Factors mediating the pressor response to isometric muscle contraction: An experimental study in healthy volunteers during lower body negative pressure.

Niels A Stens,Einar Stranden,Espen F Bakke,Lars Øivind Høiseth,Jonny Hisdal,Narinder Kaur,Dick H J Thijssen,Archana Sharma

doi:10.1371/journal.pone.0243627

Abstract

Whilst both cardiac output (CO) and total peripheral resistance (TPR) determine mean arterial blood pressure (MAP), their relative importance in the pressor response to isometric exercise remains unclear. This study aimed to elucidate the relative importance of these two different factors by examining pressor responses during cardiopulmonary unloading leading to step-wise reductions in CO. Hemodynamics were investigated in 11 healthy individuals before, during and after two-minute isometric exercise during lower body negative pressure (LBNP; -20mmHg and -40mmHg). The blood pressure response to isometric exercise was similar during normal and reduced preload, despite a step-wise reduction in CO during LBNP (-20mmHg and -40mmHg). During -20mmHg LBNP, the decreased stroke volume, and consequently CO, was counteracted by an increased TPR, while heart rate (HR) was unaffected. HR was increased during -40 mmHg LBNP, although insufficient to maintain CO; the drop in CO was perfectly compensated by an increased TPR to maintain MAP. Likewise, transient application of LBNP (-20mmHg and -40mmHg) resulted in a short transient drop in MAP, caused by a decrease in CO, which was compensated by an increase in TPR. This study suggests that, in case of reductions of CO, changes in TPR are primarily responsible for maintaining the pressor response during isometric exercise. This highlights the relative importance of TPR compared to CO in mediating the pressor response during isometric exercise.

Highlights

Isometric handgrip exercise is known to elicit increases in mean arterial blood pressure (MAP) [1,2,3]
No significant differences were observed in the blood pressure response during reduced preload induced by mild (-20mmHg) or moderate lower body negative pressure (LBNP) (-40mmHg) prior to the exercise period despite a significant reduction in cardiac output (CO) (Table 1)
Our results indicate that the muscles and skin in the resting arm do not play an important role in increasing total peripheral resistance (TPR) during isometric handgrip in supine position, but do make a contribution during reduced preload caused by application of LBNP

Summary

Introduction

Isometric handgrip exercise is known to elicit increases in mean arterial blood pressure (MAP) [1,2,3]. Despite the powerful response of this mechanism being observed a decade ago, its hemodynamic mechanisms remain to be fully elucidated. It remains debatable whether the pressor response is due to increases in cardiac output (CO) [5,6,7,8,9], total peripheral resistance (TPR) [10,11,12,13,14] or both [15,16,17,18]. CO changes during IHG is primarily driven by an elevated heart rate (HR), whilst the stroke volume (SV) is slightly reduced due to the tachycardia and increased afterload, or even maintained following both augmented ventricular contractility [19, 20] and constant or elevated preload via central blood volume mobilization [21]

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