P Wave

Under normal circumstances, the sinoatrial (SA) node has the most rapid rate of spontaneous depolarization and therefore is the dominant cardiac pacemaker. From the SA node, the impulse spreads through the right and left atria. Specialized tracts can conduct the impulse to the atrioventricular (AV) node, but they are not essential. On the ECG, depolarization of the atria is represented by the P wave. The initial depolarization primarily involves the right atrium and predominantly occurs in an anterior, inferior, and leftward direction. Subsequently, it proceeds to the left atrium, which is located in a more posterior position.

PR Interval

Once the wave of depolarization has reached the AV node, a delay is observed. The delay permits contraction of the atria and allows supplemental filling of the ventricular chambers. On the ECG, this delay is represented by the PR interval.

QRS Complex

After passing through the AV node, the electrical impulse is conducted along the ventricular conduction pathways, consisting of the common bundle of His, the left and right bundle branches, the distal bundle branches, and the Purkinje fibers. The QRS complex represents the progress of the depolarization wave through this conduction system. After terminal depolarization, the ECG normally returns to baseline.

ST Segment and T Wave

Repolarization of the ventricles, which begins at the end of the QRS complex, consists of the ST segment and T wave. Ventricular depolarization occurs along established conducting pathways, but ventricular repolarization is a prolonged process that occurs independently in every cell. The T wave represents the uncanceled potential differences of ventricular repolarization. The junction of the QRS complex and the ST segment is called the J junction. The T wave is sometimes followed by a small U wave, the origin of which is unclear. Prominent U waves are characteristic of hypokalemia (as well as hypothermia, hypomagnesemia, and hypocalcemia) and sometimes also observed after cerebrovascular accidents. Very prominent U waves may be seen in patients taking medications such as sotalol or one of the phenothiazines. Negative U waves may appear with positive T waves, an abnormal finding that has been noted in left ventricular hypertrophy and myocardial ischemia.

Standard Limb and Precordial Leads

The small electric currents produced by the electrical activity of the heart spread throughout the body, which behaves as a volume conductor, allowing the surface ECG to be recorded at any site. The standard leads are bipolar leads because they measure differences in potential between two electrodes. The electrodes are placed on the right arm, the left arm, and the left leg. The leads are formed by the imaginary lines connecting the electrodes, and the polarities correspond to the conventions of Einthoven’s triangle. They are labeled leads I, II, and III. By convention, lead I is formed by connecting the right arm and left arm electrodes, with the left arm being positive; lead II is formed by connecting the right arm and left leg, with the left leg being positive; lead III is formed by connecting the left arm and left leg, with the left leg being positive. If the three electrodes of the standard leads are connected through resistances of 5000 ohms each, a common central terminal with zero potential is obtained. When this common electrode is used with another active electrode, the potential difference between the two represents the actual potential. On a standard 12-lead ECG, three unipolar limb leads usually are recorded: aVR, aVL, and aVF. The a indicates that the limb leads are augmented and were obtained via Goldberger’s modification, in which the resistors are removed from the lead wires and the exploring electrode is disconnected from the central terminal. Goldberger’s modification produces larger voltage deflections on the ECG.

Additional information on the heart’s electrical activity is obtained when electrodes are placed closer to the heart or around the thorax. In the precordial lead system ( Fig. 13-2 ), the neutral electrode is formed by the standard leads, and an exploring electrode is placed on the chest wall. The ECG normally is recorded with the exploring electrode in one or more of six precordial positions. Each lead is indicated by the letter V followed by a subscript numeral from 1 to 6, which indicates the location of the electrode on the chest wall.

Multiple-lead electrocardiographic system consisting of four extremity electrodes: right arm ( RA ), left arm ( LA ), right leg ( RL ), left leg ( LL ), and V ₅ leads.

Three-Electrode System

As the name implies, the three-electrode system uses only three electrodes to record the ECG. In such a system, the ECG is observed along one bipolar lead between two of the electrodes; the third electrode serves as a neutral lead, and a selector switch allows the user to alter the designation of the electrodes. Three ECG leads can be examined in sequence without changing the location of the electrodes. Although the three-electrode system has the advantage of simplicity, its use is limited in the detection of myocardial ischemia because it provides a narrow picture of myocardial electrical activity.

