Extended Total Cardiopulmonary Bypass in Puppies Brit Litchford, M.D., Marko Turina, M.D., Marcos Intaglietta, Ph.D., and Nina S. Braunwald, M.D.

ABSTRACT A servo-controlled perfusion system with a low priming volume intended for total cardiopulmonary bypass has been developed and was tested in puppies. The automaticity of the system is provided by two feedback circuits which depend on the negative pressure in the venous return line and the perfusion flow.

The system was tested by using closed-chest total cardiopulmonary bypass in puppies for periods of 12 to 24 hours. Ten puppies were perfused for 24 hours, and 4 survived. Once it was ascertained that almost all the deaths resulted from fluid overload and pulmonary edema, the overload was avoided by continuous monitoring of the pulmonary artery pressure, which was kept below 4 mm. Hg. Thereafter, 5 consecutive puppies undergoing total bypass for a 12-hour period survived with an actual reduction in body weight.

A without

result of continued improvement in the technology associated with the use of heart-lung machines, it is now possible to place patients on total cardiopulmonary bypass for periods of up to 6 hours difficulty. As more prolonged periods of support are employed,

however, the morbidity and mortality rise significantly. This is especially true in neonates, in whom the disproportion between the priming volumes required to fill the system and the tolerances for intracellular and extracellular fluid volume shifts and electrolyte imbalances become more critical. Many newborn infants with complex forms of congenital heart disease as well as those with hyaline membrane disease might benefit from a system which would provide more extended periods of cardiopulmonary support, if this could be effected safely.

Previous experiments with a compact, low-prime heart-lung machine developed in our laboratory employed a membrane oxygenator in a closed circuit, since this type of oxygenator is known to be less traumatic to blood than the conventional disc or bubble variety [l-3, 51. The regulation of perfusion in the original unit was dependent in large part on manual control. As the duration of bypass was extended, however, the technical

From the Department of Surgery, Peter Bent Brigham Hospital, Boston, Mass., the Department of Surgery and Bioengineering, University of California at San Diego School of Medicine, San Diego, Calif., and Surgical Clinic A, Kantonsspital, Zurich, Switzerland.

Supported by National Institutes of Health Grant HL 15631-01. Accepted for publication May 15, 1973. Address reprint requests to Dr. Eraunwald, Department of Surgery, Peter Bent Brigham

Hospital, Boston, Mass. 02115.

VOL. 16, NO. 4, OCTOBER, 1973 349

LITCHFORD E T AL.

demands on personnel became quite impractical and led to the development of recent modifications in the system to permit the substitution of a servo- control mechanism for the manual one. This report describes the experience gained in a series of puppies undergoing prolonged periods of total cardiopulmonary bypass of 12 to 24 hours with this system.

Description of PeTfusiun System Venous blood is actively suctioned into the bypass unit by a roller pump

which also propels the blood through a membrane oxygenator (1 sq. m. Land&-Edwards), a small heat exchanger (GO ml. prime), a filter bubble trap, and back into the arterial system. Blood flow is measured by an electromagnetic flowmeter. A servo pump transfers blood into or out of the perfusion circuit from a reservoir in order to regulate blood volume (Fig. 1 ) . The entire unit (Fig. 2A) is mounted on a stand placed on the operating table and requires only a 400 ml. priming volume.

The control system of the perfusion unit is dependent upon two electrically discrete feedback circuits which combine to give an integrated

I ARTERIAL PRESSURE -20 ! -10 LIMITER

PERFUSION FLOW FEEDBACK SYSTEM

. +lo- 6 '-+I, REFERENCE PRESSURE

mmHg

VENOUS RETURN PRESSURE FEEDBACK SYSTEM

FIG. I . Diagram of the servo-control circuit. T h e venous return feedback loop (lower left) uses the negative venous return line Pressure as input information. T h e negativity is sensed by the transducer (T,) and fed into a subtractor. T h e actual negative pressure is compared with the preselected reference pressure, and pump speed is modulated to minimize the dilference between them. Th i s results in the m a x i m u m flow for the volume available. T h e perfusion flow feedback system (upper right) moves uolume between the perfusion circuit and the reservoir in response to flow rates below or above the preselected limits. (T, = venous pressure transducer; T, I arteyinl pressurc trunsdurer.)

