Fig. 15.1
Pressure curves (top left), filling curves (top right) and cardiac output curves (bottom) displayed on the SynCardia “Companion 2” driver
If this peak is not existent because at the end of the ejection cycle the membrane is still moving, there could be an imbalance. The hazard of right to left mismatch may result in lung edema or low output. The immediate reaction would be to increase systolic driving pressures, and possibly to adjust pump rate upwards or extend the relative systolic operation period. So long as the driving pressures are high enough to overcome the needs to secure an empty pump in the range of expected systolic blood pressures, displacement pumps are afterload independent. Thus, blood pressure shall be controlled to avoid episodes of high blood pressure.
Filling of the pump during diastole is ideally passive. A small vacuum has to be applied to quickly remove the air from the air chamber through the long and narrow drivelines because of their resistance. The necessary pressure difference (vacuum) could be reduced by bigger diameters of the driveline, which on the other hand heightens the necessary pneumatic displacement volume of the driver because a bigger dead volume has to be filled to create the desired systolic driving pressures.
With the extracorporeal VADs, a higher vacuum may be useful to compensate for pressure losses along the long and narrow venous cannulas, especially with the pediatric settings. If the negative pressures in accordance to the available diastolic time, defined by the pumping rate and relative diastolic operation time, are adjusted correctly, this will result in a near total filling of the ventricle (please note that proper operation for SynCardia TAH requires a filling of about 75 percent only). Pneumatic devices are preload respondent. A sudden decrease of the filling volume thus indicates rather a medical problem (hypovolemia, obstruction by tamponade) than an increased need for vacuum.
Box
In ◘ Fig. 15.1, the curves from the screen of the Companion driver can be seen. The red lines represent the left ventricle and the blue lines the right ventricle. The top left curves show the pressure of air inside the ventricles during systole (blood inside the ventricles is being ejected). The top right curves show the velocity of air flow coming outside the ventricles during diastole (ventricle fills with blood). The bottom curves are the cardiac output (in liters per minute) and evolve over time by adding a dot every minute. A reminder: the ventricles of the SynCardia TAH system have no sensors. The system is in balance as long as both ventricles full eject and partial fill. Full eject can be seen on the pressure curves (top left in ◘ Fig. 15.1). In the beginning, the ventricles are filled with blood. The driver starts to build up air pressure rapidly. When the air pressure gets higher than the arterial blood pressure, the outflow valve opens (at point “A” in ◘ Fig. 15.1), and the membrane starts to move pushing out the blood from the ventricle. During the movement of the membrane, the air pressure is not increasing (from “A” to “B” in ◘ Fig. 15.1). When the membrane reached the end position (at point “B”), the blood from the ventricle is fully ejected. The pressure rises up again to reach the programmed maximum driving pressure (e.g., 210 mmHg for left and 110 mmHg for right driving pressure, seen in ◘ Fig. 15.1). This pressure increase at the end of the curve is called the full eject flag. After systole, a valve in the driver opens and relieves the air from the ventricles. The air from the ventricles travels through the drivelines and a flow sensor before exiting the driver. The sensor can be simplified as a wheel spinning when air goes through. The top right curves in ◘ Fig. 15.1 are showing the speed of the wheel spinning during diastole. In the beginning, the ventricles decompress, meaning the high pressure is released to ambient air pressure. The wheel accelerates fast and eventually comes down. When the pressure inside the ventricles is lower than the filling blood pressure, the inflow valve opens (at point “C” in ◘ Fig. 15.1). The membrane moves while blood is coming into the ventricles. The air that is displaced by the blood let the wheel spinning at constant speed (indicating a smooth filling). The diastolic phase is stopped before the ventricle is completely filled with blood, so the curves at point “D” in ◘ Fig. 15.1 don’t touch the zero line. The beginning of the systolic phase is seen at the end of the curves as sharp increase in wheel spin. No further explanation is needed for the cardiac output curves on the bottom of ◘ Fig. 15.1.
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