Cardiovascular Response to Autonomic Stress Tests and High Carbohydrates Meals Ingestion in Healthy Young and Old Participants

Abstract

The basis of testing cardiovascular reflex control is to disturb the circulatory system and observe its subsequent recovery. Deep breathing, standing and handgrip exercise are the most commonly used monitoring techniques for assessing cardiovascular function. Aims: Develop a method of assessing cardiovascular function using continuous measurements with a Finometer, whilst combining autonomic stress tests and thermal stress/high-carbohydrate meals. The responses to deep breathing, standing and handgrip exercise at different levels (20%, 40% and 60%) of MVC were assessed. The experiments in Chapter 4 aimed to evaluate the non-invasive Finometer technique for cardiovascular monitoring during variations in local temperature. The continuous cardiovascular responses and intermittent limb blood-flow responses were assessed for two hours after high-carbohydrate meal (solid vs liquid) ingestion, with water ingestion used as a control. Methods: All laboratory visits were randomised according to each study protocol using the website Randomization.com. A Finometer was used in all studies for measurements of continuous cardiovascular responses over time. An adapted refrigerator was used for local heating and cooling of the Finometer and non-Finometer hands. Blood flow of the left lower leg was measured using venous occlusion plethysmography. The meals (high-carbohydrate) were freshly prepared on the day of each study. The liquid and solid meals contained 2.1 MJ, of which 15 % was protein, 14 % fat and 71 % carbohydrate. Results: The cardiovascular responses to the autonomic tests were greater for men than women. In males at 60% MVC, blood pressure, cardiac output and heart rate increased from 124/75mmHg, 6.3L/min, 69beats/min to 147/93 mmHg, 8.4L/min, 88.8beats/min and in women from 128/76mmHg, 5.5L/min, 68beats/min to 144/91mmHg, 6.9L/min, 87beats/min. When both the Finometer and non- Finometer hands were subjected to 15oC, there was a reduction in the skin temperature, which was due to local vasoconstriction. This was reflected by a significant increase (P≤0.05) of blood pressure on some occasions (systolic blood pressure before the 20% of MVC handgrip exercise when the Finometer hand was cooled, and diastolic blood pressure before the 20% and 40% of MVC handgrip exercises when the non-Finometer hand was cooled) compared with the pre-cooling baseline. The cardiovascular responses to the autonomic stress tests did not change significantly with heating or cooling of either hand. There was also no significant difference between the sexes in the Finometer measurements made at rest and during performance of autonomic stress tests under thermal stress. At the first hour of feeding, blood pressure showed small reductions (P≤0.05) after the high-carbohydrate solid meal (systolic blood pressure=-3.53, diastolic blood pressure= -3.35 mmHg) in healthy young participants and a large reduction (P≤0.05) in older participants (systolic blood pressure =-18.92, diastolic blood pressure= -14.68mmHg). The older participants did not have a significant heart rate or cardiac output response to either meal despite the drop in blood pressure, which might have been due to age-related changes in vagal tone. Conclusions: The cardiovascular parameters responded significantly to autonomic stress tests, and the mean values of the cardiovascular parameters were greater for men than women. The autonomic stress tests were combined with different interventions, such as local heating/cooling and high-carbohydrate meals in two forms (liquid and solid), to determine whether the cardiovascular responses to autonomic stress tests were affected by these interventions. The difference between the cardiovascular variable responses after liquid and solids meals was likely due to greater and more sustained gastric and intestinal haemodynamic responses to the solid than the l