Electrocardiogram Evaluation

Chapter 211 Electrocardiogram Evaluation



Matthew S. Mellema, DVM, PhD



KEY POINTS



• The electrocardiogram (ECG) is an extremely useful and cost-effective monitoring tool.

• ECG monitoring is indicated for nearly all critically ill patients.

• Rather than a limited study of multiple leads, continuous monitoring of a single lead is the basis of most ECG monitoring in the critically ill patient.

• ECG interpretation should be systematic and thorough to gain the most benefit from its use.

• Trends in ECG alterations may alert the clinician to changes in the patient’s condition even when the absolute values of those parameters still fall within the normal ranges.

• Electrolyte abnormalities, hypoxemia, effusions, and pain may cause acute detectable ECG changes without necessarily altering the underlying rhythm.


INTRODUCTION


Disorders of cardiac rhythm and conduction are encountered frequently in critically ill veterinary patients. Arrhythmias may be encountered in patients with primary cardiac disease or may be one manifestation of systemic illness. The severity of rhythm and conduction disturbances can range from inconsequential to acutely life threatening and can progress rapidly from one extreme to the other in some patients.


The electrocardiogram (ECG) is the diagnostic and monitoring tool used to confirm, detect, and define cardiac conduction and rhythm disturbances. In addition, the ECG provides the clinician with continuous real-time data regarding the patient’s heart rate and rhythm, which can be informative even in the absence of gross abnormalities. This chapter will focus on the use of the ECG as a monitoring tool. For details of the recognition and treatment of specific cardiac rhythm disorders the reader is referred to other sections of this book (see Chapters 45 to 47, Bradyarrhythmias and Conduction Abnormalities, Supraventricular Tachycardia, and Ventricular Tachyarrhythmias, respectively).



INDICATIONS


The ECG is an extraordinarily cost-effective and useful monitoring tool. In veterinary intensive care the ECG may be second only to serial, well-performed physical examinations in terms of its usefulness in overall patient monitoring. Although a brief multiple lead evaluation of a patient’s ECG is an important part of any diagnostic workup of suspected intrathoracic disease, in the intensive care setting continuous monitoring of cardiac rate and rhythm (typically one or a few leads at a time) is of greatest utility.


Some might argue that all critically ill animals warrant continuous ECG monitoring, and such statements may be true. However, some patients may have conditions that preclude continuous ECG monitoring and may instead mandate that intermittent evaluations be performed instead. One example of such a patient is a dog with central nervous system disease that is exhibiting circling. In this patient’s case, ECG lead wires may represent a significant tangling or tripping hazard to the patient. Also, patients with diffuse dermatologic disease may not tolerate typical lead placement. With such exceptions in mind, one can state that most critically ill patients may benefit from continuous ECG monitoring. In particular, any patient with an irregular rhythm, increased heart rate, or decreased heart rate detected on physical examination should have ECG monitoring.



ELECTROCARDIOGRAPHIC PRINCIPLES


During depolarization and repolarization of the myocardium, the heart generates an electrical field that can be detected at the surface of the body by ECG leads. The system used in clinical practice consists of a series of positive and negative leads that when placed around the heart (roughly in the frontal plane either on the trunk or the limbs) will record complexes associated with the various phases of the cardiac electrical cycle. The ECG detects the sum of all the electrical impulses generated by the individual myocytes during each cycle. When a positive deflection is seen on the ECG tracing it signifies that the sum of the heart’s electrical impulses was moving toward the positive electrode of that lead. A negative deflection signifies that the sum of the impulse was moving away from the positive electrode at that time. Impulses traveling perpendicular to an electrode do not cause a deflection in the tracing. When these deflections are plotted over time, a series of waveforms (P, QRS, and T) are detected.


The standard leads used in veterinary practice include the three bipolar leads (I, II, and III) and the augmented leads (aVR, aVL, and aVF). Each lead can produce a tracing of the heart’s electrical activity from a different angle. In combination, the information obtained from multiple leads can aid in the diagnosis of rhythm and conduction disturbances. When measurements of the P-QRS-T waveforms are performed, these should be done using a lead II tracing.



TECHNIQUE


Many lead attachment systems are available. When selecting a system it is important to bear in mind that high-quality ECG recordings require good contact between the leads and the patient’s skin. If commercially available self-adhesive lead pads are to be used, then it is advised that the hair be clipped and the skin cleaned and dried before application. Generally, lead pads placed over the lateral thorax on each side and a third pad in the left inguinal region is sufficient to obtain good quality tracings. Alligator clips are not advised for continuous monitoring, because their prolonged use can damage the patient’s skin and cause discomfort. Once the lead pads are placed and lead wires attached, it is often helpful to place a mesh stockinette shirt around the patient’s trunk so that all leads can be collected into a single “stalk” exiting the mesh shirt dorsally. This can enhance patient comfort, prevent lead detachment, and reduce obstacles to patient repositioning.


When selecting a lead to display during continuous ECG monitoring one should select the one that the caregiver believes provides the most easily recognizable waveforms. Lead II is used for rhythm evaluation in cardiac examinations, because in most patients this lead lies well within the mean electrical axis of the heart and will produce easily recognizable waveforms. However, in the critically ill patient the caregiver may need to evaluate several leads to find the one which gives the most robust signal.


If one is relying on the monitor to calculate heart rates automatically, one will often get more accurate readings if a lead is picked in which the QRS amplitude is markedly different from that of the P and T waves (otherwise double or triple counting may occur, giving erroneously high heart rate readings). It should always be noted in the patient’s record which lead is being monitored. It is essential that the clinician and nursing staff bear in mind that the normal values for canine and feline ECGs are obtained from still animals in right lateral recumbency. During continuous monitoring the patients are seldom, if ever, in the ideal position and changes in waveform amplitude are to be expected relative to normal values. The utility of the continuous ECG is predominantly in monitoring heart rate and rhythm; however, it can also alert the clinician as to whether a standardized recording of all six leads and a rhythm strip should be performed.

Related posts:

  1. Deteriorating Mental Status
  2. Hyperthermia and Fever
  3. Ventilator-Associated Lung Injury
  4. Allergic Airway Disease in Dogs and Cats and Feline Bronchopulmonary Disease

Stay updated, free articles. Join our Telegram channel
Tags:
Sep 10, 2016 | Posted by in SMALL ANIMAL | Comments Off on Electrocardiogram Evaluation

Full access? Get Clinical Tree
Get Clinical Tree app for offline access