Gynecol. Invest. 8 : 233-245 (1977)
Determinants of Venous Return in the Fetal Lamb R aymond D. G ilbert Division of Perinatal Biology, Loma Linda University, School of Medicine, Loma Linda, Calif.
Key Words. Venous return • Fetal sheep • Mean systemic pressure • Resistance to venous return Abstract. Measurements have been made of the peripheral vascular factors in fluencing venous return, and hence cardiac output, in the fetus. These measurements show the fetus maintains a high venous return by means of a high mean systemic pressure, or driving pressure for venous return, and a low resistance to venous return. Increases in fetal blood volume cause increases in mean systemic pressure with no change in resistance to venous return and, as a result, venous return is increased. These studies show the possible importance of peripheral vascular factors in the con trol of cardiac output.
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The fetal animal characteristically demonstrates a high cardiac output compared to the adult [1, 2, 4, 5, 9, 10, 25, 26], Such high output is neces sary, both for adequate exchange of substances such as 0 2, C 0 2 and nutrients across the placenta and for delivery of these substances to and from fetal tissues. For example, the fetal cardiac output can support an oxygen consumption 1V2-2 times adult levels, compared on a body weight basis, in spite of a low arterial P02 [6, 22], Reduction in cardiac output might leave vital areas insufficiently oxygenated, resulting in suboptimal fetal development. However, the factors that regulate fetal cardiac output remain largely unknown. Studies of adult animals, on the other hand, have shown that adequate cardiac output depends on adequate venous return. Specific factors affect ing venous return such as mean systemic pressure (Pms), resistance to venous return (Rvr), and changes in vascular tone or blood volume have proven to be important determinants of cardiac output in the adult [16-19],
234
G ilbert
Pms is the static pressure in the systemic vasculature when the cardiac output is zero. It constitutes the upstream driving pressure for return of blood to the right heart. Although the absolute value of Pms is low in adults (about 7 mm Hg), small changes in Pms effect large changes in cardiac out put [15]. Pms is determined largely by the tone of compliant vessels and blood volume. Changes in either of these dramatically affect venous return and cardiac output [15]. The purpose of this study was to investigate normal venous return in the fetal sheep. Pms was measured and the venous return curves plotted from which Rvr was calculated. The effects of increased fetal blood volume on venous return, Pms and Rvr were also measured. The resulting data demonstrate the importance of these peripheral factors in maintaining high fetal cardiac output.
Methods
Total Heart-Lung Bypass The fetal chest was opened and a total heart-lung bypass incorporated into the fetal circulation as depicted in figure 1. The circuit comprised these five parts, listed in direction of blood flow: (1) right atrial cannula; (2) Starling resistor (thin-walled Penrose tubing); (3) occlusive roller pump; (4) flow transducer (Micron); (5) aortic cannula. First, the large bore cannula that initiated the circuit was placed into the right atrium through its appendage for the purpose of collecting all the blood returning to the heart, and another cannula was placed in the ascending aorta. While these were inserted, the fetal heart still circulated the blood. When the extracorporeal circuit was completely primed with blood, the roller pump was slowly turned on until all venous return was directed through the extra corporeal circuit. Thus the transfer was accomplished without interrupting blood flow into the aorta. When the speed of the roller pump was sufficiently high, the Starling resistor was partially collapsed so that right atrial pressure (Pra) could be controlled by adjusting the hydrostatic level of the Starling resistor relative to the right atrium.
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Preliminary Procedures 14 pregnant ewes at term were given spinal anesthesia (pontocaine), sedated with pentobarbitol (10 mg/kg) and laid on their sides. The fetuses were delivered onto a warmed table next to the mother and wrapped in plastic to prevent heat loss. Their body temperatures were maintained at 38 “C and their umbilical cords left untouched and wrapped in cotton soaked in warm saline. All surgical procedures on the fetuses were carried out under local xylocaine anesthesia. A catheter was passed through one fetal femoral artery into the descending aorta to the level of the diaphragm for taking blood samples and for measuring aortic pressure.
