Morgan & Mikhail Chap 5 (Cardiovascular Monitoring) Flashcards
Pages 75-86
Arterial Blood Pressure
The rhythmic contraction of the left ventricle, ejecting blood into the arterial tree, results in pulsatile arterial pressures. The peak left ventricular end-systolic pressure (in the absence of aortic valve stenosis) approximates the systolic arterial blood pressure (SBP); the lowest arterial pressure during diastolic relaxation is the diastolic blood pressure (DBP). Pulse pressure is the difference between the systolic and diastolic pressures. The time-weighted average of arterial pressures during a pulse cycle is the mean arterial pressure (MAP). MAP can be estimated by application of the following formula:
MAP = (SBP)+ 2(DBP)/3
Arterial blood pressure varies depending upon where within the vasculature the pressure is measured. As a pulse moves peripherally through the arterial tree, wave reflection distorts the pressurewaveform, leading to an exaggeration of systolic
and pulse pressures (Figure 5–1).
For example, radial artery systolic pressure is usually greater than aortic systolic pressure because of its more distal location.
Difference in BP
The level of the sampling site relative to the heart affects the measurement of blood pressure
because of the effect of gravity (Figure 5–2). In patients with severe peripheral vascular disease, there may be significant differences in blood pressure measurements
among the extremities. The greater value should be used in these patients.
Noninvasive Arterial Blood Pressure Monitoring
Indications
The use of any anesthetic is an indication for arterial blood pressure measurement. The techniques and frequency of pressure determination will depend on the patient’s condition and the type of surgical procedure. A noninvasive blood pressure measurement every 3 to 5 minutes is adequate in most cases.
Contraindications
Although some method of blood pressure measurement is mandatory, techniques that
rely on a blood pressure cuff are best avoided in extremities with vascular abnormalities (eg, dialysis shunts) or with intravenous lines. It rarely may prove impossible to monitor blood pressure in patients (eg, those who have burns) who have no accessible site from which the blood pressure can be safely recorded.
Palpation
SBP can be determined by (1) locating a palpable peripheral pulse, (2) inflating a blood pressure cuff proximal to the pulse until flow is occluded, (3) releasing cuff pressure by 2 or 3 mm Hg per heartbeat, and (4) measuring the cuff pressure at which pulsations are again palpable. This method tends to underestimate systolic pressure, however, because of the insensitivity of touch and the delay between flow under the cuff and distal pulsations. Palpation does not provide a diastolic pressure or MAP. The equipment required is simple and inexpensive.
Doppler Probe
When a Doppler probe is substituted for the anesthesiologist’s finger, arterial blood pressure measurement becomes sensitive enough to be useful in obese patients, pediatric patients, and patients in shock (Figure 5–3). The Doppler effect is the shift in the frequency of sound waves when their source moves relative to the observer. For example, the pitch of a train’s whistle increases as a train approaches and decreases as it departs. Similarly, the reflection of sound waves off of a moving object causes a frequency shift. A Doppler probe transmits an ultrasonic signal that is reflected by underlying tissue. As red blood cells move through an artery, a Doppler frequency shift will be detected by the probe. The difference between transmitted and received frequency causes the characteristic swishing sound, which indicates blood flow.
Because air reflects ultrasound, a coupling gel (but not corrosive electrode jelly) is applied between the probe and the skin. Note that only systolic pressures can be reliably determined with the Doppler technique.
Auscultation
Inflation of a blood pressure cuff to a pressure between systolic and diastolic pressures will partially collapse an underlying artery, producing turbulent flow and the
characteristic Korotkoff sounds. These sounds are audible through a stethoscope placed under—or just distal to—the distal third of the blood pressure cuff. The clinician
measures pressure with an aneroid or mercury manometer.
Occasionally, Korotkoff sounds cannot be heard through part of the range from systolic to diastolic pressure. This auscultatory gap is most common in hypertensive patients and can lead to an inaccurate diastolic pressure measurement. Korotkoff sounds are often difficult to auscultate in noisy patient care environments and during episodes of hypotension or marked peripheral vasoconstriction.