Modified Three-Electrode System

Numerous modifications of the standard bipolar limb lead system have been developed. Some of these are displayed in Figure 13-3 . They are used in an attempt to maximize P-wave amplitude for the diagnosis of atrial arrhythmias or to increase the sensitivity of the ECG for the detection of anterior myocardial ischemia. In clinical studies, these modified three-electrode systems have been shown to be at least as sensitive as the standard V ₅ lead system for the intraoperative diagnosis of ischemia.

Modified precordial lead arrangements. RA, right arm; LA, left arm; LL, left leg.

(From Thys DM, Kaplan JA, editors: The ECG in anesthesia and critical care . New York, 1987, Churchill Livingstone.)

Five-Electrode System

The use of five electrodes permits the recording of the six standard limb leads—I, II, III, aVR, aVL, and aVF—as well as one precordial unipolar lead. In general, the unipolar lead is placed in the V ₅ position, along the anterior axillary line in the fifth intercostal space. With the addition of only two electrodes to the ECG system, up to seven different leads can be monitored simultaneously. This allows several areas of the myocardium to be monitored for ischemia, which is helpful in the establishment of a differential diagnosis between atrial and ventricular arrhythmias.

In 1976, Kaplan and King suggested monitoring lead V ₅ as the best choice for the detection of intraoperative ischemia. London, and colleagues demonstrated that in high-risk patients undergoing noncardiac surgery, when a single lead was used the greatest sensitivity was obtained with lead V ₅ (75%), followed by lead V ₄ (61%). Combining leads V ₄ and V ₅ increased the sensitivity to 90%, whereas the standard combination of leads II and V ₅ produced a sensitivity of only 80%. They also suggested that if three leads (II, V ₄ , and V ₅ ) could be examined simultaneously, the sensitivity would increase to 98%.

Recently, Landesberg and colleagues investigated the usefulness of analyzing data obtained from continuous online 12-lead ECG monitoring for the detection of myocardial ischemia. During 11,132 patient-hours of monitoring, the investigators reported that V ₄ was most sensitive to ischemia (83.3%), followed by V ₃ and V ₅ (75% each). Combining two precordial leads increased the sensitivity for detecting ischemia (97.4% for V ₃ + V ₅ and 92.1% for either V ₄ + V ₅ or V ₃ + V ₄ ) and infarction (100% for V ₄ + V ₅ or V ₃ + V ₅ and 83.3% for V ₃ + V ₄ ). They found that the baseline preanesthesia ST segment was above isoelectric in V ₁ through V ₃ and below isoelectric in V ₅ through V ₆ . Lead V ₄ was closest to the isoelectric level on the baseline ECG, rendering it most suitable for ischemia monitoring. The authors recommended that two or more precordial leads were necessary to approach a sensitivity of greater than 95% for the detection of perioperative ischemia and infarction.

However, in an editorial entitled “Multilead Precordial ST-segment Monitoring: ‘The Next Generation?’” London concluded that multilead precordial ST-segment monitoring is not practical. It presents a significant logistical problem, particularly if a patient needs to be mobilized quickly. It is also likely to result in a high rate of false-positive responses and artifacts. Instead, London recommended the use of true V ₄ or V ₅ leads, control of heart rate and pain, and the use of β-blockers as tolerated for all patients at risk (diabetics or patients with left ventricular hypertrophy).

Invasive Electrocardiography

The electrical potentials of the heart can be measured not only from a surface ECG but also from body cavities adjacent to the heart (esophagus or trachea) or from within the heart itself.

