Blunted sympathetic neurovascular transduction during normotensive pregnancy.

Abstract

Normotensive pregnancy is associated with elevated sympathetic nervous system activity yet normal or reduced blood pressure. It represents a unique period of apparent healthy sympathetic hyperactivity. The present study models the blood pressure and heart rate (ECG R-R interval) responses to fluctuations in sympathetic nervous system activity aiming to understand neurocardiovascular transduction. The reported data clearly demonstrate that transduction of sympathetic nervous system signalling to systemic cardiovascular outcomes is reduced in normotensive pregnancy. These data are important for understanding how blood pressure regulation adapts during normotensive pregnancy and set the foundation for exploring similar mechanisms in hypertensive pregnancies. Previously, we described sympathetic nervous system hyperactivity yet decreased blood pressure responses to stress in normotensive pregnancy. To address the hypothesis that pregnant women have blunted neurocardiovascular transduction we assessed the relationship between spontaneous bursts of sympathetic nerve activity (SNA) and fluctuations in mean arterial blood pressure and R-R interval. Resting SNA, blood pressure and ECG were obtained in pregnant (third trimester, n=18) and non-pregnant (n=18) women matched for age and pre-/non-pregnant body mass index. Custom software modelled beat-by-beat pressure (photoplethysmography) and R-R interval in relation to sequences of SNA bursts and non-bursts (peroneal microneurography). Sequences were grouped by the number of bursts and non-bursts [singlets, doublets, triplets and quadruplet (four or more)] and mean blood pressure and R-R interval were tracked for 15 subsequent cardiac cycles. Similar sequences were overlaid and averaged. Peak mean pressure in relation to sequences of SNA was reduced in pregnant vs. non-pregnant women (doublets: 1.6±1.1mmHg vs. 3.6±3.1mmHg, P<0.05; triplets: 2.4±1.2mmHg vs. 3.4±2.1mmHg, P<0.05; quadruplets: 3.0±1.0mmHg vs. 5.5±3.7mmHg, P<0.05). The nadir R-R interval following burst sequences was also smaller in pregnant vs. non-pregnant women (singlets: -0.01±0.01s vs. -0.04±0.04s, P<0.05; doublets: -0.02±0.03s vs. -0.05±0.04s, P<0.05; triplets: -0.02±0.01s vs. -0.07±0.04s, P<0.05; quadruplets: -0.01±0.01s vs. -0.09±0.09s, P<0.05). There were no differences between groups in the mean arterial pressure and R-R interval responses to non-burst sequences. Our data clearly indicate blunted systemic neurocardiovascular transduction during normotensive pregnancy. We propose that blunted transduction is a positive adaptation protecting pregnant women from the cardiovascular consequences of sympathetic hyperactivity.