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Use of an intraaortic balloon pump as a pneumatic ventricular assist device controller
Use of an Intraaortic Balloon Pump as a Pneumatic Ventricular Assist Device Controller D. J. Macrae, BMSc, FFARCS, B. Glenville, BSc, FRCSE, T. McCarthy, BSc, L. Cooper, HND, D. Guerreiro, BSc, and D. N. Ross, FRCS Departments of Anaesthesia and Perfusion, Brompton Hospital, and Department of Surgery, The Cardiothoracic Institute, London, United Kingdom
Using a circulatory analogue, we investigated sequentially the performance of a dedicated ventricular assist device driver and an intraaortic balloon pump when driving a pneumatic ventricular assist device. Each drive device was compared under identical pumping conditions at rates of 40 to 120 cycles/min against two resistances. Our preliminary study showed that a modified
intraaortic balloon pump could drive a pneumatic ventricular assist device as effectively as its dedicated driver. The necessary modifications to and possible further development of the intraaortic balloon in this role are discussed.
P
Material and Methods
ulsatile ventricular assist devices (VADs) have been in limited clinical use for several years. These devices are employed in some centers in the management of cardiogenic shock after operation [l, 21 or myocardial infarction [3] when other supportive measures have failed. Ventricular assist devices also have a role in the support of a failing heart as a bridge to cardiac transplantation [4,51. In the United Kingdom, clinical experience with pulsatile VADs is limited to a very small number of patients. A major constraint on the further application of this support technique is the cost of the necessary equipment. A typical VAD system consists of a dedicated driver, costing around $35,000, and the VAD components themselves (the VAD shell, valves, blood sac, and conduits), costing $20,000 per patient. Few cardiac surgical centers in the United Kingdom could contemplate the regular use of such expensive devices, despite their possible lifesaving role [ 6 ] . Our group considered the possibility of using an intraaortic balloon pump (IABP) to power a pneumatic VAD. Most cardiac surgical centers have access to several IABPs. If VAD driving with an IABP were feasible, it could reduce the capital cost of introducing a VAD program by eliminating the need to purchase a costly dedicated VAD driver. In pneumatic and control terms, there are many similarities between the function of both drive devices. We therefore set out to investigate the feasibility of pumping a VAD with an IABP in a simple circulatory analogue, and compared its performance with that of a dedicated VAD controller.
Accepted for publication Nov 30, 1988. Address reprint requests to Dr Macrae, Department of Anaesthesia, Brompton Hospital, Fulham Rd, London SW3 6HP, United Kingdom.
0 1989 by The Society of Thoracic Surgeons
(Ann Thorac Surg 1989;47:752-5)
A two-chambered circulatory loop capable of emulating the flows and pressures seen in the systemic circulation in humans was constructed (Fig 1).A Pierce-Donachy pneumatic VAD (Thoratec, Berkeley, CA) was connected either to a dedicated VAD driver (Thoratec) or to a modified IABP (Datascope, Paramus, NJ). The IABP (Datascope System 80) was modified as follows. The slave chamber was removed and the exhaust system altered to permit either the dumping of exhaust gas directly to the atmosphere or the application of up to 30 mm Hg vacuum. This modification necessitated disconnection of the IABP ”low vacuum” alarm. With the VAD connected to the circulatory loop, two experiments were performed.
CRT MONITOR
I l l Ilzz--l Fig 1 . Components of the circulatoy analogue and its connection to the ventricular assist device WAD). (A = arterial chamber; C = interchangeable constriction; CRT = cathode ray tube; DP = deairing point; F = flow measurement point; IABP = intraaortic balloon pump; PI = “arterial” pressure transducer; P2 = driveline pressure trunsducer; R = reservoir; V = venous chamber.) 0003-4975/89/$3.50
MACRAE ET AL VENTRICULAR ASSIST DEVICES
Ann Thorac Surg 1989;47752-5
IABP DLP
1,'I
V A D driver
bmHg)
Fig 2. Typical "arterial" (AP) and driveline pressure (DLP) traces of both devices while operating at a rate of 80 cycleslmin. (IABP = intraaortic balloon pump; VAD = ventricular assist device.)
