Blood Pressure

Fundamental to providing comprehensive care to patients is the ability to obtain an accurate medical history and carefully perform and interpret a physical examination. The optimal selection of further tests, treatments, and use of subspecialists depends on well-developed skills for taking patient history and a physical diagnosis. An important part of a normal physical examination is obtaining a blood pressure reading and auscultation of the heart tones, which both represent critical cornerstones in evaluating a patient's hemodynamic status and diagnosing and understanding physiological and anatomical pathology.

Naive ideas about circulation and blood pressure date as far back as ancient Greece. It took until the 18th century for the first official report to describe an attempt to measure blood pressure, when Stephen Hales published a monograph on "haemastatics" in 1733. He conducted a series of experiments involving cannulation of arteries in horses and invasive direct blood pressure measurement; unfortunately, his method was not applicable for humans at that time. There were many subsequent contributions to the art of measuring and understanding blood pressure during the next two centuries. One of the greatest of these was described in a publication in Gazetta medica di Torino called "A New Sphygmomanometer" by Dr. Riva-Rocci in 1896; it is recognized as the single most important advancement in practical noninvasive methods for blood pressure estimation in humans.

From: Handbook of Cardiac Anatomy, Physiology, and Devices Edited by: P. A. Iaizzo © Humana Press Inc., Totowa, NJ

In 1916, French physician Rene Laennec invented the first stethoscope, which was constructed from stacked paper rolled into a solid cylinder. Prior to his invention, physicians around the world would place one of their ears directly on the patient's chest to hear heart or lung sounds. After Dr. Laennec's initial success, several new models were produced, primarily of wood. His stethoscope was called a "monaural stethoscope." The "binaural stethoscope" was invented in 1829 by a physician named Camman in Dublin and later gained wide acceptance; in the 1960s, the Camman stethoscope was considered the standard for superior auscultation.

It is essential that health care professionals and bioengi-neers understand how these important diagnostic parameters are obtained, their sensitivities, and how best to interpret them.

1.1. Physiology of Blood Pressure

Blood pressure is the force applied on the arterial walls as the heart pumps blood through the circulatory system. The rhythmic contractions of the left ventricle result in cyclic changes in the blood pressure. During ventricular systole, the heart pumps blood into the circulatory system, and the pressure within the arteries reaches its highest level; this is called systolic blood pressure. During diastole, the pressure within the arterial system falls and is called diastolic blood pressure.

The mean of the systolic and diastolic blood pressures during the cardiac cycle represents the time-weighted average arterial pressure; this is called mean arterial blood pressure. Alternating systolic and diastolic pressures create outward and inward movements of the arterial walls, perceived as arterial pulsation or arterial pulse. Pulse pressure is the difference between systolic and diastolic blood pressures.

Blood pressure is measured in units called millimeters of mercury (mmHg). A "normal" systolic blood pressure is less than 140 mmHg; a "normal" diastolic blood pressure is less than 90 mmHg. Blood pressure higher than normal is called hypertension, and one lower than normal is called hypotension. Hence, normal mean arterial pressure is between 60 and 90 mmHg. Mean arterial pressure is normally considered a good indicator of tissue perfusion and can be measured directly using automated blood pressure cuffs or calculated using the following formulas:

MAP = DBP + PP/3 or MAP = [SBP + (2 x DBP)]/3 where PP = SBP - DBP; MAP is mean arterial pressure, DBP is diastolic blood pressure, PP is pulse pressure, and SBP is systolic blood pressure.

Blood flow throughout the circulatory system is directed by pressure gradients. By the time blood reaches the right atrium, which represents the end point of the venous system, pressure has decreased to approx 0 mmHg. The two major determinants of blood pressure are: (1) cardiac output, which is the volume of blood pumped by the heart per minute; and (2) systemic vascular resistance, which is the impediment offered by the vascular bed to flow. Systemic vascular resistance is controlled by many factors, including vasomotor tone in arterioles, terminal arterioles, or precapillary sphincters. Blood pressure can be calculated using the formula

BP = CO x SVR where BP is blood pressure, CO is cardiac output, and SVR is systemic vascular resistance.

Blood pressure decreases by 3-5 mmHg in arteries that are 3 mm in diameter. It is approx 85 mmHg in arterioles, which accounts for approx 50% of the resistance of the entire systemic circulation. Blood pressure is further reduced to around 30 mmHg at the point of entry into capillaries and then becomes approx 10 mmHg at the venous end of the capillaries.

