Effect of External Positive and Negative Pressure on Venous Flow in an Experimental Model

Positive external pressure is said to decrease transmural pressure; negative pressure in the pleural cavity is widely believed to result in negative pressure in systemic chest veins. The discrepancy between erect column height and foot venous pressure has been explained on this basis.

These core concepts rest on static closed models that may not be appropriate. This study examined the effects of external pressures in a dynamic open model that may better reflect in vivo conditions. Flow in a Penrose drain enclosed in a chamber that could be positively or negatively pressurized was used. Input and output reservoirs with pressures in the physiological range provided flow. Flow and pressure were monitored in horizontal and erect models with modifications to suit particular experiments.

The discrepancy between foot venous pressure and erect venous column height was shown in this experimental model to be a result of two flows in opposite directions (superior and inferior vena cavae) meeting at the zero reference level at the heart; the upper column pressure therefore does not register at the foot. Positive external pressure results in slowing of velocity with conversion to pressure. Internal and transmural pressures therefore do not decrease. Negative external pressure has only a marginal effect on flow; importantly, internal pressure does not become negative. In an experimental set-up it was shown that negative pressure in chest veins was not necessary for air embolism to occur.

Persistent negative pressure in systemic chest veins probably does not occur. The reason for the discrepant foot venous pressure is likely to be a result of dynamic flow and not negative pressure in chest veins. External positive pressure results in slowing of velocity but the transmural pressure remains largely unchanged.