753
Table I . Mean "Arterial" Pressures Generated in Experiment 1 Under R1 and R2 Conditions" Rate (cycleslmin)
Mean "Arterial" Pressure (mm Hg) R1
R2
40
38
40
50
49
51
60 70
60
67
73
82
80
79
80
90 100
85 86
93
120
79
86
92
P
The R1 and R2 conditions were resistances of double-ta ered constrictions with minimum diameters of 0.178 cm' and 0.045 cm , respectively; compliance was constant.
Experiment 1 Loop conditions were determined with resistances consisting of double-tapered constrictions with minimum diameters of 0.178 cm2 (Rl) and 0.045 cm2 (R2) while compliance was held constant. The two drive devices were alternated at trigger rates of 40 to 120 cycleslmin in steps of 10 cycles/min. Driveline pressures delivered to the VAD were adjusted to maintain a difference of less than 5 mm Hg at the midsystolic point between the two drive devices for each paired comparison. A constant "venous" pressure was maintained, and the timing controls were then adjusted to achieve maximal VAD output while maintaining identical systolic and diastolic time intervals with both drive devices. "Arterial" and driveline pressures were recorded (Fig 2), and pump outputs were measured volumetrically .
Experiment 2 Experiment 1 was repeated with the IABP driver under R1 conditions as described except that a minimum systolic interval of 20% was permitted. Controls on the Thoratec VAD driver limit the minimum allowable systolic interval to 30%.
Results Experiment 1 Performance of the system was identical with both drive units under the conditions in this experiment. Figure 3 illustrates the flow rates achieved with the IABP and VAD drive units. Mean pressures generated within the loop are shown in Table 1. At rates higher than 60 cycles/min under R1 conditions and higher than 50 cycles/min under R2 conditions, the stroke volume of the VAD was reduced (reflected by the flattening of the flow rate curves in Figure 3).
Experiment 2 The IABP achieved optimal stroke volumes at higher pump rates with systolic intervals shortened to 20%, resulting in a diastolic filling interval of 80% of each cycle (Fig 4). This was not possible with the Thoratec driver, which permits reduction of the systolic time interval to only 30%,thus limiting the maximum diastolic filling time to 70% of a cycle.
Fig 3 . Flows obtained with the drive devices between rates of 40 to 120 cycleslmin (min-') in experiment 1 . (0 = intraaortic balloon pump at R1; 0 = intraaortic balloon pump at R2; = ventricular assist device at R1; 0 = ventricular assist device at R2.)
51
Rate (min-1)
754
MACRAE ET AL VENTRICULAR ASSIST DEVICES
Ann Thordc Surg 1989;47752-5
Fig 4 . Flows achieved with the intraaortic balloon pump in experiment 2 . (W = systolic interval of 30%; 0 = systolic interval of 20%.)
0
40
70
80
1
90
100
120
Rate (min-1)
Comment It appears from our study that a modified IABP can successfully drive a pneumatic VAD. Under conditions presented to the VAD in our study, VAD stroke volume fell at higher rates (see Figs 3, 4). Our observations during the study lead us to the conclusion that this was partly due to relatively slow filling of the VAD during pump diastole, as increasing the diastolic interval to 80% in experiment 2 led to higher flows. In clinical practice, VADs are likely to be run at rates that would allow longer diastolic filling times to occur. Also, application of a small amount of vacuum during diastole may improve VAD filling. Our exclusion of the slave or safety chamber from the pneumatic circuit of the IABP merits comment. In the intended role of an IABP, the slave chamber is essential, as it isolates the patient to some extent from the gas cpmpressor and permits use of a fixed volume of carbon dioxide or helium to fill the secondary (ie, patient) circuit. However, there is no advantage in retaining the slave chamber when pumping VADs, and indeed the PierceDonachy and similar VADs isolate the drive gas from the blood sac with a double envelope, thereby effectively providing a built-in safety layer. A leak in either layer leads to reduced VAD function, and is usually very obvious on visual inspection. Our initial experience while setting up this study demonstrated that adequate control of a VAD using an IABP in which the standard-volume or a large-volume (100-mL) slave chamber was retained was difficult to achieve. There could also be problems refilling a slave gas compartment, as clearly it is not desirable to interrupt VAD pumping for longer than a few seconds, and except in those IABP models with "auto-fill'' facilities, longer interruptions would be likely. We are, however, aware that a VAD driving system utilizing a "slave" arrangement is currently under development. Further development of IABPs as VAD drivers will lead to expansion of control and alarm systems to provide for the specific needs of VAD driving. Facilities for monitor-
ing the driveline pressure waveform [7, 81 and VAD stroke volume [9] are highly desirable. Alarm systems tailored to VAD driving requirements and fail-safe internal and external triggering will be essential for long-term use of an IABP in the VAD driving mode. We have constructed an additional control unit incorporating some of these features; it is currently being tested. In conclusion, a minimally modified IABP functioned satisfactorily as a VAD driver in a circulatory loop. We have conducted animal studies using the support system for periods of longer than 36 hours, and the results are encouraging. While we await the results of a more detailed evaluation, we believe that an IABP can be used as a backup for a dedicated pneumatic VAD driver. In the future, combined IABP and VAD drive devices may prove to be cost-effective replacements for the older separate devices. ~~
We thank the staff of the Heart-Lung and Medical Electronics Departments of Brompton Hospital for their cooperation during the study. M. Self of Datascope BV, Holland, and Datascope Medical Ltd, UK, provided advice and arranged partial funding of the project.
