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Design a simple experiment to examine the cardiovascular and metabolic response to an incremental exercise test to exhaustion

Introduction

Humans are homoeothermic, which means their internal body temperature of 36.1-37.8oC is kept relatively constant throughout life, with fluctuations of about 1oC. It is only during extreme environmental conditions of hot or cold, illness or prolonged exercise that the body will deviate from this range, with exercise often creating dangerously high internal temperatures of 40oC. At rest an individual will be able to dissipate heat by conduction and convection 20%), radiation (60%) and evaporation (20%). During exercise, evaporation contributes up to 80% of total heat loss. Environmental conditions play a major part in heat loss and a hot and humid climate (35.5oC and 60% relative humidity (RH) will limit heat loss and sweat evaporation.

During exercise at ambient temperatures (20oC) the heart rate (HR) rises from a resting level of about 60 beats per minute (bpm) to nearly 200 bpm, whilst stroke volume (SV) the volume of blood pumped per contraction, can rise from 80 ml/beat to 150 ml/beat. In combination these adjustments can increase cardiac output (Q = HR x SV) from 5 L/min to 30 L/min. the circulatory system. The increased Q causes the systolic blood pressure (SBP) to rise from 120 to 200 mm Hg at exhaustion, whilst the diastolic blood pressure (DBP) does not change (80 mm Hg). Blood volume tends to fall during exercise due to loss of plasma volume (PV), resulting from osmotic movement of water into the active muscle. PV is directly related to exercise intensity (VO2max), at 70% VO2max PV falls by 15% within few minutes of exercise and may be restored, depending on fluid consumption and sweat loss. This loss of PV results in an increase in concentration of red blood cells (RBC) and haemoglobin and therefore an increase in the oxygen carrying capacity of the blood.

Comparison of the two hyperthermic inducing events saw time to exhaustion dramatically decreased in the subjects wearing the full neoprene wet suit and this was largely due to the fact as the subjects increased work load, demand of blood to the periphery was significantly increased to dissipate the heat but due to the 5-8mm thickness of the wetsuit, heat was not able to dissipate, resulting in a rapid increase in the skin temperature and concurrent increase in core body temperature. Plasma volume was significantly greater in the full wetsuit condition compared to the liquid conditioning garment, due to the earlier increases in temperature and thus increases in sweating. Blood lactate levels were similar in both hyperthermic events and all subjects fatigued at the same rectal temperature (40.1 - 40.2oC), and CV strain (HR = 196-198 bpm, Q = 19.9-20.8 l/min), which corroborates work by Gonzalez-Alonso and colleagues (1999).

The relevance of this study was to try and understand the CV response to heat stress of normal recreational athletes during a maximal exercise test and compare this to the main body of published data that has focussed on highly trained endurance athletes. Further work could now concentrate on manipulating the two hyperthermic situations discussed here using recreational athletes, by increasing temperatures, relative humidity, varying VO2max, and using ECG measures and observing the various responses.

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