Fetal Venous Return
235
Fig.l. Schematic of heart bypass circuit used in fetal animals. RV = Right ven tricle; LV = left ventricle; P ra = right atrial pressure, P;, = aortic pressure. When all venous return was flowing through the bypass circuit, the heart and lungs were isolated from the system with these four ligatures: (1) around the ascend ing aorta to prevent any regurgitation through the aortic valve; (2) around the pul monary artery and ductus arteriosus to prevent backflow of blood into the lungs; (3) around the A-V junction of the heart so the ventricles would not fill and empty during the various measurements; (4) around the right atrium to close the foramen ovale. Since all venous return was collected from the right atrium and directed into the ascending aorta, thus completely bypassing the heart and pulmonary vasculature, all measurements reflect characteristics of the systemic vascular system alone. Measurement of Cardiac Output In five fetuses, flow in the main trunk of the pulmonary artery was measured after the chest was opened but before the bypass circuit was installed. A cuff-type flow transducer was placed around the main trunk of Ihe pulmonary artery and flow measured for 10-20 min. From this flow, essentially right ventricular output, cardiac output was estimated for later comparison with maximum venous return in the by pass circuit.
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Measurement of Control Pms Control Pms was measured simply by stopping the pump and simultaneously clamping the tube leading from the right atrium to exclude the starling resistor; its large compliance would have introduced an error into the Pms measurement. When the flow was stopped, usually for about 10 sec, aortic pressure (Pa) and Pra equi librated within 3-4 sec and remained equal and constant until flow was restored (fig. 7). The equilibrium value was taken as Pms. Pa was measured from the catheter
236
G ilbert
in the femoral artery and Pra was measured from a catheter introduced into the right atrium through the right atrial cannula. All pressures were referenced to the A-V valve in the right atrium. Construction of Venous Return Curve by Two Methods After measurement of Pms, the flow was restored and the animal allowed 10 min to stabilize. Then a venous return curve was plotted. Pra was increased (by lifting the Starling resistor) slowly from some low value, usually -10 Hg, until venous return had ceased. This required about 35 sec. Curves were plotted as flow (Qvr) versus Pra. Flow was measured by the flow transducer in the bypass circuit. Between curves, the Pra was held at -5 to 0 mm Hg. For comparison, curves were also plotted in six fetuses by methods used in pre vious experiments [17], Pra was raised rapidly to a given level and held there until flow and pressure had equilibrated (6-10 sec), then lowered to -5 to 0 mm Hg and held there until a steady state was reached before it was raised to a different level again. This procedure was repeated until enough points were obtained to plot the venous return curve. This method required 3-6 min to plot each curve. These curves were compared to the curves generated in the same fetuses by slowly raising Pra. Effects of Increased Blood Volume The effects of increased blood volume on the venous return curve were investi gated in six fetuses by injecting 50 ml maternal blood into each fetus. Pms was measured and a venous return curve plotted within 1 min after the injection to minimize fluid loss from the vascular compartment. The addition of 50 ml blood produced an average 11.1% increase in fetal systemic volume, estimating total fetal blood volume as 134.7 ml/kg body weight [7] and assuming the heart and lungs con tain no more than 15% of the total fetal blood volume.
Results
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The 14 fetuses used in this study ranged in weight from 2.7 to 6.0 kg, the average being 4.2 (mean) ± 0.2 kg (1 SE). Control Pms was 15.2 ± 0.3 mm Hg. Blood gas measurements sampled from the aorta during the con trol period were: P02 = 22.7 ±1.3 mm Hg, PC02= 42.2 ±2.3 mm Hg and pH = 7.31 ± 0.01. Normal venous return curves were plotted for all 14 ani mals (fig. 2) with venous return normalized to fetal body weight. Maximal venous return was 266 ± 12 ml/min/kg body weight, measured at right atrial pressures between -5 and 0 mm Hg. Mean P., was measured im mediately prior to obtaining venous return curves and averaged 64 ± 2 mm Hg. Blood flow measured in the main trunk of the pulmonary artery be fore the bypass circuit was installed was 154 ± 7 ml/min/kg body weight
Fetal Venous Return
237
PRA, mm Hg
for five fetuses. Maximal venous return in these fetuses, obtained after the heart and lungs had been replaced with the bypass circuit, was 305 ± 16 ml/min/kg body weight. If the right ventricle is assumed to put out ap proximately one half the total cardiac output, the value for flow in the main trunk of the pulmonary artery may be doubled to obtain an estimate of total cardiac output. Thus, for these five fetuses, cardiac output was approximately 310 ml/min/kg compared to a maximal venous return of 305 ml/min/kg after the animals were on the bypass circuit. As G uyton et al. [17] have demonstrated, the downsloping portion of the venous return curve may be approximated with a straight line. The reciprocal of this line’s slope is termed Rvr. For the normal venous return curves of 14 fetuses (fig. 2), the downsloping portions of the curves (Pra>0) were fitted with straight lines by least squares regression analysis. The in verse of the slopes were averaged to calculate Rvr. Correlation coefficients were all 0.97 or above. Rvr averaged 0.072 + 0.003 mm Hg/ml/min • Kg. In six animals, venous return curves were obtained by two different methods, G uyton ’s original method of raising Pra repeatedly to different levels [17], and the method of slowly raising Pra. Venous return curves were the same in all six animals regardless of method. Both curves for one animal are shown in figure 3. For each animal, paired analysis was used to compare the slopes and intercepts of the straight lines fitted to the curve
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Fig. 2. Normal venous return curves of 14 fetuses. Qvr = Venous return; P ra = right atrial pressure.
