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I E E E T R A N S A C T I O N S O N B I O M E D I C A L E N G I N E E R I N G . V O L . 37. N O

2. F E B R U A R Y 1990

A Closed-Loop Control Scheme for Intraaortic Balloon Pumping

Abstract-The beneficial hemodynamic effects of IABP are critically dependent on balloon timing relative to the diastolic phase of the cardiac cycle. A microprocessor-based controller has been developed to implement real-time automation of IABP using P-R intervals to regulate balloon deflation and systolic time intervals to trigger balloon inflation in a semi-automatic fashion. Experiments were performed on anesthetized open-chest dogs. Simultaneous measurement of aortic pressure and flow, coronary flow, and left ventricular pressure were recorded. Muscle segment lengths in normal and ischemic border zones were also measured from implanted pairs of endocardial ultrasonic dimension gages. P-waves were obtained from atrial cardiograms, and heart sounds were detected using a special filtering circuit. Both signals were input together with ECG to automate IABP timing. Systolic time intervals were calculated in real-time. IABP efficacy was assessed from changes in aortic flow, coronary flow, tension time index, end diastolic pressure, and the endocardial viability ratio. Comparisons were made between automated and manual timing set by a certified technician. Results indicate that automated timing yielded equivalent hemodynamic enhancement with greater ease of adjustment. A closed-loop control scheme is proposed which allows complete automatic device operation and the capability to rapidly achieve the optimum of any directly measurable hemodynamic variable.

I . INTRODUCTION HE intraaortic balloon pump (IABP) is an in-series cardiac assist device that utilizes the principal of counterpulsation by displacing a fixed volume of blood in synchrony but out of phase with left ventricular (LV) ejection. This arterial counterpulsation aids the failing heart by elevating mean aortic diastolic pressure (MDP) and reducing ventricular afterload by reduction of aortic end diastolic pressure (EDP). The salutary effects of IABP have been extensively studied in both animal experiments and clinical applications [ 11. The device has demonstrated the potential to improve hemodynamics [2] by increasing cardiac output ( Q A ) and overall systemic perfusion and decreasing left ventricular afterload via reduction of aortic input impedance [3] and EDP. Myocardial oxygen consumption ( MVO, ) is decreased by lowering mean systolic pressure (MSP), and the hemodynamic power output of the left ventricle is improved [3]. The increase in MDP augments coronary perfusion ( Q , ) [4]-[7], which

T

Manuscript received December 9, 1988; revised March 21, 1989. J . A . Zelano is with the Department ofElectrica1 Engineering, The New Jersey Institute of Technology, Newark, NJ 07102. J . K . - J . Li and W . Welkowitz are with the Cardiovascular Research Laboratory, Department of Biomedical Engineering, Rutgers University, Piscataway, NJ 08854. IEEE Log Number 8932201.

enhances impaired contractile function of the left ventricle [8] and also limits infarct size [9]-1111 when IABP is instituted early in myocardial infarction. These beneficial effects are due to an increase of the LV myocardial oxygen supply-consumption ratio ( S / C ) [22]. Originally intended for patients who became moribund and could not be salvaged by any other means, IABP has become a routine treatment modality for both pre- and postoperative support of patients undergoing open heart surgery [ 121, [ 131. The efficacy of IABP counterpulsation is affected by a number of physical and physiological factors, but device phasing is the most predominant factor which determines IABP effectiveness [ 141, [ 151. Optimal device phasing is achieved when inflation slightly precedes the dicrotic notch (S,)and deflation borders on isovolumetric systole. These conditions yield the greatest increase in systemic and coronary perfusion while minimizing LV afterload. Commercial units currently operate by using a reference trigger to mark the beginning of the cardiac cycle. The balloon is normally deflated for a small time delay (systolic delay), which endures for most of systole and is then inflated for a period of time (the primary interval). The reference trigger may either be derived from the ECG R-wave or the derivative of the systolic edge of aortic pressure ( P A) prior to ejection. A trained technician monitors the console continuously and manually adjusts timing. Heart rate variability will affect balloon timing since both timing intervals are preset for a specific heart rate. Short periods of tachycardia or bradycardia as well arrhythmias will result in incorrect timing yielding nonoptimal inflation and deflation. This will reduce the benefits obtained with IABP and possibly cause damage if prolonged inflation during systole occurs. These practical problems can be easily corrected via real-time computer control. Previous investigators have attempted to implement systems [ 161, [ 171 which would automate IABP timing. These worked reasonably well but lacked the capability to respond dynamically to arrhythmias because they used R-wave timing queues coupled with regression equations to determine the systolic time interval (STI). Similarly, these systems also lacked the ability to anticipate ventricular ejection since the R-wave was the only electrophysiological signal available. This study uses a different approach in that it derives the second heart sound ( & ) from aortic pressure and uses this

