INTRODUCTION

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FETAL RIGHT VENTRICULAR function differs markedly from that of the adult in several important aspects. Before birth, the ductus arteriosus allows the fetal heart to function as two pumps in parallel, with the dominant right ventricle (RV) pumping two times more blood than the left ventricle (LV) (19). Only a small portion of RV output passes into the pulmonary vascular bed, whereas the majority of RV output bypasses the fetal lung and empties into the aorta via the ductus arteriosus. In addition to RV output exceeding LV output, a characteristic midsystolic notch, or plateau, in fetal pulmonary trunk blood flow also differentiates fetal pulmonary blood flow from aortic blood flow (Fig. 1) (16). Although the origin of this plateau is unknown, it is unique to the fetal circulation and is not normally evident after birth. It has been postulated that this plateau arises from the unique pattern of the fetal circulation, with the ductus arteriosus allowing LV blood flow to impede RV outflow (16). Alternatively, the plateau may arise because of an interaction between the RV and the highly constricted pulmonary vasculature.

View larger version (11K): [in this window] [in a new window]   Fig. 1.   Velocity tracings in pulmonary trunk and ascending aorta obtained by electromagnetic flow transducer implantation in fetus. Note characteristic midsystolic plateau in pulmonary trunk velocity tracing. This plateau is unique to pulmonary trunk as it does not occur in ascending aorta. Moreover, it is unique to fetal circulation as it is not evident shortly after birth. (Reproduced with permission from Ref. 16.)

Recently, wave-intensity analysis has been proposed as a novel approach to the study of ventricular-vascular interactions (7-9). As an instantaneous measure of the direction and the magnitude of energy wave transmission in a vessel, wave-intensity analysis may provide insights into ventricular-vascular coupling in the fetus that are not obtainable by using conventional measures of cardiac function, such as volume and contractility indexes. Thus wave-intensity analysis may enable the dynamic nature of ventricular-vasculature interactions in the fetus to be clarified. In this study, we employed wave-intensity analysis to investigate ventricular-vascular interactions in the fetal heart to better understand the mechanisms that act to modify ventricular function during fetal life. Specifically, we sought to determine whether the characteristic midsystolic plateau observed in the pulmonary trunk blood flow of the fetus results from an interaction between the LV and the RV via the ductus arteriosus or whether the plateau arises from an interaction between the RV and the pulmonary vasculature.

	METHODS

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All surgical and experimental procedures were performed in accordance with the guidelines established by the National Health and Medical Research Council of Australia and were approved by the Monash Medical Centre's Committee on Ethics in Animal Experimentation.

Eight fetal lambs (141 days gestation; term = 147 days) were studied. Pregnant ewes (Merino-Border Leicester cross) were anesthetized (5 mg/kg ketamine and 100 mg/kg -chloralose iv for induction, followed by 25 mg · kg¹ · h¹ -chloralose) and then intubated and ventilated while supine (2-5 cmH₂O positive end-expiratory pressure, 60-100% O₂). Each fetal lamb [4.7 ± 0.3 (SE) kg] was partially delivered by caesarean section. We placed the head of the fetus into a saline-filled bag to prevent air breathing. The upper body of the fetus was then delivered and positioned supine on the ewe's abdomen, with care taken not to interrupt the umbilical circulation.

The fetal sternum was then split, and the ribcage and lungs were retracted. Two small incisions were made in the pericardium, one (20 mm) along the atrioventricular sulcus and a second (10 mm) perpendicular to this incision along the pulmonary trunk. We measured the circumference of the ascending aorta [37 ± 1 (SE) mm] and of the pulmonary trunk (43 ± 2 mm) before positioning Transonic ultrasonic flow probes on each of these vessels (probe model S, flowmeter model T208, Transonic Systems, Ithaca, NY) (Fig. 2). We determined a calibration factor for the pulmonary trunk flow probes in each of the fetuses we studied by recording both LV and RV stroke volume and by assuming that the stroke volume of the RV in the fetus is twice that of the LV (18, 19).