Oscillometry
Arterial pulsations cause oscillations in cuff pressure. These oscillations are small if the cuff is inflated above systolic pressure. When the cuff pressure decreases to systolic
pressure, the pulsations are transmitted to the entire cuff, and the oscillations markedly increase. Maximal oscillation occurs at the MAP, after which oscillations decrease.
Because some oscillations are present above and below arterial blood pressure, a mercury or aneroid manometer provides an inaccurate and unreliable measurement.
Automated blood pressure monitors electronically measure the pressures at which the oscillation amplitudes change (Figure 5–4). A microprocessor derives systolic, mean, and diastolic pressures using an algorithm. Machines that require identical consecutive pulse waves for measurement confirmation may be unreliable during arrhythmias (eg, atrial fibrillation). Oscillometric monitors should not be used on patients on cardiopulmonary bypass. Nonetheless, the speed, accuracy, and versatility of oscillometric devices have greatly improved, and they have become the preferred noninvasive blood pressure monitors in the United States and worldwide.
Arterial Tonometry
Arterial tonometry measures beat-to-beat arterial blood pressure by sensing the pressure required to partially flatten a superficial artery that is supported by a bony
structure (eg, radial artery). A tonometer consisting of several independent pressure transducers is applied to the skin overlying the artery (Figure 5–5A). The contact stress between the transducer directly over the artery and the skin reflects intraluminal pressure. Continuous pulse recordings produce a tracing very similar to an invasive arterial blood pressure waveform. Limitations to this technology include sensitivity to
movement artifact and the need for frequent calibration.
Finger Cuff Method
The finger cuff method uses an inflatable finger cuff and an infrared light detector measure the changing finger diameter to generate a pressure waveform (Figure 5–5B).
These devices apply pressure to the finger to determine MAP and generate a waveform from which a cardiac output (CO) measurement is calculated. A 2018 study demonstrated that patients who had continuous noninvasive blood pressure monitoring experienced reduced duration and severity of intraoperative hypotension compared with patients who had intermittently monitored blood pressure.
Clinical Considerations
Adequate oxygen delivery to vital organs must be maintained during anesthesia.
Unfortunately, instruments to monitor specific organ perfusion and oxygenation are complex, expensive, and often unreliable, and for that reason, an adequate arterial blood pressure is assumed to predict adequate organ blood flow.
However, flow also depends on vascular resistance:
Flow = Pressure/Resistance
Even if the pressure is high, when the resistance is also high, flow can be low. Thus, arterial blood pressure should be viewed as an indicator—but not a measure—of organ
perfusion.
Cuff Sizing
The accuracy of any method of blood pressure measurement that involves a blood pressure cuff depends on proper cuff size (Figure 5–6). The cuff’s bladder should
extend at least halfway around the extremity, and the width of the cuff should be 20% to 50% greater than the diameter of the extremity.
Automated blood pressure monitors, using one or a combination of the methods
described previously, are frequently used in anesthesiology. A self-contained air pump
inflates the cuff at set intervals. Incorrect placement or too-frequent cycling of these automated devices has resulted in nerve palsies, and when they are placed on the same
extremity as an intravenous catheter, there can be extravasation of intravenously administered fluids or blood products.
Invasive Arterial Blood Pressure Monitoring
Indications
Indications for invasive arterial blood pressure monitoring by catheterization of an artery include current or anticipated hypotension or wide blood pressure deviations,
end-organ disease necessitating beat-to-beat blood pressure regulation, and the need for
multiple arterial blood gas or other blood analyses.
Contraindications
If possible, catheterization should be avoided in smaller end arteries lacking collateral blood flow or in extremities where there is a suspicion of preexisting vascular
insufficiency.
Selection of Artery for Cannulation
- The radial artery is commonly cannulated because of its superficial location and substantial collateral flow (in most patients, the ulnar artery is larger than the radial,
and there are connections between the two via the palmar arches). Five percent of patients have incomplete palmar arches and lack adequate collateral blood flow.
The Allen test is a simple but not reliable method for assessing the safety of radial artery cannulation.
In this test, the patient exsanguinates their hand by making a fist.
While the operator occludes the radial and ulnar arteries with fingertip pressure, the patient relaxes the blanched hand. Collateral flow through the palmar arterial arch is confirmed by flushing of the thumb within 5 seconds after pressure on the ulnar
artery is released. Delayed return of normal color (5–10 s) indicates an equivocal test or insufficient collateral circulation (>10 s).
