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

Extracorporeal membrane oxygenation uses peripherally placed cannulas and a streamlined circuit without a venous reservoir. This study tests the flow dynamics of venous catheters connected without a reservoir directly to a centrifugal pump. During in vitro testing, a 30 cm segment of collapsible 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 vena cava from peripheral sites. Catheter tip design (four types) does not affect flow within a simulated vein in vitro. Maximum pump flow is independent of filling 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 collapsible segment or in the inferior vena cava, maximal flow is significantly influenced by filling pressure (6 to 18 mm Hg) and by the ratio of catheter outer diameter to venous diameter. At all filling pressures, maximal flow in vivo is significantly reduced when this ratio is greater than 0.5. During extracorporeal membrane oxygenation, central venous pressure and catheter/vein ratio, not catheter size alone, control flow through peripheral venous catheters.

Changes in left ventricular systolic wall stress during biventricular circulatory assistance

Extracorporeal membrane oxygenation (ECMO) reduces the systolic stress integral (SSI) in the normal left ventricle. We tested the hypothesis that the SSI does not decrease in poorly contracting, dilated, ejecting hearts during ECMO. In 14 sheep, four pairs of ultrasonic crystals measured changes in left ventricular (LV) wall thickness and three LV diameters. Volume calculations were validated by balloon distention of the ventricles after death (slope = 0.85; r = 0.85). SSI was measured during ECMO flows of 20 to 100 ml/kg/min in both normal and dilated, poorly contracting hearts produced by 30 minutes of warm ischemia. After warm ischemia, end-systolic elastance, an index of contractility, decreased from 8.3 +/- 0.6 mm Hg/ml to 2.9 +/- 0.4 mm Hg/ml (p = 0.001) and peak systolic pressure decreased from 47.4 +/- 0.7 mm Hg to 37.5 +/- 0.08 mm Hg (p = 0.01). In normal hearts, as ECMO flow increased, SSI decreased from 10.5 +/- 2.2 mm Hg.sec to 7.7 +/- 0.8 mm Hg.sec at 60 ml/kg/min (p = 0.001). However, in postischemic hearts, SSI progressively increased from 6.6 +/- 0.3 mm Hg.sec before ECMO to 12.4 +/- 1.8 mm Hg.sec at ECMO = 100 ml/kg/min. These studies indicate that the initial effect of ECMO on the poorly contracting, dilated heart increases LV wall stress and that the increase in stress is proportional to ECMO flow. The increase in stress is primarily due to an increase in afterload, which more than offsets decreases in systolic and diastolic volumes.