Flow dynamics of peripheral venous catheters during extracorporeal membrane oxygenation with a centrifugal pump

J

THoRAc CARDIOVASC SURG

1988;96:478-84

Flow dynamics of peripheral venous catheters during extracorporeal membrane oxygenation with a centrifugal pump ExtracorporeaI membrane oxygenation uses peripheraUy placed cannulas and a streamlined circuit without a venous reservoir. This study tests the flow dynamics of venous catheters cormected without a reservoir directly to a centrifugal pump. During in vitro testing, a 30 cm segment of coUapsible tubing interposed between the reservoir and pump simulates the vein. In five sheep, flow was measured between catheters placed in the right atrium and inferior venacava from peripheral sites. Catheter tip design(four types) does not affect flow within a simulated vein in vitro. Maximum pump flow is independent of tiDing pressures (6 to 21 mm Hg) in vitro and in vivo when the catheter tip is in a tank reservoir or the right atrium. However, when the catheter tip is within a coUapsible segment or in the inferior vena cava, maximal flow is significantly influenced by tiDing pressure (6 to 18 mm Hg) and by the ratio of catheter outer diameter to venous diameter. At aU tiDing pressures, maximal flow in vivo is significantly reduced when this ratio is greater than 0.5. During extracorporeaI membraneoxygenation,central venous pressure and catheterJvein ratio, not catheter size alone, control flow througb peripheral venous catheters.

Robert K. Wenger, MD, Joseph E. Bavaria, MD, Mark B. Ratcliffe, MD, Daniel Bogen, MD, PhD, and L. Henry Edmunds, Jr., MD, Philadelphia, Pa.

Extracorporeal membrane oxygenation (ECMO), first used in the treatment of adult respiratory distress syndrome.I' is now recommended for neonatal respiratory failure' and for rapid initiation of circulatory support.' In contrast to left ventricular assist devices" and the artificial heart," ECMO is inexpensive, widely available, and can be rapidly implemented without thoracotomy." When used for circulatory assistance, a simplified perfusion circuit containing a centrifugal pump and no venous reservoir is preferred.v" Previous studies have defined the flow dynamics of centrally placed venous catheters.v" but there are no data that describe the flow dynamics of peripherally placed venous catheters connected in series with a centrifugal pump. The purpose of this study is to define the flow

From the Harrison Department of Surgical Research, University of Pennsylvania, Philadelphia, Pa. Supported by National Institutes of Health Grant HL 36308. Received for publication July 30, 1987. Accepted for publication Jan. II, 1988. Address for reprints: L. Henry Edmunds, Jr., MD, Department of Surgery, Hospital of the University of Pennsylvania, 34th and Spruce Sts., Philadelphia, PA 19104.

478

limitations of peripheral venous catheters when used with a centrifugal pump without a venous reservoir.

Methods Two types of venous catheters suitable for use in a simplified ECMO circuit were studied in vitro and in sheep. Straight catheters, sizes 14F to 24F, were supplied by Polystan AjS (Ballerup, Denmark) and Bard Cardiopulmonary (Santa Ana, Calif.) (Fig. I). In addition, four Bard 28F catheters with different tip designs were also tested (Fig. 2). The Polystan catheters are 40 em in length and are not wire reinforced. Bard (USCI) catheters are also 40 em in length; these catheters are wire reinforced. All sizes refer to outer diameters; wall thickness and therefore inner diameter vary between manufacturers. The in vitro test circuit, a modified "Starling resistor" (Fig. 3), uses a I-cubic-foot Plexiglas acrylic reservoir with a 1.8 cm inner diameter outflow port. A 30 cm length of thin-walled latex tubing (circumference 4.7 em, diameter L5 ern) is attached to the outflow port to simulate a collapsible peripheral vein. The tubing is not stretched. Test catheters are inserted into the latex tubing so that the catheter tip is precisely 10 ern from the downstream end of the latex tubing. Catheters are connected by polycarbonate connectors to the inflow port of a centrifugal pump (model 520C, Bio-Medicus, Inc., Eden Prairie, Minn.) by a 100 em length of 5fs-inch inner diameter polyvinyl tubing. The outlet port of the pump is attached to a 100 cm length of polyvinyl tubing that contains an in-line electromagnetic flow probe (model BL-615, Biotron-

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Extracorporeal membrane oxygenation

September 1988

479

o USCI

1967

USCI

1956

USCI 1854

USCI 1840

Fig. 2. Tested tip designs of four USCI 28F catheters.

