Hemodynamic Monitoring in Thoracic Surgical Patients

Oscillometry

Oscillometric blood pressure measurement is by far the most popular method of measuring blood pressure in the operating room (OR) in developed countries—at academic medical centers in the United States, 84% of surgical patients receive oscillometry alone. The engineering principles behind oscillometry were first described in 1876 by Marey. With the oscillometric technique, a cuff is inflated to a pressure higher than the systolic blood pressure (SBP), which occludes all flow to the extremity. At this occlusion pressure, no (Korotkoff) sounds are produced on auscultation, and pulsatile pressure transmitted to the cuff is minimal ( Fig. 12.1 ). Gradually (or stepwise), the cuff is deflated until the pressure is below the diastolic blood pressure (DBP). As the pressure within the cuff is deflated, blood begins to flow into the extremity, which produces oscillations in the cuff pressure, which can be measured and recorded. As noted by Posey et al., the pressure within the cuff at which oscillations are maximal is closely correlated with mean arterial pressure (MAP). The algorithms used to determine SBP and DBP are propriety to each manufacturer.

The introduction of oscillometric blood pressure measurement offered to the anesthesiologists many advantages in an OR environment when compared with the classic auscultatory noninvasive method: it can be automated (freeing up the anesthesia provider to perform other tasks), it requires significantly less skill and training to use, it can be more forgiving in patient positions in which frequent access to a distal artery for auscultation would prove difficult or impossible, and it is easier to use in a noisy OR environment.

Unlike intra arterial catheters, which are site independent (measured blood pressure does not change with movement of the extremity as long as the level of the transducer is not changed), oscillometric cuffs measure blood pressure at a specific location, irrespective of its location relative to the right atrium. The anesthesiologist caring for the thoracic surgical patient must therefore take care to account for cuff position relative to the organ of interest when interpreting blood pressure values. Given the frequent use of lateral positioning during thoracic procedures, the extremity on which the blood pressure cuff is placed may be higher or lower than the heart and brain, which are the primary organs of interest for blood pressure management. The arterial system is valve-less, and the pressure within it will therefore fall linearly with increasing height against gravity. However, given that blood pressure is measured in mm Hg and mercury is far denser than blood, a correction factor (0.74 mm Hg lower for every 1 cm) must be applied to measure the impact of these changes. Moving the blood pressure cuff from the dependent to nondependent arm in a patient in the lateral position can significantly change the reported blood pressure because of hydrostatic effects, despite the fact that actual blood pressure in the areas of interest (brain, heart) has not changed at all ( Fig. 12.2 ).

Oscillometry has been extensively compared with the gold standard of intraarterial catheterization. A single center, retrospective analysis of 15,310 noncardiac anesthetics in which the patient had both intraarterial catheters placed as well as oscillometry used found that the standard deviation of MAP was ± 12.5 mm Hg around a MAP of 75 mm Hg, when oscillometry was most accurate—this equates to 95% confidence intervals of approximately ± 25 mm Hg (Bland-Altman analysis). When MAP deviated substantially, the performance of the oscillometric technique decreased even further.

Tonometry

In 1963, Pressman and Newgard published the first description of what would become artery applanation tonometry. This was the first development of a continuous form of noninvasive blood pressure monitoring, and the concept was advanced by O’Rourke. Applanation tonometry involves the flattening (applanation) of a superficial artery, most commonly the radial, with a pressure sensing device. Only arteries that can be compressed (e.g., a radial artery) can be assessed. When the artery is compressed enough to reach a flattened state, the pressure sensed by the detector is similar to the intraarterial pressure, and the resulting pressure waveform closely mimics the true intraarterial pressure waveform. Tonometers are incapable of making absolute pressure measurements and thus need to be calibrated against another device (e.g., blood pressure cuff). The use of frequency domain mathematical analysis (e.g., fast Fourier transformation) allows a peripheral artery tracing to be “transformed” into an estimate of central arterial blood pressure using what is referred to as a transfer function.

If measurements are taken at a “large” artery, applanation tonometry is relatively resistant to confounding variables that can disrupt other noninvasive continuous blood pressure measurement methodologies (see later) such as vasoconstriction. Applanation tonometry has been validated in over 1600 patients, including over 15,000 individual blood pressure measurements but is clinically impractical in the OR because it requires unrestricted access to the wrist for placement of the pen-like “tonometer.”

Because the underlying technology is sound, but the practical implementation is cumbersome, efforts have been made to miniaturize applanation tonometers for utility in the OR environment. Testing by multiple groups has revealed relatively low bias (6.3 mm Hg, range 4.8–12.6 mm Hg) and limits of agreement (approximately ± 12.4 mm Hg, range ± 9.4 to ± 24.7 mm Hg) that are at least comparable, if not slightly better than noninvasive devices generally. A recent large metaanalysis has cast some doubt onto the accuracy of tonometers in patients who have cardiovascular disease. “The results from this metaanalysis found that inaccuracy and imprecision of continuous noninvasive arterial pressure monitoring devices are larger than what was defined as acceptable. This may have implications for clinical situations where continuous noninvasive arterial pressure is being used for patient care decisions.”

