By Erika Zanabria, M.S., and Gregory L. Welch, M.S.

Hypertension or high blood pressure, a common disease in industrial societies, has reached epidemic proportions. Approximately 50 million Americans are hypertensive and, though it can affect anyone, the greatest incidence occurs among middle-aged and older individuals. In addition, generally more men than women and African Americans than Caucasians have hypertension (ACSM, 1993). Results from epidemiologic studies have associated low levels of physical fitness with hypertension, independent of body mass or obesity (Lesniak and Dubbert, 2001). Often referred to as the silent killer, hypertension is a leading risk factor for stroke, myocardial infarction, chronic heart and renal failure (Stewart, 2000).

Defining Hypertension

Hypertension is defined as blood pressure equal to or greater than 140/90mmHg. Thus, hypertension can result from either increased systolic pressure (the first number of the two values), diastolic pressure (the second number) or both. An increase in either raises the risk of medical complications, thus the higher the increase, the greater the risk (Haskell, 2001). Individuals with a blood pressure reading over 160/95mmHg have a 150 to 300 percent higher annual incidence rate of coronary artery disease (CAD), chronic heart failure, intermittent claudication and stroke than individuals with normal blood pressure (ACSM, 1993).

Hypertension is generally classified into one of two categories: essential and secondary. Although the cause of essential hypertension is unknown, it is believed to develop in individuals with certain hereditary variations in genes. In contrast, secondary hypertension is a consequence of a known etiology (i.e., disease process), thus results from another disease (e.g., renal artery stenosis, coarctation of the aorta, adrenocortical or benign tumors and hypokalemia) (ACSM, 1997).

Blood Pressure Response

Blood pressure (i.e., the force exerted by blood against the arterial walls) is indirectly measured with a sphygmomanometer or blood pressure cuff. Blood pressure is determined by how much blood the heart pumps and the resistance to blood flow. Systolic pressure (SBP) is produced as the heart ejects blood during ventricular systole. Diastolic pressure (DBP) is created during ventricular relaxation (i.e., the period in which the ventricles fill with blood) (Powers and Howley, 1996). A normal blood pressure reading for a person at rest is 120/80mmHg for males and 110/70mmHg for females.

Dynamic exercise (which consists of alternating muscle contraction and relaxation, such as in walking, running and cycling) produces a different blood pressure response than static or resistance exercise (in which the muscle contraction is held for more than a few seconds before relaxing, such as in strength training and isometric exercise). During dynamic exercise the systolic rate should rise steadily as exercise intensity increases, while the diastolic rate should vary minimally. Consequently, a single session of dynamic exercise usually evokes a normal rise in SBP from baseline levels in a hypertensive unmedicated person. During vigorous dynamic exercise, a typical systolic rate range is between 160mmHg and 220mmHg. However, if the systolic rate becomes greater than 240mmHg, does not increase as exercise intensity increases and/or drops below resting levels, then the cardiovascular system is not responding appropriately and the exercise bout should be stopped. The same should be done if the diastolic rate increases 20mmHg above resting value or reaches 115mmHg (Haskell, 2001).

On the other hand, during static or heavy resistance exercise, the pressure within the muscle increases and causes the small blood vessels (i.e., arterioles and capillaries) in it to collapse. Whenever this occurs, oxygen rich blood cannot reach the working muscle. This hypoxia (i.e., lack of oxygen) rapidly increases SBP and DBP during the contraction. The increased blood pressure is believed to be the bodyÕs attempt to send oxygen to the working muscles by forcing open the arterioles (Haskell, 2001). The speed and magnitude of the rise in systolic and diastolic rates are greater as the contraction intensity and duration increase. However, this can be avoided if the contraction lasts only a few seconds or there is a rest period of a few seconds before the muscle contracts again. Additionally, exercisers should avoid ValsalvaÕs maneuver (i.e., holding their breaths during the exertion phase of an exercise). This maneuver reduces blood flow to the heart, thus decreases the amount of blood the heart pumps and potentially limits blood flow to the brain. A good lifting technique is to exhale on the exertion (i.e., lifting) phase and inhale upon the relaxation phase.

Medication

Although antihypertensive drugs reduce blood pressure, some may also dampen exercise performance. Hypertension control through beta-blockers and, to a lesser degree, the calcium antagonists diltiazem and verapamil, reduces the heart rate response to sub-maximal exercise. Beta-blockers blunt exercise-mediated increases in heart rate and cardiac output (Q) and may reduce exercise performance. This reaction is more pronounced with non-selective beta-blockers (e.g., propranolol). Conversely, dihydropyridine (i.e., derivative calcium antagonists) and direct vasodilators may increase heart rate response to sub-maximal exercise (ACSM, 1996). Vasodilators, alpha-adrenergic blocking drugs and calcium channel blockers do not suppress cardiac output or exercise capacity.

Exercise as Therapy

Non-pharmacologic interventions can serve as definite therapy for select hypertensive patients and adjunctive therapy for many others. Aerobic exercise and diet-induced weight loss have emerged as the most effective and physiologically desirable approaches (Chintanadilok and Lowenthal, 2002). Studies have also indicated exercise training lowers blood pressure in individuals with essential hypertension and those taking hypertensive medications. Stewart (2000) cites studies by Arroll and Beaglehole (1992) as well as Kelly and McClellan (1994) which show moderate intensity exercise can reduce both systolic and diastolic blood pressure by 7mmHg. A review by the National Institutes of Health (NIH) demonstrated that in 70 percent of all exercising subjects, blood pressure lowered an average of 10.5/8.6mmHg from an average starting level of 154/98mmHg (Hagberg, 1995). Additionally, Renolds, et al., (2002) concluded that in a population of older hypertensive females, aerobic exercise training improved insulin sensitivity and lowered blood pressure without a reduction in plasma tumor necrosis factor (TNF) levels.