References 1. Pennock JL, Pierce WS, Wisman CB, et al. Survival and
2. 3. 4.
5.
complications following ventricular assist pumping for cardiogenic shock. Ann Surg 1983;198:469-78. Pierce WS, Grant VS, Parr GVS, et al. Ventricular assist pumping in patients with cardiogenic shock following cardiac operations. N Engl J Med 1981;305:1606-10. Pae WE, Pierce WS. Temporary left ventricular assist pumping in acute myocardial infarction and cardiogenic shock. Rationale and criteria for utilization. Chest 1981;79:692-5. Pennington DG, Codd JE, Merjavy JP, et al. The expanded use of ventricular bypass systems for severe cardiac failure and as a bridge to cardiac transplantation. J Heart Transplant 1984;3: 170-5. Farrar DJ, Hill JD, Gray LA, et al. Heterotopic prosthetic ventricles as a bridge to cardiac transplantation. A multicenter study in 29 patients. N Engl J Med 1988;318:33340.
MACRAE ET AL VENTRICULAR ASSIST DEVICES
Ann Thorac Surg 1989;47752-5
6. Glenville B, Ross D. Ventricular assist devices. Br Med J 1986;292:361-2. 7. Coleman SJ, Bornhorst WJ, Lafarge CG, Carr JG. Pneumatic
waveform diagnostics of implanted ventricular assist pumps. Trans Am SOCArtif Intern Organs 1972;18:17M. 8. Rosenberg G, Landis DL, Phillips WM, et al. Determining
755
arterial pressure, left atrial pressure and cardiac output from the left pneumatic drive line of the total artificial heart. Trans Am SOCArtif Intern Organs 1978;24:341-4. 9. Willshaw P, Neilsen D, Nanas J, et al. A cardiac output monitor and diagnostic unit for pneumatically driven hearts. Artif Organs 1984;8:215-9.
REVIEW OF RECENT BOOKS
Manual of Postoperative Management in Adult Cardiac Surgery By Carlos E. Moreno-Cabral, R . Scoff Mitchell, and D . Craig Miller Baltimore, Williams b Wilkins, 1988 102 p p , illustrated, $21.95 Reviewed by Karl E . Karlson, M D This pocket-size book describes in detail the principles and practice of postoperative care of cardiac patients in the Stanford University Medical Center. The manual is meant for the instruction and guidance of medical students, junior surgical residents, and intensive care unit nurses. In addition to a brief text (63 pages), there are copies of numerous forms and routine doctor’s order sheets that are used at Stanford. Various forms used by nurses, including Kardexes for sets of postoperative orders, are illustrated. There is a table of infusion rates for cardiovascular drugs. Antiarrhythmic drugs are tabulated with route of admin-
istration, dose, indications, and toxicity given. A table pertaining to drug therapy in renal failure lists over 100 drugs, with the half-life and dosage adjustments necessary. Brief explanations of cardiopulmonary bypass and common cardiac operations are given, together with pertinent pathophysiology, to form a basis for postoperative routines. The rationale for treatment of postoperative problems is discussed in sections devoted to bleeding, hypotension, low cardiac output, arrhythmias, pacing, hypertension, fever, postpericardiotomy syndrome, respiratory insufficiency, renal failure, and neurological complications. This is a comprehensive, yet short and concise, handbook which is a quick guide for intensive care unit care. The tables in the appendix summarize most of the current drugs for handy reference.
