Lack of positive correlation between placental blood flow and mean arterial pressure in the non-anaesthetised guinea pig near term; an indication for autoregulation?

The objective of the present study was to evaluate whether placental blood flow (PBF) decreases or increases, in response to a fall or rise in mean arterial pressure (MAP), respectively, in an unanaesthetised animal model with a haemochorial placenta. Such a response would support the absence of autoregulation of placental blood flow for changes in mean arterial pressure. To this end, 31 late-pregnant guinea pigs were subjected to isovolemic haemodilution, a sham procedure or isovolemic haemoconcentration, after adequate recovery from instrumentation. The percentage change from baseline in PBF (delta PBF, %), was compared between animals with a fall in MAP, in response to haemodilution or the sham procedure (MAP-fall, n = 8, mean delta MAP -11 +/- 7%) and those with a rise in MAP, following haemoconcentration (MAP-rise, n = 10, mean delta MAP +15 +/- 8%). In addition, the percentage change in the blood flow to the myocardium, gastrointestinal tract, adrenals, kidneys, brain, skin, carcass and myometrium was compared between the MAP-fall and the MAP-rise group. The direction of the change in PBF did not differ between the MAP-fall and the MAP-rise group. In this respect, the distribution of delta PBF over the two groups was comparable with that observed for the myocardium, gastrointestinal tract, adrenals, skin, carcass and myometrium. The blood flow to the kidneys and the brain varied in response to the change in haematocrit. The results of this study show that PBF varies independently from MAP. This observation supports the idea that the blood flow to the labyrinthine placenta of the awake and healthy guinea pig near term may be autoregulated for hydrostatic inflow pressure, at least over the spectrum of arterial pressures observed in the present study.