350 THE ANNALS OF THORACIC SURGERY

Extended Total Cardiopulmonary Bypass in Puppies

FIG. 2. (A) T h e perfusion unit. T h e membrane oxygenator is positioned below a metal plat- form, on which rests the pump used for venous return and reser- voir infusion and withdrawal. T h e heat exchanger and reservoir are on the right; the filter bubble trap is on the left. A 12-inch ruler is placed below for comparison of size. (B) T h e servo-control unit. T h e perfusion flow feedback is on the left with the flowmeter display and flow limit. The ve- nous return feedback is in the center with its display of negative pressure in the venous line. On the right is the high-pressure cutoff mechanism.

B

physiological control; it has been described previously in detail [4]. A venous return feedback controls the speed of the pump, which produces suction in the venous cannula. It responds to changes in negative pressure in the venous return line that are sensed by a strain-gauge transducer. A desired amount of negative pressure or suction, usually in the range of -12 to -15 mm. Hg, is preset on the control panel (Fig. ZB), and the pump adjusts its speed to achieve this value. This setting is usually determined by

VOL. 16, NO. 4, OCTOBER, 1973 351

LITCHFORD ET AL.

observing the actual negative pressure reading on the control panel attached to the strain gauge that produces the required flow, prior to making the final adjustment on the control dial. Thereafter the pump slows as the negativity increases and accelerates as it decreases to control venous return automatically.

The second control device is a perfusion flow feedback which controls a second bidirectional roller pump connected to the reservoir (see Fig. 1). If the flow drops below a preselected level, the pump is activated to infuse blood into the system. If it rises above that level, it removes blood. Flow limits are set on the control panel, and in order to prevent the introduction of air an automatic low-level photocell indicator provides an automatic shutoff if the reservoir level falls below a safe limit.

Both feedback systems operate on two types of operational modes, manual and automatic. Therefore, it is possible at all times for an operator to override these controls. Additionally, since some membrane oxygenators can be damaged by excessively high pressure, a pressure transducer has been introduced in the circuit which records the inflow pressure proximal to the oxygenator and activates an automatic venous pump shutoff mechanism whenever the pressure exceeds the preset level.

Experimental Methods and Materials Fifteen mongrel puppies weighing between 2.2 and 7.0 kg. were

anesthetized with 1 mg. per kilogram of body weight of Phensyclidine and Sparine intramuscularly. Atropine, 0.5 mg. per kilogram, and succinylcho- line, 10 mg., were administered. The animals were intubated and placed on a I-Iarvard respirator (18 respirations per minute, tidal volume 20 cc. per kilogram). Thereafter anesthesia was supplemented with 0.5y0 Fluothane in 100% oxygen. Following heparinization (330 units per kilogram), a metal arterial cannula 3 to 3.5 mm. in diameter was inserted into the femoral artery, and another thin-walled metal cannula was positioned in the right atrium through the right internal jugular vein for venous return. A plastic catheter (polyethylene 240) was used to drain the urinary bladder.

The oxygenator was rinsed three times prior to use, twice with 5% dextrose in water and once with Ringer’s lactate, and it was then connected to the circuit, which was primed with a mixture of 400 ml. of canine plasma and blood adjusted to a hematocrit of 30%. The electrocardiogram, the rectal and esophageal temperatures, the arterial and venous pressures, and the pump flow were continuously monitored throughout the experiment. In addition, the arterial Po2; Pco,; pH; serum Na, K, Ca, glutamic oxaloacetic transaminase (SGOT), glutamic pyruvic transaminase (SGPT), lactic de- hydrogenase (LDH), bilirubin, and creatinine; blood urea nitrogen; total protein: colloid osmotic pressure [7]; glucose; plasma; hemoglobin; hemato- crit; white blood count; and platelet count were measured prior to going on

352 THE ANNALS OF THORACIC SURGERY

Extended Total Cardiopulmonary Bypass in Puppies

bypass and at regular intervals throughout the period of total cardiopulmo- nary bypass and the postoperative period. All puppies were weighed before being anesthetized and again at the end of the operation.

