Abstract
We studied hydraulic input power and input impedance changes in the left pulmonary arterial circulation (LPAC) before and after birth because they account for the relative importance of pulsatile and steady power and provide insight into the physiologic mechanisms of accomodating to each. Pulsatile pressure and flow were measured by a Millar micromanometer and electromagnetic flowmeter in 5 near term fetal lambs. Instantaneous power (IP) was calculated as the product of pressure and flow with time. The apparent power was calculated as the root mean square (RMS) of IP after the steady power was subtracted. Input impedance analysis allowed partitioning of pulsatile apparent power into average and reactive (RMS) components. Power values were calculated from 4 epochs of 48 seconds before and after birth in each animal.
The calculated Potential Hydraulic Input Power (milliwatts) was:
Although the steady component of power increased 3 fold after birth, the magnitude of apparent power remained unchanged and was greater than that due to steady power. When apparent power was partitioned, in the fetal state, reactive power was greater than average power while it was less in the newborn. Hemodynamic transmission line theory suggests that average power is the portion of the apparent power dissipated into heat by the viscous properties of the blood and vessel walls, whereas reactive power is given to elastic wall expansion. Since the apparent power associated with pulsatile blood pressure and flow represents a substantial magnitude of the total power delivered to the LPAC, especially in the fetus, how it is partitioned may play a role in normal or abnormal pulmonary vascular development.
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Bennett, S., Milstein, J. & Goetzman, B. Potential Hydraulic Input Power Changes in the Left Pulmonary Arterial Circulation at Birth. Pediatr Res 18 (Suppl 4), 119 (1984). https://doi.org/10.1203/00006450-198404001-00157
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DOI: https://doi.org/10.1203/00006450-198404001-00157