Fig. 1. Tip design of test catheters. USCI catheter (left) is wire-wrapped; Polystan catheter (right) is not. ex Laboratory, Inc., Kensington, Md.) 30 ern from the outlet port. A 60 em vertical column provides constant afterload; the end of the tubing discharges into the reservoir, which is filled with a 0.9N saline solution. Pressure within the system is measured by a pressure transducer at the reservoir outlet (Gould DTX disposable transducer, Gould Inc., Cleveland, Ohio). Filling pressures between 6 and 18 ern H 20 are adjusted by changing the saline level in the reservoir. Pump speed is gradually increased and maximum flow is determined either by reaching maximal speed (4500 rpm) or by collapse of the tubing around the catheter tip. In all studies, maximum pump flow is a function of fluid ingress and not pump output, which can exceed 8 Lzmin. Each catheter is tested five times both with the collapsible latex tubing in place and also with the catheter tip placed directly into the reservior. The electromagnetic flow probe is calibrated several times during each test session by timed measurements into a 2 L graduated cylinder. Five adult Blackface sheep between 30 and 50 kg (mean 42 kg) were used for in vivo studies. All animals received humane care in compliance with the "Principles of Laboratory Animal Care" as formulated by the National Academy of Sciences and published by the National Institutes of Health (NIH Publication No. 80-23, revised 1978). Anesthesia was induced in a fluoroscopy suite intravenously by thiopental 25 rng/kg and maintained with halothane (0.25% to 1.0%) in air. The animals were intubated and the lungs were mechanically ventilated. Left femoral arterial pressure and central venous

pressure at the level of the diaphragm were monitored (Gould DTX disposable transducer) and displayed on a strip chart recorder (Gould electrostatic recorder ESloooB). The left internal jugular vein and right common iliac vein were exposed. After heparin administration (15,000 U intravenously supplemented by 10,000 U at I hour), a 28F catheter was placed in the right atrium from the jugular vein and a 14F catheter was placed via the right iliac vein in the inferior vena cava just cephalad to the confluence of renal veins. Position of the catheters was verified with a fluoroscope. The two venous catheters were connected to a simple extracorporeal perfusion system, which consisted of a centrifugal pump (model 520C, Bio-Medicus, Inc.), a venous reservior, and an in-line electromagnetic flow probe (BL-615, Biotronex Laboratory, Inc.). The system was primed with a solution of 6% hetastarch in 0.9% saline (American Hospital Supply Corp., Evanston, 111.). The flow probe was calibrated by timed measurements with a blood-starch solution. The animals' blood volume and the priming starch solution were thoroughly mixed before studies began. The central venous pressure was adjusted to 6 mm Hg by removal of perfusate volume. Blood was then pumped from the inferior vena cava catheter to the right atrium until maximal flow was achieved. Each test was repeated three times; the caval catheter was then changed to a larger size and the test repeated. Catheters between 14F and 24F were tested. All catheters were tested at 6 mm Hg central venous pressure before the pressure was raised to 9 mm Hg by volume loading. Each catheter size was tested at central venous pressures of 6, 9, 12, 15, 18, and 21 mm Hg. After tests of inferior caval catheters, a 24F catheter was placed in the inferior vena cava in three sheep. Catheters of different sizes were then inserted into the jugular vein and advanced until the tip was within the right atrium. Maximal flow was measured for each catheter size and at each central venous pressure with the direction of flow reversed (i.e., right atrium to inferior vena cava). After the completion of tests, the animals were killed. The electromagnetic flow probe was calibrated by direct volume measurements, and calibration factors were calculated and applied to flow probe readings if necessary. The segment of inferior cava just cephalad to the confluence of the renal veins was excised. The circumference of this segment was measured, and the diameter was calculated and used to determine the ratio of catheter outer diameter to caval diameter (10.2 to 14.3 mm). The mean maximal flow rate measured for each catheter diameter for each central venous pressure during cavoatrial

480

The Journal of Thoracic and Cardiovascular

Wenger et al.

Surgery

..

60cm

pressure

(or'r- 30cm----1 H

I 1

..

..

r----L.r-------

IOcm

Fig. 3. Diagram of in vitro test circuit. Catheter tip is placed 10 ern into 30 ern length of thin-walled, unstretched latex rubber tubing (1.5 em diameter). Catheter is connected directly to centrifugal pump. Filling pressure is regulated by height of fluid within Plexiglas acrylic reservoir.

MOll.imum

Flow

(L/min)

Fig. 4. Maximum flow versus reservoir pressure for USCI and Polystan catheters in the in vitro test circuit. Numbers above each curve refer to catheter outer diameter in French units (1 mm = 3F).

pumping was plotted against the catheter/caval ratio for each filling pressure.

