Interpretation of the Electrocardiogram in Small Animals

Nick A. Schroeder

• Editorial: Wiley-Blackwell
• Año: 2021
• ISBN:
  9781119763109
• Edición: 1
• Categoría: Medicine
• Categoría: Veterinary Medicine

Interpretation of the Electrocardiogram in Small Animals

Explore this easy-to-follow clinical manual on how to interpret electrocardiograms in canine and feline patients

Interpretation of the Electrocardiogram in Small Animals delivers a simplified and accessible approach to electrocardiography interpretation for veterinarians treating dogs and cats. This practical manual covers a comprehensive list of arrhythmias and associated phenomena using an easy-to-follow format, with hundreds of electrocardiograms and ladder diagrams supporting the text.

The book focuses on the clinical applications of electrocardiographic interpretation, with less emphasis on physics and subcellular mechanisms, making the book a useful patient-side resource. Interpretation of the Electrocardiogram in Small Animals offers readers an up-to-date understanding and knowledge of common and rare phenomena alike.

Beginning with a description of the single cardiac cycle as illustrated by surface EKG, the book moves on to describe variations on the P-QRS-T complex that occur in response to chamber enlargement and hypertrophy. Readers will also benefit from the inclusion of:

• A detailed discussion of aberrancy and its differentiation from ventricular ectopy
• A thorough exploration of arrhythmias and conduction disorders including escape mechanisms, atrioventricular block, extrasystoles, accelerated rhythms, tachycardias, flutter and fibrillation
• An examination of pacemaker basics, as well as a few common abnormalities and pacemaker problems, including undersensing, oversensing, and failure to capture

Interpretation of the Electrocardiogram in Small Animals is a convenient, one-stop reference for the interpretation of electrocardiography in small animals for veterinary students, residents, and specialists as well as for small animal general practitioners.

Part I
The P-QRS-T Complex

Interpreting an electrocardiogram (EKG) is a frequently confusing task to the new veterinarian. Some EKGs are very unclear and can really be interpreted in different ways, depending on the cardiologist you talk to. If you look at an EKG the same way, every time, in a systematic manner, you will be able to figure most of them out.

Novice veterinarians often want to be able to glance at an EKG and simply spit out a diagnosis. This is a bad habit and a good way to miss subtle abnormalities. If you take the time to sit down and do everything you are supposed to, there is a much better chance you will figure it out. Before you can determine if an EKG is abnormal or not, you need to know what is normal. This cannot be overstated. First of all, you need to know just what an EKG is. An EKG is a two-dimensional representation of electrical events within the three-dimensional heart that a machine senses from the skin via the use of electrodes and leads. A lead is an electrical perspective. It is like looking at the shadow of something using a light. If you change where the light is, the shadow changes and your perspective changes.

The P-QRS-T complex represents one cardiac cycle. A lot of information may be gleaned from the examination of a single complex. Axis changes, enlargement patterns, and abnormalities secondary to physiologic/metabolic pathology may all manifest on the EKG. Equally as important, it is imperative to remember that a normal EKG does not necessarily exclude serious heart disease. The simple act of calculating a heart rate from the EKG is an oft undervalued skill and gives the clinician critical information regarding the hemodynamic status of the patient in question.

The P-QRS-T: Basics

• Leads
• Waveforms/intervals
• Amplitudes
• Paper speeds
• Mean electrical axis
• False poling
• Situs inversus

The P-QRS-T: Some Abnormalities

• Right atrial abnormality/P pulmonale
  • P pseudo-pulmonale
• Left atrial abnormality/P mitrale
  • Interatrial conduction defect
  • P pseudo-mitrale
• Biatrial abnormality/bilateral atrial abnormality/P biatriale
  • P pseudo-biatriale
• Right ventricular enlargement/hypertrophy
• Left ventricular enlargement/hypertrophy
• Biventricular enlargement
• Horizontal QRS axis
• Vertical QRS axis
• Indeterminate QRS axis
• Early repolarization

The P-QRS-T: Trouble

• P-R segment changes
• Q-T segment changes
  • The prolonged QRS
  • Hypokalemia
  • Other changes
    • Calcium
    • Magnesium
    • Q-Ti prolongation
    • Hypothermia
  • Digoxin effects
  • Cushing's response
  • S-T segment elevation/depression
    • Myocardial infarction/ischemic changes
• Low-voltage QRS
  • Electrical alternans
  • Pericarditis

1
The P-QRS-T: Basics

CHAPTER MENU

• Leads
• Waveforms/intervals
  • P-R interval (P-Ri)
  • S-T segment
  • Q-T interval (Q-Ti)
• Amplitudes
• Paper speeds
• Mean electrical axis
• False poling
• Situs inversus

Leads

In the frontal plane, that is, the plane that divides the dorsal portion of the animal from the ventral portion, there are six leads that are used to generate an electrocardiogram (EKG, ECG). These are the leads used to create a mean electrical axis (MEA) for the QRS complex. Leads I, II, and III are the bipolar limb leads, and the augmented (unipolar) leads are aVL, aVR, and aVF. The lead attached to the R rear limb (color-coded green) is simply a ground and can really be placed anywhere on the animal. The electrodes may be attached anywhere on the limb, as long as they are positioned approximately equidistant from the heart. The further the leads are placed toward the paws, the greater the chance of baseline artifact from movement. The augmented leads use two of the other leads as one, with the negative pole intermediate between them. Electrodes are placed at the right (R, color-coded white) and left (L, color-coded black) forelimbs and the left rear limb (LL, color-coded red) to get these leads. Leads II, III, and aVF are termed the “inferior” leads (inferior being analogous to caudal in small animals). Leads I and aVL are termed the left lateral leads. The V leads (precordial leads, the left of which are the only ones used with any frequency) are along the horizontal plane – a human term, which is why it is confusing in small animals (the analogue is the transverse plane) – which divides the cranial portion of the animal from the caudal portion. The point at which the QRS complexes change from a rS to Rs pattern (isoelectric RS wave) in the precordial leads is known as the transition zone (normally in V3 or V4).

• Lead I: the negative electrode at the R forelimb, the positive electrode is the L fo...