00 18-9294/90/0200-01 82$0 1 .OO 0 1990 IEEE

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I N T R A A O R T I C BALLOON PUMPING

signal to measure STI in real-time. This enables the processor to perform inflation prior to S, and allows the CPU to dynamically respond to arrhythmias. The P-wave is detected in order to measure P-R intervals which enables the CPU to deflate the balloon before the onset of ejection. These two time intervals together with appropriate timing delays have been used to completely specify the variation of hemodynamic parameters as a function of IABP timing and define criteria for optimum automated control. 11. EXPERIMENTAL MATERIALS AND METHODS Procedures were performed on ten mongrel dogs. All animals were anesthetized with 20 mg/kg Nembutal and placed on a positive pressure respirator. Left lateral thoracotomy was performed and the heart was suspended in the pericardium cradle. Segmental muscle shortening was monitored by inserting pairs of ultrasonic dimension gages (Schuessler LTZ-5 MHz, Cardiff by the Sea, CA 92007) in the normal and ischemic border zones of the myocardium. Aortic ( P A ) and left ventricular ( P L v ) pressures were measured with Millar (PC-350, Houston, TX 77023) catheters. Pulsatile coronary and aortic flows were measured with electromagnetic flow probes placed around the respective vessels. The signals were filtered using Biotronex (BL-610, Kensington, MD 20895) flowmeters at 50 Hz. Surface ECG was continuously monitored and atrial cardiograms were taken by inserting a pacing catheter (Cordis 370-110, Coral Gables, FL 33130) into the right atrium. Second heart sounds (S2) were detected by a filtering and peak detection circuit applied to PA[21]. Balloon rise ( TRlsE) and fall ( TFALL)times were measured in vivo with a Millar pressure catheter and storage oscilloscope using a nontraumatic clamp to occlude the aorta. The TRrsE was 20 ms with a delay past inflation trigger of 50 ms yielding a total delay of 70 ms. TFALL was 30 ms with a 40 ms delay past deflation trigger yielding a total delay of 70 ms. Helium was used as the balloon driving gas under all conditions. Myocardial ischemia was induced by occluding the left anterior descending coronary artery just distal to the first major branching. A single chambered polyurethane balloon was placed in the descending aorta 2-4 cm below the aortic arch junction. Balloon counterpulsation was initiated one hour after occlusion using a Datascope (Model 80, Paramus, NJ 07652) system that had modified pressure and vacuum modules to minimize TRIsEand TFALL. The three timing signals, S2, atrial P-waves, and ECG, were fed into individual hardware detectors. All detectors were greater than 99 percent accurate when the ECG and P-wave detector were tested in the abscence of pacer spikes. All detectors produced logic level pulses upon signal detection that were priority encoded interrupt inputs to a Motorola (MC68000 KDM, Phoenix, AZ 85036) single board microcomputer which controlled balloon pump timing. The microprocessor control program used several algorithms to adjust IABP timing. Two algorithms were used to trigger balloon inflation: initiation of balloon in-

183

flation upon detection of S,, and triggering of balloon inflation prior to S2 by using the running average of the systolic time intervals (STI) measured from the twenty previous beats and subtracting a inflation delay time that is manually entered by the operator. The term inflation delay time ( TI ) is defined as the time period prior to S2at which the pressure solenoid in the IABP is activated. It is important to note that this time period is an anticipation time even though it is designated a delay. In order to take into account rapid excursions in heart rate, the computer compares the most recent STI with the running average and if they differ by more than 15 percent, the most recent STI is used to set the timing. In all cases, detection of S2 immediately triggers balloon inflation. Balloon deflation is triggered by two modes: deflation is initiated at a time delay past the P-wave equal to the average P-R interval measured from the 20 previous beats and subtracting a manually entered balloon deflation delay time. The term deflation delay time (To) specifies the time period prior to the R-wave at which the exhaust solenoid of the IABP is activated. This “delay” is also an anticipation time. This mode is the normal operating mode since it constrains deflation to occur only in the PR interval. The second mode allows deflation to occur past the R-wave by specifying a R-wave deflation delay time (TR). Hence deflation is initiated at a time TR past detection of the R-wave. This mode is primarily used for experimental purposes. Variations of the P-R interval on a beat to beat basis are taken into account by comparing the average with the P-R interval measured from the most recently acquired cycle. If the two differ by more than 15 percent, the most recent value is used to set deflation. Both schemes make it possible to deflate at any time in the interval beginning from the P-wave and ending at a specified time in late systole. Data were taken under computer control and stored for analysis. Control segments were taken for at least eight to ten beats and the pump was then activated for another 10-15 beats to collect data. Pumping was terminated for at least 50 beats to allow the cardiovascular system to return to its control state. Two sets of timing experiments were performed to test inflation and deflation timing. During inflation experiments, To was set to 60 ms as this was determined to yield the minimum EDP. Inflation was started at S2 and T, was changed in 15 ms intervals up to 180 ms early inflation. Deflation timing runs were performed in the following way: TI was set to 60 ms for all deflation runs since this value yielded the greatest increment in mean diastolic pressure without systolic loading. To was varied in the P-R interval from zero (R-wave deflation) to 135 ms preceding the R-wave in 15 ms increments. Another set timed deflation to occur at a time TR past the R-wave up to a maximum of 135 ms, hence deflation data were obtained for a 270 ms time interval in 15 ms increments with the center deflation time set at the R-wave. Data were analyzed from the second and third cycles following initiation of IABP because the first cycle was