View larger version (27K): [in this window] [in a new window]   Fig. 2.   Schematic illustration of fetal circulation and instrumentation utilized to record blood flow and pressure in pulmonary trunk (pa and Ppa) and aorta (ao and Pao), respectively. Distance between aortic and pulmonic valves via ductus arteriosus is ~45 mm. a, Location of main pulmonary artery ligation (see Fig. 6).

Pulmonary trunk blood pressure was recorded with a transducer-tipped catheter (model TC-510, Millar Instruments, Houston, TX) positioned in the pulmonary trunk distal to the flow probe (Fig. 2). Care was taken to ensure that the tip of the catheter remained within the pulmonary trunk (within 10 mm of the flow probe) and did not pass into the ductus arteriosus. The zero for this transducer was adjusted to equal that recorded from the catheter's central fluid-filled lumen. To record aortic pressure, we advanced a transducer-tipped catheter (model MPC-500, Millar Instruments) into the LV (via the left carotid artery) and then pulled the catheter back until its tip rested just proximal to the aortic flow probe (Fig. 2). The zero for this transducer was adjusted to equal that recorded from a fluid-filled catheter positioned in the ascending aorta. Blood samples were also withdrawn from this fluid-filled catheter for blood-gas and pH analysis (Radiometer ABL 500 blood-gas analyzer, Radiometer, Copenhagen, Denmark).

We connected the two fluid-filled catheters to calibrated strain-gauge manometers (Transpac IV disposable transducer, Abbott Critical Care Systems, Sligo, Ireland). All pressures were referenced to the midplane of the heart. The strain-gauge manometers, transducer-tipped catheters, and flow probes were connected to a signal conditioner (Cyberamp 380, Axon Instruments, Foster City, CA) and low-pass filtered at 100 Hz. All physiological signals were recorded on a thermal chart recorder (model 7758A, Hewlett-Packard, Waltham, MA) and on computer at a sampling rate of 200 Hz, using an analog-to-digital-converting board (ADAC 4801/16, ADAC, Woburn, MA) and data-acquisition software (CVSOFT, Odessa Computer Systems, Calgary, AB).

Protocol. Experiments began 15-30 min after the completion of the surgery. Blood samples were collected from each fetus and analyzed for blood-gas and pH status. Hemodynamic data were then collected for a minimum of 1 min and were subsequently analyzed off-line. In one fetus, data were also collected for 1-min periods before, during, and after a brief total ligation of the main pulmonary artery (several minutes were allowed for a steady state to be reached in each condition before data collection). At the completion of the study, the fetuses were killed (150 mg/kg pentobarbitone sodium), and the aorta and pulmonary trunk were dissected to determine the average thickness of the vessel wall.

Data analysis. Wave-intensity analysis requires knowledge of the blood pressure and blood flow velocity within a given vessel (8). Blood flows in the aorta and the pulmonary trunk were converted to velocities (m/s) by dividing flow by the internal cross-sectional area of the vessel. Blood pressures were converted to newtons per square meter (1 mmHg = 133.4 N/m²). Net wave intensities were then determined as the product PU, where P and U were measured as the incremental differences in pressure and velocity, respectively, over 5-ms intervals. By convention, PU is positive for waves traveling away from the ventricle (forward-traveling waves) and negative for waves traveling toward the ventricle (backward-traveling waves). The changes in pressure and velocity recorded at a particular time are the resultants of forward-traveling and backward-traveling waves that coincide there. Thus PU is the algebraic sum of these forward-traveling (PU+) and backward-traveling (PU) wave intensities [PU = (PU+) + (PU)]. PU+ and PU were calculated as follows