The puppies were placed on bypass, the respirator was turned off, and total-body perfusion was instituted at normothermia while maintaining a flow of 140 ml. per kilogram per minute. The animals were then rapidly cooled to 30°C. and the flow reduced to 100 ml. per kilogram per minute. The venous return line pressure was adjusted so that it ranged from -15 to -40 cm H20, depending to some extent on flow and cannula position.

Fluothane was administered directly into the circuit as needed to keep the systemic arterial pressure between 80 and 100 mm. Hg. Increments of 10 mg. of succinylcholine were given if the puppies began to breathe spontaneously.

The arterial and venous Po2 levels were monitored continuously with in-line Po2 sensor probes. Flow was increased slightly if the pH began to fall below 7.35, and carbon dioxide values were maintained at normal levels by the addition of 2yo C 0 2 into the gas mixture. To prevent atelectasis, once every hour the lungs were inflated with room air by turning on the respirator. A constant record of the fluid intake and output was kept throughout the experiment. The hematocrit was maintained at 30y0 by additions of whole blood or plasma to the reservoir as needed. Serum potassium determinations were made every 2 hours, and 0.5 mEq. increments of KC1 were given if the values were less than 3 mEq. per liter until sequent serum determinations indicated a reversion to normal levels. Additional heparin (160 units per kilogram) was given every 2 hours together with 250 mg. of 50% dextrose in water every 2 hours to meet caloric requirements. Ampicillin, 125 mg., was given every 4 hours during the perfusion and twice daily thereafter for five days postoperatively.

Ten puppies placed on total cardiopulmonary bypass were perfused for a period of 24 hours and 5 for 12 hours. In the latter group, a Swan-Ganz catheter (5F) was floated into the pulmonary artery through a branch of the left external jugular vein for continuous pressure measurements and was used to regulate the pulmonary artery pressure, which was not allowed to rise above 4 mm. Hg at any time. The systemic venous pressure was kept in the range of -2 to +2 mm. Hg throughout the perfusion.

At the conclusion of the perfusion, the animals were rewarmed and given 1 to 2.5 mg. of Lasix. The flow was reduced over a five- to ten-minute period to allow the animal’s heart to resume function gradually. Bypass was terminated when the systolic arterial blood pressure was in the range of 100 mm. Hg.

The puppies were maintained on a respirator until they were fully awake and ventilating adequately as determined by arterial Po, values. The arterial and venous lines were removed, the incisions closed, and the puppies placed in an oxygen tent for 12 to 24 hours.

VOL. 16, NO. 4, OCTOBER, 1973 353

LITCHFORD ET AL.

Results Of the 10 puppies placed on cardiopulmonary bypass for 24 hours

during the early phase of the investigations, 4 survived and were well several months later.

Most of the deaths were associated with the development of progressive pulmonary edema, which was noted to appear after about 12 to 24 hours of perfusion. Once colloid replacement was substituted for crystalline fluid replacement in the reservoir, this problem was controlled; and the colloid osmotic pressure thereafter averaged 24 cm. H20 as measured with a membrane transducer csmometer [7]. In 3 puppies death was due to technical error which included air embolism and calcium and potassium overdose.

All 5 puppies maintained on total bypass for 12 hours and in which the pulmonary artery pressure was kept below 4 mm. Hg throughout the perfusion were long-term survivors. At the conclusion of the experiment they were found to weigh an average of 200 to 300 gm. less than before bypass, representing a loss of 5.4% of their original body weight. Animals in this group required support on the respirator for 1 '/2 to 4 hours.

The Po, values at all times were in the range of 150 to 300 mm. Hg (Fig. 3) during the course of perfusion. There was a moderate fall in arterial Po, to an average of 80 mm. Hg with a range of 60 to 100 mm. Hg during the rewarming phase. The temperature-corrected CO, values averaged 30 mm. Hg and ranged from 25 to 37 mm. Hg at 6 hours. The pH remained above 7.38 throughout the 12-hour perfusions in all except 1 animal, in which it fell to 7.24; in this instance it responded to a single dose of 0.67 THAM buffer shortly after beginning the perfusion. The average colloid osmotic pressure in the donor blood was 27 cm. H,O; after circulating through the oxygenator, the value fell to 13 cm. H,O; and during the per- fusion the mean value rose to 24 cm. H20 . The serum protein values av- eraged 6 gm. per 100 ml. in the donor blood, falling to 3.6 gm. per 100 ml. during precirculation and rising to an average of 5.6 gm. per 100 ml. at the end of the 12- to 24-hour perfusions.

