All patients undergoing vascular surgical procedures should receive continuous perioperative electrocardiogram (ECG) monitoring. Continuous ECG monitoring is the penultimate example of an ideal monitoring system. The primary physiologic data (i.e., heart rate, cardiac rhythm, and the presence of ischemic changes) are of unquestioned relevance and the technology required is noninvasive, inexpensive, ubiquitous, and requires minimal training or experience to be applied effectively. While prospective evidence for effectiveness is lacking, the high prevalence of cardiac disease in this population dictates a pragmatic approach to ECG monitoring.

Tachycardia is probably the most common and physiologically important abnormality detected by continuous ECG monitoring. The etiology of tachycardia is clearly multifactorial (i.e., hemorrhage, hypovolemia, hypoxia, inadequate analgesia, and so on). Nevertheless, the crucial importance of timely exclusion or correction of these various contributory factors (as well as the adverse consequences of failing to do so) should be readily apparent to even the casual observer. Similarly, cardiac arrhythmias are both common and potentially lethal peri-operative events. Timely detection and appropriate pharmacologic or electrophysiologic management of cardiac arrhythmias are of self-evident importance to ensuring optimal outcomes following vascular surgical procedures.

Perhaps the most controversial role for continuous ECG monitoring is in the detection of myocardial ischemia. In addition to an extremely high prevalence of CAD, numerous peri-operative factors affecting myocardial oxygen supply and demand contribute (alone or in combination) to the high incidence of myocardial ischemia in these patients. These include (among others) pain, tachycardia, hypoxia, anemia, hypertension, hypotension, fluid overload, and vasoactive drugs. Prospective evidence demonstrating the specificity of continuous ECG monitoring to exclude myocardial ischemia in this setting is lacking. Direct observation of the ST segment for the appearance of depression (subendocardial ischemia) or elevation (transmural ischemia) is a logical (albeit nonspecific) way to identify patients at risk. Sensitivity is low with three-lead continuous ECG systems but increases markedly with five-lead systems. Thus, routine five-lead ECG monitoring with continuous monitoring of leads II and V5 is currently recommended. Positional effects (i.e., lateral decubitus), right and left bundle branch blocks, left ventricular hypertrophy with strain, tachyarrhythmias, and pacemaker activity significantly limit the utility of direct ST segment analysis in up to 15% of vascular surgical patients. More recently, computer software is now available for real-time peri-operative ECG analysis; the ultimate utility of this technology is yet to be demonstrated.

Continuous monitoring of arterial hemoglobin saturation by pulse oximetry has probably made the largest impact on patient safety of any peri-operative monitoring technology and should be considered in all patients. Like continuous ECG monitoring, pulse oximetry has many of the attributes
of an ideal physiologic monitor in that it provides information continuously, noninvasively, and inexpensively, and requires minimal expertise or training to be applied effectively. A pulse oximeter provides continuous information on arterial hemoglobin saturation (Sao₂) and pulse rate by measuring light absorption in peripheral blood. Pulse oximetry uses a light source emitting two wavelengths (red and infrared) that shine through a tissue bed (usually a finger or ear lobe). A photodiode opposite the light source measures the transmitted light intensity in a manner similar to a laboratory co-oximeter. The pulse oximeter measures the ratio of the pulsatile component of red light absorbed to the pulsatile component of the infrared light absorbed. This ratio varies directly with the arterial oxyhemoglobin saturation. Arterial oxygen saturation and heart rate determination as measured by pulse oximetry obviously require a pulsatile distribution of blood flow and may be falsely depressed by vasoconstriction, hypothermia, hypotension, severe peripheral vascular disease, and alpha agonists. In addition, the presence of methemoglobin and carboxyhemoglobin may result in falsely elevated values for arterial oxygen saturation. Despite these potential drawbacks, pulse oximetery is quite accurate in a wide variety of patients with a tremendous variation in pulse amplitude.

If the pulse oximetry data are to be used most effectively, they must be interpreted in the context of the other factors responsible for systemic oxygen delivery; namely, arterial oxygen content and cardiac output. Oxygen content is dependent upon both hemoglobin saturation and concentration in arterial blood. Thus, efforts to maintain and improve oxygenation will have the most beneficial effect to the extent that they are coupled with efforts to ensure adequate oxygen carrying capacity (i.e., correcting anemia) and blood flow (i.e., optimizing cardiac output).