184

IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING. VOL. 37. NO. 2. FEBRUARY 1990 CONTROL

'

s2 TIMING

100 '\

A'

LN

Fig. 1. Experimental waveforms under unassisted conditions. P-wave = right atrial cardiogram; R-wave = ECG, LH = border zone segment length; L, = normal zone segment length; Sz = second heart sound; PA = aortic pressure; timing = IABP solenoid driving signal; P , = left ventricular pressure; Q,- = coronary blood flow: Q, = aortic blood flow; Pressure is in mm Hg

mean (MDP) values of aortic diastolic pressure over control without loading the heart during systolic. Increased MDP will increase Q , which leads to enhanced O2 delivery. Balloon deflation is timed so that the drop of aortic EDP is the same as PLVat the border of isovolumetric systole. The rapid balloon deflation causes a substantial decrease in EDP which decreases ventricular afterload. This systolic unloading causes a decrease in peak ( Vpp) and mean PLVwhich decreases MV02. Hence the oxygen supply-consumption ratio has increased during IABP. Upon examining the border zone segment length ( L B ) , increased segmental shortening is observed indicating increased contractility which results from a combined increase in S/C and decrease in afterload. End diastolic segment lengths (EDL) in LB and the normal zone ( L N ) are reduced indicating less distension of the left ventricle 111. RESULTS and reduced preload. The decrease in systolic loading toFig. 1 illustrates typical data in the absence of cardiac gether with the increased Q , increases S / C , improves QA, assistance. The ECG R-wave and atrial P-waves are sharp and enhances overall ventricular performance. These and easy to detect. Ventricular segment lengths in both changes are typical of the beneficial hemodynamic effects normal and border zones show differential shortening. The of well-timed IABP. control waveforms of Q , and QA as well as PLVand PA Fig. 3 shows experimental waveforms during IABP are at normal levels. The filtered P A waveform shows when S2 is used to trigger inflation. In this case To was sharp transitions at S2 facilitating detection of this signal. equal to 35 ms preceding the R-wave which is a later deFig. 2 shows waveforms during IABP when the timing flation than To equal to 60 ms. The pump timing trace is set manually by a trained technician. When compared indicates that detection of S2occurs near the leading edge to Fig. 1 , several changes in the hemocjnamic variables of this waveform. The later deflation has resulted in eleare readily apparent. Balloon inflation occurs rapidly and vated EDP which implies that the minimum EDP must lie slightly precedes S2.This increases the maximum and the at an earlier deflation setting. Higher EDP increases sys-

only partially affected by the pump. Analyzing the cycles immediately following control eliminates the influence of any long term changes of the cardiovascular system due to IABP. The values for Q A , Q,, EDP, and segment shortening in normal ( L N )and border ( L B )zones were calculated for unassisted conditions and during IABP. Tension time index (TTI) [22], the integral over systole of PLV,was computed to estimate MV02 and the endocardial viability ratio (EVR) was computed to estimate the ratio of oxygen supply to oxygen consumption ( S / C ). EVR was computed as the integral of diastolic PAdivided by TTI. Percentage changes between assisted and unassisted conditions were calculated for all hemodynamic parameters to assess the effects of IABP.

ZELANO e r o l . INTRAAORTIC BALLOON P U M P I N G

185 M4NUAL DUnPING

1

e52

I

1).

i

IS0

LB s2 TIMING

100 PA

LN

Fig. 2. Same as Fig. 1 except pump timing is set by a certified technician.

HEART SOUN'I 858

TRIGGER

"M,S

150

100

50

0

I

I

1

Fig. 3. Same as Fig. 1 except balloon inflation is controlled by S, trigger. Balloon deflation is controlled by P-R intervals; To = 35 ms.

tolic loading which leads to larger EDL in LB and LN and less enhancement of fiber contraction. In addition, QA is decreased because ventricular ejection is hindered by increasing loading. Augmentation of Q , is less than in the manual pumping case due to late inflation from the S, trigger which causes reduced augmentation of MDP. Al-

though this timing condition is not as optimal as the manual case, substantial improvements in hemodynamic parameters are apparent when compared to Fig. 1. This pumping mode demonstrates the advantage of the computer automated pumping for tracking patients with erratic heart beats.

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IEEE T R A N S A C T I O N S ON BIOMEDICAL E N G I N E E R I N G . VOL 37. NO 1. F k B R U A R Y I Y Y O

150

LB s2 TIMING 100 PA

50

1

1

l

j

Fig. 4. Same as Fig. 1 except STI are used to control balloon inflation; T, = 60 ms. Balloon deflation is controlled by P-R intervals; T, = 60 ms.

