collateral blood flow among animals. Seven of 16 dogs treated with nitrous oxide and 7 of 8 dogs treated with nitrogen survived coronary occlusion and reperfusion ( P < 0.05). Coronary artery occlusion produced paradoxical systolic bulging in the ischemic zone in both groups of experiments. After reperfusion, segment shortening gradually returned toward control levels but remained depressed from the preocclusion state after 3 h in the nitrogen-treated control group. Similar results were observed after
R
eperfusion after brief periods of coronary artery occlusion is associated with delayed recovery of contractile function despite return of blood flow to normal levels (1). This reversible postischemic myocardial dysfunction, termed "stunned' myocardium (2), may be prolonged but occurs without tissue necrosis (3,4). This phenomenon appears to play an important role in the natural history of ischemic heart disease because spontaneous reperfusion of ischemic myocardium after thrombosis or coronary vasospasm is known to occur frequently in This work was supported by U.S. Public Health Service Grants HL 36144 and HL 32911, Anesthesiology Research Training Grant GM 08377, and VA Medical Research Funds. Accepted for publication April 30, 1992. Address correspondence to Dr. Warltier, Medical College of Wisconsin, MFRC, Room A1000, 8701 West Watertown Plank Road, Milwaukee, WI 53226. 01992 by the International Anesthesia Research Society 0003-2999/92./$5.00
reperfusion in the nitrous oxide group; however, segment function in the ischemic region was significantly ( P < 0.05) depressed throughout the 3-h reperfusion period compared with the control group. Transmural coronary collateral blood flow during occlusion was not significantly different (P > 0.05) between groups, indicating that differences in recovery of contractile function observed between groups could not be attributed to differences in myocardial oxygen supply. In addition, the similarity in systemic hemodynamics between the nitrous oxide and control groups indirectly suggests that differences in recovery of function could not be attributed to differences in myocardial oxygen demand. The results indicate that 70% nitrous oxide produces greater mortality after coronary artery occlusion and reperfusion and reduces functional recovery of postischemic, reperfused myocardium compared with 70% nitrogen in open-chest, acutely instrumented dogs. (Anesth Analg 1992;75:53943)
patients with coronary artery disease. In addition, patients with acute myocardial ischemia often undergo coronary artery reperfusion through interventions, such as thrombolytic therapy, angioplasty, and emergency bypass surgery, in attempts to salvage myocardium at risk. The pathogenesis of stunned myocardium remains unclear despite extensive study (3). Although tissue reperfusion is necessary to preserve myocardial integrity, several ischemia-induced cellular abnormalities have been implicated in the subsequent development of reperfusion injury (1,3-7). Intracellular calcium overload (8), depletion of high-energy phosphates (9-ll), production of oxygen-derived free radicals (12,13), and uncouphg of myocyte excitation and contraction (14) have all been suggested as contributing etiologies for the contractile dysfunction observed in stunned myocardium. Pharmacologic Anesth Analg 1992;75:53948
539
540
SIKER ET AL. NITROUS OXIDE AND STUNNED MYOCARDIUM
therapy has focused on modulation of these diverse pathophysiologic abnormalities and a variety of treatment modalities, including calcium channel blockers (15-18), vasodilators (15,19), P-adrenergic antagonists (20), specific bradycardic agents (21), and superoxide dismutase and catalase (22), have all been shown to enhance recovery. The effects of anesthetics on contractile function recovery in postischemic, reperfused myocardium have been examined only recently. Volatile anesthetics have been shown to improve functional recovery of stunned myocardium when these agents are administered in vivo (23) and in vitro (24) before and during, but not after (25), brief periods of myocardial ischemia. The effects of nitrous oxide on stunned myocardium have not been previously described. Use of this anesthetic gas in patients with ischemic heart disease remains controversial. Studies in experimental models of coronary artery constriction have suggested that nitrous oxide may contribute to development of myocardial ischemia (26-31). Most clinical investigations have been unable to confirm these findings in patients with coronary artery disease (32-38). Accordingly, systemic and coronary hemodynamics, regional contractile function, and intramyocardial blood flow, as evaluated by the radioactive microsphere technique, were recorded during brief coronary artery occlusion (15 min) and 3 h of reperfusion in control (70% nitrogen) and nitrous oxide (70%)groups. Because dogs have varying degrees of coronary collateral circulation, retrograde coronary flow was diverted during occlusion to eliminate the variability of endogenous collateral blood flow among animals. The present investigation was performed to characterize actions of nitrous oxide on recovery of function of stunned myocardium in open-chest, barbiturate-anesthetized dogs.