The average white blood cell count in the donor blood was 10,000 per cubic millimeter. This fell to 4,000 during precirculation and rose again to around 7,000 by the end of a 12-hour bypass. Platelets fell from 200,000 to 50,000 per cubic millimeter during precirculation and rose during the perfusion to the range of 150,000 to 200,000 (Fig. 4).

Serum sodium values ranged from 142 to 148 mEq. per liter during perfusion. The potassium values ranged between 4 and 5 mEq. per liter with occasional values as low as 3.5 mEq. per liter during periods when urine output was excessive. Potassium replacement during the course of a 12-hour perfusion averaged 5.7 mEq. KC1 per kilogram of body weight.

Blood urea nitrogen determinations averaged 10 2 3 mg. per 100 ml.

354 THE ANNALS OF THORACIC SURGERY

Extended Tota l Cardiopulmonary Bypass in Puppies

PO, 300 mmHg 200 :: I00 50 0

40 PCO2

mmHg 30

20

- lt-p?i;' -

-

-

before bypass and 16 2 4 mg. per 100 ml. after bypass. The average urine output was 102 ml. per kilogram during a 12-hour perfusion and ranged from 60 to 161 ml. per kilogram. Plasma hemoglobin rose to a mean value of 243 mg. after 12 hours (Fig. 4). The average SGOT was 38 units before bypass and 95 afterward. The LDH rose from 100 to 350 units, and SGPT exhibited very little if any significant rise (from 20 to 28 units). After 12 hours of perfusion the average total bilirubin rose from 0.12 mg. to 0.9 mg. per 100 ml. The addition of a glucose solution (50y0) maintained the blood glucose levels between 95 and 185 mg. per 100 ml.

Analysis of the performance of the servo-control mechanism indicated that with high degrees of suction, as manifested by readings in the venous line in the range of -60 mm. Hg, there was periodic collapse of the walls of the large veins that impeded venous return. Between -40 and -60 mm. Hg, perfusion flow was relatively constant. As the negative pressure approached 0 mm. Hg, the perfusion flow decreased progressively to a range of 80 ml. per minute. There was a concomitant fall in aortic pressure with the expected rise in inferior vena cava pressure.

I t was possible to achieve total cardiopulmonary bypass in this

VOL. 16, NO. 4, OCTOBER, 19'73 355

LITCHFORD E T AL.

16

12 wBcxl000

4 -

0 -

500

400

PLATELETS 300 XlOOO 200

100

0 -

- - :

- - - - -

T

400-

PLASMA Hbg mg% 200

100

0 -

I

- -

T

I-- BYPASS -4 I ,

DONOR PUMP PUPPY O H R S 6 HRS 12 HRS 2 HRS BEFORE BEFORE POST BYPASS BYPASS BYPASS

FIG. 4 . T h e white blood cell count, platelets, and plasma hemoglobin in 5 puppies undergoing 12 hours of cardiopulmonary bypass and 2 hours after coming off the circuit. T h e white blood count and platelet value decreased during the passage through the oxygenator, as a result of either dilution with the remaining rinse solution or adherence of these elements to the membranes. A recovery in the values was noted during the course of the perfusion. Plasma hemoglobin rose to 243 mg. per 100 ml. after 12 hours but continued to increase even after bypass. During this latter period, volume losses were replaced with donor canine packed cells or whole blood.

experiment with the combination of a single thin-walled cannula positioned in the right atrium through the jugular vein and suction by means of a pump. As flow increased to the range of 600 ml. per minute (Fig. 5), the systemic and pulmonary arterial pressures became nonpulsatile. Both the right atrial and pulmonary artery pressures fell to the range of 2 to 4 mm. Hg. At this time, the Po, values measured from blood samples taken from the pulmonary artery were higher than systemic mixed venous samples, suggesting a reversal of flow in the pulmonary vessels.