Fig. 4 shows experimental data for IABP using STI to control inflation and P-R intervals to control deflation. T/ and To are both 60 ms in this experiment. It is clear that this pumping mode yields identical results with manual timing control after comparing waveforms with Fig. 2. Hence, the real-time control scheme described here is a viable method of controlling IABP timing. It is noteyielded values that worthy that measured TRIsEand TFALL were larger than the optimal timing settings and therefore set limits on TI and To. It was easier and faster for an operator to arrive at optimal timing conditions using the computer than with a conventional balloon pump console. Fig. 5 summarizes percent changes from unassisted conditions of the hemodynamic parameters Q,, QA, and EDP as a function of T I . To was chosen as 60 ms since this value yielded minimum EDP. The data indicate the EDP is not affected by changes of inflation delay timing as is expected since EDP is determined primarily by balloon deflation prior to systole. Q, and QA show definite trends as inflation becomes progressively earlier. When the S, is used to trigger inflation ( T I = 0 ) the changes in these parameters are large but not maximum. This demonstrates that S2 inflation does yield substantial beneficial changes in hemodynamic parameters. As inflation becomes progressively earlier, Q, and QA become larger and reach a maximum at T/ = 60 ms. If inflation is initiated even earlier ( T / = 90), the percentage change in the parameters begins to decrease. Therefore an optimum inflation time exists for maximum improvement in Q, and QA. The manual timing adjustment sets inflation for T/ equal to 60 ms. Subjectively, an operator tries to obliterate the oscillation of S2 in the PA waveform and make the curve

smooth and continuous without excessively loading the heart. This condition yields the maximum improvement in Qc and QA. Fig. 6 shows the percentage change from unassisted conditions of EDP, Qc and QA as a function of To. It is evident that each hemodynamic parameter has a specific To at which its maximum (or minimum) occurs. TI was chosen as 60 ms because this yielded optimal inflation. If balloon deflation is early, none of the parameters reach their extrema. At To equal to 90 ms, EDP is negative, QA and Q, are positive but not maximum. At To equal to 60, EDP has decreased to its minimum while Q, and QA are still increasing. This condition is identical to the timing setting chosen by the technician. Trained technicians try to minimize EDP while increasing QAas much as possible by early inflation. If deflation is initiated later ( T o = 45 ms), QA is maximum and Q, continues to rise. EDP begins to increase indicating elevated systolic loading. Still later deflation at To equal to 30 yields maximum Q, and decreased QA with increasing EDP. Finally, when deflation is triggered at the R-wave, Q , and QA are both decreasing and EDP is much larger indicating additional ventricular loading. Fig. 7 shows the percentage changes from control in TTI, and EVR as a function of balloon inflation preceding S,. The behavior of TTI for early inflation follows a linear variation proportional to the delay during which the balloon remains inflated in systole 1211. For small values of T I , blood propagation delays through the arterial system cause the balloon pressure pulse to arrive at the aortic valve after end systole. The heart is already isolated from the aorta and no loading occurs. Since the balloon remains

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INTRAAORTIC BALLOON PUMPING

GROUP AVERAGES OF PERCENTAGE CHANGE FROM CONTROL OF HEMODYNAMIC PARAMETERS vs BALLOON INFLATION PRECEEDING THE SECOND HEART SOUND ( S p )

+-a

50

30

IQC I

Coronary Blood Flow

Aortic Blood Flow

TD = 6 0 m s e c

IQA I

End Diastolic Pressure l E D P l

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150

135

120

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90

75

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15

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lmsecl

Fig. 5. Percentage change of hemodynamic parameters versus balloon inflation time ( T I )preceding the second heart sound ( & ) . Q, = coronary blood flow; QA = aortic flow; EDP = end diastolic pressure; TD = 60 ms .

GROUP AVERAGES OF PERCENTAGE CHANGE OF HEMODYNAMIC PARAMETERS vs BALLOON DEFLATION TIME

*--*

Coronary Blood Flow I Aortic Flow IOAI

IQC!

T, = 60 msec

End Diastolic Pressure lEOPl

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Fig. 6. Percentage change of hemodynamic parameters versus balloon deflation time ( T D ) .Q , = coronary blood flow; Q, = aortic flow; EDP = end diastolic pressure; T, = 60 ms.

1

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IEEE ' T R A N S A C T I O N S ON B I O M E D I C A L E N G I N E E R I N G . VOL. 37. N O . 2. F E B R U A R Y I Y Y O

GROUP AVERAGES OF PERCENTAGE CHANGE FROM CONTROL OF TENSION TIME INDEX ITTI] AND ENDOCARDIAL VIABILITY RATIO [EVR I vs DELAY OF BALLOON INFLATION PRECEDING THE SECOND HEART SOUNDIS21 To =60ms

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Fig. 7. Percentage change from unassisted conditions of tension time index (TTI) and endocardial viability ratio ( E V R ) versus delay o f balloon deflation ( T, ) preceding the second heart sound ( S , ) . T,, = 60 ms.

inflated in systole for a fraction of the STI, the total loading effect is averaged over the systolic period by computing TTI. Ventricular afterload affects M V 0 2 and therefore the S I C ratio of the LV. The endocardial viability ratio (EVR) is computed by taking the ratio of the diastolic time index (integral of diastolic pressure over diastole) and TTI. TTI is traditionally considered a measure of LV work ( O2 consumption) and MDP is usually proportional to Q , which is directly related to left ventricular O2 supply. Hence EVR is a direct measure of left ventricular

s/c.

