Electrocardiography in Exotic Animal Species

Key Points

  • Electrocardiography monitors the electrical activity of cardiac muscle cells and is particularly useful during long-term anesthesia when disturbances in acid-base and electrolyte balance can lead to arrhythmias.
  • Alligator clips are generally not attached directly to small patients or in species with delicate skin. Instead clips are attached to the tips of small-gauge hypodermic needles or stainless-steel suture that has been passed through the skin and subcutaneous space.
  • Adhesive electrocardiogram (ECG) patches or button electrodes can be placed on the foot pads of small mammals or directly onto the skin of mammals or large, smooth reptiles.
  • The normal ECG tracing in the bird can resemble ventricular tachycardia, primarily because of a large negative S wave.
  • The normal reptile ECG tracing can include an SV wave, which represents depolarization of the sinus venosus, low amplitude waveforms, and longer ST and QT intervals.
  • Always use caution when using ECG as an indicator of life in reptiles as the heart can continue to contract for long periods of time following death. Always use an additional monitoring technique to confirm circulation like capnography or Doppler ultrasound flow.
  • This article is part of a series on anesthetic monitoring in exotic animal patients. Additional topics available include:  blood pressure, capnometry, pulse oximetry, and monitoring vital signs.

Introduction

The electrical activity of cardiac muscle cells can be monitored by an electrocardiogram (ECG) tracing.5,15 The ECG tracing consists of three primary complexes:  P wave, QRS complex, and T wave, which represent atrial depolarization and ventricular depolarization, respectively (Fig 1). Electrocardiography is particularly useful during long-term anesthesia when disturbances in acid-base and electrolyte balance can lead to arrhythmias or during procedures in which disturbances in cardiac function or rhythm can be anticipated, such as thoracic surgery.5

The classic ECG tracing consists of a P wave (atrial depolarization), QRS wave (ventricular depolarization), and T wave (ventricular repolarization)

Figure 1. The classic ECG tracing consists of a P wave (atrial depolarization), QRS wave (ventricular depolarization), and T wave (ventricular repolarization). Illustration from Anatomy & Physiology, Connexions Web site via Wikimedia Commons.

Veterinary instruments that accurately monitor rapid heart rates and are capable of displaying low-voltage ECG signals are best in small exotic animals (Fig 2).5,7 Ideally, the ECG should also have freeze capability and recording speeds up to 100 mm/s.7,8,9

Electrocardiography tracing in a rat

Figure 2. Electrocardiogram tracing in a rat (Rattus norvegicus). Photo credit: Katrina Lafferty, CVT, VTS. Click image to enlarge.

Exotic companion mammals

Due to the delicate nature of exotic mammal skin, alligator clips are not attached directly to the skin but instead the clips are attached to the tips of small-gauge hypodermic needles or stainless-steel suture that has been passed into the subcutaneous space and through the skin.6 Alternatively, adhesive ECG patches or button electrodes can be placed on the foot pads. Large patches can easily be cut down to size but take care as the adhesive can be irritating to delicate skin.5

The ECG can be used to detect and diagnose arrhythmias and conduction abnormalities:

  • A pronounced sinus arrhythmia is often observed in the normal ferret (Mustela putorius furo). Second degree atrioventricular block can also be a normal finding in ferrets.14 Sinus tachycardia, and atrial or ventricular premature complexes are commonly seen in ferrets with cardiac disease. Sinus bradycardia may develop secondary to hypoglycemia and insulinoma.
  • Rabbits (Oryctolagus cuniculus) can present with cardiovascular concerns related to thymoma, congestive heart failure, or valvular disease. An ECG can be used to detect abnormal rhythms associated with these conditions, including atrial fibrillation, ventricular premature complexes, ventricular tachycardia, and supraventricular tachycardia.11

Birds

Although the rapid heart rate of the bird can challenge most ECG equipment, electrocardiograms have been described in a number of avian species.2,4,5 A bipolar ECG reading can be obtained by placing the right atrial lead (sometimes labeled “right arm” or RA) on the right side of the cranial sternum and the left atrial lead on the left side of the caudal sternum.16 Alligator clips are typically attached to needles or stainless-steel suture as described above in exotic companion mammals.17

The normal ECG tracing in the avian patient can have the appearance of ventricular tachycardia, primarily because of the large negative S wave (Fig 3).13 This QRS morphology is caused by the high density of sympathetic and parasympathetic nerve fibers in the avian ventricles and atria when compared to mammals.18

A monitoring screen with electrocardiography from an anesthetized whooping crane

Figure 3. Monitoring screen from an anesthetized whooping crane (Grus americana). Note the prominent S wave and rapid rate (arrow), normal for many avian species. Photo credit:  Katrina Lafferty, CVT, VTS. 

Reptiles

The thick, scaly skin of snakes and lizards limits the sensitivity of ECG leads. Effective signal conduction requires alligator clips be attached to needles or stainless-steel suture.17 Adhesive ECG patches can also be placed directly on smooth-scaled reptiles, like large snakes and chelonians (Fig 4).3 Electrocardiograph leads can be placed in either a standard three-limb or base-apex configuration.17 The latter simply requires the “right arm” lead be placed 10-12 scutes cranial to the heart (one-third to one-quarter down the length of the body) and the “left arm” lead is placed 10-12 scutes caudal to the heart.17 Esophageal base-apex ECG lead configurations are also commercially available that can be combined within the tubing of an esophageal stethoscope.17

Electrocardiography patches placed on an Aldabra tortoise

Figure 4. Electrocardiogram patches placed on an Aldabra tortoise (Aldabrachelys gigantea). Photo credit: Katrina Lafferty, CVT, VTS. Click image to enlarge.

The use of electrocardiography in reptiles is hampered by a lack of normal values, however the typical ECG tracing may include an SV wave that precedes the P wave.1,3,12 The SV wave represents depolarization of the sinus venosus, which serves as a pacemaker of the three-chambered heart in most reptiles.1 The reptilian ECG also has relatively low amplitude waveforms, a much longer repolarization phase (longer ST and QT intervals), and a considerably shorter TP interval.17,10

Although the presence of an ECG waveform does not rule out cardiac arrest due to pulseless electrical activity, the reptile heart often continues to contract for long periods of time following death, so use caution interpreting ECG findings as an indication of life.3 Always use an additional monitoring technique that ensures spontaneous circulation, such as capnography or Doppler ultrasound flow.15

Summary

Electrocardiography can be used to detect and diagnose arrhythmias and conduction abnormalities, particularly during long-term anesthesia. When monitoring the ECG in exotic animals, alligator clips are generally not attached directly to the patient, particularly those of small size or with delicate skin. Instead clips are attached to the tips of small-gauge hypodermic needles or stainless-steel suture loops that have been passed through the skin and subcutaneous space. Adhesive ECG patches can be used in mammals or large, smooth reptile patients. The normal reptile ECG tracing may include an SV wave, low amplitude waveforms, and a relatively long repolarization phase (represented by longer ST and QT intervals). The normal avian ECG tracing can resemble ventricular tachycardia, primarily because of a large negative S wave.

 

References