Cardiovascular Monitoring in Noncardiac Surgery

Electrocardiogram

The ECG is a mainstay for monitoring cardiac status. Continuously monitoring cardiac electrical activity, it provides heart rate and rhythm data, as well as assessment of cardiac conduction (PR interval, QRS duration) and repolarization (ST segment, T-wave morphology, and QT interval). The normal morphologies of the ECG signal and the ECG intervals are shown in Fig. 9.1 .

A three-lead system, using three or four electrodes (right arm, left arm, left leg, ground), allows monitoring of limb leads I, II, or III, providing primarily rhythm and conduction data. This can suffice for healthy patients, but a five-electrode system (right arm, left arm, left leg, precordial, ground) is usually used perioperatively and in intensive care units. This system allows simultaneous monitoring of a limb lead (usually lead II) and a precordial “V” lead that enhances the detection of myocardial ischemia. The sensitivity for detecting myocardial ischemia when using a combination of leads II and V ₅ has been reported to be 80%. The V lead can be placed according to the particular area of interest, ranging from anterior (V ₁ ) to lateral (V ₆ ) ( Fig. 9.2 ), with V ₃ to V ₅ generally being the most sensitive for anterior-lateral myocardial ischemia (lead II is used for inferior wall ischemia).

Myocardial ischemia most often manifests as ST-segment depression, although elevated ST segments, change in T-wave morphology, new conduction defects, or frequent premature ventricular contractions may also be signs of myocardial ischemia. ECG monitoring systems have automated digital signal processing to continuously display heart rate, QT interval, and ST-segment depression or elevation, as well as alarm systems for these parameters.

Abnormal rhythms, such as sinus bradycardia and tachycardia, junctional rhythms, atrial fibrillation, right and left bundle branch blocks, and heart block, are not uncommon in cardiac patients. All these abnormalities can be detected using a five-electrode system ( Tables 9.2 and 9.3 ). Whereas a limb lead such as lead II is preferred for conduction and rhythm detection, the precordial leads are preferred for diagnosis of myocardial ischemia, infarction, and bundle branch blocks.

Table 9.3

Electrocardiographic Morphology of Common Abnormalities Encountered Perioperatively in Cardiac Patients

ECG Diagnosis	Example	Comments
Atrial fibrillation		Narrow QRS, irregularly irregular
Atrial flutter		Regular, flutter sawtooth waves, narrow QRS
Complete heart block		No conduction through AV node; P waves unassociated with QRS complexes
AV dissociation		Regular, atrial and ventricular unrelated, QRS duration depends on ventricular source, ventricular rate faster than atrial
Left bundle branch block		V 1 , V 2 , V 3; QRS >0.12 s, regular; ST segment and T deflection opposite that of QRS; rate <100 beats/min; signifies significant coronary disease
Inferior wall myocardial infarction		ST segment elevation in inferior leads (II, III, aVF)
Anterior wall myocardial infarction		ST-segment elevation in anterior precordial leads
Myocardial ischemia		ST-segment depression
Hyperkalemia		Peaked T waves
Premature ventricular contractions		Bizarre QRS, compensatory pause
Ventricular tachycardia		100–250 beats/min, wide QRS
Ventricular fibrillation		Rate and rhythm absent
Torsades de pointes		Rate 150–250 beats/min, phasic variation of QRS; associated with prolonged QT interval
Ventricular pacing		Single pacing spike followed by QRS
Dual-chamber pacing		Two pacing spikes per QRS

AV, Atrioventricular; ECG, electrocardiographic.

Cardiac patients often present with cardiac implantable electrical devices (CIEDs). Paced rhythms (pacing spikes) can be detected best in a limb lead such as lead II, and the timing and number can often allow identification of the type of pacing. CIEDs are discussed in Chapter 4 .

Electrolyte abnormalities can cause various conduction and repolarization changes in the heart, with examples being peaked, high T waves in hyperkalemia and U waves in hypokalemia. Leads II and precordial leads are sufficient for detecting these abnormalities.