Esophageal Electrocardiography

The concept of esophageal ECG is not new, and numerous studies have demonstrated the usefulness of this approach in the diagnosis of complicated arrhythmias. A prominent P wave usually is displayed in the presence of atrial depolarization, and its relationship to the ventricular electrical activity can be examined. The esophageal electrodes are incorporated into an esophageal stethoscope and are welded to conventional ECG wires ( Fig. 13-4 ). To record a bipolar esophageal ECG, the electrodes are connected to the right and left arm terminals, and lead I is selected on the monitor. In one study of 20 cardiac patients, 100% of atrial arrhythmias were correctly diagnosed with the esophageal lead (intracavitary ECG was used as the standard); lead II led to a correct diagnosis in 54% of the cases, and V ₅ led to a correct diagnosis in 42% of the cases. In addition, the esophageal ECG may be helpful in the detection of posterior wall ischemia because of its proximity to the posterior aspect of the left ventricle. Jain described the use of esophageal ECG and compared it with surface ECG (SECG) in patients undergoing coronary bypass grafting. He found that the recognition and measurement of all the PQRST waves could be improved and automated by simultaneous use of esophageal ECG and SECG. The P-wave amplitude was greater in esophageal ECG than in SECG, which might facilitate the identification of supraventricular versus ventricular arrhythmias. ST-segment deviation in the unipolar esophageal ECG was not suitable for the routine detection of ischemia because of excessive noise.

The cardioesophagoscope. The esophageal leads are embedded in plastic. The electrocardiogram wires are connected to the right arm and left arm leads, and lead I is selected on the monitor.

To minimize the risk of esophageal burn injury, an electrocautery protection filter capable of filtering radio frequencies greater than 20 kHz should be inserted between the ECG cable and the esophageal lead.

Intracardiac Electrocardiography

For many years, long central venous catheters filled with saline have been used to record the intracardiac ECG (IC-ECG). To best illustrate an IC-ECG, 1) attach a plastic adapter to the hub of the central venous catheter (CVC), whose most distal port is at the low superior vena cava or superior region of the right atrium; 2) instill saline via a syringe needle inserted through the rubber head of the adapter, ensuring by previous aspiration that no blood clots are present and air bubbles are cleared; 3) connect the needle of the syringe with one of the six precordial cables of a standard ECG machine using an alligator clamp; and 4) record a standard ECG, which will show the IC-ECG substituting for the selected V lead. Lead V ₁ is used most often, but any of leads V ₁ through V ₆ can be used.

Chatterjee and colleagues described the use of a modified balloon-tipped flotation catheter for recording intracavitary electrograms. The multipurpose pulmonary artery catheter presently available has all the features of a standard pulmonary artery catheter. In addition, three atrial and two ventricular electrodes have been incorporated into the catheter ( Fig. 13-5 ). These electrodes allow the recording of intracavitary ECGs and the establishment of atrial or AV pacing. The diagnostic capabilities of this catheter are great because atrial, ventricular, and AV nodal arrhythmias and conduction blocks can be demonstrated. The large voltages obtained from the intracardiac electrodes are relatively insensitive to electrocautery interference and are therefore useful for intraaortic balloon pump triggering. Other pulmonary artery catheters have ventricular and atrial ports that allow passage of pacing wires. These catheters also can be used for diagnostic purposes or for therapeutic interventions (pacing).

The multipurpose pacing pulmonary artery catheter. Three atrial and two ventricular electrodes are shown.

Benzadon and colleagues compared the amplitude of the P wave obtained by IC-ECG with those of the P waves obtained by esophageal ECG and SECG. They found that IC-ECG and esophageal ECG made it possible to register P waves larger than those registered by SECG but reported no difference between IC-ECG and esophageal ECG.

Tracheal Electrocardiography

Tracheal ECG allows monitoring when it is impractical or impossible to monitor the SECG. The tracheal ECG consists of a standard tracheal tube in which electrodes have been embedded. In a recent report, a tracheal tube was described with two coiled-wire stainless steel electrodes embedded in the tube’s cuff ( Fig. 13-6 ). The same safety precautions as for esophageal ECG should be followed for tracheal ECG.

Schematic representation of an endotracheal tube with cuff electrodes, spiral reference electrode, and electrocardiograph ( ECG ) amplifier connector.

(From Hayes JK, Peters JL, Smith KW, et al: Monitoring normal and aberrant electrocardiographic activity from an endotracheal tube: comparison of the surface, esophageal, and tracheal electrocardiograms. J Clin Monit 1994;10:81-90.)