The speed of the advancing pressure wave during each cardiac cycle far exceeds the actual blood flow velocity. In the aorta, the pressure wave speed may be 15 times faster than the flow of blood. In an end artery, the pressure wave velocity may be as much as 100 times the speed of the forward blood flow.

As the pressure wave moves peripherally through the arterial system, wave reflection distorts the pressure waveform, causing an exaggeration of systolic and pulse pressures. This enhancement of the pulse pressure in the periphery causes the systolic blood pressure in the radial artery to be 20-30% higher than the aortic systolic blood pressure and the diastolic blood pressure to be approx 10-15% lower than the aortic diastolic blood pressure. Nevertheless, the mean blood pressure in the radial artery will closely correspond to the aortic mean blood pressure.

1.2. Methods of Measuring Blood Pressure

Arterial blood pressure can be measured both noninvasively and invasively; these methods are described next.

1.2.1. Noninvasive Methods 1.2.1.1. Palpation

Palpation is a relatively simple and easy way to assess systolic blood pressure. A blood pressure cuff containing an inflat able bladder is applied to the arm and inflated until the arterial pulse felt distal to the cuff placement disappears. Then, the pressure in the cuff is released at a speed of approx 3 mmHg per heartbeat until the arterial pulse is felt again. The pressure at which the arterial pulsations start is the systolic blood pressure. Diastolic blood pressure and mean arterial pressure cannot be readily estimated using this method. Furthermore, the measured systolic blood pressure using the palpation method is often an underestimation of the true arterial systolic blood pressure because of the insensitivity of the sense of touch and the delay between blood flow below the cuff and the appearance of arterial pulsations distal to the cuff.

1.2.1.2. Doppler Method

The Doppler method is a modification of the palpation method and uses a sensor (Doppler probe) to determine blood flow distal to the blood pressure cuff. The Doppler effect is the shift of the frequency of a sound wave when a transmitted sound wave is reflected from a moving object. When a sound wave shifts (e.g., as caused by blood movement in an artery), it is detected by a monitor as a specific swishing sound. The pressure of the cuff, at which blood flow is detected by the Doppler probe, is the arterial systolic blood pressure.

This method is more accurate (less subjective) in estimating systolic blood pressure compared to the palpation method. It has also been quite a useful method in detecting systolic blood pressure in patients who are in shock, have low-flow states, are obese, or are pediatric patients. Disadvantages of the Doppler method include: (1) inability to detect diastolic blood pressure; (2) necessity for sound-conducting gel between the skin and the probe (because air is a poor conductor of ultrasound); (3) likelihood of a poor signal if the probe is not applied directly over an artery; and (4) potential for motion and electrocautery unit artifacts.

1.2.1.3. Auscultation (Riva-Rocci Method)

The auscultation method uses a blood pressure cuff placed around an extremity (usually an upper extremity) and a stethoscope placed above a major artery just distal to the blood pressure cuff (e.g., the brachial artery if using the cuff on the upper extremity). Inflation of the blood pressure cuff above the systolic blood pressure flattens the artery and stops blood flow distal to the cuff. As the pressure in the cuff is released, the artery becomes only partially compressed, which creates conditions for turbulent blood flow within the artery and produces the so-called Korotkoff sounds, named after the individual who first described them. Korotkoff sounds are caused by the vibrations created when blood flow in partially flattened arteries transforms from laminar into turbulent, and they persist as long as there is an increased turbulent flow within a vessel. Systolic blood pressures are determined as the pressures of the inflated cuffs at which Korotkoff sounds are first detected. Diastolic blood pressures are determined as the cuff pressure at which Korotkoff sounds become muffled or disappear.

Sometimes, in patients with chronic hypertension, there is an "auscultatory gap" that represents disappearance of the normal Korotkoff sounds in a wide pressure range between the systolic and the diastolic blood pressures. This condition will lead to inaccurately low blood pressure assessments. Korotkoff sounds can also be difficult to detect in patients who are in low-flow states or in those with marked peripheral vasoconstriction. The use of microphones and electronic amplification of such signals can greatly increase the sensitivity of this method. Yet, considerations for systematic errors include motion artifact and elec-trocautery interference.

1.2.1.4. Oscillometry

Oscillometry is defined as the blood pressure measurement method that uses automated blood pressure cuffs. Arterial pulsations cause oscillations in the cuff pressure. These oscillations are at their maximum when the cuff pressure equals the mean arterial pressure and decrease significantly when the cuff pressures are above the systolic blood pressure or below the diastolic blood pressure. Advantages of this approach are the ease and reliability of use. Some of the potential technical problems include motion artifacts, electrocautery unit interference, and inability to measure accurate blood pressure when patients elicit arrhythmias.

