Optimizing venous drainage using an ultrasonic flow probe on the venous line

Abstract

The use of smaller cannulae for minimally invasive surgery techniques and/or aggressive miniaturization of the cardiopulmonary bypass (CPB) circuitry has necessitated the need to augment venous drainage to achieve adequate flow rates. Vacuum assisted venous drainage (VAVD) has become the dominant method to augment venous drainage. VAVD, however, has been associated with a number of known side effects including increased transmission of gaseous microemboli to the patient, venous line chatter, and increased arterial to venous shunts in the circuit. Historically, our practice has been to monitor the arterial output flow rate and to monitor VAVD by observing venous line chatter and changes in the venous reservoir level. In 2008 our pediatric cardiothoracic service began monitoring venous line flow rates by using a second ultrasonic flow probe placed on the venous line. After 12 months, our staff perfusionists reviewed the impact of monitoring venous line flow rates on VAVD and its known side effects on daily clinical practice. When monitoring venous line flow rates, empiric observation revealed that less overall vacuum pressure was needed for our CPB cases. This novel approach to monitoring venous drainage has aided us in providing optimal vacuum levels and therefore, may reduce some of the known side effects experienced with excessive VAVD.

Figure 1.

Arterial and venous transonic flow probe measurement devices (Transonic Systems Inc., Ithaca, NY).

Figure 2.

Arterial (top) and venous (bottom) H7XL ultrasound transit time flow probe transducers (Transonic Systems, Inc., Ithaca, NY). “Ultrasound %” displayed shows the amount of ultrasound signal being transmitted based on the manufactured specifications. Small changes in signal quality can be improved by adding ultrasound gel to enhance the transmission of the sound wave. A greater than 10% loss in transmission quality may require the device to be recalibrated.

Figure 3.

Schematic of the UTHSCSA pediatric CPB circuit. Notice that the HT 110 Transonic flow probes are prior to the MUF circuit on the venous line and distal to all shunts present in the arterial line to account for exact flow measurements to and from the patient.

Figure 4.

This is a conceptual graph depicting the relationship between vacuum pressure and venous line flow. Notice the box highlighting the small area of optimal flow versus vacuum pressure.