Providence, Rhode Island
Using a circulatory analogue, we investigated sequentially the performance of a dedicated ventricular assist device driver and an intraaortic balloon pump when driving a pneumatic ventricular assist device. Each drive device was compared under identical pumping conditions at rates of 40 to 120 cycles/min against two resistances. Our preliminary study showed that a modified
intraaortic balloon pump could drive a pneumatic ventricular assist device as effectively as its dedicated driver. The necessary modifications to and possible further development of the intraaortic balloon in this role are discussed.
P
Material and Methods
ulsatile ventricular assist devices (VADs) have been in limited clinical use for several years. These devices are employed in some centers in the management of cardiogenic shock after operation [l, 21 or myocardial infarction [3] when other supportive measures have failed. Ventricular assist devices also have a role in the support of a failing heart as a bridge to cardiac transplantation [4,51. In the United Kingdom, clinical experience with pulsatile VADs is limited to a very small number of patients. A major constraint on the further application of this support technique is the cost of the necessary equipment. A typical VAD system consists of a dedicated driver, costing around $35,000, and the VAD components themselves (the VAD shell, valves, blood sac, and conduits), costing $20,000 per patient. Few cardiac surgical centers in the United Kingdom could contemplate the regular use of such expensive devices, despite their possible lifesaving role [ 6 ] . Our group considered the possibility of using an intraaortic balloon pump (IABP) to power a pneumatic VAD. Most cardiac surgical centers have access to several IABPs. If VAD driving with an IABP were feasible, it could reduce the capital cost of introducing a VAD program by eliminating the need to purchase a costly dedicated VAD driver. In pneumatic and control terms, there are many similarities between the function of both drive devices. We therefore set out to investigate the feasibility of pumping a VAD with an IABP in a simple circulatory analogue, and compared its performance with that of a dedicated VAD controller.
Accepted for publication Nov 30, 1988. Address reprint requests to Dr Macrae, Department of Anaesthesia, Brompton Hospital, Fulham Rd, London SW3 6HP, United Kingdom.
0 1989 by The Society of Thoracic Surgeons
(Ann Thorac Surg 1989;47:752-5)
A two-chambered circulatory loop capable of emulating the flows and pressures seen in the systemic circulation in humans was constructed (Fig 1).A Pierce-Donachy pneumatic VAD (Thoratec, Berkeley, CA) was connected either to a dedicated VAD driver (Thoratec) or to a modified IABP (Datascope, Paramus, NJ). The IABP (Datascope System 80) was modified as follows. The slave chamber was removed and the exhaust system altered to permit either the dumping of exhaust gas directly to the atmosphere or the application of up to 30 mm Hg vacuum. This modification necessitated disconnection of the IABP ”low vacuum” alarm. With the VAD connected to the circulatory loop, two experiments were performed.
CRT MONITOR
I l l Ilzz--l Fig 1 . Components of the circulatoy analogue and its connection to the ventricular assist device WAD). (A = arterial chamber; C = interchangeable constriction; CRT = cathode ray tube; DP = deairing point; F = flow measurement point; IABP = intraaortic balloon pump; PI = “arterial” pressure transducer; P2 = driveline pressure trunsducer; R = reservoir; V = venous chamber.) 0003-4975/89/$3.50
MACRAE ET AL VENTRICULAR ASSIST DEVICES
Ann Thorac Surg 1989;47752-5
IABP DLP
1,'I
V A D driver
bmHg)
Fig 2. Typical "arterial" (AP) and driveline pressure (DLP) traces of both devices while operating at a rate of 80 cycleslmin. (IABP = intraaortic balloon pump; VAD = ventricular assist device.)
753
Table I . Mean "Arterial" Pressures Generated in Experiment 1 Under R1 and R2 Conditions" Rate (cycleslmin)
Mean "Arterial" Pressure (mm Hg) R1
R2
40
38
40
50
49
51
60 70
60
67
73
82
80
79
80
90 100
85 86
93
120
79
86
92
P
The R1 and R2 conditions were resistances of double-ta ered constrictions with minimum diameters of 0.178 cm' and 0.045 cm , respectively; compliance was constant.