where the density of blood  was assumed to be 1,000 kg/m³, and aortic and pulmonary trunk wave speeds (c, m/s) were estimated as the ratio of P/U during early systole, when wave travel was assumed to be unidirectional (12). The average values (±SE) for c in the pulmonary trunk and the aorta were 2.6 ± 0.3 and 4.5 ± 0.5 m/s, respectively. In addition to determination of the direction of a traveling wave, wave-intensity analysis also determines whether the wave is a compression or expansion wave. Using the analogy of a straw, "blowing" generates a compression wave and "sucking" an expansion wave. These wave types are used to characterize both forward- and backward-traveling waves. For forward-traveling waves, if P+ is positive, the wave is a compression wave and if P+ is negative, the wave is an expansion wave. Similarly, backward-traveling waves are compression waves when P is positive and expansion waves when P is negative. Wave-intensity analysis was performed on 10 sequential cardiac cycles recorded from each of the fetuses studied, and values are expressed as means ± SE. Values were compared by using a paired t-test, and P < 0.05 was assumed to be statistically significant.

	RESULTS

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Blood-gas and pH data (pH = 7.33 ± 0.02, arterial PO₂ = 28 ± 2 Torr, arterial O₂ saturation = 70 ± 4%, arterial PCO₂ = 49 ± 2 Torr, Hb = 13 ± 1 gm/dl, and base excess = 1 ± 1 mM) were consistent with those obtained in fetal lambs studied under similar experimental conditions (3, 5, 6) and indicated a stable physiological preparation. Figure 3 illustrates the measurements of pressure and blood flow recorded from the ascending aorta and pulmonary trunk in a fetus. Blood flow patterns displayed characteristics similar to those described by Rudolph (16). During early systole, pulmonary flow increased rapidly. Subsequently, flow in the pulmonary trunk began to decrease even though pulmonary trunk pressure continued to increase. A plateau developed in midsystole, corresponding approximately with the peak of pulmonary trunk pressure. This plateau continued as pulmonary trunk pressure began to fall. In contrast, aortic flow peaked during early systole and subsequently declined without developing a plateau.

View larger version (20K): [in this window] [in a new window]   Fig. 3.   Ppa and pa (A and B) and Pao and ao (D and E) data recorded from pulmonary trunk and ascending aorta, respectively, of a fetal lamb and corresponding wave intensities [PUpa (C) and PUao (F)], respectively.

Two positive peaks of wave intensity, as described by Jones and Sugawara (8), were present in both the aorta and the pulmonary trunk (Figs. 3 and 4). One positive peak occurred early in systole (a forward-traveling compression wave) and one late in systole (a forward-traveling expansion wave). The forward-traveling expansion wave late in systole was more prominent in the pulmonary trunk (35 ± 6 W/m²) than in the aorta (15 ± 3 W/m², P < 0.01). During midsystole, a negative peak of wave intensity was also observed in both the pulmonary trunk and in the aorta. These negative peaks were backward-traveling compression waves as P was positive. The magnitude of the backward-traveling compression wave was significantly greater in the pulmonary trunk (10 ± 2 W/m²) than in the aorta (4 ± 1 W/m², P < 0.01). Moreover, the magnitude of the backward-traveling compression wave relative to the preceding forward-traveling compression wave was significantly greater in the pulmonary trunk than in the aorta (18 ± 2 vs. 11 ± 1%, respectively, P < 0.01).

View larger version (21K): [in this window] [in a new window]   Fig. 4.   Blood flow velocity (Upa) and Ppa data (A) recorded from pulmonary trunk and from ascending aorta (Uao and Pao, respectively; B) of fetal lamb and corresponding wave intensity (PU). Two positive peaks of wave intensity were observed in both aorta and pulmonary trunk: 1 early in systole (a forward-traveling compression wave; FCW) and 1 late in systole (a forward-traveling expansion wave; FEW). During midsystole, a negative peak of wave intensity was also observed (a backward-traveling compression waves; BCW). Magnitude of BCW relative to preceding FCW wave was significantly greater in pulmonary trunk than in aorta.