Results In vitro testing of four tip designs of 28F USCI catheters (9.3 nun diameter) placed within a 16.2 nun diameter segment of latex tubing shows no relationship between maximal achieved flow and tip design. Maximal flow in these four Bard 28F catheters ranges from a mean of 3.1 ± 0.1 Ljmin at 6 nun Hg filling pressure to 5.5 ± 0.05 Ljmin at 21 nun Hg and does not vary measurably among catheters. When the catheter tip is placed directly into the Plexiglas acrylic reservoir, filling pressure does not affect maximum flow (data not shown). Maximum flow varies directly with catheter diameter. Polystan cathe-

ters, which have thinner walls, permit I9% to 26% higher flows than comparably sized USCI catheters. When catheters were inserted into the right atrium from the internal jugular vein in three sheep, changes in central venous pressure between 6 and 2I mm Hg did not alter maximum flow. Maximum flow varies directly with catheter size (Table I) and is higher with Polystan catheters. The graph on the left of Fig. 4 represents the maximum flows of USCI catheters (l4F to 22F) as a function of filling pressure when tested in the in vitro circuit. The l4F catheter with an outer diameter of 4.7 nun maintains a constant flow at varying filling pressures, and the data are identical to those obtained when the catheter tip is placed directly in the reservoir. The l6F catheter, with an outer diameter of 5.3 mm, shows the effect on flow of filling pressures between 6 and 9 nun Hg; above this point, filling pressures has no effect on flow (Fig. 4, left graph). For catheters with outer diameters greater than 5.3 nun, maximal flow is affected by the interaction between the collapsible latex segment and the catheter tip. Maximum flow actually decreases as catheter size increases, and this effect is only partially overcome by increasing filling pressures. A similar pattern is observed with Polystan catheters (Fig. 4, right graph). When maximal flow is reached, the centrifugal pump generates negative pressures as low as -300 torr (measured at the pump inlet). Flow decreases rapidly until pump flow is reduced and the latex tubing and catheter tip separate. The importance of the catheter-vein interaction on maximum flow is clearly illustrated by plotting maximum flow as a function of the ratio of catheter outer diameter to the diameter of the collapsible latex segment (Fig. 5). For USCI catheters, maximum flow at all filling pressures occurs when this ratio is 0.56. Deviation

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Extracorporeal membrane oxygenation

September 1988

7.0

USC I in vitro

481

Polystan in vitro

6.0 18mmHg

15mmHg c

E

5.0

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Fig. 5. Maximum flow plotted against ratio of catheter outer diameter/latex diameter (1.5 em). Data are from measurements made in the in vitro circuit. o.D., Outer diameter.

Table I. Maximum flow from right atrium to inferior vena cava USCI

Polystan

Catheter size

Mean max. flow (Lfmin)

SD

14F 16F 18F 20F 22F

2.6 3.2 4.2 4.8 5.4

0.25 0.29 0.26 0.62 0.78

Catheter size

Mean max. flow (Lfmin)

SD

14F 16F 18F 21F

3.2. 3.5 4.5 6.0

0.24 0.55 0.70 0.42

Studies in three sheep. In each sheep. flows were measured three times. at 6. 9. 12. 15. 18. and 21 mm Hg central venous pressure. Since filling pressure did not measurably affect maximal flow. the data were pooled to calculate means and standard deviations (SD).

from this ratio decreases maximum achievable flow. Polystan catheters achieve maximum flow at a catheter/latex diameter ratio of 0.63, and variance from this number similarly decreases maximum flow. When flow is established from the inferior vena cava to the right atrium, the importance of caval diameter, collapsible caval wall, and catheter diameter is apparent. As observed in vitro, the ratio between catheter outer diameter and caval diameter affects maximal flow (Fig. 6). For both USCI and Polystan catheters, peak flow occurs near a ratio of 0.5. In vivo, the differences in wall thickness between catheters produces less effect on maximal flow than observed in vitro. As expected, central venous pressure greatly affects maximal flow. At maximal flow in vivo, the lowest pressure recorded at the right atrial-inferior caval junction is zero. Flow

rate falls rapidly and staccato flow at rates well below maximum occurs if pump speed is not reduced. Thinwalled Polystan catheters sometimes collapse. A reduction in pump speed restores continuous flow. We did not observe bubble formation during staccato flow and did not observe damage to the excised vein wall adjacent to the catheter tip.