Direct monitoring of respiratory rate and tidal volume documents the presence of tidal gas flow but not the adequacy of ventilation. By definition, effective ventilation is occurring if and when arterial carbon dioxide tension (PaCO₂) is 40 mmHg. Capnometry is the measurement of the CO₂ concentration at the airway, and it provides a continuous monitor of the effectiveness of ventilation. The peak concentration of CO₂ in mixed-expired gas occurs at endexpiration (end-tidal). End-tidal CO₂ can be monitored continuously at the airway using mass or infrared spectroscopy. Capnography is the graphic display of the end-tidal CO₂ curve.

By providing this information on a breath-to-breath basis, capnography can be used as a continuous monitor of both the integrity of the respiratory circuit and the integrity of the cardiovascular system. Any acute decrease in cardiac output will necessarily result in a corresponding decrease in pulmonary blood flow and thus an acute drop in end-tidal CO₂. This same principle allows for the detection of acute pulmonary emboli by capnography. In fact, the only catastrophic cardiopulmonary problem that is not detected immediately by capnometry is acute arterial desaturation (which is detected by continuous pulse oximetry).

Capnometry is extremely useful in confirming the correct position of the endotracheal tube, as well as in facilitating weaning from mechanical ventilation. In fact, using a combination of pulse oximetry and capnometry, many patients can be successfully weaned from mechanical ventilation without the need to obtain arterial blood gases. It becomes readily apparent that the noninvasive combination of capnometry and pulse oximetry can provide continuous, beat-to-beat and breath-to-breath monitoring of the adequacy of oxygenation, ventilation, and circulation.

Hypothermia is extremely common in vascular surgical patients, due to anesthesiainduced alterations in thermoregulatory control, and due to the prolonged and complex nature of the procedures performed and the occasional requirement for massive transfusion and intravenous fluid administration. Major complications of hypothermia include coagulopathy and arrhythmias, as well as an increased risk of adverse cardiac events and wound infections. Other complications include electrolyte imbalances, metabolic acidosis, and altered pharmacokinetics. Core temperature should be monitored in all patients using a tympanic membrane, esophageal, nasopharyngeal, pulmonary artery catheter, or bladder thermistor. Efforts should be made to achieve peri-operative normothermia through the aggressive use of forced air warmers and resistive heating blankets and by the judicious warming of ventilator circuits and intravenous fluids.

Indirect, noninvasive monitoring of arterial blood pressure using a pneumatic cuff and oscillometer is indicated for all patients undergoing vascular surgical procedures. Like continuous ECG monitoring, this technology is ubiquitous, automated, inexpensive, and very reliable. Direct arterial catheterization is necessary for continuous monitoring of arterial blood pressure. It is important to remember that the numerical values that the monitor system derives from a peripheral arterial catheter are not necessarily synonymous with aortic root pressure and therefore vital organ perfusion. The periodic complex wave that is displayed on the monitor is the product of multiple harmonics initiated by left ventricular contraction and transmitted down a theoretically continuous fluid column in a compliant chamber from the left ventricle to the cathetertransducer-monitor system. Therefore, the magnitude and the morphology of the arterial pressure waveform depend not only upon the characteristics and integrity of this fluid column, but also on the natural frequency and dampening of the transducer, the length and compliance of the connecting tubing, and the reflectance of the arterial tree. Reflectance of the arterial tree is affected by vascular calcification, anesthetic agents, and the use of vasoactive drugs. Moreover, tranducer-monitor systems are subject to calibration, zeroing, and leveling errors, as well as problems due to overextension (adding additional compliant tubing) or overdampening (due to air bubbles, blood clots, stopcocks, and so on). Despite these limitations, direct measurement of arterial pressure is considered the gold standard for arterial pressure monitoring and provides the surgeon with continuous reassurance of pulsatile arterial blood flow. Relative indications for direct, invasive monitoring of arterial blood pressure using an intra-arterial catheter and transducer-monitoring system are listed in Table 8-1.

The radial artery is the most frequent site of access, although the ulnar, brachial, subclavian, femoral, and even dorsalis pedis arteries have been used in the event that radial artery cannulation is impossible or contraindicated. Direct, catheter-over-needle or wire-guided access techniques are most commonly used. A modified Allen test is recommended to assess for completeness of the palmar arch is recommended prior to radial artery catheterization and considered to be reasonably accurate. Noninvasive
assessment of digital artery perfusion increases specificity but is time consuming and expensive and therefore inefficient for routine use prior to instituting invasive monitoring. Fortunately, complications such as thrombosis and digital artery embolism are rare but may cause ischemic necrosis of the digits. Less frequent complications include nerve injury, hematoma, pseudoaneurysm, and infection.