As PU is the algebraic sum of forward (PU+) and backward-traveling (PU) wave intensities (Fig. 5), we utilized PU+ and PU to assess the temporal relationship between wave intensity and blood flow. The onset of the backward-traveling compression wave observed in the pulmonary trunk closely corresponded to the rapid decrease in pulmonary trunk blood flow, and the end of the backward-traveling compression wave subsequently corresponded to the onset of the plateau in blood flow velocity (Fig. 5).

View larger version (14K): [in this window] [in a new window]   Fig. 5.   Wave-intensity analysis data performed on Ppa and Upa data (A) recorded from a fetus. PU (B) is algebraic sum of forward (PU+) and backward-traveling (PU) wave intensities (C). FCW corresponds to increase in both pressure and flow (A). Onset of BCW closely corresponded to rapid decrease in Upa, and end of BCW subsequently corresponded to onset of plateau in blood flow velocity.

To determine whether the backward-traveling compression waves in the pulmonary trunk emanated from waves generated by the LV that cross the ductus arteriosus, or from waves generated by the RV that are reflected from the pulmonary circulation, we calculated the distance that the forward-traveling compression waves in the aorta and pulmonary trunk could travel within the time period between the peaks of these forward-traveling compression waves and the peak of the backward-traveling compression wave in the pulmonary trunk (Table 1). On the basis of the wave speeds recorded, the forward-traveling compression wave in the aorta would travel 170 ± 26 mm, whereas the forward-traveling compression wave in the pulmonary trunk would travel only 94 ± 11 mm.

Ligation of the main pulmonary artery proximal to the ductus arteriosus (See a in Fig. 2) eliminated the midsystolic plateau in pulmonary trunk blood flow velocity (Fig. 6). In addition, the backward-traveling compression wave occurred much earlier in systole after ligation; this markedly altered pulmonary blood flow velocity by introducing a brief shoulder, or plateau, early in systole. After removal of the ligation, the midsystolic plateau reappeared.

View larger version (19K): [in this window] [in a new window]   Fig. 6.   Wave-intensity analysis performed before (A-C), during (D-F), and immediately after (G-I) a brief ligation of main pulmonary artery in fetus (see a in Fig. 2) confirms that BCW responsible for generation of midsystolic plateau in pulmonary blood flow and velocity arises from an interaction between right ventricle and pulmonary circulation. Before occlusion of main pulmonary artery, a characteristic plateau exists in Upa of pulmonary trunk (@, A). Closely associated with this plateau is BCW that occurs ~0.035 s after peak of FCW (PU+; @, C). This BCW is evident in both net wave intensity (i.e., PU; B) and in backward-traveling WAVE intensity (PU; C). Ligation of main pulmonary artery eliminated midsystolic plateau in velocity (D) and shifted reflection site closer to right ventricle. During ligation, BCW was no longer evident in PU (E) but remained evident in PU (#, F) and introduced a brief plateau, or shoulder, in blood flow and velocity early in systole (#, D). During ligation, BCW occurred much earlier in systole, just 0.005 s after peak of FCW [distance to reflection site was reduced from ~60 to ~10 mm (on basis of a wave speed of 3.5 m/s in this lamb)]. Removing ligation restored pulmonary circulation as major reflection site, eliminated early systolic plateau in pulmonary trunk blood flow, and reintroduced midsystolic plateau (G-I).

	DISCUSSION

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Our assessment of ventricular-vascular interactions by using wave-intensity analysis provides new insight into the control of the fetal circulation and into the interactions that occur between the heart and the pulmonary vasculature of the fetus. As in the adult (7-10, 12), two positive peaks of wave intensity dominate the analysis. In both the ascending aorta and the pulmonary trunk, a forward-traveling compression wave occurred early in systole, followed by a forward-traveling expansion wave in late systole. This forward-traveling expansion wave was larger in the pulmonary trunk than in the aorta, which, as we discuss below, may correspond to the right-side dominance of the fetal heart. Wave-intensity analysis also revealed the presence of a backward-traveling compression wave that immediately followed the forward-traveling compression wave. This backward-traveling compression wave was significantly more prominent in the pulmonary trunk than in the ascending aorta. The termination of the backward-traveling compression wave corresponded temporally to the beginning of the plateau observed in flow profile recorded from the pulmonary trunk.

