PROCEDURE 58 • Understanding of the anatomy and physiology of the cardiovascular system, basic rhythm interpretation, and electrical safety is necessary. • Advanced cardiac life support knowledge and skills are needed. • Familiarity with principles of electrophysiology is needed. • The right ventricular (RV) leads V1R through V6R and left posterior leads V7 through V9 are unipolar leads in which the chest electrode serves as the “exploring” electrode or positive pole of the lead. These precordial leads view the heart from the vantage point of their electrode positions on the chest, similar to the standard precordial leads V1 through V6. • For recordings of RV or left posterior leads, the three limb electrodes (right arm [RA], left arm [LA], left leg [LL]) also are required to create a central terminal (negative pole); one limb electrode (right leg [RL]) serves as the ground lead and is used to stabilize the electrocardiographic (ECG) recording. • Accuracy in identification of anatomic landmarks for location of electrode sites and knowledge of the importance of accurate electrode placement are needed. Nurses must locate accurately the electrode positions for the standard 12-lead ECG because the same anatomic landmarks are used to locate the RV and left posterior leads. Accurate ECG interpretation is possible only when the recording electrodes are placed in the proper positions. Slight alterations of the electrode positions may distort significantly the appearance of the ECG waveforms and can lead to misdiagnosis.9 Reliable comparison of serial (more than two ECGs recorded at different times) ECG recordings relies on accurate and consistent electrode placement. An indelible marker is recommended for use with clear identification of the electrode locations to ensure that the same electrode locations are selected when serial ECGs are recorded. • Nurses should be aware of body positional changes that can alter ECG recordings. Serial ECGs should be recorded with the patient in a supine position to ensure that all recordings are done in a consistent manner. Side-lying positions and elevation of the torso may change the position of the heart within the chest and can change the waveforms on the ECG recording.2,3 If a position other than supine is clinically necessary, notation of the altered position should be made on the tracing. • Right precordial leads are useful in diagnosis of a RV myocardial infarction (MI). v. These RV leads are important because they enable clinicians to identify patients with an acute MI who are at high risk of atrioventricular (AV) conduction disturbances,4 to predict the site of coronary artery occlusion, and to guide appropriate hemodynamic monitoring and interventions. Left posterior leads are used to aid in the detection of posterior wall MI and to facilitate timely reperfusion treatment. Recording of left posterior leads also can help in the differential diagnosis of tall R waves in lead V1 and V2.6 • Nurses should be able to operate the 12-lead ECG machine. Calibration of 1 mV equals 10 mm and paper speed of 25 mm/s are standards used in clinical practice. For ST-segment analysis, filter settings of 0.05 to 100 Hz are recommended by the American Heart Association.12 Any variation used for particular clinical purposes should be noted on the tracing. Specific information regarding configuring the ECG machine, troubleshooting, and safety features is available from the manufacturer and should be read before use of the equipment. • Nurses should be able to interpret recorded ECGs for the presence or absence of myocardial ischemia, MI, and dysrhythmias so that patients can be treated appropriately. Patients with an acute inferior MI and RV involvement, determined by ST-segment elevation in the right precordial leads, are at high risk for high-degree AV block. Nurses should monitor patients closely for conduction disturbances and anticipate the need for temporary pacing. Patients with RV infarction are prone to hypotension and shock that responds to treatment with fluid resuscitation. • Indications for recording a right precordial ECG are as follows: v. Evaluation and treatment of suspected acute MI, especially patients with inferior wall MI (ST-segment elevation in leads II, III, and augmented vector foot or aVF) v. Evaluation of the risk for AV node conduction disturbances and anticipation of treatment plans v. Prediction of the site of coronary artery occlusion (RV infarction occurs with proximal right coronary artery [RCA] occlusion) v. Determination of the risk of “volume-responsive” shock, in which case fluid resuscitation is warranted and vasodilators (e.g., nitroglycerin) are contraindicated • Indications for recording a left posterior ECG are as follows: v. Evaluation and treatment of acute or suspected MI, especially patients with isolated ST-segment depression in the precordial leads V1 through V3 and patients with a nondiagnostic ECG v. Presence of chest pain or anginal-equivalent symptoms (e.g., jaw, left shoulder or arm discomfort, or shortness of breath) or ST-segment depression in the left precordial leads V1 through V3 after percutaneous coronary interventions of the left circumflex artery v. Any of these ECG characteristics indicative of posterior MI in lead V1: R waves greater than or equal to 6 mm in height, R wave greater than or equal to 40 ms in duration, R/S ratio (R wave amplitude in mm over S wave amplitude in mm) greater than or equal to 1, or S wave less than or equal to 3 mm. In lead V2: R wave greater than or equal to 15 mm in height, R wave greater than or equal to 50 ms in duration, R/S ratio greater than or equal to 1.5, or S wave less than or equal to 4 mm.11 v. Differentiation of true posterior MI from other conditions that can cause tall R waves in lead V1, such as RV hypertrophy, right bundle-branch block, Wolff-Parkinson-White syndrome, and ventricular septal hypertrophy
Extra Electrocardiographic Leads: Right Precordial and Left Posterior Leads
PREREQUISITE NURSING KNOWLEDGE
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