For TI less than or equal to 75 ms preceding S2, TTI is relatively constant whereas MDP is increasing due to correction of propagation delays in the aorta and the longer duration of balloon inflation over the diastolic period. This causes increased Q , and maximizes the EVR at a delay of 60 ms preceding S2. This is the timing setting chosen for manual adjustment as well as computer control to create a smooth continuous aortic pressure waveform at end systole. Inflation at a delay less than 60 ms preceding S2 causes less than optimal enhancement of MDP without increase in TTI. At time delays greater than 60 ms, inflation is initiated progressively earlier into systole which increases ventricular afterload and TTI. Since MDP is relatively constant EVR will decrease for these timing set-

tings. Therefore, EVR is a good index for judging the efficacy of IABP timing for early inflation. Fig. 8 shows changes in TTI and EVR as a function of T, and TR. It is observed that TTI slightly decreases for TDvalues of 120 ms to 60 ms preceding the R-wave. The minimum of TTI is at 60 ms preceding the R-wave, which corresponds to minimum EDP. Deflation timed earlier than this setting results in increased EDP due to retrograde blood flow filling the volume originally displaced by the balloon. The ventricle ejects against a greater volume of blood for these timing conditions which causes increased afterload. Later deflation causes the balloon to remain partially or totally inflated during the subsequent ventricular ejection which linearly increases TTI as a function of the time delay past end diastole for which balloon inflation endures [2 l]. The EVR displays a maximum at a time delay of 30 ms preceding the R-wave which coincides with the maximum of Q,. Although EDP has substantially increased at this setting, TTI has increased less. The augmentation of MDP due to increased balloon inflation in late diastole has maximized Qc and also EVR. Progressively later deflation increases TTI with MDP remaining approximately constant causing a decrease in EVR. EVR is decreased for very early deflation because augmentation of MDP is less for shorter periods of balloon inflation in the diastolic phase

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INTRAAORTIC BALLOON P U M P I N G

GROUP AVERAGES OF PERCENTAGE CHANGE FROM CONTROL OF TENSION TIME INDEX AND ENDOCARDIAL VIABILITY RATIO [EVR] vs TIME DELAY OF BALLOON DEFLATION

[TTI]

32

0

1

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28

3

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4

24

5

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Fig. 8 . Percentage change from unassisted conditions of tension time index (TTI) and endocardial viability ratio ( E V R ) versus time delay of balloon deflation ( T , > ) . T, = 60 ms.

of the cardiac cycle. TTI also increases slightly for early deflation which contributes to the decrease in EVR. It appears from the data presented here that EVR is a good index for the optimization of Q, and for judging overall IABP performance. Although the maximum of EVR occurs at increased loading, the overall effect of enhanced EVR is beneficial to the ischemic LV. Since PL, is not normally available in a typical clinical setting, EVR is not a practical index for optimization of Q,. IV. DISCUSSION The hemodynamic results presented here verify previous model-based theoretical predictions regarding the timing dependence of hemodynamic parameters with respect to inflation-deflation timing [ 151, [ 181 as well as the fact that the extrema of any two hemodynamic parameters cannot be achieved simultaneously [ 191. This is the typical tradeoff encountered with IABP timing adjustment. Maximizing QAor Q, and minimizing LV afterload (i.e., EDP or TTI) cannot be accomplished simultaneously. Similarly, optimization of S / C by maximizing EVR again increases ventricular load. The choice of which parameter to optimize will depend on the clinical situation at hand. For cases where the LV is infarcted and it is necessary to preserve the viable myocardium and limit further damage, maximization of Q, or EVR will accomplish this goal. For cases after revascularization procedures, a different

approach is requested since the compromised coronary circulation has been surgically repaired. In this case, it is important to reduce EDP or TTI so that the healing heart will need to perform as little work as possible. One major result from this study has been the determination of stringent timing boundaries in which balloon inflation and deflation are to take place. The results indicate that useful inflation will take place for values of T, that range from zero ( S2 inflation) to TRIsE. If T, is equal to TRISE,then the balloon is fully inflated at the time of valve closure. The results indicate that a T, value less than this limit satisfies optimal conditions. Optimal deflation occurs within the P-R interval and should be initiated at a To equal to a value less than TFALL.T F A L L imposes another constraint on To and the optimum values of all hemodynamic parameters lie in the time interval where To is less than TFALL. The computer automation scheme described here can afford accurate reproducible control over a number of experiments. The detection of S2 allows for dynamic response to arrhythmias or other aberrant beats and the algorithm that chooses either the previous beat STI or the average STI allows adjustment to large differences in heart rate. Its one drawback is that it operates in a semi-automatic mode. Assistance occurs dynamically as timing queues are detected but the manual adjustment of To and TI are necessary. In order to implement a closed loop con-

1

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IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING. VOL. 37. NO. 2 . FEBRUARY 1990