1.2.1.5. Blood Pressure Cuff Pressure

When using blood pressure cuffs for such measurements, it is important to select them in accordance with the patient's size. Blood pressure cuffs for adult and pediatric patients come in variable sizes. An appropriate size means that the cuff's bladder length is at least 80% and the cuff width is at least 40% of the arm circumference. If the cuff is too small, it will take more pressure to occlude arterial blood flow completely, and the resultant measured pressures will be falsely elevated. If the cuff is too large, however, the pressure inside the cuff needed for complete occlusion of the arterial blood flow will be less, and the measured pressures will be falsely low.

Blood pressure is most commonly taken while the patient is seated with the arm resting on a table and slightly bent, which positions the arm at the same level as the patient's heart. This same principle should be applied if the patient is in a supine position; the blood pressure cuff should be level with the heart. If the location of the blood pressure cuff during blood pressure measurements is above or below the patient's heart level, registered blood pressure will be either lower or higher than the patient's actual blood pressure. This difference can be represented as the height of a column of water interposed between the blood pressure cuff and the heart levels. To convert centimeters of water (cmH2O) to millimeters of mercury, the measured height of the water column should be multiplied by a conversion factor of 0.74 (1 cmH2O = 0.74 mmHg).

All of the aforementioned methods for assessing blood pressure do so indirectly by registering blood flow below the blood pressure cuff. Other noninvasive methods include plethysmog-raphy and arterial tonometry.

1.2.1.6. Plethysmography

The plethysmographic method for blood pressure measurement uses the fact that arterial pulsations cause a transient increase in the blood volume of an extremity and thus in the volume of the whole extremity. A finger plethysmograph determines the minimum pressures needed by a finger cuff to maintain constant finger blood volume. A light-emitting diode and a photoelectric cell detect changes in the finger volume and rapidly adjust the cuff pressure, which can be displayed as a beat-to-beat tracing. Thus, in healthy patients, the blood pressure measured on the finger will correspond to the aortic blood pressure; this will not be true for patients with low peripheral perfusion, like those with peripheral artery disease, hypothermia or low-flow states.

1.2.1.7. Arterial Tonometry

Tonometry devices can determine beat-to-beat arterial blood pressures by adjusting the pressure required to partially flatten a superficial artery located between a tonometer and a bony surface (e.g., radial artery). These devices commonly consist of an electronic unit and a pressure-sensing head. The pressure-sensing head includes an air chamber with adjustable air pressure and an array of independent pressure sensors that, when placed directly over the artery, assess intraluminal arterial pressures. The resultant pressure record resembles an invasive arterial blood pressure waveform. Limitations to these methods include motion artifacts and the need for frequent calibrations.

1.2.2. Invasive Methods of Blood Pressure Measurement

1.2.2.1. Indications

Indications for the use of direct blood pressure monitoring (arterial cannulation) include hemodynamic instability, intraoperative monitoring in selected patients, and use of vasoactive drugs like dopamine, epinephrine, norepinephrine, and the like.

1.2.2.2. Cannulation Sites

The arteries most often selected for cannulation are the radial, ulnar, brachial, femoral, dorsalis pedis, or axillary. Cannulation of an artery should be avoided if there is: (1) documented lack of collateral circulation, (2) a skin infection on the site of cannulation, and/or (3) a preexisting vascular deficiency (e.g., Raynaud's disease). The radial artery is the most often selected artery for invasive blood pressure monitoring because of its easily accessed superficial location and good collateral flow to the region it supplies.

1.2.2.3. Techniques

The two frequently utilized techniques for arterial cannulation are: (1) a catheter over a needle or (2) Seldinger's technique. When using the first technique, the operator enters the blood vessel with a needle that has a catheter placed over it. After free blood flow is documented through the needle, the catheter is advanced over the needle into the artery, and the needle is withdrawn. The catheter is then connected to the pressure-transducing system. When using Seldinger's technique, the operator first enters the artery with a needle. After free blood flow is confirmed through the free end of the needle, the operator places a wire through the needle into the blood vessel and withdraws the needle. Then, a plastic catheter is advanced into the artery over the steel wire, the wire is removed, and the catheter is connected to a transducer system. Both methods require sterile techniques and skilled operators.

