Prevention of caval collapse during venous drainage for CPB

Can venous cannula design influence venous return and negative pressure with a minimally invasive extracorporeal circulation?

INTRODUCTION

Recent advances to make cardiopulmonary bypass more physiological include the use of kinetic-assisted venous drainage but without a venous reservoir. Despite manipulation of intravascular volume and patient positioning, arterial flow is frequently reduced. Negative venous line pressures can be generated, which may elicit gaseous microemboli. We investigated the influence of venous cannula design on venous return and negative venous line pressures.

METHODS

In a single-centre, single-surgeon, prospective, randomized, double-blind trial, 48 patients undergoing isolated coronary artery, aortic valve or combined coronary artery and aortic valve surgery, with a minimally invasive circuit, were randomized to a conventional two-stage (2S) or three-stage venous cannula (3S), or to a three-stage venous cannula with additional 'fenestrated' ridges (F3S). Blood flow, venous line pressures and gaseous microemboli number and size were measured.

RESULTS

The pump flow achieved was the same between groups, but in each case fell below the target range of 2.2-2.4 L min^-1 m^-2. The three-stage cannula recorded significantly lower negative pressure than the other cannulae. The total count and volume of gaseous emboli detected with the F3S cannulae was very high in some cases, with wide heterogeneity.

DISCUSSION

The low negative pressures recorded with three-stage cannula, despite having a larger drainage orifice area, suggest that negative pressure may be more influenced by lumen diameter and vena cava collapse rather than drainage hole size. The additional fenestrations resulted in flow characteristics and negative pressures similar to the larger two-stage cannula but are associated with generation of gaseous microemboli.

New Dual Lumen Self-Expanding Catheter Design Requiring Less Suction

Contribution of venovenous extracorporeal membrane oxygenation (v-v ECMO) to gas transfer is flow dependent. Catheter design is a key factor for optimal pressure/flow rate relationship. This study was designed for the assessment of a new self-expanding dual lumen catheter design versus the current standard. Outlet pressure/flow rate and inlet pressure/flow rate for a new Smart catheter with self-expanding dual lumen design constricted to 27 F with 5 mm long constrictor corresponding to the percutaneous path versus Avalon 27 F catheter (control) were compared on a flow bench with a Biomedicus centrifugal pump. Flow, pump inlet pressure and outlet pressure were determined at 500, 1,000, 1,500, 2,000, and 2,500 revolutions per minute (RPM). At 500 RPM and with a 5 mm long constrictor (1,000; 1,500; 2,000; and 2,500 RPM), catheter outlet pressure values were -0.13 ± 0.07 mm Hg (-2.55 ± 0.06; -7.38 ± 0.14; -15.03 ± 0.44; -26.46 ± 0.39) for self-expanding versus -2.93 ± 0.23* (-10.60 ± 0.14; -22.74 ± 0.34; -38.43 ± 0.41; -58.25 ± 0.40)*: p < 0.0001* for control. The flow values were 0.61 ± 0.01 L/min (1.64 ± 0.03, 2.78 ± 0.02; 4.07 ± 0.04; 5.37 ± 0.02) for self-expanding versus 1.13 ± 0.06*; (2.19 ± 0.04; 3.30 ± 0.03; 4.30 ± 0.03; 5.30 ± 0.03)*: p < 0.0001* for control. The corresponding catheter inlet flow rates of the self-expanding catheter were slightly more than that of the control. For the given setup, our evaluation demonstrated that the new dual lumen self-expanding catheter requires lower catheter outlet pressures for higher flows as compared to the current standard.

How to prevent venous cannula orifice obstruction during extracorporeal circulation

Venous cannula orifice obstruction is an underestimated problem during augmented cardiopulmonary bypass (CPB), which can potentially be reduced with redesigned, virtually wall-less cannula designs versus traditional percutaneous control venous cannulas. A bench model, allowing for simulation of the vena cava with various affluent orifices, venous collapse and a worst case scenario with regard to cannula position, was developed. Flow (Q) was measured sequentially for right atrial + hepatic + renal + iliac drainage scenarios, using a centrifugal pump and an experimental bench set-up (afterload 60 mmHg). At 1500, 2000 and 2500 RPM and atrial position, the Q values were 3.4, 6.03 and 8.01 versus 0.77*, 0.43* and 0.58* l/min: p<0.05* for wall-less and the Biomedicus® cannula, respectively. The corresponding pressure values were -15.18, -31.62 and -74.53 versus -46.0*, -119.94* and -228.13* mmHg. At the hepatic position, the Q values were 3.34, 6.67 and 9.26 versus 2.3*, 0.42* and 0.18* l/min; and the pressure values were -10.32, -20.25 and -42.83 versus -23.35*, -119.09* and -239.38* mmHg. At the renal position, the Q values were 3.43, 6.56 and 8.64 versus 2.48*, 0.41* and 0.22* l/min and the pressure values were -9.64, -20.98 and -63.41 versus -20.87 -127.68* and -239* mmHg, respectively.

At the iliac position, the Q values were 3.43, 6.01 and 9.25 versus 1.62*, 0.55* and 0.58* l/min; the pressure values were -9.36, -33.57 and -44.18 versus -30.6*, -120.27* and -228* mmHg, respectivly. Our experimental evaluation demonstrates that the redesigned, virtually wall-less cannulas, allowing for direct venous drainage at practically all intra-venous orifices, outperform the commercially available control cannula, with superior flow at reduced suction levels for all scenarios tested.