TABLE I GROUP AVERAGES AND S T A N D A R D DEVIATIONS O F C O R O N A R Y BLOODFLOW INDEX (ICOR) A N 0 P E R C E N I A G E C H A N C E I N H E M O D Y N A M I C PARAMETERS AS A FUNCTION OF BALLOON INFLATION DELAY, r,, USING STI TO COKTROL INFLATION T,

EDP

Q,

O(S2) 30 60 90

-11.39 f 3.11 1 I .79 k 2.70 -10.08 2.47 -10.59 +_ 2.86

23.04 k 3.71 25.72 2.51 29.59 f 3.89 22.04 f 3.29

~

*

ICOR

QA

10.75 14.49 18.49 12.34

*

8.82 k 0.97 9.95 & 1.03 10.09 1.13 6.70 0.89

+_ 2.96

k 3.03 k 3.02 k 3.47

*

F ' , = 60 rns

trol it is necessary to automatically adjust TD and TI on a beat to beat basis. The adjustment of these delays will be based on real-time evaluation of performance indexes or hemodynamic parameters. Optimal search procedures [ 171 may be used to rapidly approach the extremum of an index by appropriate variation of TI and TD which will achieve optimal closed-loop control. To this end, the first requirement is to determine TRrsE and TFALL in the patient. TRisE can be measured by performing an S2 inflation and observing PA directly. TFALL can be measured by performing a P-wave deflation and measuring the time between 10-90 percent of minimum P A . This sets upper limits for T D and T I . TI can be initialized to TRrsE. This is nonoptimal but will not excessively load the heart by inflating too early and will provide a suitable starting point for a search procedure. The initial value for TD can be estimated with a P-wave deflation. The computer will measure the time from solenoid closing to the minimum of P A . The time from R-wave detection to EDP for an unassisted beat is measured next in order to provide an estimate of the electrical to mechanical activation delay of the LV. In order for minimum EDP to occur at the border of isovolumetric systole, the two quantities should be subtracted. Using data obtained from Fig. 1 and P-wave deflations, a starting value of 60 ms is obtained which corresponds to the optimum value of T D obtained from the data. Once initial time delays are obtained, it is necessary to optimize an index for a particular clinical context. Each appropriate index must be evaluated on a beat to beat basis and an iterative search procedure can be used to converge to the optimum value. If afterload is to be minimized, EDP can be determined directly from PAand be minimized via adjustment of To. TTI is not a practical index since P,, is not clinically available in conventional settings. If QA is to be maximized, aortic flow velocity may be monitored using a flow catheter and changes in both T, and T D can be made to optimize this quantity. S I C is a very important hemodynamic quantity to optimize since this variable measures the major goal of an assist device which is to increase cardiac O2 supply while minimizing MV02 or LV energy expenditure. Previous investigators have used the EVR as a means of measuring the overall performance and benefits of IABP [22], [23]. The results of this study indicate that S/C for the LV or EVR has a maximum that coincides with Q,. Since TTI

is not directly available from standard hemodynamic measurements, EVR is not a practical quantity to evaluate clinically. EVR, however, does display extremely high sensitivity in the region of maximum Q , and would be useful in implementing tight control. A possible solution to this dilemma is to approximate EVR using the integral of PA over the STI in lieu of TTI. This would yield an estimate of TTI over the ejection phase which may be sufficient to implement tight control on Q,. Direct measurement of Q , is possible by placing flow probes around the coronary arteries. This can be done experimentally in animals or clinically during open-heart surgery. Another possible alternative is to derive an index proportional to Q , which may be evaluated practically. After close examination of experimental data, it was observed that Q , is proportional to MDP but will decrease when LV systolic pressure increases because of extravascular compression of the coronary arteries. Since PAis always available clinically, an empirical index of the following form was constructed that includes both MDP and MSP ICOR

=

K1 * MSP/MSPo

+ K2 * MDP/MDPo

where MSP, is a control value of MSP and MDP, is a control for MDP. The constants K1 and K2 are weighting factors that assign some relative importance to each term. Because the weighting of MDP and MSP are not equal, the K ' s were chosen to be the fraction of the cardiac cycle over which each pressure interval endures or KI = -HR

K2

=

HR

* STI

* DTI

where DTI is the diastolic time interval and HR is the heart rate. K 1 is negative because increased systolic loading will decrease Q,. The index ICOR was calculated using percentage changes in MDP and MSP from experimental data. The results are summarized in Tables 1 and IT for each timing condition. In general, the maximum of ICOR coincides with the maximum of Qc for both inflation and deflation timing settings. It does not have extremely high sensitivity; however, it follows the correct trend and may be useful in implementation of effective closed-loop control. Studies are currently being conducted to evaluate the usefulness of ICOR and a modified EVR to estimate Q , during closed loop operation.