Case report A 28-year-old woman weighing 43 kg with congenital cardiomyopathy and unstable hemodynamics was transferred for cardiac transplantation. Although hemodynamically unstable, she had pneumonia in the lower lobe of the left lung. After cardiac arrest, she was resuscitated with the aid of an intraaortic balloon pump. After recurrent episodes of ventricular fibrillation, she received heparin I rug/kg, and an 18F USCI catheter was inserted into the right internal jugular vein

The Journal of Thoracic and Cardiovascular Surgery

4 8 2 Wenger et al.

5.0

USCI

in vivo

Polys tc n in vivo

(n= 5)

(n= 5)

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Fig. 6. Maximum flow plotted against ratio of catheter outer diameter/inferior vena caval diameter during perfusion at central venous pressures between 6 and 21 mm Hg in five sheep. Three measurements were made at each pressure in each sheep. Ratios were calculated after measurement of caval circumference and collated into IO groups 0.025 apart. Thus each curve is drawn between IO points for each of the six different pressures. Standard errors ranged between 0 and 0.63 L/min for the 60 flow points that were calculated for the two different catheter designs. OiD.. Outer diameter; I. V.C. inferior vena caval.

and advanced into the right atrium (Fig. 7). An l8F Bard catheter was inserted into the right femoral artery. The perfusion circuit consisted of a centrifugal pump (Bio-Medicus 530), a spiral coil membrane oxygenator (1.5 m', SciMed Life Systems, Inc., Minneapolis, Minn.), and an in-line electromagnetic flowmeter and functioned in parallel with the natural heart, which continued to eject when unloaded by the intraaortic balloon pump. The perfusion circuit maintained uninterrupted flow between 3.3 and 4.0 L/min at pulmonary arterial diastolic pressures ranging from 12 to 38 mm Hg (mean 23 mm Hg) for 26 hours, at which time the diagnosis of brain death was made.

Discussion There is a need to provide temporary circulatory assistance beyond that provided by the intraaortic balloon pump for patients with reversible, life-threatening cardiac dysfunction (e.g., stunned myocardium") and for those who require immediate cardiac transplantation.? Magovern, Park, and Maher" have estimated that 1% of patients having cardiac operations will require postoperative mechanical circulatory support beyond the intraaortic balloon pump and that approxi-

Fig. 7. Portable chest roentgenogram of 28-year-old woman after cannulation of right internal jugular vein with l8F USCI catheter. Dashed line traces distal portion of catheter within enlarged right atrium. A Swan-Ganz catheter is in right pulmonary artery.

mately 50,000 patients a year die of acute cardiogenic shock despite maximal medical therapy. ECMO provides several advantages over other methods of mechanical circulatory assistance. Unlike the intraaortic balloon pump, ECMO actually circulates blood; unlike left ventricular assist devices and the artificial heart, ECMO does not require thoracotomy. For long-term applications, the centrifugal pump is superior to the roller pump." Because peripheral vessels are cannulated, ECMO is rapidly implemented.' Other advantages include wide availability, low cost, and ease of operation. ECMO requires only low-dose heparin and does not require full-time technical support; it can be run safely by the nurse at the bedside. The major disadvantages of ECMO are the need for heparin, damage to blood elements, the potential for sepsis around transcutaneous catheters, and inadequate venous return from peripherally placed catheters. This study addresses the latter problem. When catheter tips are within large reservoirs (e.g., tank or right atrium), filling pressures between 6 and 21 mm Hg have no measurable effect on maximal flow. Under these conditions, interaction between catheter tip and reservoir walls is minimal and flow is largely a function of catheter internal diameter.

Volume 96 Number 3

Extracorporeal membrane oxygenation

September 1988

Peripherally placed catheters do not tap unlimited venous reservoirs. Flow is limited by the amount of blood that reaches the vein around the catheter tip and the interaction between the collapsible vein wall and catheter tip. The in vitro model nicely simulates this interaction and shows that filling pressure and the ratio of catheter outer diameter to "vein" diameter are the primary determinants of flow. The importance of filling pressure is obvious, because higher filling pressures distend venous tributories, effectively increase the volume of blood around the catheter tip, and reduce the interaction between caval wall and catheter tip. Unfortunately, patients do not tolerate sustained high filling pressures, particularly during extracorporeal perfusion. The massive fluid infusions and the release of vasoactive peptides from activated blood elements result in massive accumulations of interstitial fluid, which undoubtedly compromise organ function. For temporary circulatory support, ECMO and other mechanical assist devices must maintain central venous pressure near preassistance values. The catheter/vein ratio determines the interaction between catheter tip and vein wall and, like filling pressure, is a major determinant of pump flow in vitro and in vivo. Our data clearly show the dynamics of the Starling effect and disprove the axiom that larger peripheral catheters mean larger flows. When the ratio is low (e.g., Fig. 5, left graph), flow is a function of catheter resistance and the effect of both filling pressure and catheter-vein interaction is minimal. As the ratio increases and catheter resistance decreases, the importance of both filling pressure and catheter-vein interaction becomes apparent. Like roller pumps, centrifugal pumps develop negative pressure when inflow is obstructed. We have measured pressures at the pump inlet at less than -300 rom Hg in vitro. In vivo flow rates fall rapidly and staccato flow develops as incoming blood transiently separates catheter tip and vein wall. Continuous flow can be restored at a lower flow rate or by increasing venous pressure. The staccato flow phenomenon preempts the need for a venous reservoir and streamlines the circuit to the two essential elements: pump and oxygenator. The streamlined system reduces heat loss and simplifies operation so that a technician is not required. Maximum flows in vivo are uniformly less than those obtained in vitro. This observation is probably due to in vivo limitations of venous return into the inferior vena caval reservoir from the venous tributaries of the animal. This tributary flow is an anatomic variable that is