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G ilbert
Fig. 3. Comparison in one fetus of venous return curves obtained by slowly raising right atrial pressure (--------------- ) and by raising right atrial pressure to various static levels (----- ).
PRA, mm Hg
Fig. 4. Comparison in six fetuses of downslopes of venous return curves obtained by slowly raising right atrial pressure (--------------- ) and by raising right atrial pres sure to various static levels (----- ). This figure shows the mean regression lines and 95% confidence limits, demonstrating that curves generated by both methods are in deed the same.
Downloaded by: Univ. of California Santa Barbara 128.111.121.42 - 3/6/2018 11:05:09 AM
downslopes. There was no statistical difference in either the slopes or the intercepts for the six fetuses, as demonstrated in figure 4. Increasing fetal blood volume by 11% in six fetuses increased Pms sig nificantly (p
Determinants of Venous Return in the Fetal Lamb R aymond D. G ilbert Division of Perinatal Biology, Loma Linda University, School of Medicine, Loma Linda, Calif.
Key Words. Venous return • Fetal sheep • Mean systemic pressure • Resistance to venous return Abstract. Measurements have been made of the peripheral vascular factors in fluencing venous return, and hence cardiac output, in the fetus. These measurements show the fetus maintains a high venous return by means of a high mean systemic pressure, or driving pressure for venous return, and a low resistance to venous return. Increases in fetal blood volume cause increases in mean systemic pressure with no change in resistance to venous return and, as a result, venous return is increased. These studies show the possible importance of peripheral vascular factors in the con trol of cardiac output.
Downloaded by: Univ. of California Santa Barbara 128.111.121.42 - 3/6/2018 11:05:09 AM
The fetal animal characteristically demonstrates a high cardiac output compared to the adult [1, 2, 4, 5, 9, 10, 25, 26], Such high output is neces sary, both for adequate exchange of substances such as 0 2, C 0 2 and nutrients across the placenta and for delivery of these substances to and from fetal tissues. For example, the fetal cardiac output can support an oxygen consumption 1V2-2 times adult levels, compared on a body weight basis, in spite of a low arterial P02 [6, 22], Reduction in cardiac output might leave vital areas insufficiently oxygenated, resulting in suboptimal fetal development. However, the factors that regulate fetal cardiac output remain largely unknown. Studies of adult animals, on the other hand, have shown that adequate cardiac output depends on adequate venous return. Specific factors affect ing venous return such as mean systemic pressure (Pms), resistance to venous return (Rvr), and changes in vascular tone or blood volume have proven to be important determinants of cardiac output in the adult [16-19],
234
G ilbert
Pms is the static pressure in the systemic vasculature when the cardiac output is zero. It constitutes the upstream driving pressure for return of blood to the right heart. Although the absolute value of Pms is low in adults (about 7 mm Hg), small changes in Pms effect large changes in cardiac out put [15]. Pms is determined largely by the tone of compliant vessels and blood volume. Changes in either of these dramatically affect venous return and cardiac output [15]. The purpose of this study was to investigate normal venous return in the fetal sheep. Pms was measured and the venous return curves plotted from which Rvr was calculated. The effects of increased fetal blood volume on venous return, Pms and Rvr were also measured. The resulting data demonstrate the importance of these peripheral factors in maintaining high fetal cardiac output.