Milnor (11) defined afterload as arterial impedance. Although impedance analysis provides information about how the contracting heart and the arterial tree interact to produce the observed flow, because it is a calculation in the frequency domain, beat-to-beat-time domain comparisons with conventional hemodynamic parameters are difficult (1). Wave-intensity analysis provides a measure of the power per unit area (i.e., energy flux) carried by pressure and velocity waves in the cardiovascular system and provides information regarding both upstream and downstream events, i.e., factors arising from the heart and the vasculature. It is the time-domain nature of wave-intensity analysis that has allowed us to determine that the backward-traveling compression wave markedly decreases pulmonary trunk blood flow before the onset of the plateau. Moreover, the beginning of the plateau in the fetal pulmonary trunk flow waveform corresponds to the end of a backward-traveling compression wave.

Backward-traveling compression waves result from energy reflections in the peripheral circulation (8, 12). This reflected energy acts to increase pressure and, at the same time, impede blood flow (and velocity) out of the ventricle (8, 10). Given the presence of the ductus arteriosus, and the parallel nature of the fetal circulation (Fig. 2), the backward-traveling compression wave observed in the pulmonary trunk could arise from 1) the forward-traveling compression wave generated by the LV passing directly across the ductus arteriosus into the pulmonary trunk, 2) the forward-traveling compression wave generated by either the LV or RV being reflected from a site in the systemic circulation and then crossing the ductus arteriosus, 3) the forward-traveling compression wave generated by the RV being reflected at the ductus arteriosus, and 4) the forward-traveling compression wave generated by the RV being reflected from a site in the pulmonary vasculature. On the basis of our analysis of the wave speed, the time at which the backward-traveling compression waves arrive, and the distance over which these waves could travel in this time period, it is unlikely that the backward-traveling compression wave observed in the pulmonary trunk arises from a forward-traveling compression wave generated by the LV. We estimate that the forward-traveling compression wave generated by the LV would travel 170 mm in the time available, much further than the actual distance that exists between the aortic and pulmonic valves via the ductus arteriosus (46 ± 2 mm, measured in 5 fetal sheep of similar gestation). Moreover, it is unlikely that the backward-traveling compression wave in the pulmonary trunk results from waves generated by either the LV or the RV that were reflected from the systemic peripheral circulation, given that the backward-traveling compression wave was so much smaller in the aorta than in the pulmonary trunk. Finally, it is unlikely that the backward-traveling compression wave in the pulmonary trunk results from the forward-traveling compression wave generated by the RV being reflected at the ductus arteriosus. We estimate that the forward-traveling compression wave generated by the RV would travel 94 mm in the time available, more than double the actual distance (40 mm) that exists from the pulmonic valve to the ductus arteriosus and back.

Thus it is likely that the backward-traveling wave has its origin in the RV as a forward-traveling compression wave that is reflected back toward the RV from a site in the pulmonary vasculature. That this backward-traveling wave was a compression wave indicates that the fetal pulmonary circulation acts as a closed-ended reflection site, a situation unique to the fetus. In the adult, the pulmonary vasculature acts as an open-ended reflection site, and, as a result, backward-traveling expansion waves return to the adult RV and act to augment flow while pressure falls (7), in a manner exactly opposite to what we observed in the fetus. The closed-ended reflection site of the fetus is most likely associated with the high pulmonary vascular resistance that exists in the fluid-filled lung (18, 19), because hypoxia, and its associated pulmonary vasoconstriction, is known to produce large backward-traveling pressure and flow waves in the pulmonary trunk of calves (20). These hypoxia-induced backward-traveling waves produce a plateau in pulmonary artery blood flow that is very similar to the plateau that characterizes fetal pulmonary trunk blood flow. Moreover, our observation that ligating the main pulmonary artery moved the reflection site closer to the RV and eliminated the midsystolic plateau in pulmonary trunk blood flow (Fig. 6) confirms that the reflection site lies within the pulmonary vasculature.