Experiment 1 Loop conditions were determined with resistances consisting of double-tapered constrictions with minimum diameters of 0.178 cm2 (Rl) and 0.045 cm2 (R2) while compliance was held constant. The two drive devices were alternated at trigger rates of 40 to 120 cycleslmin in steps of 10 cycles/min. Driveline pressures delivered to the VAD were adjusted to maintain a difference of less than 5 mm Hg at the midsystolic point between the two drive devices for each paired comparison. A constant "venous" pressure was maintained, and the timing controls were then adjusted to achieve maximal VAD output while maintaining identical systolic and diastolic time intervals with both drive devices. "Arterial" and driveline pressures were recorded (Fig 2), and pump outputs were measured volumetrically .
Experiment 2 Experiment 1 was repeated with the IABP driver under R1 conditions as described except that a minimum systolic interval of 20% was permitted. Controls on the Thoratec VAD driver limit the minimum allowable systolic interval to 30%.
Results Experiment 1 Performance of the system was identical with both drive units under the conditions in this experiment. Figure 3 illustrates the flow rates achieved with the IABP and VAD drive units. Mean pressures generated within the loop are shown in Table 1. At rates higher than 60 cycles/min under R1 conditions and higher than 50 cycles/min under R2 conditions, the stroke volume of the VAD was reduced (reflected by the flattening of the flow rate curves in Figure 3).
Experiment 2 The IABP achieved optimal stroke volumes at higher pump rates with systolic intervals shortened to 20%, resulting in a diastolic filling interval of 80% of each cycle (Fig 4). This was not possible with the Thoratec driver, which permits reduction of the systolic time interval to only 30%,thus limiting the maximum diastolic filling time to 70% of a cycle.
Fig 3 . Flows obtained with the drive devices between rates of 40 to 120 cycleslmin (min-') in experiment 1 . (0 = intraaortic balloon pump at R1; 0 = intraaortic balloon pump at R2; = ventricular assist device at R1; 0 = ventricular assist device at R2.)
51
Rate (min-1)
754
MACRAE ET AL VENTRICULAR ASSIST DEVICES
Ann Thordc Surg 1989;47752-5
Fig 4 . Flows achieved with the intraaortic balloon pump in experiment 2 . (W = systolic interval of 30%; 0 = systolic interval of 20%.)
0
40
70
80
1
90
100
120
Rate (min-1)
Comment It appears from our study that a modified IABP can successfully drive a pneumatic VAD. Under conditions presented to the VAD in our study, VAD stroke volume fell at higher rates (see Figs 3, 4). Our observations during the study lead us to the conclusion that this was partly due to relatively slow filling of the VAD during pump diastole, as increasing the diastolic interval to 80% in experiment 2 led to higher flows. In clinical practice, VADs are likely to be run at rates that would allow longer diastolic filling times to occur. Also, application of a small amount of vacuum during diastole may improve VAD filling. Our exclusion of the slave or safety chamber from the pneumatic circuit of the IABP merits comment. In the intended role of an IABP, the slave chamber is essential, as it isolates the patient to some extent from the gas cpmpressor and permits use of a fixed volume of carbon dioxide or helium to fill the secondary (ie, patient) circuit. However, there is no advantage in retaining the slave chamber when pumping VADs, and indeed the PierceDonachy and similar VADs isolate the drive gas from the blood sac with a double envelope, thereby effectively providing a built-in safety layer. A leak in either layer leads to reduced VAD function, and is usually very obvious on visual inspection. Our initial experience while setting up this study demonstrated that adequate control of a VAD using an IABP in which the standard-volume or a large-volume (100-mL) slave chamber was retained was difficult to achieve. There could also be problems refilling a slave gas compartment, as clearly it is not desirable to interrupt VAD pumping for longer than a few seconds, and except in those IABP models with "auto-fill'' facilities, longer interruptions would be likely. We are, however, aware that a VAD driving system utilizing a "slave" arrangement is currently under development. Further development of IABPs as VAD drivers will lead to expansion of control and alarm systems to provide for the specific needs of VAD driving. Facilities for monitor-
ing the driveline pressure waveform [7, 81 and VAD stroke volume [9] are highly desirable. Alarm systems tailored to VAD driving requirements and fail-safe internal and external triggering will be essential for long-term use of an IABP in the VAD driving mode. We have constructed an additional control unit incorporating some of these features; it is currently being tested. In conclusion, a minimally modified IABP functioned satisfactorily as a VAD driver in a circulatory loop. We have conducted animal studies using the support system for periods of longer than 36 hours, and the results are encouraging. While we await the results of a more detailed evaluation, we believe that an IABP can be used as a backup for a dedicated pneumatic VAD driver. In the future, combined IABP and VAD drive devices may prove to be cost-effective replacements for the older separate devices. ~~
We thank the staff of the Heart-Lung and Medical Electronics Departments of Brompton Hospital for their cooperation during the study. M. Self of Datascope BV, Holland, and Datascope Medical Ltd, UK, provided advice and arranged partial funding of the project.
