Hemodynamic Monitoring

Definitions of Blood Pressure

Accompanying the development of various methods of blood pressure determination was controversy over the actual definition of systolic, diastolic, and mean blood pressures. For invasive methods that produce a pulsatile waveform, the definitions are straightforward: systolic pressure is the maximum instantaneous pressure, diastolic pressure is the minimum instantaneous pressure, and mean pressure is the area under the waveform-time curve divided by the time interval for one or more beats, a quantity easily determined by software.

Blood pressure determinations are highly dependent on the anatomic site where it is being measured. Usually, there is an increase in systolic values and a decrease in diastolic values as blood pressure is measured more peripherally in the vascular tree of healthy subjects. Because of the opposite changes of the systolic and diastolic values, mean blood pressure normally remains relatively constant as the measurement site changes. In patients with vascular disease and resultant restricted arterial flow, further errors are introduced that usually produce decreases in systolic, diastolic, and mean flow at more distal locations. Despite these well-known predictable errors, the radial arterial pressure—determined by a small cannula inserted near the wrist, combined with an electric transducer and digital display system—has become the de facto clinical gold standard for human blood pressure determinations. Nearly all published methodology comparisons and so-called accuracy studies use the radial arterial pressure as the reference standard. This is done despite the fact that the choice of the radial artery is more one of safety and convenience than of scientific validity. Much less affected by arterial system variables is the central aortic root pressure, probably a much more reliable standard, although measurement of aortic root pressure in humans generally involves unacceptable risk.

Instrumentation and Units of Measure

Several different types and models of automated noninvasive blood pressure instrumentation have become available in the United States in recent years, and their use has become ubiquitous in anesthesia over much of the world. Each approach measures different physical quantities, from which the values for systolic, diastolic, and mean blood pressure are derived. Noninvasive blood pressure readings never correlate exactly with measured invasive radial arterial blood pressure, irrespective of construction and calibration precision. It is always hoped, however, that the accuracy of any method is such that differences between readings are of little clinical significance. In general, this is true for most commercial oscillometric instruments, although other noninvasive methods do not consistently perform as well in all situations. Reliability of modern automated noninvasive oscillometric equipment has reached the point where it is unnecessary to validate the automated unit with an older method, such as auscultation, because the manual approach is less reliable and more subjective than the automated method and most often represents a step-down in accuracy.

The standard unit of measure for blood pressure in the United States is millimeters of mercury (mm Hg), or torr , in which 760 mm Hg equals 1 standard atmosphere of pressure at sea level. Elsewhere in the world, the kilopascal (kPa) often is the standard unit of pressure measurement (1 kPa = 7.5 mm Hg). Most commercial digital blood pressure instrumentation provides a readout to within 1 mm Hg, although the implied significance considerably exceeds the actual precision and repeatability of even the best invasive units and certainly does not provide meaningful additional clinical information. The actual precision of the best noninvasive devices is approximately 5 to 10 mm Hg. Calibration accuracy of noninvasive blood pressure devices is most often measured and adjusted by the manufacturer through comparison with radial arterial blood pressure in healthy human subjects. This method obviously involves limitations, not the least of which is using average values without adjustment for anatomic differences, such as body habitus.

As mentioned, radial arterial pressure correlates well with central aortic pressure in healthy subjects, but the two values may disagree by a considerable amount, especially in hypertensive and hyperdynamic patients and in patients with peripheral vasoconstriction or vascular disease. In addition, every noninvasive method measures blood pressure indirectly, by inference from measured physical quantities, such as cuff air-pressure oscillations; the correlation with invasive pressure is never perfect, even under the best of circumstances. This should be kept in mind when interpreting noninvasive blood pressure readings.

Reference Points

If the aortic root is taken as the desired reference point for blood pressure, all measurement techniques must take into account the effect of gravity and the water column hydrostatic pressure that results from a difference in height between the aortic root and the location of the transducer. This amounts to a difference of approximately 7.5 mm Hg for every 10 cm difference in vertical height from the aortic root. The effect is small for a brachial cuff, but it can be large (>50 mm Hg) if, for example, the pressure transducer is accidentally positioned improperly, or if an ankle cuff is used on an individual in a sitting position. Under these circumstances, an accurate pressure can still be obtained, but the operator must add or subtract a fixed amount to both noninvasive and invasive instruments.

Manual (Riva-Rocci) Measurement Technique

The measurement of arterial blood pressure with an air-inflatable cuff placed on the proximal arm, listening with a stethoscope over the brachial artery for Korotkoff sounds as cuff pressure is slowly decreased, remains the most common and inexpensive method of blood pressure determination. This method was originally described by Scipione Riva-Rocci in the mid-nineteenth century. Five distinct sound phases are heard as pressure decreases from above systolic to below diastolic: in phase I, clear tapping sounds are heard; in phase II, sounds become softer and longer; in phase III, they become crisper and louder; in phase IV, sounds become muffled and softer; and in phase V, sounds completely disappear. Systolic blood pressure is measured at the onset of phase I, and diastolic is measured at the onset of phase V. With this technique, mean blood pressure (BP) is not specifically measured, but it is often approximated as follows:

where DP is diastolic pressure and SP is systolic pressure. This formula assumes that approximately two-thirds of the cardiac cycle is spent in diastole with the remaining one-third in systole. The accuracy of the formula will be affected when diastolic filling time is reduced in conditions such as tachycardia.

The advantages of the Riva-Rocci (auscultatory) technique are numerous and include low cost, simplicity, lack of dependence on electricity, and ruggedness. This method suffers from imperfect correlation with invasive measurement of blood pressure because of numerous factors, such as ambient noise, auditory acuity of the clinician, atherosclerotic vascular changes, obesity, and cuff size in relation to the limb. In healthy patients, however, the clinical accuracy is high. Riva-Rocci blood pressure is generally biased low (10 to 30 mm Hg) for systolic pressure, and high (5 to 25 mm Hg) for diastolic pressure, especially in hypertensive patients. The precision (scatter) is approximately ± 20 mm Hg compared with invasive radial arterial pressure. The errors are exacerbated by obesity, edema, and vascular disease. In critically ill hypotensive patients, it is often impossible to obtain a reliable auscultatory blood pressure without resorting to Doppler flow-sensing devices to detect the arterial blood flow. With the increased use of pulse oximetry, systolic pressure can be reliably measured with a much improved sensitivity over manual palpation or Korotkoff sounds by noting the point of occlusion of pulsatile flow in the finger through the observation of the corresponding pulse waveform on the oximeter display. Despite a lack of accuracy and high subjectivity compared with automated and invasive methods, manual auscultatory measurement of blood pressure is commonly used for healthy, nonsurgical patients because of the low cost and the unimportance of small errors in the healthy population.

Multiple techniques exist for the physical measurement of cuff pressure throughout deflation. Mercury sphygmomanometers are the more traditional approach, but aneroid and hybrid devices are most common since actual mercury manometers have mostly disappeared from developed countries because of environmental concerns with liquid mercury. Hybrid devices generally use an electronic pressure gauge that is digitally displayed, replacing the mercury column and circular scale of Bourdon tube mechanical gauges. Hybrid devices usually have the option for the displayed pressure to stop decreasing when significant systolic, diastolic, and mean pressure levels are reached. All pressure techniques are comparable when used properly, although devices that are more modern are easier to use. Automated devices using the Riva-Rocci technique were developed in the past but were more complicated when compared with oscillometric devices and are no longer manufactured.