Fetal in vivo continuous cardiovascular function during chronic hypoxia

Abstract

Although the fetal cardiovascular defence to acute hypoxia and the physiology underlying it have been established for decades, how the fetal cardiovascular system responds to chronic hypoxia has been comparatively understudied. We designed and created isobaric hypoxic chambers able to maintain pregnant sheep for prolonged periods of gestation under controlled significant (10% O2) hypoxia, yielding fetal mean P(aO2) levels (11.5 ± 0.6 mmHg) similar to those measured in human fetuses of hypoxic pregnancy. We also created a wireless data acquisition system able to record fetal blood flow signals in addition to fetal blood pressure and heart rate from free moving ewes as the hypoxic pregnancy is developing. We determined in vivo longitudinal changes in fetal cardiovascular function including parallel measurement of fetal carotid and femoral blood flow and oxygen and glucose delivery during the last third of gestation. The ratio of oxygen (from 2.7 ± 0.2 to 3.8 ± 0.8; P < 0.05) and of glucose (from 2.3 ± 0.1 to 3.3 ± 0.6; P < 0.05) delivery to the fetal carotid, relative to the fetal femoral circulation, increased during and shortly after the period of chronic hypoxia. In contrast, oxygen and glucose delivery remained unchanged from baseline in normoxic fetuses. Fetal plasma urate concentration increased significantly during chronic hypoxia but not during normoxia (Δ: 4.8 ± 1.6 vs. 0.5 ± 1.4 μmol l(-1), P<0.05). The data support the hypotheses tested and show persisting redistribution of substrate delivery away from peripheral and towards essential circulations in the chronically hypoxic fetus, associated with increases in xanthine oxidase-derived reactive oxygen species.

Figure 1. Isobaric hypoxic chambers and the CamDAS system

Each chamber was equipped with an electronic servo‐controlled humidity cool steam injection system to return the appropriate humidity to the inspirate (i). Ambient $P_{{\mathrm{O}}_2}$, $P_{\mathrm{C}{\mathrm{O}}_2}$, humidity and temperature within each chamber were monitored via sensors (ii). For experimental procedures, each chamber had a double transfer port (iii) to internalise material and a manually operated sliding panel (iv) to bring the ewe into a position where daily sampling of blood could be achieved through glove compartments (v). Each chamber incorporated a drinking bowl on continuous water supply and a rotating food compartment (vi) for determining food intake. A sealed transfer isolation cart could be attached to a side exit (vii) to couple chambers together for cleaning. Waste was disposed off via a sealable waste pipe (viii). The CamDAS system was contained in a custom‐made sheep jacket able to hold a box containing the Transonic flow probe connectors on one side (ix) and the pressure acquisition system box (x) on the other. Cables (xi) connected the two boxes together and also linked to two battery packs able to power the system for 24 hours. Measurements made using the data acquisition were transmitted wirelessly to a laptop kept outside the chamber room (xii) via Bluetooth (xiii), thereby making it possible to view continuous recordings of the maternal and fetal cardiovascular data.

Figure 2. Maternal blood gas, acid base and metabolic status

Values are mean ± SEM for pregnant sheep undergoing normoxic (◯, n = 6) or chronic hypoxic (, n = 6) pregnancy. Maternal blood gas values were corrected to 38 °C. pH, arterial pH; $P_{\mathrm{aC}{\mathrm{O}}_2}$, arterial CO₂ partial pressure; $P_{\mathrm{a}{\mathrm{O}}_2}$, arterial O₂ partial pressure; Sat[Hb], percentage saturation of haemoglobin; ABE, acid base excess; Htc, haematocrit; Glucose, blood glucose concentration; Lactate, blood lactate concentration; (N), normoxic recovery. The x‐axis shows time in hours (hr) and days (d). Significant differences (P < 0.05): *differences indicating a significant main effect of time compared with baseline; ^†differences indicating a significant main effect of treatment compared with normoxic pregnancy (two‐way repeated‐measures ANOVA + Tukey test). For Htc, the comparison of slopes was achieved with the Student's t test for unpaired data.

Figure 3. Fetal blood gas, acid base and metabolic status

Values are mean ± SEM for fetal sheep undergoing normoxic (◯, n = 6) or chronic hypoxic (, n = 6) pregnancy. Fetal blood gas values were corrected to 39.5 °C. pH, arterial pH; $P_{\mathrm{aC}{\mathrm{O}}_2}$, arterial CO₂ partial pressure; $P_{\mathrm{a}{\mathrm{O}}_2}$, arterial O₂ partial pressure; Sat[Hb], percentage saturation of haemoglobin; ABE, acid base excess; Htc, haematocrit; Glucose, blood glucose concentration; Lactate, blood lactate concentration; (N), normoxic recovery. The x‐axis shows time in hours (hr) and days (d). Significant differences (P < 0.05): *differences indicating a significant main effect of time compared with baseline; ^†differences indicating a significant main effect of treatment compared with normoxic pregnancy (two‐way repeated‐measures ANOVA + Tukey test).

Figure 4. Fetal cardiovascular responses to chronic hypoxia

Values are mean ± SEM for the change from baseline in cardiovascular variables in fetal sheep undergoing normoxic (◯, n = 6, left) or chronic hypoxic (, n = 6, right) pregnancy. CBF, carotid blood flow; FBF, femoral blood flow; BPM, beats per minute; (N), normoxic recovery. The x‐axis shows time in days (d). Significant differences (P < 0.05): *differences indicating a significant main effect of time compared with baseline; ^†differences indicating a significant main effect of treatment compared with normoxic pregnancy (two‐way repeated‐measures ANOVA + Tukey test). For descending aortic pressure, the two‐way ANOVA represents a comparison of slopes. For FBF and CBF, the two‐way ANOVA represents a comparison of areas under the curve.

Figure 5. Fetal carotid and femoral arterial oxygen and glucose delivery in the chronically hypoxic fetus

Values are mean ± SEM for the change from baseline in oxygen and glucose delivery in the ascending and the descending aorta and the ratio of these values in fetal sheep undergoing normoxic (◯, n = 6) or chronic hypoxic (, n = 6) pregnancy. (N), normoxic recovery. The x‐axis shows time in hours (hr) and days (d). Significant differences (P < 0.05): *differences indicating a significant main effect of time compared with baseline; ^†differences indicating a significant main effect of treatment compared with normoxic pregnancy (two‐way repeated‐measures ANOVA + Tukey test). For the ratio of ascending/descending oxygen delivery, the two‐way ANOVA represents an analysis of the area under the curve.

Figure 6. Fetal and maternal vitamin C and urate levels in the chronically hypoxic fetus

Values are mean ± SEM for the change from baseline in vitamin C and urate in pregnant ewes and fetal sheep undergoing normoxic (◯, fetus; □, ewe; n = 6) or chronic hypoxic (, fetus; ■, ewe; n = 6) pregnancy. (N), normoxic recovery. The x‐axis shows time in hours (hr) and days (d). Significant differences (P < 0.05): *differences indicating a significant main effect of time compared with baseline; ^†differences indicating a significant main effect of treatment compared with normoxic pregnancy (two‐way repeated‐measures ANOVA + Tukey test). For the fetal urate levels the two‐way ANOVA represents an analysis of the area under the curve.