Saturday, June 30, 2012
Singapore - Under pressure
Do you really have high blood pressure?
High blood pressure is an increasingly common condition, especially in a population with a higher proportion of elderly people. Yet, many, especially the elderly, do not understand what really constitutes high blood pressure or hypertension.
What is high blood pressure?
When the heart pumps blood out into the aorta (the major artery that supplies blood from the heart to the body) during contraction of the left lower heart chamber (LV), the highest pressure generated is termed as the systolic blood pressure (SBP). Subsequently, the LV relaxes and expands in size, the pressure falls and the lowest recorded blood pressure is termed as the diastolic blood pressure (DBP). When the SBP consistently exceeds 140 mm Hg ( millimetres mercury which is the unit of measurement of blood pressure) and/or the DBP exceeds 90 mm Hg, the person is considered to have high blood pressure.
In the young, the aorta is an elastic and distensible vessel which is able to distend when blood enters the vessel during contraction of the LV. As the blood flows from the aorta to other organs during relaxation of the LV, it recoils to its non-distended resting state. An analogy will be to think of the aorta as a large elastic rubber tubing. As such, it is able to absorb part of the pressure generated during LV contraction and with its recoil, it will prevent the DBP from dropping too low. The net effect is that the difference between the SBP and the DBP, also called pulse pressure, is usually kept about 40 mm Hg. Hence, it is common in young individuals to record blood pressure readings of 100/60, 110/70 or 120/80.
Just as a rubber tubing hardens and becomes less elastic over time, the aorta also becomes stiffer with increasing age. As a result, the aorta is less able to absorb the blood pressure generated during LV contraction resulting in higher SBP values. An interesting effect is that the aorta also has less elastic recoil and hence, the DBP tends to be lower during relaxation of the heart chamber. Therefore, as age increases, elevation of the SBP becomes more prevalent and the pulse pressure often increases beyond 140 mm Hg.
This SBP elevation is particularly more pronounced in women. Data from the Framingham heart study in the USA showed that for older women 65 years or more, the incidence of hypertension was 78 per cent. With increasing age, the prevalence increased markedly and was 85 per cent for those 60 to 79 years, and 94 per cent for those 80 years of age or more.
Is low blood pressure harmful?
One of the common findings in the elderly is the presence of a high SBP associated with a low DBP. While elevation of blood pressure is harmful, the often asked question is whether a low DBP is harmful? For the elderly, a DBP of less than 70 mm Hg carries an increased risk of heart disease similar to that associated with elevation of the DBP of more than 90 mm Hg. A likely explanation is the decreased blood flow to the heart muscles if the DBP drops too low.
About 80 per cent of the blood flowing into the heart muscle occurs during relaxation of the heart muscles. For an elderly individual with SBP elevation who exercises, the heart has to work harder to pump blood into the stiff aorta resulting in an increased demand for oxygen and yet the faster heart rate means that there is less time for the heart to relax, and hence less time for blood to flow into the heart muscles. The lower DBP associated with a stiff aorta also means that the pressure may drop below the 60 mm Hg value which is the minimal pressure required for adequate flow into the heart muscle. It is like a tap with low pressure and the flow becomes slower and volume lower. The net effect is insufficient oxygen supply to the heart muscles and this is aggravated if the individual also has a significant blockage of the heart artery.
Underestimating and overestimating blood pressure
Accurate measurement of the blood pressure in the elderly requires an understanding of the age-related changes in the regulatory mechanisms and the vessels. When evaluating blood pressure, always use the arm with the highest blood pressure measurement as the reference arm. A common cause of underestimating SBP in the elderly is the failure to pump the pressure to a sufficiently high level before starting to auscultate or listen for sounds to detect SBP. This may result in underestimating SBP in the elderly where an "auscultatory gap", as defined by the period during which sounds indicating true SBP fade away and reappear at a lower pressure point, is more commonly seen.
Blood pressure is often overestimated in the elderly as the prevalence of the "white-coat effect" ( transient elevation of the blood pressure in a clinic or hospital environment) may be as high as 25 per cent. Home blood pressure monitoring is increasingly seen as similar or better than office blood pressure readings in being able to reflect the true blood pressure readings. This is especially so in the elderly where home monitoring will prevent over diagnosis of hypertension resulting from the "white-coat effect".
While there is little that can be done to prevent age-related changes in the aorta, attention to lifestyle habits can make a difference. Heart disease is increased by up to three times in those with hypertension and smoking increases this risk by an additional two- to three-fold. For every additional 10 cigarettes smoked per day, mortality from heart disease increases by 18 per cent in men and 31 per cent in women. Contrary to common misconception, It is never too late to benefit from smoking cessation.
For those who drink, the bad news is that it does not matter whether you drink beer, wine or hard liquor; all alcoholic drinks are significantly associated with high blood pressure and this is especially so if it is taken without meals.
Many elderly consume painkillers for various ailments especially joint problems. Nonsteroidal anti-inflammatory drugs (NSAIDs) are commonly prescribed for elderly patients as painkillers. In the elderly, use of NSAIDs may not only have an adverse impact on blood pressure control but may be also be associated with deterioration of kidney function.
Hence, it is important to bear in mind that hypertension in the elderly is not quite the same as hypertension in the younger population. The key points to remember are making sure it is truly hypertension, avoiding under or overtreatment, and avoiding detrimental lifestyle habits and drugs that aggravate the control of high blood pressure.
Dr Michael Lim
Dr Lim is medical director at the Singapore Heart, Stroke & Cancer Centre. He is also editor-in-chief, Heart Asia; British Medical Journals Publishing Group, chairman; Scientific Advisory Board, Asia Pacific Heart Association honorary professor and senior medical adviser, Peking University Heart Centre.
This article was first published in The Business Times.