EFFECTS OF TEMPERATURE AND EXERCISE ON LIMB BLOOD FLOW IN MAN

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EFFECTS OF TEMPERATURE AND EXERCISE ON LIMB BLOOD FLOW IN MAN

ABSTRACT

The influence of temperature on the hemodynamic adjustments to direct passive heat stress within the leg’s major arterial and venous vessels and compartments remains unclear. Fifteen healthy young males were tested during exposure to either passive whole body heat stress to levels approaching thermal tolerance [core temperature (Tc) + 2°C; study 1; n = 8] or single leg heat stress (Tc + 0°C; study 2; n = 7). Whole body heat stress increased perfusion and decreased oscillatory shear index in relation to the rise in leg temperature (Tleg) in all three major arteries supplying the leg, plateauing in the common and superficial femoral arteries before reaching severe heat stress levels. Isolated leg heat stress increased arterial blood flows and shear patterns to a level similar to that obtained during moderate core hyperthermia (Tc + 1°C). Despite modest increases in great saphenous venous (GSV) blood flow (0.2 l/min), the deep venous system accounted for the majority of returning flow (common femoral vein 0.7 l/min) during intense to severe levels of heat stress. Rapid cooling of a single leg during severe whole body heat stress resulted in an equivalent blood flow reduction in the major artery supplying the thigh deep tissues only, suggesting central temperature-sensitive mechanisms contribute to skin blood flow alone. These findings further our knowledge of leg hemodynamic responses during direct heat stress and provide evidence of potentially beneficial vascular alterations during isolated limb heat stress that are equivalent to those experienced during exposure to moderate levels of whole body hyperthermia.

CHAPTER ONE

INTRODUCTION

Background of the study

Local tissue and blood temperature (TB) increases with elevations in skeletal muscle metabolism and heat production during dynamic exercise (Sproule & Archer, 1959; Saltin & Hermansen, 1966; Gonzalez-Alonso ´ et al. 1999, 2000). However, the influence of temperature on perfusion to exercising and non-exercising human limbs is not established. In humans, the increases in local perfusion and oxygen uptake (V˙ O2 ) during leg exercise are much greater than during arm exercise, reflecting differences in muscle mass and work capacity (Secher et al. 1977; Knight et al. 1992; Volianitis & Secher, 2002; Calbet et al. 2004; Mortensen et al. 2005). Limb tissue and blood temperatures depend upon the balance between heat production and endogenous heat transfer. In exercising limbs, heat is transferred from the working muscles to the neighbouring tissues and the overlying skin as well as to the body core. This is made possible via the flowing blood (convective heat transfer) and direct intercellular heat conduction (conductive heat transfer; Barcroft & Edholm, 1943; Pennes et al. 1948; Gonzalez-Alonso ´ et al. 2000). Differences in V˙ O2 and thus metabolic heat production between the exercising lower and upper limbs could affect the increase in TB and the relationship between TB and perfusion in exercising and non-exercising limbs if the differences in heat production are not matched by proportional changes in endogenous heat transfer. To date, no study has examined the relationships amongst limb TB, perfusion and aerobic metabolism during separate and combined lower and upper limb exercise to determine whether a coupling between TB and limb perfusion is still apparent when accounting for differences in metabolism and heat production. Understanding of thermoregulation during exercise is largely based on the regulation of skin blood flow and sweating in resting limbs (Johnson et al. 2014). Yet the local thermal stimuli modulating these key thermoregulatory responses during exercise are likely to be different in the exercising and non-exercising limbs, unless increases in temperature in the exercising limbs lead to similar elevations in blood and tissue temperature in the non-exercising limbs. The net heat transfer from the exercising limbs to the trunk and head results in increased core and brain temperatures (Saltin et al. 1966; Nybo et al. 2002; Kenny et al. 2003; Trangmar et al. 2014). In non-exercising limbs during prolonged leg exercise, however, forearm venous TB and muscle temperature are lower and do not increase to the same extent as core and active leg muscle and blood temperatures (Gonzalez-Alonso ´ et al. 1999; Jay et al. 2007). To shed light on the mechanisms of temperature and limb blood-flow regulation, it is timely to investigate the impact of haemodynamic and thermodynamic events in exercising limbs on central and non-exercising limb perfusion and TB. Temperature is one of a congregate of metabolic byproducts proposed to contribute to regulation of limb tissue perfusion (Barcroft & Edholm, 1943). In support of a role for hyperthermia, increases in local blood and muscle temperatures are associated with similar elevations in limb perfusion during both isolated leg and whole-body heat stress (Pearson et al. 2011; Heinonen et al. 2011; Chiesa et al. 2015), irrespective of differences in systemic temperature and haemodynamic responses between conditions (Chiesa et al. 2015).

1.2 Statement of the problem

Although small compared with exercise hyperaemia, this hyperthermia-mediated limb hyperaemia is maintained during combined heat stress and one-legged knee-extensor exercise (Pearsonet al. 2011; Chiesa et al. 2015). The thermal hyperaemia in resting limbs is associated only in part with metabolic vasodilatation, because the concomitant elevation in limb V˙ O2 is too small to account for the increase in perfusion (Pearson et al. 2011; Chiesa et al. 2015). Thus, hyperthermia induces vasodilatation of the limb tissue vascular beds through other mechanisms, which may be temperature sensitive. Along these lines, hyperthermia is associated with elevations in intravascular concentration of the potent vasoactive substance ATP (Pearson et al. 2011), accompanying skeletal muscle, skin and bone vasodilatation (Heinonen et al. 2011; Pearson et al. 2011). During exercise in normal environmental conditions, plasma ATP also increases in the forearm and leg circulations (Forrester & Lind, 1969; Forrester, 1972; Gonzalez-Alonso ´ et al. 2002; Mortensen et al. 2011), possibly in response to alterations in erythrocyte oxygen binding and other adjuvant metabolic, thermal and mechanical stimuli (Bergfeld & Forrester, 1992; Ellsworth et al. 1995; Sprague et al. 1998; Wan et al. 2008; Kalsi & Gonzalez-Alonso, 2012).

1.3 Objectives of the study

1. To understand the impact of temperature and exercise on limb blood flow in man

2. To identify the relationship level between exercise activities, temperature and limb blood flow in man

1.4 Research Questions

1. What is the impact of temperature and exercise on limb blood flow in man

2. What is the relationship level between exercise activities, temperature and limb blood flow in man

1.5 Research Hypothesis

H0: There is no relationship level between exercise activities, temperature and limb blood flow in man

H1: There is a relationship level between exercise activities, temperature and limb blood flow in man

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