What are the criteria and implications of a persistent S wave on an electrocardiogram (ECG)?

Persistent S Wave on ECG: Criteria and Implications

Persistent S waves on ECG are most commonly associated with right ventricular hypertrophy, pulmonary embolism, or left bundle branch block, and require evaluation in the clinical context of the patient's presentation.

Definition and Recognition of Persistent S Waves

• A persistent S wave refers to the presence of an S wave (negative deflection after the R wave) across multiple leads where it would not normally be expected, particularly in lateral leads (I, aVL, V5, V6) 1
• In normal ECGs, S waves typically diminish or disappear in the lateral leads, so their persistence across these leads is considered abnormal 1
• The depth and width of the S wave are important characteristics to note, as deeper S waves (>1mm) in lateral leads are more likely to be pathological 1

Clinical Significance and Pathological Associations

Pulmonary Embolism

• New onset S wave in lead I is a recognized finding in acute pulmonary embolism 2
• S wave typically disappears after successful treatment (such as thrombolysis), making it a potentially useful marker for both diagnosis and monitoring response to therapy 2
• This finding is often accompanied by other ECG changes such as T wave inversions in right precordial leads 1

Right Ventricular Hypertrophy (RVH)

• Persistent deep S waves in leads I, V5, and V6 are characteristic findings in RVH 1
• Pediatric criteria for RVH include age-specific values for S wave depth in V6 (ranging from 10mm in infants to 4mm in children >5 years) 1
• In adults with suspected RVH, S waves in V5 or V6 combined with right axis deviation are highly suggestive of RVH 1

Left Bundle Branch Block (LBBB)

• In LBBB, persistent deep S waves are commonly seen in right precordial leads (V1-V3) 1
• The sum of S wave in V2 and R wave in V6 exceeding 4.5 mV has been shown to have high sensitivity (86%) and specificity (100%) for diagnosing left ventricular hypertrophy in the presence of LBBB 3
• More recent research suggests that parameters based on S wave amplitude in V2 or V3 combined with R wave amplitude in leads aVL, V5, or V6 have the highest diagnostic accuracy for LVH in LBBB patients 4

Acute Myocardial Ischemia

• During acute anterior transmural myocardial ischemia, significant decreases in S wave amplitude (including complete loss of S waves in some cases) have been observed 5
• S wave changes correlate inversely with ST segment elevation and may reflect acute changes in ventricular depolarization during ischemia 5
• The magnitude of S wave reduction correlates with hemodynamic changes, potentially offering prognostic information 5

Differential Diagnosis

• Athlete's heart: Highly trained athletes may show increased S wave voltage that can mimic pathological conditions like hypertrophic cardiomyopathy 6
• Left ventricular hypertrophy: Often associated with deep S waves in right precordial leads 1
• Biventricular hypertrophy: May present with tall R waves and deep S waves in leads V2-V4 with combined amplitude greater than 60mm 1
• Normal variant: Some individuals may have persistent S waves as a normal variant, particularly if other ECG parameters are normal 1

Evaluation Approach

• Compare with previous ECGs when available, as new onset S waves are more likely to represent pathology than longstanding findings 1
• Obtain ECG during symptomatic episodes when possible and compare with tracings taken when symptoms have resolved 1
• Consider the clinical context - persistent S waves in a patient with acute dyspnea should raise suspicion for pulmonary embolism, while those in a patient with hypertension may suggest ventricular hypertrophy 1, 2
• Evaluate for other ECG abnormalities that may accompany persistent S waves, such as right axis deviation, P pulmonale, or ST-T changes 1

Pitfalls and Caveats

• Isolated persistent S waves without other ECG or clinical abnormalities may represent a normal variant and should not be over-interpreted 1
• The presence of bundle branch block significantly alters QRS morphology and requires specific criteria for interpreting S waves 1, 3
• Athletic training can produce ECG changes that mimic pathological conditions, including increased S wave voltage 6
• A completely normal ECG during an episode of significant chest pain should direct attention to non-cardiac causes rather than assuming cardiac pathology 1