Electrocardiographic abnormalities more likely represent pathology in patients with cardiac disease than in healthy patients, so understanding and close monitoring of the ECG waveform is particularly important in cardiac patients.

Arterial Blood Pressure Monitoring

Intermittent noninvasive arterial blood pressure (NBP) monitoring is typically provided using an automated cuff that detects arterial pulsations using oscillometry. Close monitoring of blood pressure is often required in cardiac patients, and the frequency of measurements can be adjusted to as often as every minute. Care should be exercised, however, because pressure injury can result from prolonged, frequent NBP measurements. An emerging practical alternative is continuous noninvasive blood pressure monitoring, with examples being the Edwards Clearsight and the LiDCO Rapid devices. These systems consist of a finger cuff with mild inflation (“volume clamp method”) with a high-resolution bladder and sensor system to detect pulsatile pressure. Because they provide a continuous measurement, they are particularly useful during procedures that are associated with rapid changes in blood pressure (e.g., carotid endarterectomy, airway surgery). A significant advantage of these systems is that they also provide cardiac output and dynamic parameter information as well (vide infra). Because the cuff is very distal, they may be inaccurate in cases of severe peripheral vascular disease or vasoconstriction. In those cases, invasive, intra-arterial pressure monitoring should be considered. Another possible method to obtain continuous noninvasive blood pressure measurements is tonometry. This technique has been used at the wrist (Tensys TLine).

Promising experimental techniques include systems using pulse-wave velocity and pulse-wave transit time. In the near future, technology providing noninvasive continuous blood pressure monitoring will almost certainly be widely adopted, supplanting invasive arterial measurement in many cases.

Intraarterial pressure monitoring is indicated for high-risk patients and surgeries, particularly when periodic arterial blood samples will be desired. An arterial wave of high fidelity, with appropriate damping, frequency response, and morphology, is necessary for accurate monitoring. The waveform results from a sine-like pressure wave generated by the heart, with superimposed reflections from the vascular tree ( Fig. 9.3 ).

Arterial access is typically obtained with a catheter in the radial artery, brachial artery, or femoral artery. The dorsalis pedis, ulnar, and axillary arteries have also been used. The radial artery is generally preferred because of easy access and the low incidence of complications. Brachial artery catheterization can usually be performed when radial artery attempts are unsuccessful and likewise has a low incidence of complications. Ulnar artery catheterization has been reported to be safe, but it may be ill-advised because in most cases, it provides the bulk of blood flow to the hand. It should not be used if there have been previous unsuccessful attempts at the ipsilateral radial artery or if an Allen text contraindicates its use. If central arterial pressure is required, as in cases in which there is a large central–to–peripheral arterial pressure gradient, the femoral artery can be used. Complications of femoral arterial catheterization include increased risk of infection and retroperitoneal hematoma. Femoral catheterization should be done under strict sterile conditions. When femoral catheterization is performed, surface ultrasound is recommended so as to avoid nearby nerve damage and deep injuries leading to retroperitoneal hematoma.

Pulse Oximetry

Pulse oximetry not only provides arterial oxygen saturation but also heart rate, an index of peripheral perfusion, and a dynamic parameter (pleth variability index [PVI]). These can be useful in assessing circulatory status and potential response to fluid challenge. Like other dynamic parameters such as pulse pressure variation (PPV), stroke volume variation (SVV), and systolic pressure variation (SPV), PVI results from cyclical changes in left ventricular filling associated with the respiratory cycle. Large changes indicate the likelihood that volume infusion will increase the cardiac output. Dynamic parameters such as PVI, when used during positive-pressure ventilation with tidal volumes of 8 mL/kg or greater and in the absence of frequent arrhythmias, are far more sensitive for potential volume responsiveness than any other clinically available metric, including central venous pressure (CVP) and pulmonary artery pressure. The primary mechanism for dynamic fluctuations in cardiac output is depicted in Fig. 9.4 .