The Allen test is of such questionable utility that many practitioners routinely avoid it.
Alternatively, blood flow distal to the radial artery occlusion can be detected by palpation, Doppler probe, plethysmography, or pulse oximetry. Unlike the Allen test, these methods of determining the adequacy of collateral circulation do not require patient cooperation.
- Ulnar artery catheterization is usually more difficult than radial catheterization because of the ulnar artery’s deeper and more tortuous course. Because of the risk of
compromising blood flow to the hand, ulnar catheterization would not normally be considered if the ipsilateral radial artery has been punctured but unsuccessfully
cannulated. - The brachial artery is large and easily identifiable in the antecubital fossa. Its proximity to the aorta provides less waveform distortion. However, being near the
elbow predisposes brachial artery catheters to kinking. - The femoral artery is prone to atheroma formation and pseudoaneurysm but often
provides excellent access. The femoral site has been associated with an increased incidence of infectious complications and arterial thrombosis. Aseptic necrosis of
the head of the femur is a rare but tragic complication of femoral artery cannulation in children. - The dorsalis pedis and posterior tibial arteries are some distance from the aorta and therefore have the most distorted waveforms.
- The axillary artery is surrounded by the axillary plexus, and nerve damage can result from a hematoma or traumatic cannulation. Air or thrombi can quickly gain access to the cerebral circulation during vigorous retrograde flushing of axillary
artery catheters. Nevertheless, in extensively burned patients, the axillary artery may
be the best option.
Radial Artery Cannulation Technique
Clinical Considerations of Intraarterial Cannulation
Because intraarterial cannulation allows continuous beat-to-beat blood pressure measurement, it is considered the optimal blood pressure monitoring technique. The
quality of the transduced waveform, however, depends on the dynamic characteristics of the catheter–tubing–transducer system. False readings can lead to inappropriate
therapeutic interventions.
Transducer on Intraarterial Cannulations
Catheter–tubing–transducer systems must also prevent hyperresonance, an artifact
caused by reverberation of pressure waves within the system. A damping coefficient (β) of 0.6 to 0.7 is optimal. Arterial blood pressure measurements are improved by
minimizing tubing length, eliminating unnecessary stopcocks, removing air bubbles, and
using low-compliance tubing.
Although smaller-diameter catheters lower natural
frequency, they improve under-dampened systems and are less apt to result in vascular
complications.
Transducers contain a diaphragm that is distorted by an arterial pressure wave. The
mechanical energy of a pressure wave is converted into an electric signal. Most transducers are resistance types that are based on the strain gauge principle: stretching
a wire or silicone crystal changes its electrical resistance. The sensing elements are arranged as a “Wheatstone bridge” circuit so that the voltage output is proportionate to the pressure applied to the diaphragm (Figure 5–8C).
Lead II and V5
The electrical axis of lead II is approximately 60° from the right arm to the left leg,
which is parallel to the electrical axis of the atria, resulting in the largest P-wave voltages of any surface lead. This orientation enhances the diagnosis of arrhythmias and
the detection of inferior wall ischemia. Lead V5 lies over the fifth intercostal space at the anterior axillary line; this position is a good compromise for detecting anterior and lateral wall ischemia. A true V5 lead is possible only on operating room ECGs with at least five lead wires, but a modified V5 can be monitored by rearranging the standard three-limb lead placement (Figure 5–9). Ideally, because each lead provides unique information, leads II and V5 should be monitored simultaneously. If only a singlechannel machine is available, the preferred lead for monitoring depends on the location
of any prior infarction or ischemia and whether arrhythmia or ischemia appears to be the greater concern.
Common ECG Findings
Commonly accepted criteria for diagnosing myocardial ischemia require that the ECG be recorded in “diagnostic mode” and include a flat or downsloping ST-segment
depression exceeding 1 mm, 80 msec after the J point (the end of the QRS complex), particularly in conjunction with T-wave inversion. ST-segment elevation with peaked T
waves can also represent ischemia. Wolff–Parkinson–White syndrome, bundle-branch blocks, extrinsic pacemaker capture, and digoxin therapy may preclude the use of ST segment information.