ZELANO

PI

d . : INTRAAORTIC BALLOON P U M P I N G

TABLE I1 GROUP AVERAGES A N D STANDARD DEVIATIOVS OF CORONARY BLOODFLOWI N D E X (ICOR) A N I ) PERCENTAGE I N HEMODYNAM PARAMETERS IC AS A FUNCTION OF BALLOON DEFI.ATION CHANGE INFLATION DELAY,T,,, USING STI TO CONTROL

TD 0 30 45 60 90 T,

Q,

EDP 9.83 2.34 - 1 I .34 -12.58 -9.68 =

*

f 3.86 f 3.57 f 4.42 k 4.22 3.85

ICOR

QA

32.92 4.28 55.27 i 4.92 36.89 f 4.03 28.77 k 4.85 13.41 2.89

*

** *

12.93 3.83 17.63 3.52 26.87 4.07 20.92 k 3.77 15.25 4.08

17.78 18.12 16.81 15.91 14.45

f 2.34 f 1.99

k 1.78 1.63

k 1.57

60 ms

V. CONCLUSIONS In summary, the automated system described here has demonstrated the ability to control pump timing with the same accuracy as that of a certified technician. Adjustment of this system in an open-loop mode is easier than with a conventional balloon pump console. The system has the ability to compensate for changes in pump timing due to erratic heart rates because of its ability to detect S, in real-time and dynamically adjust inflation according to changes in heart rate. The use of the P-R interval provides discernible timing boundaries in which balloon deflation is to take place. The ability to detect all three signals in real-time provides the ability to sense ventricular arrhythmias and terminate pumping automatically during the beat in which the arrhythmia occurs. The major drawbacks of the system is that it operates in a semi-automatic mode. A plausible closed-loop control scheme has been discussed that will enable the system to optimize any chosen hemodynamic parameter using real-time control on a beat to beat basis.

Balloon fall time Inflation delay time R-wave deflation delay Balloon rise time Tension time index Peak ventricular pressure

Aortic end diastolic pressure Endocardial viability ratio Intra-aortic balloon pump Coronary flow index Border zone segment length Normal zone segment length Left ventricle Aortic mean diastolic pressure Aortic mean systolic pressure Control aortic mean diastolic pressure Control aortic mean systolic pressure Myocardial oxygen consumption Oxygen Aortic pressure Left ventricular pressure Aortic blood flow Coronary blood flow Second heart sound Oxygen supply consumption ratio Systolic time interval Deflation delay time

[2]

[3] [4]

[5]

[7]

[8]

[9] [IO]

[ll]

PLV QA

[I21

Qc

s2 s/c STI TD

TRISE TTI VPP

[ I ] S. Scheidt, M. Collins, J. Goldstein, and J. Fisher, “Mechanical cir-

[6]

EDP EVR IABP ICOR LB LN LV MDP MSP MDPO MSPO MV02 0, PA

TR

REFERENCES

TABLEOF ABBREVIATIONS Parameter

TFALL TI

[I31

culatory assistance with the intra-aortic balloon pump and other counterpulsation devices.” Prog. Cardiovasc. Dis., vol. 35, pp. 55-76, 1982. W. J. Powell, W. M. Daggett, A. R. Magro et a l . , “Effects of intraaortic balloon counterpulsation on cardiac performance, oxygen consumption and coronary blood flow in dogs,” Circ. R e s . , vol. 26, p. 753, 1970. D. Jaron, “Left ventricular afterload and systemic hydraulic power during in-series cardiac assistance: Studies using intraaortic balloon pumping in dogs,” Ann. Biomed. Eng., vol. 5, p. 95, 1977. J. T . Watson, J. T. Willerson, D. E. Fixler et al., “Temporal changes in collateral coronary blood flow in ischemic myocardium during intraaortic balloon pumping,” Circulation, vol. 49, 50 suppl. 11, p. 249, 1974. C. C. Gill, A. S . Wechsler, G . E. Newman et al., “Augmentation and redistribution of myocardial blood flow during acute ischemia by intraaortic balloon pumping,” Ann. Thorac. Surg., vol. 16, p. 445. 1973. R. R. Limet, J . N . Ross, I. Hojevac et al., “Effects of intraaortic balloon counterpulsation (IABCP) on the distribution of coronary blood flow in experimental ischemic left ventricular failure,” J . Cardiovas. Surg., vol. 22, p. 305, 1971. V . K. Saini, W. B. Hood, H. B. Hechtman et al., “Nutrient myocardial blood flow in experimental myocardial ischemia,” Circulation, vol. 5 2 , p. 1086, 1975. S . Sasayama, G. Oskada, M. Takahashi et al., “Effects of intra-aortic balloon counterpulsation on regional myocardial function during acute coronary occlusion in the dog,” Amer. J . Cardiol., vol. 43, p. 59, 1979. W. L. Sugg, W. R. Webb, and R. R. Ecker, “Reduction of extent of myocardial infarction by counterpulsation,” Ann. Thorac. Surg., vol. 7, p. 311, 1969. P. R. Maroko, E. G. Berstein, P. Libby et al., “Effects of intra-aortic counterpulsation on the severity of myocardial ischemic injury following acute coronary occlusion,” Circulation, vol. 45, p. 1150, 1972. A. J. Roberts, D. R. Alonso. J. R. Combes et al., “Role of delayed intraaortic balloon pumping in treatment of experimental myocardial infarction,” Amer. J . Cardiol., vol. 41, p. 1202, 1978. D. Bregman, E. N . Parodi, R. N . Edie er al., “Intraoperative unidirectional intraaortic balloon pumping in the management of left ventricular power failure,” J . Cardiovasc. Surg., vol. 70, p. 1010, 1975. M. J. Buckley, R. C. Leinbach, J. A. Kastor, J. D. Laird, A. R. Kantrowitz, P. N . Madras, C . A. Sanders, and W. G. Austen, “Hemodynamic evaluation of intra-aortic balloon pumping in man,” Circulation, vol. 46, suppl. 11, p. 130, 1970.

