Arterial Blood Pressure in the Unanaesthetized Fetal Lamb After Changes in Fetal Blood Volume and Haematocrit

Abstract

Fourteen sheep fetuses in the third trimester of gestation were surgically prepared with intravascular catheters and five were nephrectomized. All experiments were done on unanaesthetized fetuses in utero, 3 to 9 days after surgery. Mean arterial blood pressure was recorded while fetal blood volume was changed by injection or withdrawal of blood, or infusion of Ringer's or dextran solutions. Fetal arterial blood gases and pH changed little during the experiments and were shown not to have affected the results. Arterial blood pressure was strongly influenced by changes in fetal blood volume and was also found to depend on haematocrit. There were no detectable differences between the responses of normal and nephrectomized fetuses or between normal fetuses and fetuses treated with hexamethonium. In all preparations central venous pressure depended on blood volume. Heart rate did not depend on blood volume in normal fetuses, but an increase in heart rate with an increase in blood volume was seen after hexamethonium. The open loop gain of baroceptor control of arterial blood pressure could not be proved to differ significantly from zero. Venoconstrictor responses and heart rate responses of reflex origin did not occur when arterial blood pressure was changed. The absence of detectable baroceptor control appears to explain the great sensitivity of arterial blood pressure to blood volume in the unanaesthetized fetal lamb. Previously we found that fetal placental blood flow is almost proportional to fetal blood volume [Faber, Gault, Green and Thornburg, 1973]. Since placental resistance to flow is not known to be under fetal control, changes in placental blood flow were thought to be secondary to changes in arterial blood pressure, an explanation which would require that arterial blood pressure itself is also proportional to blood volume. Studies that demonstrate some control of arterial blood pressure by the baroceptors of the fetus [Brinkman, Ladner, Weston and Assali, 1969] and the neonate [Downing, 1960; Downing, Gardner and Rocamora, 1969] seem to argue against this supposition and we therefore investigated the relationship between arterial blood pressure and blood volume in unanaesthetized fetal lambs under near normal conditions. The contributions of the baroceptors, and of renal mechanisms, to the regulation of arterial blood pressure were assessed by experiments on fetuses with blocked ganglionic transmission or previous bilateral nephrectomy. Recordings of the pH, P02, and PCO2 of fetal arterial blood were made to monitor the condition of the preparations and to eliminate the possibility that the chemo-ceptors affected arterial blood pressure in these experiments. METHODS Animals Pregnant western mixed breed and Dorset ewes were starved for 24 hr prior to surgery and operated on under nitrous oxide-halothane anaesthesia, after induction 241

Relationship of arterial pressure and heart rate in fetal, new‐born and adult sheep.

“…Baroreflex mechanisms could not be shown to moderate these changes. The fetal blood volume per unit weight is high (Creasy, Drost, Green & Morris, 1970), and blood volume is an important determinant of fetal blood pressure and placental blood flow (Faber & Green, 1972;Faber et al 1974). Vatner, Boettcher, Hendryx & McRitchie (1975) have shown in the conscious dog that volume loading results in a decrease of baroreflex sensitivity.…”

“…1969;Ponte & Purves, 1973) but it is not clear if the baroreceptors are active in utero in the absence of endocrine and other changes induced by operation. Faber et al (1974) calculated the open-loop gain of the baroreflex of fetal lambs in utero to be zero. They altered blood pressure by varying the blood volume and found that arterial and central venous pressures and heart rate increased when the blood volume was increased.…”

“…Use of pressor drugs gave estimates of baroreflex sensitivity of approximately 8 msec/mmHg Ismay et al 1979) while the use of an aortic balloon or cuff gave values between 1-38 and 3-3 msec/mmHg (Shinebourne et al 1972;Maloney et al 1977). However Faber, Green & Thornburg (1974) estimated the baroreflex sensitivity to be zero when they altered blood pressure by changing the fetal blood volume.…”

Hypotension in Fetal and Newborn Lambs; Different Patterns of Reflex Heart Rate Control Revealed by Autonomic Blockade

“…However, others [6] using similar methods have contended that the baroreflex does not operate at resting arterial pressures in a normal fetus in utero. Fetal responses to hemorrhage have sup ported the presence of functional baroreflexes [2] but this too is controversial [7], This study examined responses to arterial hypotension in fetal and newborn lambs. Few studies have examined fetal cardiovascular responses to reductions of arterial pressure and these have provided conflicting results.…”

Physiological Role of Placental Proteases: Interaction Between Pregnancy-Induced Bioactive Peptides and Proteases

“…The regulation of blood pressure is mainly influenced by autoregulation, the autonomic nervous system and the humoral system, but the placenta (uteroplacental circulation) is not capable of autoregulation [49]; it appears to be totally dependent on perfusion pressure. Although the autonomic nervous system in the fetus develops with advancing gestation, fetal end organ response to the autonomic nervous system (in both blood vessels and the heart) is of a much smaller magnitude than the adult response [50,51]. Blood pressure homeostasis may be more important in the fetus than in the adult, as it determines the level of placental perfusion which then determines the extent of respiratory gas exchange.…”