References 1. Pennock JL, Pierce WS, Wisman CB, et al. Survival and
2. 3. 4.
5.
complications following ventricular assist pumping for cardiogenic shock. Ann Surg 1983;198:469-78. Pierce WS, Grant VS, Parr GVS, et al. Ventricular assist pumping in patients with cardiogenic shock following cardiac operations. N Engl J Med 1981;305:1606-10. Pae WE, Pierce WS. Temporary left ventricular assist pumping in acute myocardial infarction and cardiogenic shock. Rationale and criteria for utilization. Chest 1981;79:692-5. Pennington DG, Codd JE, Merjavy JP, et al. The expanded use of ventricular bypass systems for severe cardiac failure and as a bridge to cardiac transplantation. J Heart Transplant 1984;3: 170-5. Farrar DJ, Hill JD, Gray LA, et al. Heterotopic prosthetic ventricles as a bridge to cardiac transplantation. A multicenter study in 29 patients. N Engl J Med 1988;318:33340.
MACRAE ET AL VENTRICULAR ASSIST DEVICES
Ann Thorac Surg 1989;47752-5
6. Glenville B, Ross D. Ventricular assist devices. Br Med J 1986;292:361-2. 7. Coleman SJ, Bornhorst WJ, Lafarge CG, Carr JG. Pneumatic
waveform diagnostics of implanted ventricular assist pumps. Trans Am SOCArtif Intern Organs 1972;18:17M. 8. Rosenberg G, Landis DL, Phillips WM, et al. Determining
755
arterial pressure, left atrial pressure and cardiac output from the left pneumatic drive line of the total artificial heart. Trans Am SOCArtif Intern Organs 1978;24:341-4. 9. Willshaw P, Neilsen D, Nanas J, et al. A cardiac output monitor and diagnostic unit for pneumatically driven hearts. Artif Organs 1984;8:215-9.
REVIEW OF RECENT BOOKS
Manual of Postoperative Management in Adult Cardiac Surgery By Carlos E. Moreno-Cabral, R . Scoff Mitchell, and D . Craig Miller Baltimore, Williams b Wilkins, 1988 102 p p , illustrated, $21.95 Reviewed by Karl E . Karlson, M D This pocket-size book describes in detail the principles and practice of postoperative care of cardiac patients in the Stanford University Medical Center. The manual is meant for the instruction and guidance of medical students, junior surgical residents, and intensive care unit nurses. In addition to a brief text (63 pages), there are copies of numerous forms and routine doctor’s order sheets that are used at Stanford. Various forms used by nurses, including Kardexes for sets of postoperative orders, are illustrated. There is a table of infusion rates for cardiovascular drugs. Antiarrhythmic drugs are tabulated with route of admin-
istration, dose, indications, and toxicity given. A table pertaining to drug therapy in renal failure lists over 100 drugs, with the half-life and dosage adjustments necessary. Brief explanations of cardiopulmonary bypass and common cardiac operations are given, together with pertinent pathophysiology, to form a basis for postoperative routines. The rationale for treatment of postoperative problems is discussed in sections devoted to bleeding, hypotension, low cardiac output, arrhythmias, pacing, hypertension, fever, postpericardiotomy syndrome, respiratory insufficiency, renal failure, and neurological complications. This is a comprehensive, yet short and concise, handbook which is a quick guide for intensive care unit care. The tables in the appendix summarize most of the current drugs for handy reference.
Providence, Rhode Island