Effects of maternal hypoxia on fetal cardiovascular hemodynamics

The pulmonary vein Doppler flow velocity waveform: feature analysis by comparison of in vivo pressures and flows with those in a computerized fetal physiological model

OBJECTIVES

Doppler flow velocity waveforms (FVW) in fetal veins that discharge into the atria show fluctuations related to atrial events. Pulmonary veins are of particular interest because both ends (atrial and collecting venule) are within the intrathoracic pressure environment reducing fetal breathing artifacts. Indices, such as pulsatility index for veins (PIV), have been suggested to classify FVWs and relate them to fetal well being. We wished to examine the relationship between function and FVW in circumstances which cannot ethically be examined in vivo, by studying the mechanisms which produced altered 'flows' in a detailed fetal computer model. We then related these findings to current flow indices.

METHODS

A computer model of the feto-placental unit, responding to changes in organ oxygenation and regional flow is briefly described. In vivo intracardiac pressures and FVWs obtained from other studies were used to extend detail in the model until matching 'pressures' and 'flows' resulted. The effects of flow redistribution in the hypoxic fetus on pulmonary vein 'Doppler' flow velocity waveforms were then studied.

RESULTS AND CONCLUSIONS

Flow reversal in pulmonary veins during atrial contraction indicates hypoxia, but change of shape of the FVW envelope reflects the changes in the pressure waveform of the left atrium. Of the major veins the pulmonary vein Doppler FVW gave the truest representation of atrial pressure response to both intracardiac and systemic vascular status. Although current indices indicate general fetal condition, more specific indices are needed if pulmonary venous flow is to be used as an end-point. A pulmonary vein pressure gradient index is suggested.

Hemodynamic responses to asphyxia in spontaneously breathing newborn term and premature lamb

The comparative effects of asphyxia were studied in spontaneously breathing premature and full term newborn lambs. The premature lambs exhibited significantly higher baseline blood flow to several organs. Asphyxia induced similar changes in regional organ blood flow in preterm and term newborn lambs. Regional differences were found in baseline brain blood flow with an increase from cephalad to caudad. Asphyxia resulted in preferential increases in blood flow to the lower brain structures (cerebellum, medulla, midbrain, and spinal cord). Blood flow to most organs returned to baseline 140 min after recovery from asphyxia.

Blood flow to fetal organs as a function of arterial oxygen content

In a sheep preparation the blood flow to fetal organs was studied 3 to 10 days after surgery by means of the microsphere technique over a range of fetal arterial O2 content from 6 to 1 mM. Blood flows to neural tissues (cerebrum, cerebellum, brain stem), heart, and the adrenals increased in inverse relation to arterial O2 content. As a result the arterial supply of O2 to these organs tended to remain constant over the O2 range studied. Blood flow to the fetal lungs decreased progressively with hypoxia. The blood flow to kidneys, digestive tract, pancreas, and carcass had a tendency to remain constant or increase gradually in the transition from high to moderately low levels of arterial O2 content and then to decrease abruptly in more severe hypoxia. Umbilical blood flow did not change systematically in relation to arterial O2 content.

Diminished limb blood flow in infants with transposition of the great vessels: An adaptation to chronic hypoxia?

Calf blood flow was measured by venous occlusion plethysmography using a mercury in rubber strain gauge in infants with transposition of the great vessels. (TGV), and in comparable infants free from cardiopulmonary disease. Resting calf blood flow in the infants with TGV was 3.6 +/- 0.8 ml/100 ml/min, while in the control group flow was 6.8 +/- 2.3 ml/100 ml/min, a highly significant difference. We postulate that newborns with TGV decrease their resting calf flow in response to chronic hypoxia.

Acute reduction of uterine blood flow and fetal heart rate changes in pregnant sheep near term

Five merino sheep near term were used for acute preparations to investigate the influence of basal uterine blood flow (UBF), basal fetal oxygenation and basal fetal heart rate (FHR) on FHR changes elicited by acute reduction of UBF. The ewes were anaesthesized with pentobarbital and Alloferin was given for relaxation. Ventilation was maintained via a tracheal tube by a Starling pump. Maternal heart rate, arterial pressure and intrauterine pressure as well as fetal heart rate and umbilical artery pressure were recorded continously. Initially blood flow was recorded by cuff flow meter sequentially in both uterine arteries for a short time in order to determine total UBF and the ratio of flows. During the UBF reduction sequence flow was recorded in the artery of the gravid horn continously and measurement was corrected according to the initial ratio of flows in both arteries, presuming that this ratio would remain constant throughout the course of the experiment. Repetitive and progressive acute reduction of UBF (approximately 25%, 50% and 100% from basal UBF value) was achieved three times, lasting 120 sec each and each approximatly 12 min apart, by flow meter controlled partial or total occlusion of the aorta abdominalis of the ewe with a balloon catheter inserted into the aorta. (In some cases the lateral abdominal section was left open and the aorta was compressed manually). Before, 90 sec after the beginning and 10 min after the end of UBF reduction blood samples were taken from the fetal catheters (Fig. 3). In two preparations this sequence was repeated once after complete recovery of FHR.