The physiological stress of exercise elicits a predictable cascade of responses on the ECG. Analysis of the ECG during and postexercise should always include the following:
- Heart rate and the relationship with exercise intensity
- Heart rhythm
- Measurements and morphology including QRS complexes, ST-segment changes, and QT interval
The Heart Rate and Workload Relationship
A consistent and reproducible linear relationship exists between heart rate (HR) and workload (heart rate increases linearly with exercise intensity up to the maximum heart rate, or HRmax). By plotting heart rate against workload, the practitioner can observe the chronotropic response of the heart. A sudden increase in heart rate during or postexercise may indicate the development of a tachyarrhythmia. In contrast, a rapid fall in heart rate immediately following exercise, together with a dramatic fall in blood pressure, may precipitate presyncope or syncope associated with a vasovagal response. A failure of heart rate to rise or an abnormally slow increase during exercise (termed chronotropic incompetence) may indicate electrical conduction pathway disease. An abnormal heart rate response during recovery is a strong predictor of all-cause mortality in a clinical population (meaning they have been referred). (Apparent chronotropic incompetence may also be drug induced—for example, beta-blockers or non-dihydropyridine calcium channel antagonists).
The increase in heart rate during exercise causes a shortening of diastole and systole. Despite these changes, sinus rhythm is maintained in the normal heart. Recognition of abnormal rhythms is important in identifying brady- and tachyarrhythmias during exercise.
A number of expected ECG changes occur during exercise in the normal heart (see the section, Expected ECG Changes in the Normal Heart). Differentiating normal from abnormal changes assists in the diagnosis of underlying cardiovascular disease.
Exercise-induced arrhythmias can be determined by answering similar questions to those posed when evaluating an ECG at rest:
- What is the atrial rhythm? In the normal heart, P waves should be clearly identifiable because they occur prior to every QRS complex. Identification of a P wave may be difficult at high heart rates during exercise because the P wave may be superimposed on the T wave of consecutive beats.
- What is the ventricular rhythm? In the normal heart the duration of the PR interval and the QRS complex are shortened.
- Is the AV conduction normal? QRS complexes should always be preceded by a fixed PR interval.
- Are there any unusual complexes? The morphology of some ECG parameters may change throughout exercise in a predetermined fashion (discussed later); however, P waves, QRS complexes, and T waves will have the same nomenclature in a single lead at any given time.
- Is the rhythm dangerous? Dangerous rhythms that indicate the need to stop exercise immediately during stress testing include ventricular fibrillation (VF), sustained ventricular tachycardia (VT), and ST-segment elevation (≥1 mm) in leads without diagnostic Q waves (other than V1 or aVR). Relative ECG stop test indicators include ST or QRS changes such as excessive ST-segment depression (>2 mm of horizontal or downsloping ST-segment depression) or marked axis shift; arrhythmias other than VT including multifocal PVCs, triplets, supraventricular tachycardia (SVT), heart block, and bradyarrhythmias; and the development of bundle branch block (see table 5.1 on p. 100 for a full listing of exercise stress test stop test indicators).