Dynamic characterization and hemodynamic effects of pulmonary waves in fetal lambs using cardiac extrasystoles and beat-by-beat wave intensity analysis

Abstract

Smolich JJ, Mynard JP, Penny DJ. Dynamic characterization and hemodynamic effects of pulmonary waves in fetal lambs using cardiac extrasystoles and beat-by-beat wave intensity analysis. Am J Physiol Regul Integr Comp Physiol 297: R428 -R436, 2009. First published June 3, 2009 doi:10.1152/ajpregu.00174.2009.-Steadystate wave intensity (WI) analysis indicates that characteristic midsystolic falls in fetal pulmonary trunk (PT) and artery (PA) blood flow are due to an extremely large backward-running compression wave (BCWms) that 1) originates from the pulmonary microvasculature by a combination of cyclical pulmonary vasoconstriction and vascular reflection of the forward-running compression wave (FCWis) associated with impulsive right ventricular ejection, and 2) is transmitted into the PT. However, no information is available about the dynamic properties of PA BCW ms and its contribution to beat-to-beat regulation of pulmonary hemodynamics. Accordingly, beat-by-beat WI analysis was performed during brief increases in ventricular contractility accompanying an extrasystole (ES) in nine anesthetized lategestation fetal sheep instrumented with PT and left PA micromanometer catheters to measure pressure (P) and transit-time flow probes to obtain blood velocity (U). WI was calculated as the product of P and U rates of change. At steady state, the magnitude of PA BCWms, and its associated P and U changes (⌬P and ⌬U, respectively), were similar to those of FCWis. The PA FCWis and BCWms, and their accompanying ⌬P and ⌬U, were all transiently potentiated after an ES. Beat-by-beat PA FCWis-BCWms wave area, ⌬P and ⌬U relationships were highly linear (R 2 Ն 0.91) with slopes of 1.36 -1.47 (P Ͻ 0.001), consistent with the presence of a vasoconstrictor component in PA BCWms. PA-PT BCWms area and ⌬P and ⌬U relationships were also linear (R 2 Ն 0.77) with slopes of 0.23-0.64 (P Ͻ 0.001). These results indicate that the fetal PA BCWms contributes to beat-to-beat regulation of not only PA but also PT hemodynamics.fetal pulmonary blood flow; fetal pulmonary blood pressure; postextrasystolic potentiation IN THE FETUS, ONLY ϳ10% OF the right ventricular (RV) output entering the pulmonary trunk (PT) passes into the pulmonary arterial (PA) bed, with the remainder crossing the ductus arteriosus into the descending aorta (11,37,38,44). In association with this low level of pulmonary perfusion, the fetal PA blood flow profile demonstrates forward flow in early systole, but flow then falls abruptly in midsystole, despite PA blood pressure continuing to rise (25,38). In a recent study (43), we examined the physiological basis of this unusual flow profile using wave intensity (WI) analysis, an approach based on the premise that cardiovascular function is accompanied by the propagation of infinitesimal wavefronts defined by their pressure (P) and velocity (U) effects (4, 30), with the product of changes in P and U in the time domain (i.e., WI) related to the instantaneous energy carried by the wavefronts. Steady-state WI analysis indicated th...

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“…a beat with a long diastole) was initiated with a brisk tap to the right ventricular infundibulum using forceps [41]; this beat was then used for analysis. Signals were filtered using a 48 Hz low-pass filter and flow was converted to velocity using the nominal size of the flow probe [7,11,12,41,42]. Wave speed (c) was calculated using the pu-loop method [28,43,44].…”

Ductus arteriosus wave intensity analysis in fetal lambs: midsystolic ductal flow augmentation is due to antegrade pulmonary arterial wave transmission

“…Despite the close anatomical proximity, however, the blood flow profile of the DA is strikingly different from that of the PT or PA. Thus, both the PT and PA demonstrate a flow peak in early systole but, particularly in the main and large branch PA, flow then falls abruptly in midsystole, even though pulmonary blood pressure continues to rise (19,29,36,37). By contrast, DA flow also rises in early systole but then increases further in midsystole to a delayed peak (31).…”

“…This approach is founded on the premise that cardiovascular function is accompanied by the propagation of infinitesimal wave fronts defined by their pressure (P) and velocity (U) effects (2,23), with the product of changes in P and U in the time domain (i.e., "WI") related to the instantaneous energy carried by the wave fronts. Using WI analysis (36,37), we recently demonstrated that the midsystolic fall in fetal PA blood flow was caused by an unusually large PA backward-running compression wave (BCW ms ) that originated in the pulmonary microvasculature and was of similar or greater magnitude than the forward-running compression wave (FCW is ) associated with impulsive RV ejection of blood. Furthermore, retrograde transmission of ϳ25% of PA BCW ms energy into the PT (36) produced the midsystolic fall evident in the flow/velocity profile of this vessel (10,29).…”

“…Using WI analysis (36,37), we recently demonstrated that the midsystolic fall in fetal PA blood flow was caused by an unusually large PA backward-running compression wave (BCW ms ) that originated in the pulmonary microvasculature and was of similar or greater magnitude than the forward-running compression wave (FCW is ) associated with impulsive RV ejection of blood. Furthermore, retrograde transmission of ϳ25% of PA BCW ms energy into the PT (36) produced the midsystolic fall evident in the flow/velocity profile of this vessel (10,29). Although the various fates of the PA BCW ms are yet to be fully elucidated, this relatively minor degree of PA-to-PT transmission raises the possibility that another portion of the PA BCW ms passed into the adjacent DA.…”

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“…WIa performed in the beats surrounding an ectopic beat, showed that there is a close relationship between the BCWpulm_foet_pa and the preceding FCWpulm on a beat-to-beat basis, with matching transient potentiation in both waves after an ectopic beat. This relationship highlighted the possible role of the BCWpulm_foet_pa in the regulation of pa and pT hemodynamics 66 . This wave is also transmitted into the Da as a FCW (FCWpulm_foet_Da) and is responsible for augmentation in Da mid-systolic blood flow, a mechanism that supports foetal right-to-left flow at a period in the cardiac cycle where pT and pa flows are decreasing 67 .…”