Volume Clamp Technique

First described by Penaz in 1973, volume clamping is a method of noninvasive blood pressure measurement that can provide beat-to-beat information in real time. It involves placement of an ultrafast, inflatable cuff upstream of a finger-based infrared sensor and emitter. By using a feedback loop between the cuff and the infrared sensor, the cuff is inflated and deflated cyclically such that the intensity of the infrared radiation is nearly constant (i.e., the blood volume of the finger is unchanged). In theory, if the pressure in the cuff exactly matches the arterial blood pressure, changes in finger blood volume (and absorbance of infrared light) are minimal. Thus the pressure waveform required to maintain minimal variation in infrared absorbance is thought to closely approximate arterial blood pressure ( Fig. 12.3 ).

These devices need to be periodically calibrated by taking advantage of the fact that detected pulsatility distal to a cuff is maximal near MAP (discussed earlier during oscillometry). When external cuff pressure is close to blood pressure, the arterial wall experiences minimal tension and is said to exist in an “unloaded” state. Commercially available devices that use the volume-clamp technique differ in the algorithms used to achieve vascular unloading.

Given the dependence of the volume clamp technique on small vessel, distal extremity perfusion, it is susceptible to periods of hypoperfusion ^, and poor blood flow as can be seen in shock, and hypothermia will impact the accuracy of the blood pressure measurements.

Individual studies looking at agreement between volume clamping and applanation tonometry in a variety of settings have been mixed; and in 2014, a metaanalysis of both of these techniques demonstrated an accuracy that was slightly worse than noninvasive technique overall. Of course, this decrease in accuracy needs to be counterbalanced by the continuous measurements provided by these devices, which is an important advantage. As the technology improves, its potential to provide beat-to-beat blood pressure values noninvasively is tantalizing and has significant implications in large surgeries with significant fluid shifts and hemodynamic swings, such as thoracic surgery.

Intraarterial Catheterization

When continuous, accurate blood pressure monitoring is indicated, intraarterial catheterization is most widely used. This technique is particularly useful in thoracic surgery patients because it also permits sampling for arterial blood gas analysis. A detailed review of arterial physiology is beyond the scope of this chapter and is available elsewhere. Three key points related to intraarterial blood pressure monitoring deserve mentioning.

First , a basic understanding of the engineering principles behind intraarterial blood pressure monitoring is necessary to use the technology properly. Intraarterial blood pressure monitors use a “Wheatstone bridge” strain gauge to estimate the intraarterial pressure. This pressure is transmitted through an uninterrupted column of fluid that begins with an intraarterial catheter and extends to the strain gauge, where the column terminates. ^, The physical characteristics (length, diameter, distensibility) of the tubing used to connect the catheter to the strain gauge impact the appearance of the pressure waveform.

To accurately measure relevant features of the waveform (systolic and diastolic pressure), it is essential that the measurement system has an appropriate dynamic response—this can be determined by measuring the natural frequency (f _n , the oscillating frequency after application of a pressure pulse) and the damping coefficient (ζ , the time it takes an oscillating waveform to decay). Both f _n and ζ can be measured using the “flush test”—f _n is the inverse of the time between two peaks (in s ^-1 ), and ζ is the ratio of two successive amplitudes (A ₂ /A ₁ ). By plotting f _n and ζ on a “dynamic response map,” the adequacy of the measurement system can be ensured ^, ( Fig. 12.4 ).

In general, clinicians should use rigid, short tubing systems to connect the pressure monitor to the intraarterial catheter, and take great care to avoid air bubbles. The compressible air bubbles results in damping of the arterial tracing. Of note, the MAP reading will not be affected by inadequate dynamic response, only the shape of the waveform and the resultant estimates of systolic and diastolic pressure.

Second , the shape of the blood pressure waveform is highly dependent on the physical characteristics of the cardiovascular system, as well as the measurement location ( Fig. 12.5 ). Rigid, noncompliant vessels will alter the arterial waveform, which is a combination of forward and backwards (from reflection sites) traveling wavefronts. A common clinical mistake is to see an extremely high systolic pressure and attribute it to “whip” (i.e., an underdamped system), when in reality the dynamic response of the arterial measurement system is fine. This is common in patients with significant cardiovascular disease and atherosclerosis, especially when pressure is measured at a peripheral site (e.g., radial artery).

Third , although the infection rate of intraarterial catheters is low (approximately 1.7 per 1000 catheter days), it is comparable to the infection rate of nontunneled, centrally inserted central venous catheters (2.7 per 1000 catheter days). The risk of infection can be lowered through sterile insertion technique and the use of a chlorhexidine-impregnated dressing.