Chapter 21
Common Arrhythmias in Anesthetized Patients
Is that ECG normal?
Benjamin M. Brainard and Gregg S. Rapoport
College of Veterinary Medicine, University of Georgia, USA
- Q. What is the appropriate placement for ECG leads in a small animal?
- A. For purposes of anesthesia, the main utility of ECG monitoring is to display a rhythm strip so the anesthetist can evaluate the rate, regularity, and pattern of cardiac conduction. Classically, this is accomplished in small animal patients by following a lead II trace, with the right arm (white) lead placed in the right axilla, or on the right leg, of the patient. The left arm (black) lead may be placed on the left leg in a similar manner. The hind limb (red) lead is placed on the left hind limb or in the inguinal area. Because the ECG is a galvanometer, it will display the electrical activity of whatever is placed between the two electrodes. In the context of surgery where a limb might not be easily available (e.g., amputation), the leads can be positioned so that the heart lies between the white and red electrode (e.g., by placing a lead on the lateral thorax), or other ECG options (e.g., lead I or III) may be evaluated to find the tracing that best displays the waves of the ECG pattern and allows for diagnosis of arrhythmias during anesthesia.
- Q. What are the parts of an ECG tracing?
- A. Each ECG tracing should have a P wave (corresponding with atrial electrical activity), a QRS complex (corresponding to ventricular electrical activity), and a T wave (representing ventricular repolarization). The general appearance of these waves, in a lead II tracing, are illustrated and labeled in Figure 21.1a.
Figure 21.1 Sinus Rhythms. A. Normal Sinus Rhythm. B. 1st degree AV Block. Note the prolonged interval between the P and QRS complex, but every P wave is followed by a QRS. C. 2nd degree AV Block. Note that some P waves are not followed by a QRS and the overall rate is slow D. 3rd degree AV block. Note that the P waves occur independently and the QRS complexes have a wide and bizarre appearance with a slow rate E. Atrial Fibrillation. Note the undulating baseline, absence of discernable P waves, and the irregularly irregular pattern to the rhythm. F. ST segment depression. Note the abnormal “slurred” appearance of the ST interval.
- Q. How can you calculate the heart rate from an ECG tracing?
- A. When the ECG is run at 25 mm/s (the usual paper speed for small animal patients), each large box on the paper is equal to 1/5 of a second, and each set of 5 large boxes (usually indicated by a black mark at the base of the grid) is equivalent to 1 second of ECG. To calculate the rate, a 3 s interval (3 sets of large boxes, or three black marks) is chosen and the number of QRS complexes in that interval is counted. This number is multiplied by 20 to calculate the beats per minute. At 50 mm/s, 3 sets of large boxes is equivalent to 1.5 s and the number of QRS complexes should be multiplied by 40 to calculate beats/min. Commercially available ECG monitors with variable speeds and printer functions are available and these allow traces to be printed out and the heart rate calculated more easily in patients with extreme tachycardia or rapidly changing rhythms.
- Q. What is a step-wise method for analyzing the ECG tracing?
- A. Calculate the rate. In awake animals, tachycardia may be considered as a rate greater than 160 beats/min in a dog and greater than 200 beats/min in a cat, while bradycardia is considered a rate less than 45 beats/min in a dog, and less than 160 beats/min in cats. Due to the influence of anesthetic drugs, animals may display relative bradycardia during anesthetic events that may be considered normal or appropriate.
- Note the regularity of the QRS complexes and characterize the rhythm as regular or irregular. Patients that have respiratory sinus arrhythmia (rhythmic variation as a consequence of changes in intrathoracic pressure that occur with breathing) have a regularly irregular rhythm, in that it is irregular in a predictable way. Dogs with atrial fibrillation have an irregularly irregular rhythm, without any discernible pattern to the irregularity.
- Evaluate the tracing for P waves. Verify that a P wave precedes every QRS complex. Also, evaluate the distance between the P wave and each QRS and verify that it is equal for all complexes. In patients with AV block, the P–R interval may vary or some P waves may not correspond reliably with a QRS complex. In patients with ventricular or junctional arrhythmias, a QRS complex will occur without an associated P wave.
- Verify that there is a QRS complex for every identified P wave. QRS complexes without P waves may be ventricular in origin or they may arise from the AV junction (the portion of the conduction systemic including the AV node plus the bundle of His).
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- A. Each ECG tracing should have a P wave (corresponding with atrial electrical activity), a QRS complex (corresponding to ventricular electrical activity), and a T wave (representing ventricular repolarization). The general appearance of these waves, in a lead II tracing, are illustrated and labeled in Figure 21.1a.
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