1.2.2.4. Considerations

Arterial cannulation provides beat-to-beat numerical information and tracing waveforms of arterial blood pressures and is considered a gold standard in blood pressure monitoring. Invasive arterial pressure monitoring systems or kits commonly include a catheter (20-gauge catheter for adults), tubing, a transducer, and an electronic monitor for signal amplification, filtering, and analysis. Such pressure transducers are usually based on the strain gauge principle: Stretching a wire of silicone crystal changes its electrical resistance. The catheter, the connective tubing, and the transducer are filled with saline. A pressure bag provides a continuous saline flush of the system at a rate of 3-5 mL/h. The system should also allow for intermittent flush boluses as needed.

The quality of the information gathered depends on the dynamic characteristics of the whole system. The complex waveform obtained from the arterial pulse can be expressed as a summation of simple sine and cosine waves using a method called "Fourier analysis." Most invasive blood pressure monitoring systems have natural frequencies of approx 16-24 Hz, which must exceed the frequency of the arterial pulse waveform to reproduce it correctly. This natural frequency is described as the frequency at which the system oscillates when disturbed. Another property of the catheter-tubing-transducer system is the "dumping coefficient." The dumping coefficient characterizes how quickly oscillations in the system will decay.

Both the natural frequency and the dumping coefficient are primarily determined by the length, size, and compliance of the catheter and tubing and by presence of air bubbles or cloths trapped in the fluid column. This chapter does not go into details of how to determine and change the system characteristics. Briefly, "underdumping" the system will exaggerate artifacts like a "catheter whip" and can result in a significant overestimation of the systolic blood pressure. "Overdumping" will blunt the response of the system and lead to an underestimation of the systolic blood pressure. In addition, systems with low natural frequencies will show amplifications of the pressure curves, causing overestimation of the systolic blood pressures. Diastolic blood pressures will also be affected by altering the above factors, but to a lesser degree. Note that system response characteristics can be optimized by using short and low-compliance tubing and by avoiding air trapping when the system is flushed.

When an invasive blood pressure system is connected to a patient, it should be zero referenced and calibrated. Zero referencing is performed by placing the transducer at the level of the midaxillary line, which corresponds to the level of the patient's heart. The system is opened to air and closed to the patient, and then the transducing system is adjusted to a 0 mmHg baseline. For this, it is not necessary for the transducer to be at the level of the midaxillary line as long as the stopcock, which is opened to air during the zero reference, is at that level. The system is then opened to the patient and ready for use.

System calibrations are separate procedures and involve connecting the invasive blood pressure systems to mercury manometers, closing the systems to the patient, and pressurizing the systems to specified pressures. Then, the gains of the monitor amplifiers are adjusted until displayed pressures equal the pressures in the mercury manometers. Recommendations are to perform zero referencing at least every clinical shift and calibration at least once daily. It should be noted that some of the more contemporary transducer designs rarely require external calibration.

When connected to the patient, such monitoring systems provide digital readings of systolic, diastolic, and mean blood pressures and, commonly, pressure waveforms. Watching the trend of the waveform and its shape can provide other important information as well. More specifically, the top of the waveform represents the systolic pressure, and the bottom is the diastolic blood pressure. The dicrotic notch is caused by the closure of the aortic valve and the backsplash of blood against the closed valve. The rate of the upstroke of the arterial blood pressure wave depends on the myocardial contractility; the rate of the downstroke is affected by the systemic vascular resistance. Exaggerated variation in the size of the waves with respiration suggests hypovolemia. Integrating the areas under the waveforms can be used for calculations of the values of the mean arterial pressures. See also Chapter 16 for more details on pressure waveforms.

1.2.2.5. Complications

Potential complications associated with arterial cannulation include bleeding, hematoma, infection, thrombosis, ischemia distal to the cannulation site, vasospasm, embolization with air bubbles or thrombi, nerve damage, pseudoaneurysm, atheroma, and inadvertent intraarterial drug injection.

1.3. Diagnoses

1.3.1. Pulsus Paradoxus

Inspiration can decrease arterial pressure by more than 10 mmHg; the inspiratory venous pressure stays relatively unchanged. Normally, the arterial and venous blood pressures fluctuate throughout the respiratory cycle, decreasing with inspiration and rising with expiration. This fluctuation in the blood pressure under normal conditions is less than 10 mmHg. Inspiration increases venous return, therefore increasing the right heart output transiently, according to the Frank-Starling law. As the blood is sequestered in the pulmonary circulation during inspiration, the left heart output is reduced transiently, accounting for the normal lower systolic pressure during this phase. The right ventricle contracts more vigorously and mechanically bulges the interventricular septum toward the left ventricle, reducing its size and accounting for the lower systolic blood pressure.