483

partially influenced by blood volume, central venous pressure, and venous tone. Usually, a flow rate of 2 to 4 L/min is needed to provide temporary circulatory support for an adult. Phillips and associates" have achieved a flow rate of 2. to 2.5 L/min using two venous catheters inserted percutaneously into the inferior vena cava. As shown in Table I, larger flow rates are obtained with smaller single catheters when the catheter tip is advanced into the right atrium. For practical purposes, 16F to 20F catheters inserted via the right internal jugular vein into the right atrium should provide adequate partial circulatory assistance for most adults at near normal venous pressures.

I.

2.

3.

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5.

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7.

8.

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10.

II.

REFERENCES Hill JD, O'Brien TG, Murray JJ, et al. Prolonged extracorporeal oxygenation for acute post-traumatic respiratory failure (shock-lung syndrome): report of a successful case using the Bramson membrane lung. N Engl J Med 1972;286:629-31. Zapol WM, Snider MT, Hill JD, et al. Extracorporeal membrane oxygenation in severe acute respiratory failure. JAMA 1979;242:2193-6. Bartlett RH, Roloff DW, Cornell GR, Andrews AF, Dillon PW, Zwischenberger JB. Extracorporeal circulation in neonatal respiratory failure: a prospective randomized study. Pediatrics 1985;76:479-87. Phillips SJ, Ballentine B, Sionine D, et al. Percutaneous initiation of cardiopulmonary bypass. Ann Thorac Surg 1983;36:223-5. Pierce WS, Parr GVS, Myers JL, Pae WE Jr, Bull AP, Waldhausen JA. Ventricular assist pumping in patients with cardiogenic shock. N Engl J Med 1981;305:160610. Rose DM, Laschinger J, Grossi E, Kreiger KH, Cunningham JNJ, Spencer Fe. Experimental and clinical results with a simplified left heart assist device for treatment of profound left ventricular dysfunction. World J Surg 1985;9:11-7. Hill JD, Farrar DJ, Hershon JJ, et al. Use of a prosthetic ventricle as a bridge to cardiac transplantation for postinfarction cardiogenic shock. N Engl J Med 1986;314:6268. Griffith BP, Hardesty RL, Kormos RC, et al. Temporary use of the Jarvik-7 total artificial heart before transplantation. N Engl J Med 1987;316:130-44. Palatianos GM, Edmunds LH Jr, Cohen DJ, Stephenson LW. Extracorporealleft ventricular assistance with prostacyclin and heparinized centrifugal pump. Ann Thorac Surg 1983;35:504-15. Harshbarger HG, Kirklin JW, Donald DE. Studies in extracorporeal circulation: IV. Surgical techniques. Surg Gynecol Obstet 1958;106:111-8. McGoon DC, Moffitt EA, Theye RA, Kirklin JW.

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Physiologic studies during high flow, normothermic, whole body perfusion. J THoRAc CARDIOVASC SURG 1960;

39:275-87. 12. Arom K, Ellestad C, Grover F, Trinkle J. Objective evaluation of the efficacy of various venous cannulas. J THORAC CARDIOVASC SURG 1981;81:464-9. 13. Bennett E, Jewel J, Barra J, Grover F, Trinkle J. Comparison of flow differences among venous cannulas. Ann Thorac Surg 1983;36:59-65.

The Journal of Thoracic and Cardiovascular Surgery

14. Braunwald E, Kloner RA. The stunned myocardium: prolonged, postischemic ventricular dysfunction. Circulation 1982;66: 1146-9. 15. Magovern GJ, Park SB, Maher TO. Use of a centrifugal pump without anticoagulants for postoperative left ventricular assist. World J Surg 1985;9:25-36.

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