1

I E E E T R A N S A C T I O N S O N B I O M E D I C A L E N G I N E E R I N G . VOL

W. Welkowitz, Engineering Hemodynamics: Application to Cardiac Assist Devices. Lexington, MA: Heath, 1977. D. Jaron, T . Moore, and P. He, “Theoretical considerations regarding the optimization of cardiac assistance by intraaortic balloon pumping,” IEEE Trans. Biomed. Eng., vol. BME-30, p. 117, 1983. M. J . Williams, “Experimental determination of optimal performance of counterpulsation assists pumping under computer control,” Compur. Biomed. Res., vol. IO, p. 545, 1977. E. Philippe, J. W. Clark, A. Lande, and J . R. Ellis, “Microprocessor control of intra-aortic balloon pumping,” Ann. Biomed. Eng., vol. 8, pp. 209-224, 1980. D. Jaron, T. W. Moore, and P. He, “Control of intraaortic balloon pumping: theory and guidelines for clinical applications,” Ann. Biomed. Eng., vol. 13, pp. 155-175, 1985. W. S . Kuklinski, “Closed loop control of intraaortic balloon pumping: Studies using a computer simulation and animal experiments, ” Ph.D. dissertation, Univ. Rhode Island, Kingston, 1979. W. Meyer, “The aortic and pulmonary components of the second heart sounds in mitral stenosis, atrial septal defect and tetralogy of fallot,” Cardiologia, vol. 50, pp. 65-74, 1967. J . A. Zelano, “The computer control of intra-aortic balloon pumping,” Ph.D. dissertation, Rutgers University, New Brunswick, NJ, 1986. E. A. Amsterdam, N. A. Awan, G. Lee, R. Low, J. A. Joye et al., “Intra-aortic balloon counterpulsation: Rationale, application, and results,” in Critical Care Cardiology, C . E. Rackley and A. N. Brest, Eds. Philadelphia, PA: Davis, 1981, p. 79. J . Jablkowski, J . Serafin, “Parametric optimization of the intra-aortic balloon pump,” Trans. ISAOIIFAC Symp., vol. 2, p. 245, 1980.

John A. Zelano (S’79-M’80) was born on September 22, 1951. He received the B.S degree in applied science from Rutgers University, New Brunswick, NJ, in 1974, the M.S degree in biophysics from the University of Chicago, Chicago, IL, in 1979, the M.S. degree in electrical engineering in 1980, and the Ph.D. in Biomedical Engineenng, both from Rutgers University, in 1980 and 1986, respectively. He is currently an Assistant Professor in the Department of Electrical Engineering at the New

37. NO 2. F E B R U A R Y 1990

Jersey Institute of Technology. He also is an adjunct Professor in the Department of Cardiothoracic Surgery at the New York Hospital-Come11 Medical Center and collaborates with several researchers in the Department of Biomedical Engineering at the New York Eye and Ear Hospital. His current research interests center around the development of microprocessor based medical instruments for cardiac assist devices and image reconstruction. Dr. Zelano is a member of Tau Beta Pi, Eta Kappa Nu, the Cardiovascular System Dynamics Society. the Association for Computing Machinery, and other engineering societies.

John K.-J. Li (S’74-M’78-SM’84) received the B Sc honors degree in physics and electronic engineering from the University of Manchester, England, in 1972, and the M.S.Eng and Ph.D degrees in bioengineering from the University of Pennsylvania, Philadelphia, in 1974 and 1978, respectively. He headed Biomedical Engineering in Cardiology at the Presbyterian-University of Pennsylvania Medical Center before joining Rutgers University as an Assistant Professor in 1979. He became an Associate Professor in 1983 and is currently a Professor of Biomedical Engineering at Rutgers and an Adjunct Professor of Surgery at UMDNJ-Robert Wood Johnson Medical School. He has published widely and chaired 5essions on cardiovascular dyndmics, cardiology. dnd biomedical instrumentation He also wrote Arterial Svstem Dvnamics (New York New York Univ Press) He is an Associate Editor of the IEEE TRANSAC TIONS ON BIOMEDICAL ENGINEERING Dr Li IS a member of the American Heart Association, the American Physiological Society and the Cardiovascular System Dynamics Society, he is also a Chairman of the EMBS Princeton Section.

Walter Welkowitz, for a photograph and biography, see this issue, p 137