According to a position stand by the American College of Sports Medicine (1993), the blood pressure lowering effects of exercise, usually about 10mmHg to 20mmHg in SBP, can be observed one to three hours after a 30- to 45-minute bout. This response can linger up to nine hours post exercise. Blood pressure changes can be seen as early as three weeks to three months after initiating exercise training, with the maximum blood pressure decrease reached after three months. Also, the DBP reduction was found to be related to the length of the training, but such is not the case for SBP. Although blood pressure can be reduced through exercise, it only lasts as long as endurance training is continued.

The mechanism by which exercise reduces blood pressure in hypertensive individuals is ambiguous. However, the pressure-lowering effects of exercise are not mediated by a sole mechanism, but several underlying factors. The sympathetic nervous system (SNS) allegedly plays an important role. Evidence suggests the blood pressure reduction is associated with a decrease in plasma norepinephrine levels. Furthermore, blood pressure reduction could be due to an increase in circulating vasodilator substances. Also, hyperinsulinemia has been proposed to be a cause for hypertension and exercise is known to amend a hyperinsulinemic state. Finally, exercise can alter renal function by regulating body sodium and, thereby, plasma volume and cardiac output as well (ACSM, 1993).

In order to manage hypertension, individuals should make lifestyle modifications in conjunction with exercise training. Some of these modifications include (ACSM, 1993):

• Weight reduction if overweight. Evidence suggests an average reduction of 15mmHg SBP and 10mmHg DBP results from a 10kg (22 pounds) average weight loss.
• Limit alcohol intake to less than an ounce per day of ethanol (e.g., 24 ounces of beer, eight ounces of wine or two ounces of 100-proof whisky).
• Reduce sodium intake to less than 2.3 grams per day. Studies have associated an 80meq/day reduction with a 5mmHg SBP and 3mmHg DBP decrease.
• Maintain adequate dietary potassium, calcium and magnesium intake. Increasing dietary potassium to 80meq/day resulted in average reductions of 8mmHg in SBP and 4mmHg in DBP.
• Stop smoking and reduce dietary fat, saturated fat and cholesterol intake.

Most, if not all, current guidelines recommend exercise as an adjunct to pharmacologic interventions for individuals with mild hypertension, including those on antihypertensive medication. A tailored exercise prescription, determined by exercise testing, can aid blood pressure reduction. Exercise test information provides some indication of risk stratification for patients with blood pressure response above the 85th percentile (Chintanadilok and Lowenthal, 2002). All hypertensive individuals who want to start an exercise program should have a resting electrocardiogram (ECG) taken. However, ACSM does not recommend an exaggerated blood pressure response to exercise as a screening test to identify individuals at high risk for developing hypertension.

Standard exercise testing methods and protocols may be used for individuals with hypertension. Graded exercise tests (GXT) can estimate the degree of blood pressure response during exercise, rate of recovery and incidence of arrhythmias during the test. When undergoing a GXT, individuals should be taking their usual medications. A resting SBP equal to or greater than 200mmHg or a DBP equal to or greater than 115mmHg is considered a contraindication to exercise testing. During the test, if SBP rises above or equals 260mmHg or DBP rises above or equals 115mmHg, the test should be terminated immediately (ACSM, 1997).

According to ACSM, 20 to 60 minutes of aerobic exercise, three to five days per week, at 50 to 85 percent of maximal oxygen uptake is appropriate for individuals with mild hypertension. However, for individuals in Stage 2 or 3 hypertension, exercise should be at 40 to 70 percent of maximal oxygen uptake after patients begin pharmacological therapy (Stewart, 2000). Resistance training is recommended as an adjunct to aerobic exercise. It should be performed independently, since research has not shown it can decrease blood pressure consistently, with the exception of circuit weight training (ACSM, 1993). This type of training should use low resistance and high repetitions (ACSM, 1997). The American Heart Association recommends mild to moderate resistance training at 30 to 60 percent of maximal effort for improving muscle strength and endurance (Stewart, 2000).

Conclusion

An abundance of evidence suggests increasing physical activity in sedentary individuals and maintaining it in active ones can significantly impact hypertension. The amount of activity required for benefit is feasible for almost everyone. Counseling by health care providers is one important, but underutilized, method of encouraging adults to engage in physical activity and exercise. Moreover, physical activity opportunities in schools and communities should be encouraged for hypertension prevention and intervention across all age groups (Lesniak and Dubbert, 2001). AF

Erika Zanabria, M.S., recently completed her graduate studies at California State University, Fullerton. She is currently a healthy lifestyle counselor at the Wellness Institute of the Downey Family YMCA in Downey, California.

Gregory L. Welch, M.S., is an exercise physiologist and president of SpeciFit; An Agency of Wellness located in Seal Beach, California. He is a faculty member at California State University, Fullerton and recognized nationally as an author and lecturer. Welch also heads the newly developed Wellness Institute for the Downey Family YMCA in Downey, California. He can be reached at (562) 862-4201.