Certain conditions drastically reduce the transmural or distending (filling) pressure of the heart and interfere with the diastolic filling of the two ventricles. In such cases, there is an exaggeration of the inspiratory fall in the systolic blood pressure, which results from reduced left ventricular stroke volume and the transmission of negative intrathoracic pressure to the aorta. Common causes for such reductions include pericardial effusion, adhesive pericarditis, cardiac tamponade, pulmonary emphysema, severe asthma, paramediastinal effusion, endocar-dial fibrosis, myocardial amyloidosis, scleroderma, mitral stenosis with right heart failure, tricuspid stenosis, hypov-olemia, and/or pulmonary embolism. Associated clinical signs include a palpable decrease in pulse with inspiration and decrease in the inspiratory systolic blood pressure more than 10 mmHg compared to the expiratory pressure.

1.3.2. Pulsus Alternans

An alternating weak and strong peripheral pulse is usually caused by alternating weak and strong heart contractions. It may be found in patients with severe heart failure, various degrees of heart block, or arrhythmias. Pulsus alternans is characterized by a regular rhythm and must be distinguished from pulsus bigeminus, which is usually irregular.

1.3.3. Bigeminal Pulse

A bigeminal pulse is caused by occurrences of premature contractions (usually ventricular) after every other beat, which results in alternation in the strength of the pulse. Bigeminal pulse can often be confused with pulsus alternans. However, in contrast to the latter, in which the rhythm is regular, in pulsus bigeminus the weak beat always follows a shorter pulse interval.

1.3.4. Pulse Deficit

Pulse deficit is the inability to detect arterial pulsations when the heart beats, as can be observed in patients with atrial fibrillation, in states of shock, or with premature ventricular complexes and the like. The easiest way to detect pulse deficit is to place a finger over the radial artery while monitoring the QRS complexes on an electrocardiogram monitor. A QRS complex without a detected corresponding pulse represents a pulse deficit. In the presence of atrioventricular dissociation, when atrial activity is irregularly transmitted to the ventricles, the strength of the peripheral arterial pulse depends on the timing of the atrial and ventricular contractions. In a patient with rapid heartbeats, the presence of such variations suggests ventricular tachycardia; with an equally rapid rate, an absence of variation of pulse strength suggests a supraventricular mechanism.

1.3.5. Wide Pulse Pressure

Wide pulse pressure, or as it is often called "water hammer pulse," is observed in cases of severe aortic regurgitation and consists of an abrupt upstroke (percussion wave) followed by rapid collapse later in systole with no dicrotic notch.

1.3.6. Pulsus Parvus et Tardus

The phenomenon of pulsus parvus et tardus is observed in cases of aortic stenosis and is caused by reductions in stroke volumes and prolonged ejection phases, which produce reductions and delays in the volume increments inside aortas. "Tardus" refers to delayed or prolonged early systolic accelerations; "parvus" refers to diminished amplitudes and rounding of the systolic peaks.

1.3.7. Bisferiens Pulse

A bisferiens pulse is characterized by two systolic peaks— the percussion and tidal waves—separated by a distinct midsystolic dip; the peaks are often equal, or one may be larger. It occurs in conditions in which a large stroke volume is ejected rapidly from the left ventricle and is observed most commonly in patients with pure aortic regurgitation or with a combination of aortic regurgitation and stenosis. A bisferiens pulse also occurs in patients with hypertrophic obstructive cardi-omyopathy. In these patients, the initial prominent percussion wave is associated with rapid ejection of blood into the aorta during early systole, followed by a rapid decline as obstruction becomes prominent in midsystole and by a tidal wave. Very rarely does it occur in individuals with a normal heart.

1.3.8. Dicrotic Pulse

Not to be confused with a bisferiens pulse, in which both peaks occur in systole, the dicrotic pulse is characterized by a second peak that is in diastole immediately after the second heart sound. The normally small wave that follows aortic valve closure (dicrotic notch) is exaggerated and measures more than 50% of the pulse pressure on direct pressure recordings. It usually occurs in conditions such as cardiac tamponade, severe heart failure, and hypovolemic shock, in which a low stroke volume is ejected into a soft elastic aorta. Rarely, a dicrotic pulse is noted in healthy adolescents or young adults.

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Essentials of Human Physiology

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