Methods
Total Heart-Lung Bypass The fetal chest was opened and a total heart-lung bypass incorporated into the fetal circulation as depicted in figure 1. The circuit comprised these five parts, listed in direction of blood flow: (1) right atrial cannula; (2) Starling resistor (thin-walled Penrose tubing); (3) occlusive roller pump; (4) flow transducer (Micron); (5) aortic cannula. First, the large bore cannula that initiated the circuit was placed into the right atrium through its appendage for the purpose of collecting all the blood returning to the heart, and another cannula was placed in the ascending aorta. While these were inserted, the fetal heart still circulated the blood. When the extracorporeal circuit was completely primed with blood, the roller pump was slowly turned on until all venous return was directed through the extra corporeal circuit. Thus the transfer was accomplished without interrupting blood flow into the aorta. When the speed of the roller pump was sufficiently high, the Starling resistor was partially collapsed so that right atrial pressure (Pra) could be controlled by adjusting the hydrostatic level of the Starling resistor relative to the right atrium.
Downloaded by: Univ. of California Santa Barbara 128.111.121.42 - 3/6/2018 11:05:09 AM
Preliminary Procedures 14 pregnant ewes at term were given spinal anesthesia (pontocaine), sedated with pentobarbitol (10 mg/kg) and laid on their sides. The fetuses were delivered onto a warmed table next to the mother and wrapped in plastic to prevent heat loss. Their body temperatures were maintained at 38 “C and their umbilical cords left untouched and wrapped in cotton soaked in warm saline. All surgical procedures on the fetuses were carried out under local xylocaine anesthesia. A catheter was passed through one fetal femoral artery into the descending aorta to the level of the diaphragm for taking blood samples and for measuring aortic pressure.
Fetal Venous Return
235
Fig.l. Schematic of heart bypass circuit used in fetal animals. RV = Right ven tricle; LV = left ventricle; P ra = right atrial pressure, P;, = aortic pressure. When all venous return was flowing through the bypass circuit, the heart and lungs were isolated from the system with these four ligatures: (1) around the ascend ing aorta to prevent any regurgitation through the aortic valve; (2) around the pul monary artery and ductus arteriosus to prevent backflow of blood into the lungs; (3) around the A-V junction of the heart so the ventricles would not fill and empty during the various measurements; (4) around the right atrium to close the foramen ovale. Since all venous return was collected from the right atrium and directed into the ascending aorta, thus completely bypassing the heart and pulmonary vasculature, all measurements reflect characteristics of the systemic vascular system alone. Measurement of Cardiac Output In five fetuses, flow in the main trunk of the pulmonary artery was measured after the chest was opened but before the bypass circuit was installed. A cuff-type flow transducer was placed around the main trunk of Ihe pulmonary artery and flow measured for 10-20 min. From this flow, essentially right ventricular output, cardiac output was estimated for later comparison with maximum venous return in the by pass circuit.
Downloaded by: Univ. of California Santa Barbara 128.111.121.42 - 3/6/2018 11:05:09 AM
Measurement of Control Pms Control Pms was measured simply by stopping the pump and simultaneously clamping the tube leading from the right atrium to exclude the starling resistor; its large compliance would have introduced an error into the Pms measurement. When the flow was stopped, usually for about 10 sec, aortic pressure (Pa) and Pra equi librated within 3-4 sec and remained equal and constant until flow was restored (fig. 7). The equilibrium value was taken as Pms. Pa was measured from the catheter
236
G ilbert
in the femoral artery and Pra was measured from a catheter introduced into the right atrium through the right atrial cannula. All pressures were referenced to the A-V valve in the right atrium. Construction of Venous Return Curve by Two Methods After measurement of Pms, the flow was restored and the animal allowed 10 min to stabilize. Then a venous return curve was plotted. Pra was increased (by lifting the Starling resistor) slowly from some low value, usually -10 Hg, until venous return had ceased. This required about 35 sec. Curves were plotted as flow (Qvr) versus Pra. Flow was measured by the flow transducer in the bypass circuit. Between curves, the Pra was held at -5 to 0 mm Hg. For comparison, curves were also plotted in six fetuses by methods used in pre vious experiments [17], Pra was raised rapidly to a given level and held there until flow and pressure had equilibrated (6-10 sec), then lowered to -5 to 0 mm Hg and held there until a steady state was reached before it was raised to a different level again. This procedure was repeated until enough points were obtained to plot the venous return curve. This method required 3-6 min to plot each curve. These curves were compared to the curves generated in the same fetuses by slowly raising Pra. Effects of Increased Blood Volume The effects of increased blood volume on the venous return curve were investi gated in six fetuses by injecting 50 ml maternal blood into each fetus. Pms was measured and a venous return curve plotted within 1 min after the injection to minimize fluid loss from the vascular compartment. The addition of 50 ml blood produced an average 11.1% increase in fetal systemic volume, estimating total fetal blood volume as 134.7 ml/kg body weight [7] and assuming the heart and lungs con tain no more than 15% of the total fetal blood volume.
