Differences in displayed pump flow compared to measured flow under varying conditions during simulated cardiopulmonary bypass

Evaluation of shunting flow differences in varied conditions in a simulated adult CPB model during normothermia

BACKGROUND

It is well documented that lower pump flow during clinical cardiopulmonary bypass (CPB) procedures may be significantly detrimental to tissues and organs. The major cause of hypoperfusion is due to shunting flow existing in the closed CPB circuits. The purpose of this investigation was to evaluate various shunting flow conditions in a simulated adult CPB model under normothermia.

MATERIALS AND METHODS

The pseudo adult patient undergoing CPB was established with an extracorporeal circuit. The pre-oxygenator flow, post-filter flow and purge line pressure were recorded. The temperature of the circuit was kept around 36°C using a heat-cooler. The pseudo patient was subjected to five pump flow rates ranging from 3000 to 5000 ml/min (500 ml/min increments), and post-filter pressure was varied from 120-220 mmHg (20 mmHg increments) using a single, static roller pump. Four different shunting states were measured with two flowmeters.

RESULTS

When the pump flow was constant, the percentage of shunting flow increased as the post-filter pressure increased in State 1. However, when the post-filter pressure was constant, the trend of the shunting flow was the opposite. Purge line pressure increased with the increase of the post-filter pressure at a constant pump flow rate in State 1. The shunting flow for State 2 was the largest of the four states. The shunting flow for State 3 was similar to State 4.

CONCLUSION

During different states of shunt opening, varying degrees of blood flow were diverted away from the arterial line. Shunting of blood flow may, therefore, result in hypoperfusion in the patient.

The influence of the residual stress in silicone tubes in the calibration methods of roller pumps used in cardiopulmonary bypass

The rotation of rollers in cardiopulmonary bypass pumps propels the blood through various devices to reach the patient. Very occlusive settings may squeeze red blood cells, whereas a nonocclusive setting may result in retrograde flow. Occlusion of roller pumps may be regulated either by measuring the drop rate or by dynamic calibration. This study evaluated the influence of silicone tubing residual stress found on pump regulation. Silicone tubes obtained from two different suppliers were used in 6-inch DeBakey roller pumps. The variations occurring over time in the measurements of drop rate, dynamic calibration, and tube residual stress were analyzed. Covariance analysis of the four linear regressions has shown a progressive and accentuated reduction in drop rate (p < 0.002). It is noticeable that the angular coefficients of the drop rate measurements of the four silicone tubes are the same (p > 0.56). This reduction in drop rate measurements may affect the regulation of the pumps before surgical procedures. One probable cause for this reduction is the residual stress found in the silicone tubes. Settings based on the dynamic calibration process tended to be repeated over time. Simple linear regression test (angular coefficient equals zero) has shown a p > 0.79 showing no interference of the silicone tubes residual stress on dynamic calibration, suggesting that one should use this method to calibrate roller pumps.