C o ?n men t In the past, prolonged periods of total cardiopulmonary bypass with

survival have been especially difficult to achieve, particularly in neonates. Certain features of the system employed in the present series of investigations were believed to contribute to achieving survival after more prolonged periods of perfusion. The use of a servo-control mechanism, instead of manual regulation, combined with a system requiring a very small priming volume assured considerable hemodynamic stability during the entire period of bypass (Fig. 6). Thus infusion and removal of as little as 25 ml. of blood

356 THE ANNALS OF THORACIC SURGERY

Extended T o t a l Cardiopulmonary Bypass in Puppies

ECG

FLOW m l f min

AORTA mmHg

I vc mmHg

PA mmHp

1

1000 j 0

200

100

0

40

0

40

EF6lN BYPASS BYPASS ~OOYPLETE P.A. OECOYPRTSSED 3 mia in BYPASS

P

-

10 SEC ' F I G . 5 . Init iation of total cardiopulmonary bypass in a 6 kg. puppy at 37°C. As flow increased to higher than 500 ml. per minute , the aortic pressure became nonpulsatile, as did tlie pulmonary artery (PA) pressure. T h e inferior uena cawa (IVC) and pulmonary urtery pressures fell lo necrr 2 rnni. H g . These obserunfions along w i f h the arterial Po, data suggest reversal of flow in the pulmonary vasculature, indicating that the entire venous return was removed through the single atrial cannula. (ECG = electrocardiogram.)

ECG

FLOW ml/min

AORTA mmHg

I V C mmHg

PA mmHg

FIG. 6.

1000

500

0 200

I00

0

40

20

0

40

20

0

10 SEC

1 H O U R 3 HOURS 6 HOURS 12 HOURS

Hernodynamic dala during 12-hour total cardiopulmonary bypass i n a 6 kg. p u p p y at 30°C. T h e flow, aortic pressure, inferior uena cauu (IVC) pressure, and pul- monary artery (PA) pressure remained very conslant throughout the 12-hour bypass. (EGG = electrocardiogram.)

VOL. 16, NO. 4, OCTOBER, 1973 357

LITCHFORD ET AL.

were achieved with little alteration of blood flow or of systemic arteriovenous or pulmonary artery pressures. Analysis of fluid balance during the course of 12 to 24 hours of servo-controlled total cardiopulmonary bypass revealed that in general the fluid volume which the perfusion flow feedback unit added to the system was equivalent to the volume depleted during bypass by urinary losses, blood samples, and so on (Fig. 7). As a result, relatively little weight change was observed in the surviving puppies even after 24 hours of perfusion.

Additionally, total cardiopulmonary bypass has been difficult to achieve in the closed-chest preparation using conventional methods of cannulation. Total bypass was accomplished in the present studies by positioning an extremely thin-walled cannula at the junction of the superior vena cava and right atrium and regulating the amount of suction on the line so that maximum venous return short of caval collapse was achieved. The facts that the aortic pressure was nonpulsatile in configuration, that the pulmonary artery pressure was in the range of 0 to 4 mm. Hg, and that P o p values obtained from samples of blood drawn from the pulmonary artery were higher than mixed venous samples, indicating reversal of flow in this vessel, all served to confirm that total bypass had in fact been achieved.

It was believed, however, that the single most important factor contributing to increased survival on a more consistent basis as the work progressed was due to initiation of continuous monitoring of the pulmonary artery pressure by means of the Swan-Ganz catheter. By meticulously avoiding any situation which led to an excessive rise in pulmonary artery pressure, even transiently (such as might be produced by excessive perfusion

350 - VOLUME L O S T ml /Kg o-o

VOLUME ADDED m l / K g 0-0 300 -

250 -

200 -

150 -

k OFF BYPASS

T I M E IN HOURS

FIG. 7. Fluid balance in a 4.3 kg. puppy during the course of 12 to 24 hours of seruo- controlled total cardiopulmonary bypass at 30°C. (hematocrit, joyo). In general, the fluid volume that the unit added to the system was equivalent to the volume lost by the puppy.

