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Posted on 12 August 2022

​With the UK heatwave rumbling on, wildfires burning and drought conditions nationwide – it’s not only the environment that’s having a hard time controlling the heat. Our bodies take a direct toll from hot conditions – beyond sunburn and dehydration. Our heart, beating every second of our lives, is a dynamic organ changing in real time to environmental conditions, external and internal changes. In the heat, our heart works hard to radiate body heat out into the world, allow precipitation of sweat, and regulate internal temperature for normal, everyday functioning. The heart is particularly under stress when the temperatures begin to soar, and this change is challenging for those with heart conditions, obesity and other illnesses. Measuring and understanding blood pressure is a great way to understand personal metrics, risk and also inform yourself of when to take time out in the shade or stay cool. Even more so, we aim to demystify blood pressure in this week’s article, as well as looking at how heat and extreme temperatures change the cardiac cycle in response.

Blood pressure is the force of the blood when moving throughout the vasculature and circulatory system, and is responsible for the transport of blood to areas where nutrients and waste products are delivered and removed. Blood pressure, in a normal individual, is incredibly variable to changes in temperature, exercise status, sleep, caffeine and other parameters which dictate the rate at which blood is distributed around the body. A majority of the pressure of the blood is dictated by the cardiac output of the heart – the force at which the left ventricle of the heart discharges blood out of major arteries to respiring organs, muscles and tissues around your body. The walls of arteries, arterioles and, to some extent – veins – are responsive to hormones and cell messengers which dictate the dilation or constriction of the vessel, which increases or decreases the diameter of the vessel, subsequently increasing or reducing the pressure at which blood flows through.

Think of it as a garden hose – a hose with a large diameter will allow more water to flow smoothly with less pressure – but placing your thumb on the hose and applying force increases the pressure, and causes a much quicker flow of water – this is a basic analogy of what happens in our vessels on a beat-to-beat basis. That’s why health bosses are viscerally aware of the pressures on healthcare services expected in the coming weeks as the UK experiences a long heatwave.

When measuring blood pressure manually or automatically, certain numbers are produced which give us information about the cardiac cycle and how the heart and vasculature are working in response to stresses and resting conditions. We often hear of the ‘top’ number and ‘bottom’ number in relation to blood pressure – such as 121/79 – the top number being the systolic, and bottom number representing diastolic blood pressure. Systolic blood pressure reflects the pressure of the blood in the arteries during a contraction of the heart, whilst diastolic blood pressure reflects the residual pressure in the vessels during relaxation of the heart in the cardiac cycle. A commonly accepted classification for blood pressure is that below 120/80mmHg is a normal, healthy blood pressure for individuals over the age of 18. There are also classifications of higher blood pressure described as ‘hypertension’, which exist in stage 1 hypertension and stage 2 hypertension, all of which describe relative risk for the development of cardiovascular disease and cardiac events such as heart attacks and strokes.

  • Elevated blood pressure: 120-129/<80 mm Hg

  • Stage I hypertension: 130-139/80-89 mm Hg

  • Stage 2 hypetension: 140>/90> mm Hg

As blood pressure changes on a beat to beat, diurnal and even day to day basis, it is a good idea to collect your blood pressure variables in the morning, lunchtime and evening, and average these over a week or so of collection to get a good idea of your average blood pressure. Even moderate increases of blood pressure of around 130-139/80-89mmHg doubles your risk for developing serious cardiovascular disease, as well significantly increasing your risk of a heart attack or stroke. At this stage, doctors now recommend lifestyle changes and the addition of an anti-hypertensive drugs. A significant risk of high blood pressure is the rupturing of atheromas (fat and cholesterol build ups in arteries and arterioles, a natural process of aging), which cause blockages and strokes, though commonly cause coronary heart attacks due to blockage of the coronary vasculature, which supplies the heart with the blood needed to maintain function.

In the heat, there are several general changes in the cardiac cycle, not least an increase in blood pressure and heart rate as more blood is directed to the skin and body surface in an attempt to lose heat and reduce body temperature. In these temperatures experienced in the recent weeks in the UK, the heart can circulate 2-4 times as much blood as a normal day in an attempt to lose heat and keep major organs from overheating. Other minerals such as sodium, potassium and other electrolytes are lost in higher quantities through sweating, all of which are involved in keeping the ionic balance of the blood from being too acidic or too alkaline. With a reduction in the number of electrolytes in the blood, or dehydration, perspiration is inhibited and blood vessels near the skin dilate even further to aid in releasing heat through radiation. Even more water may be taken out of the blood at the kidneys to increase perspiration further and thus reducing overall blood volume, meaning increased arterial pressure is needed to maintain a similar cardiac output. Take your pulse on a hot day and you’ll likely feel it racing even at rest as the hypothalamus (part of your brain that regulates cardiac function and temperature. All of these changes in tandem pose risks for the elderly, those with pacemakers, high blood pressure, obesity and other heart conditions in the increased cardiac stress in these temperatures of late.

  • Exercise and physical activity – even small amounts of aerobic exercise such as walking can reduce systolic and diastolic blood pressure by as much as 3-5mmHg in as little as 8 weeks – a greatly effective method for non-pharmacological intervention.

  • Stress management – being aware of stress levels and planning for stressful situations is a great way to manage your blood pressure – a rise in the level of stress hormones such as cortisol raises your blood pressure and can increase the risk of heart attacks and strokes.

  • Diet – managing the amount of lipids and fats you consume in your diet is also a key part of staying in a healthy range, as well as reducing your risk for coronary artery disease. Make a conscious effort to increase the amount of healthy fats you consume, as well as taking on good, complex carbohydrates, green cruciferous vegetables and to reduce the intake of saturated fat and cholesterol.

  • Weight management – there is a strong correlation between body weight and blood pressure, particularly as body mass places strain on the body and heart, as well as increasing the chance of occlusion of arteries and vessels in the lower limbs. Even modest weight loss can greatly improve your blood pressure readings, along with reducing your changes for other non-communicable diseases such as metabolic disease and diabetes.

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