Intensive Care Unit ECG Monitoring

Abstract

Perhaps the most useful means of monitoring the critically ill patient is the electrocardiogram (ECG). A 12lead ECG is useful to rule out cardiac complications in the acutely injured patient, the postoperative patient, and the septic patient [1]. Limb lead II or chest lead V may be continuously monitored for evidence of progressive cardiac disease and complicating dysrhythmias [1]. This section summarizes the usefulness and indications of the ECG in monitoring of the high-risk, critically ill patient and possibilities for future clinical use. Interpretation of the ECG waveform provides the basis for diagnosis and therapy of numerous conditions associated with disorders of the myocardium in the intensive care unit (ICU) [1]. Continuous ECG monitoring is essential for the adult patient with acute myocardial infarction because dysrhythmias are the most common life-threatening complication [1]. Continuous ECG monitoring of the postoperative general surgical patient is not as useful because the incidence of lifethreatening dysrhythmias is lower [1,2]. However, in adults and children with decreased myocardial function, hypovolemia, or hypoxemia, the ECG may show PVCs, PACs, ST segments changes, altered T-waves, and bradycardia [1]. These abnormalities may be produced by inadequate oxygen delivery to the myocardium and therefore serve to alarm the physician of potentially signi~cant pathophysiologic disturbances. One of the main uses for the ECG is the determination of heart rate or pulse. Most ICUs measure heart rate as a calculated time-averaged signal from the bedside ECG monitor. Heart rate is a very nonspeci~c cardiorespiratory variable. However, tachycardia may signal signi~cant and abrupt cardiovascular compromise such as hypovolemia, hypoperfusion, or diminished myocardial contractility in the critically ill patient [3]. The degree of tachycardia is roughly proportional to the severity of the cardiovascular impairment. Tachycardia is also present in association with infection, anxiety, stress, fever, exercise, pain, and anxiety. Bradycardia may occur with inferior myocardial infarction, arteriosclerotic heart disease, or severe systemic hypoxemia [3]. Heart rate may also be used during invasive hemodynamic monitoring with thermodilution pulmonary arterial catheters to calculate stroke volume and other hemodynamic variables. The ECG may be used directly or as an adjunctive test in the diagnosis of numerous diseases and conditions affecting the critically ill patient. Myocardial ischemia may occur in the critically ill patient with shock, even in the absence of primary coronary artery disease [1]. Myocardial ischemia ultimately results in T-wave inversion. Myocardial injury due to ischemia may cause elevation of the ST segment if the ischemia is transmural or ST segment depression if the ischemia is nontransmural and predominantly subendocardial [1]. Progressive ischemia results in myocardial infarction which is often manifested in the ECG by Qwave deepening and distortion, loss of R-wave voltage, and/or an increased R-wave amplitude [1]. However, ECG abnormalities characterizing myocardial ischemia and infarction are nonspeci~c. Serial ECGs should be followed to speci~cally diagnose and localize myocardial ischemia and infarction [1]. Table 1 lists characteristic changes in the ECG associated with other disease states commonly seen in critically ill patients. ECG changes observed in drug overdose or poisoning, and electrolyte and metabolic disturbances are shown in Tables 2 and 3. Abnormal cardiac rhythms may occur during critical illness at any time and are the most common mechanism of death in adults [1]. Automated arrhythmia detection algorithms are commercially available and in use in many ICUs. These algorithms ~lter the incoming ECG signal, detect and classify the QRS complex, identify ectopic events and ventricular rhythms, and generate alarms. For ventricular ectopic beats, the system has a reported sensitivity of up to 90% with a false positive rate of #1%. However, the algorithms may result in false positive alarms if the Tor P-waves are $50% of the R-wave amplitude, and are not useful for detection of aberrantly conducted beats, atrial ~brillation and _utter, and intermittent bundle branch block. In summary, the clinical utility and cost effectiveness of these and other automated forms of ECG monitoring in the ICU, such as automated ST segment monitoring [5] and signal-averaged electrocardiography (detailed in subsequent sections), have yet to be determined. The ECG waveform itself may be an important source of clinical information that has only recently