Results
Downloaded by: Univ. of California Santa Barbara 128.111.121.42 - 3/6/2018 11:05:09 AM
The 14 fetuses used in this study ranged in weight from 2.7 to 6.0 kg, the average being 4.2 (mean) ± 0.2 kg (1 SE). Control Pms was 15.2 ± 0.3 mm Hg. Blood gas measurements sampled from the aorta during the con trol period were: P02 = 22.7 ±1.3 mm Hg, PC02= 42.2 ±2.3 mm Hg and pH = 7.31 ± 0.01. Normal venous return curves were plotted for all 14 ani mals (fig. 2) with venous return normalized to fetal body weight. Maximal venous return was 266 ± 12 ml/min/kg body weight, measured at right atrial pressures between -5 and 0 mm Hg. Mean P., was measured im mediately prior to obtaining venous return curves and averaged 64 ± 2 mm Hg. Blood flow measured in the main trunk of the pulmonary artery be fore the bypass circuit was installed was 154 ± 7 ml/min/kg body weight
Fetal Venous Return
237
PRA, mm Hg
for five fetuses. Maximal venous return in these fetuses, obtained after the heart and lungs had been replaced with the bypass circuit, was 305 ± 16 ml/min/kg body weight. If the right ventricle is assumed to put out ap proximately one half the total cardiac output, the value for flow in the main trunk of the pulmonary artery may be doubled to obtain an estimate of total cardiac output. Thus, for these five fetuses, cardiac output was approximately 310 ml/min/kg compared to a maximal venous return of 305 ml/min/kg after the animals were on the bypass circuit. As G uyton et al. [17] have demonstrated, the downsloping portion of the venous return curve may be approximated with a straight line. The reciprocal of this line’s slope is termed Rvr. For the normal venous return curves of 14 fetuses (fig. 2), the downsloping portions of the curves (Pra>0) were fitted with straight lines by least squares regression analysis. The in verse of the slopes were averaged to calculate Rvr. Correlation coefficients were all 0.97 or above. Rvr averaged 0.072 + 0.003 mm Hg/ml/min • Kg. In six animals, venous return curves were obtained by two different methods, G uyton ’s original method of raising Pra repeatedly to different levels [17], and the method of slowly raising Pra. Venous return curves were the same in all six animals regardless of method. Both curves for one animal are shown in figure 3. For each animal, paired analysis was used to compare the slopes and intercepts of the straight lines fitted to the curve
Downloaded by: Univ. of California Santa Barbara 128.111.121.42 - 3/6/2018 11:05:09 AM
Fig. 2. Normal venous return curves of 14 fetuses. Qvr = Venous return; P ra = right atrial pressure.
238
G ilbert
Fig. 3. Comparison in one fetus of venous return curves obtained by slowly raising right atrial pressure (--------------- ) and by raising right atrial pressure to various static levels (----- ).
PRA, mm Hg
Fig. 4. Comparison in six fetuses of downslopes of venous return curves obtained by slowly raising right atrial pressure (--------------- ) and by raising right atrial pres sure to various static levels (----- ). This figure shows the mean regression lines and 95% confidence limits, demonstrating that curves generated by both methods are in deed the same.
Downloaded by: Univ. of California Santa Barbara 128.111.121.42 - 3/6/2018 11:05:09 AM
downslopes. There was no statistical difference in either the slopes or the intercepts for the six fetuses, as demonstrated in figure 4. Increasing fetal blood volume by 11% in six fetuses increased Pms sig nificantly (p