358 THE ANNALS OF THORACIC SURGERY

Extended Tota l Cardiopulmonary Bypass in Puppies

flow, for example), it was noted that pulmonary congestion was avoided; postoperative pulmonary problems were virtually eliminated thereafter, and more consistent survival on a regular basis was achieved.

In general, utilization of this unit was found to be associated with excessive hemolysis, considering the relatively prolonged duration of bypass. This was the case in spite of the use of a totally occlusive pump and creation of a moderate degree of negativity on the venous side, factors both generally thought to be responsible for the destruction of red blood cells [6].

The incorporation of a membrane oxygenator was a prerequisite for the development of the system. A completely closed blood circuit is essential for proper function of the feedback control system. Volume changes in a closed system are subject to much closer scrutiny and more accurate correction. Also, the membrane eliminates the blood-gas interface and therefore much of the trauma to blood seen with conventional oxygenators.

Analysis of the data reveals that there was an initial decrease in total protein, colloid osmotic pressure, white blood cells, and platelets after going on bypass, which might have been due in part to the use of a diluted prime as well as the adherence of some blood elements to the silicone membrane layers in the oxygenator. The fact that the volume losses incurred during the course of the perfusion were replaced with either blood or plasma, depending on the hematocrit, probably accounts for the recovery levels seen in both the plasma proteins and the colloid osmotic pressure following the termination of bypass. Crystalloid solutions were used as replacement fluid more frequently in the early experiments. They may have contributed to the extravascular fluid overload observed early in our experience in some of the puppies undergoing 24-hour perfusion which did not survive.

The data relative to acid-base balance, renal function, and survival percentage in the puppies undergoing 12 hours of perfusion substantiate the impression that the metabolic requirements of the animals were being met satisfactorily, a reflection of a consistently adequate perfusion. The relatively high arterial Po, values at the conclusion of bypass and the 5.4% weight reduction during bypass indicate minimal vascular extravasation, a function of strict volume control.

On the basis of the compactness of the unit, the relatively low prime volume, the ease of operation, and the ability of this system to deliver a consistently adequate perfusion, it is believed that this servo-controlled perfusion system has solved some of the problems associated with the use of total cardiopulmonary bypass of extended duration, particularly in subjects of small size.

References 1. Hill, J. D., DeLeval, M. R., Dontigny, L., Bramson, M. D., Osborn, J. I., and

Gerbode, F. Clinical prolonged extracorporeal circulation for respiratory insufficiency. Transplant. Proc. 3: 1437, 1971.

VOL. 16, NO. 4, OCTOBER, 1973 359

LITCHFORD E T AL.

2. Landk, A. J. , Edwards, L., Bloch, J. H., Carlson, R. G., Subramanian, V. A., Ascheim, R. S., Scheidt, S., Fillmore, S., Killip, T., and Lillehei, C. W. Clinical experience with emergency use of prolonged cardiopulmonary bypass with a membrane pump-oxygenator. Ann . Thorac. Surg. 10:409, 1970.

3. Turina, M., Houseman, L. D., Intaglietta, M., Schauble, J., and Braunwald, N. S. An automatic cardiopulmonary bypass unit for use in infants. J . Thorac. Cardiovasc. Surg. 63:263, 1972.

4. Turina, M., Litchford, B., Intaglietta, M., and Braunwald, N. S. Servo- controlled extended cardiopulmonary bypass. Trans. Am. Soc. Artzf. Intern. Organs. In press.

5. White, J. J., Andrews H. G., Risemberg, H., Mazur, D., and Haller, J. A., Jr. Prolonged respiratory support in newborn infants with a niembrane oxygenator. Surgery 70:288, 1971.

6. Yarborough, K., Mockros, L. T., and Lewis, T. H. Hydrodynamic hemolysis in extracorporeal machines. J . Thoyac. Cardzovasc. Surg. 52:555, 1966.

7. Zweifach, B. W., and Intaglietta, M. Measurement of blood plasma colloid osmotic pressure. Microvnsc. Res. 3:83, 1971.

360 THE ANNALS OF THORACIC SURGERY