Although backward-traveling compression waves occurred in both the aorta and the pulmonary trunk, these waves were only associated with a plateau in pulmonary trunk blood flow. The greater sensitivity of the RV to afterload may account for this observation (13, 14). The lack of a plateau in aortic flow may also be associated with the smaller magnitude of the backward-traveling compression wave that appears in the aorta. The difference in magnitude of the aortic and pulmonic backward-traveling compression waves may result from the pulmonary reflection site having a greater reflection coefficient, or simply from the fact that the distance from the RV to the pulmonary reflection site is less than the distance from the LV to the systemic reflection site (7). Calculations of the distance traveled by the forward-traveling compression wave generated by the LV indicate that the return distance traveled was more than double that of the forward-traveling compression wave generated by the RV (Table 1). The closer the reflection site is to the ventricle, the less the reflected wave would be attenuated as it returns to the ventricle.

The initial positive peak of wave intensity (forward-traveling compression wave) observed in the ascending aorta and the pulmonary trunk is thought to indicate the "initial ventricular impulse" (17) that accelerates blood in the aorta (8). These forward-traveling compressive waves occur in the aorta when ventricular pressure exceeds aortic pressure. The second of the two positive waves, the forward-traveling expansion wave, is also generated by the ventricle. This wave decelerates blood flow, decreases blood pressure, leads to valve closure, and may provide quantitative information about ventricular-restoring forces that contribute to diastolic filling (10). Significantly, this wave was more prominent in the fetal pulmonary trunk than in the aorta. This suggests that the restoring forces that contribute to ventricular filling may be greater in the functionally dominant fetal RV than in the adult RV. Anatomic features of the fetal RV, including a lower ventricular compliance relative to the adult and a greater ratio of RV to LV free-wall thickness relative to the adult, may contribute to the large forward-traveling expansion wave (13, 15).

The quantitative nature of our results may have been affected by the nature of our experimental preparation. It was not possible to close the pericardium and thorax in these studies, given the small size of the fetuses and given the size of the flow probes. In recognition of the importance of ventricular constraint in determining fetal cardiac function (3-6), it is likely that closing the pericardium and the chest would have reduced LV and RV flow. While our observations may differ quantitatively from in vivo conditions, our qualitative observations should not be affected.

In the fetus, the RV dominates the circulation and pumps approximately two-thirds of the combined ventricular output (19). Even so, blood flow through the lungs is minimal as the vast majority of RV output passes through the ductus arteriosus with only 8-10% of total cardiac output passing into the lungs of the near-term fetus (2). Pulmonary blood flow increases up to 10-fold within minutes of birth as pulmonary vascular resistance decreases to 10% of the fetal level (19). In addition, the plateau observed in the flow profile of the fetal pulmonary trunk is eliminated after birth, perhaps as the dilatation of the pulmonary vasculature converts the pulmonary vascular bed from a closed-ended reflection site to an open-ended reflection site. Each of these changes must substantially affect ventricular-vascular interactions at birth. Moreover, a failure of the newborn to progress naturally through these transitions, as might occur with premature birth, pulmonary hypoplasia, or persistent pulmonary hypertension, may result in ventricular-vascular interactions that are detrimental. Future studies will be required to assess these possibilities.

In summary, our study has utilized wave-intensity analysis to assess ventricular-vascular interactions in the fetus. Our results provide new insight into the functioning of the fetal cardiovascular system. Specifically, the presence of a backward-traveling compression wave in the pulmonary trunk leads to the plateau observed in the pulmonary trunk blood flow. This wave and the resulting plateau in pulmonary trunk flow arise from an interaction between the RV and the pulmonary vasculature, and not, as previously speculated, from an interaction of the LV and RV via the ductus arteriosus. Elimination of this backward-traveling compression wave may be an important adaptation of the cardiorespiratory system at birth.