Physiology, Pulse Pressure

Cellular Level

Arteries are efferent vessels that lead away from the heart. They are lined by endothelial cells and consist of three different layers, which appear in the figure below. The innermost layer, the tunica intima, consists primarily of an endothelial layer, subendothelial layer, and an internal elastic lamina. The middle layer, also called the tunica media, has concentric layers of helically arranged smooth muscle cells, as well as varying amounts of elastic and reticular fibers and proteoglycans. Some of the larger arteries also contain an external elastic lamina. Finally, the tunica adventitia also called the tunica externa, is the outermost layer made up of longitudinally oriented type-I collagen fibers. 

There are two main types of arteries in the human body. The first, which is the more prominent of the two, is the muscular artery. Muscular arteries have a thin intimal layer with a well-developed internal elastic lamina. They also have a muscular wall that can be up to forty layers thick. The primary function of these arteries is to regulate blood flow through adjustment of blood vessel caliber. The other main type of artery is the elastic artery. Elastic arteries are unique as they have elastic fibers interspersed in between the smooth muscle cells of the tunica intima, which allows elastic arteries to store kinetic energy to smooth out the surge in blood pressure that occurs during systole, known as the Windkessel effect.

Pathophysiology

An increase in pulse pressure can occur in a well-conditioned endurance runner. As he or she continues to exercise, the systolic pressure will progressively increase due to an increase in stroke volume and cardiac output. Diastolic pressure, on the contrary, will continually decrease due to a decrease in the total peripheral resistance. This effect is due to the accumulation of red (slow-twitch) muscle tissue in the arterioles instead of white (fast-twitch) tissue. As a result, the pulse pressure is going to increase; this can also occur in individuals with larger amounts of muscle mass.[4][5][6][7]

Aging impacts pulse pressure and arterial compliance. With aging, there is a decrease in the compliance of the large elastic arteries. This change is due to structural molecular changes in the arterial wall, including decreased elastin content, increased collagen I deposition, and calcification, which increases the stiffness of the wall. This process is often described as "hardening of the arteries." As the left ventricle contracts against stiffer, less compliant arteries, systolic and diastolic pressures increase and can result in a widened pulse pressure.  In response, the left ventricular tend to hypertrophy. When excessive pulse pressure transmits through the microcirculation of vital organs such as the brain and kidneys, extensive tissue damage tends to occur.[8]

A widened (or larger) pulse pressure occurs with several diseases, including aortic regurgitation, aortic sclerosis (both heart valve conditions), severe iron deficiency anemia (reduced blood viscosity), arteriosclerosis (less compliant arteries), and hyperthyroidism (increased systolic pressure). In the majority of these cases, systolic pressures increase while diastolic pressures remain near normal. In aortic regurgitation, the aortic valve insufficiency results in a backward, or regurgitant flow of blood from the aorta back into the left ventricle, so that blood ejected during systole returns during diastole. This condition leads to an increase in the systolic pressure and a decrease in the diastolic pressure, which results in increased pulse pressure. In aortic stenosis, there is a narrowing of the aortic valve, which interferes with the ejection of blood from the left ventricle into the aorta, which results in a decrease in stroke volume and a subsequent decrease in pulse pressure.

Narrow pulse pressures occur in several diseases such as heart failure (decreased pumping), blood loss (decreased blood volume), aortic stenosis (reduced stroke volume), and cardiac tamponade (decreased filling time). In the majority of these cases, systolic pressures decrease while diastolic pressures remain near normal.

Clinical Significance

The research done by Blacher et al. has shown that pulse pressure is a significant risk factor in the development of heart disease. It has even been shown to be more of a determinant than the mean arterial pressure, which is the average blood pressure that a patient experiences in a single cardiac cycle. In fact, as little as a 10 mmHg increase in the pulse pressure increases the cardiovascular risk by as much as 20%. This finding was consistent in both Caucasian and Asian populations.[9]

Pulse pressure is also independently associated with an increased risk of developing atrial fibrillation. A study done by Mitchell et al. showed that patients with a pulse pressure of 40 mmHg or less developed atrial fibrillation at a rate of 5.6%, whereas patients with a pulse pressure greater than 61 mmHg developed atrial fibrillation at a rate of 23.3%. In fact, for every 20 mmHg increase in pulse pressure, the adjusted hazard ratio for developing atrial fibrillation is 1.28. This risk is independent of mean arterial pressure.[10]

Other research has focused on helping to maintain normal pulse pressure. One of the most effective ways to do this is to increase arterial compliance. According to Thorin-Trescases et al., endurance aerobic exercise is the only intervention that has been shown to help mitigate age-related arterial stiffening by reducing age-related increases in collagen I and III and calcification. These same benefits were not seen with resistance training, such as bench press, as this decreases arterial compliance and increases the pulse pressure.[11]

In addition to aerobic exercise training, Rajkumar et al. demonstrated that one could also increase arterial compliance by increasing estrogen compounds (as in hormone replacement in post-menopausal women), increasing the consumption of n-3 fatty acids, and decreasing salt intake. There has also been some evidence that supports the notion that ACE inhibitors have beneficial arterial wall effects and may be of use.[12]

Narrowed pulse pressures are seen in dysautonomia/postural orthostatic tachycardia syndrome (POTS). Some patients drop to pulse pressure to 0 on standing. They are pulseless standing and many cannot stand which causes extremely high morbidity. Conventional non-prescription measures fail most of these patients.