Methods Experimental Preparation All experimental procedures and protocols used in this investigation were reviewed and approved by the Animal Care Committee of the Medical College of Wisconsin. Furthermore, all conformed to the Guiding Principles in the Care and Use of Animals of the American Physiologic Society and were in accordance with the Guide for the Care and Use of Laboratory Animals (DHEW [DHHS] publication No. [NIH] 85-23, revised 1985). Twenty-four adult male and female mongrel dogs, weighing 25-30 kg, were fasted overnight and anesthetized with sodium pentobarbital (25 mgkg) and sodium barbital (200 mg/kg). After tracheal intubation, ventilation was controlled (Ohio Medical Prod-
ANESTH ANALG 1992;75:53948
ucts, Arc0 Inc., Madison, Wis.) with an oxygen (30%)nitrogen (70%) mixture (1.0 L/rnin). Respiratory rate and tidal volume were adjusted to maintain arterial blood gases within normal physiologic limits (pH 7.357.45; Pco, 30-35 mm Hg; Po, 85-100 mm Hg). Temperature was monitored and maintained at 37” & 1°C (mean ? SE) by a humidified circuit, warming light, and heating pad connected to a servomechanical controller. Dual-pressure transducer-tipped catheters (SPR277, Millar, Houston, Tex.) were placed into the left ventricle and ascending aorta through the left carotid artery for measurement of continuous left ventricular and arterial pressures, respectively. Peak positive dP/dt, an index of global left ventricular contractility, was obtained by electronic differentiation of the left ventricular pressure pulse. A triangular wave signal of known slope was used to calibrate the differentiator. The right femoral vein and left femoral artery were catheterized for fluid administration and for withdrawal of reference arterial blood samples, respectively. A thoracotomy was performed at the left fifth intercostal space. The lung was gently retracted, and the heart was suspended in a pericardial cradle. A catheter was placed in the left atrium through the atrial appendage for injection of radioactive microspheres. The right carotid artery was isolated and cannulated. A 1.0-1.5-cm segment of the left anterior descending coronary artery (LAD)was isolated distal to the first diagonal branch and cannulated with a large-bore, metal-tipped catheter. Heparin (300 U/kg) was administered intravenously. Perfusion to the distal LAD was reestablished within 2 min after cannulation via a silastic shunt from the right carotid artery. A cannulating electromagnetic flow probe (Statham SP7515, Gould Instruments, Cleveland, Ohio) for measurement of coronary blood flow was placed proximal to a side arm in the shunt with a three-way stopcock. The stopcock was suspended at the level of the heart and used for passive diversion of coronary collateral blood flow during coronary artery occlusion. A steel clamp was positioned around the silastic shunt immediately distal to the flow probe for production of coronary artery occlusion. The site of LAD cannulation was chosen so that approximately equal regions of the left ventricle were subjected to ischemia in all animals. The mass of the left ventricle subjected to occlusion and reperfusion was 27.7% t 1.0% (mean -+ SE) of the total left ventricular mass in all dogs. Myocardial contractile function (percent segment shortening [%SS]) was measured in the ischemic (LAD) and normal (left circumflex coronary artery [LCCA]) regions with ultrasonic segment-length transducers (5 MHz) implanted (10-15 mm apart and 7-9 mm deep) into the subendocardium of each region parallel to fiber orientation. Segment-length
ANESTH ANALG 1992;75:53948
signals were monitored and driven by an ultrasonic amplifier (Hartley, Houston, Tex.). End-systolic segment length (ESL) was determined at maximal negative dP/dt, and end-diastolic segment length (EDL) was determined at the onset of left ventricular isovolumetric contraction. The lengths were normalized according to the method described by Theroux et al. (39). Percent segment shortening was calculated using the following equation: %SS = (EDL - ESL) x 100/EDL. All hemodynamic data were continuously recorded on a polygraph (model 7, Grass Instrument Co., Quincy, Mass.). At the end of each experiment, the heart was electrically fibrillated, immediately removed, and fixed in 10% formaldehyde for 24-48 h. India ink (5 mL) was injected distally through the LAD cannula to darken the area of myocardium subjected to coronary occlusion and reperfusion. The left ventricle and septum was separated from the remainder of the heart, divided into stained and unstained regions, and weighed to determine the mass of the left ventricle subjected to ischemic insult.
Regional Tissue Perfusion Carbonized plastic microspheres (15 2 2 pm in diameter) (New England Nuclear, Boston, Mass.) labeled with 14'Ce, lo3Ru,51Cr,or 95Nbwere used to measure regional tissue perfusion (40). Immediately before injection, the sphere suspension was ultrasonicated (E/MC Corporation, model 450) for 15 min and agitated in a vortex mixer (Cole-Parmer, model 4722) for 5 min. The microsphere injection consisted of approximately 2-3 x lo6 spheres injected into the left atrial chamber as a bolus dose over 10 s and flushed in with 10 mL of warm (37°C) sterile saline solution. A few seconds before each microsphere injection, a timed collection of reference arterial blood from the femoral arterial catheter was initiated and maintained at a constant rate of 7 mL/min for 3 min (precalibrated Harvard infusiodwithdrawal pump, model 1941). At the conclusion of each experiment, transmural myocardial tissue samples were selected for mapping of regional myocardial blood flow. The samples were subdivided into subepicardial, midmyocardial, and subendocardial layers of approximately equal thickness. Samples were weighed, placed in scintillation vials, and the activity of each isotope was determined. The activity of each isotope in the reference blood samples was also determined. The radioactivity in the tissue and reference blood samples was analyzed with a gamma counter (Minaxi 5000, Packard, Downers Grove, Ill.). Myocardial blood flow (Q,) (mL.min-l.g-l) was calculated from the following equation: Q, = Q;C,/C,, where Q, is the rate of withdrawal of the reference blood flow sample (mL/
SIKER ET AL. NITROUS OXIDE AND STUNNED MYOCARDIUM
541
min), C, is the activity (counts per minute [cpm]) of the reference blood flow sample, and C, is the activity (cpm/g) of the tissue sample.
Experimental Protocol Dogs were assigned to receive either nitrous oxide (70%)or nitrogen (70%) in combination with oxygen (30%)before and during LAD occlusion in a random fashion in separate experimental groups. After surgery was completed, the preparation was allowed to stabilize for 30 min during ventilation with room air enriched with oxygen (30%). Control hemodynamics and regional contractile function were then recorded, and a radioactive microsphere injection was completed. Hemodynamics and regional myocardial blood flow were again measured after a 30-min period of equilibration during administration of nitrous oxide (70%) or nitrogen (70%) in oxygen (30%). End-tidal gases were continuously measured by mass spectrometry (Advantage 2000, Marquette Electronics, St. Louis, Mo.). The LAD flow was then interrupted for 15 min by the mechanical occluder in both groups. During LAD occlusion, retrograde LAD coronary flow from the cannulas was allowed to drain by gravity to reduce collateral perfusion within the ischemic zone. After 12 min of LAD occlusion, hemodynamics and segment function were measured, and another set of radioactive microspheres was administered. The occlusion was removed after 15 min, and reperfusion was allowed to occur. All dogs (in both control [nitrogen] and nitrous oxide groups) received lidocaine (1 mg/kg IV) and procainamide (5 mg/kg IV) immediately before reperfusion. Nitrous oxide (70%) or nitrogen (70%) was continued throughout the occlusion and 15 min into the reperfusion period in separate groups. Nitrous oxide was then discontinued, and room air enriched with oxygen (30%)was reinstituted. Hemodynamics and regional myocardial function were recorded at 5, 15, 30, 60, 120, and 180 min during the reperfusion period. A final set of radioactive microspheres was administered at the completion of the 3-h reperfusion period. Thus, systemic hemodynamics, regional myocardial function, and myocardial tissue perfusion before and during acute coronary artery occlusion and during 3 h of reperfusion were studied in control (70% nitrogen) and nitrous oxide (70%)-exposed groups. Twentyfour dogs were used to provide seven experiments in each group.
Statistical Analysis Statistical analysis of data within and between groups before and during coronary artery occlusion and reperfusion was performed using analysis of variance
542
ANESTH ANALG 1992;75:53948
SIKER ET AL. NITROUS OXIDE AND STUNNED MYOCARDIUM
Table 1. Hemodynamics During Coronary Artery Occlusion and Reperfusion in Control Experiments in Dogs Preocclusion control
HR (beatdmin) M A P (mm Hg) LVSP (mm Hg) LVEDP (mm Hg) RPP (beats.min mm Hg x lo3) dP/dt (mm Hg/s) LAD MCBF (mL/min) LAD SS (%) LCCA SS (%)
Coronary occlusion
Reperfusion (min)
5
121 5 6 105 t 10 125 2 10 4tl 14.82 2.0
110% 5" 107 t 4" 8426" 82 t 6" 105 2 5" 99 2 6" 7 2 2" 7 2 1" 10.5 t 0.4 11.2 t 1.0"
2040 2 70 35 t 5 19.5 2 3.3 10.4 t 2.7
1660 2 57 t -3.3 t 1.5" 9.4 2 14.5 2 3.2 12.0 2 1690 2 110" O t o "
15
30
111 t 4"
114 5 89 5 7 107 2 7" 722 12.32 1.2"
87 2 6 107 2 5" 7 t 2" 11.6f 0.9"
*
60
120
180
114 2 5 92 11 116 +. 9 622 12.6 2 1.8
123 2 6 99 2 10 124 2 9 622 14.22 1.8
121 2 5 102 f 8 122 f 9 7f2 14.1 f 12.7
*
130" 1750 2 1W 1800 2 120" 1860 t 120 27 t 3 29 t 3 36 2 6 7" 2.6" 8.1 2 2.4 8.9 2 2.9" 10.1 2 2.7' 8.9 2 2.4 9.5 2 2.7 3.3 10.5 t 3 3
1840 2 29 2 9.8 2 8.4 2
140 1820 2 120 29 f 5 4 1.9" 7.9 t- 1.9" 9.3 2 2.3 1.9
HR, heart rate; MAP, mean arterial pressure; LVSP, left ventricular systolic pressure; LVEDP, left ventricular end-diastolic pressure; RPP, rate-pressure product; LAD, left anterior descending coronary artery, MCBF, mean coronary blood flow; SS, segment shortening; LCCA, left circumflex coronary artery Data are mean 3- SE ( n = 7). "Significantly (P < 0.05) different from preocclusion control values
(ANOVA) with repeated measures, followed by application of Bonferroni's modification of the t-test. Changes from control within and between groups were considered statistically significant when the probability (P) value was
