Pathogenesis of PE and its ECG manifestations
Pulmonary embolism (PE) is an acute occlusion of the main pulmonary trunk or its branches by a thromboembolus, most commonly migrating from the deep veins of the lower extremities via the inferior vena cava system into the right heart chambers and further into the pulmonary circulation.
The pulmonary trunk originates from the right ventricle and divides into the right and left pulmonary arteries. Occlusion of one of the branches dramatically increases pressure within the pulmonary arterial system. The right ventricle is forced to overcome the increased afterload, leading to its acute overload and dilation, as well as right atrial overload. It is these hemodynamic changes that produce the electrocardiographic pattern, which in many ways resembles right ventricular hypertrophy, but occurs acutely.
Key ECG signs of PE
Classically, five groups of changes are distinguished; none of them is pathognomonic on its own, but their combination in the appropriate clinical context is highly informative:
- Sinus tachycardia is the most frequent and early sign; heart rate frequently exceeds 100–150 bpm.
- Right bundle branch block (complete or incomplete) as a reflection of acute right ventricular overload.
- Negative T waves in leads V1–V3, as well as in II, III, and aVF.
- Rightward axis deviation resulting from right heart overload.
- Signs of right atrial overload — P-pulmonale: a tall peaked P wave (greater than 2.5 mm) in lead II and a dominant positive P in V1.
The S1Q3T3 pattern
A separate entity is the so-called McGinn–White pattern, or S1Q3T3. It includes three components:
- A deep S wave in lead I.
- A pathological Q wave in lead III.
- A negative T wave in lead III.
Despite its widespread recognition, this pattern occurs in only approximately 15% of PE cases and should not be considered an absolute criterion. Its absence does not exclude the diagnosis, and its presence requires correlation with the clinical picture (sudden dyspnea, chest pain, hypoxemia, risk factors for venous thromboembolism).
Algorithm for ECG analysis in suspected PE
When evaluating the tracing, it is advisable to sequentially check for the following signs:
- assess rhythm and heart rate — identify sinus tachycardia;
- analyze the QRS complex in the right precordial leads for right bundle branch block (QRS widening, rSR' pattern in V1, wide S wave in I and V6);
- assess the polarity of T waves in V1–V3, II, III, and aVF;
- determine the electrical axis of the heart — marked rightward deviation is characteristic;
- assess P wave morphology (P-pulmonale) and the presence of a dominant R wave in V1;
- specifically look for the S1Q3T3 pattern.
It should be remembered that the electrocardiographic signs of acute overload and chronic right ventricular hypertrophy look very similar, so interpretation is always performed taking into account the medical history, acuteness of symptoms, and imaging data.
Clinical significance of the ECG in PE
The ECG in PE is not a method for verifying the diagnosis: definitive confirmation is provided by CT pulmonary angiography, while risk assessment is complemented by D-dimer levels, echocardiography, and scoring systems (Wells, Geneva scores). However, the ECG remains a rapid, accessible, and mandatory test that allows suspecting PE in a patient with sudden dyspnea and tachycardia, as well as ruling out alternative causes of chest pain, primarily acute myocardial infarction. The dynamics of ECG changes (disappearance of right heart overload signs after reperfusion) serve as an indirect marker of treatment efficacy.
Key points
- The ECG in PE reflects acute right heart overload against the background of pulmonary hypertension.
- The most frequent sign is sinus tachycardia, often exceeding 100–150 bpm.
- Right bundle branch block and negative T waves in V1–V3, II, III, and aVF are characteristic.
- The electrical axis of the heart deviates to the right, and P-pulmonale develops.
- The S1Q3T3 pattern occurs in only ~15% of patients and is not mandatory.
- No single sign is pathognomonic; the diagnosis is made based on the totality of data.
- PE is verified using CT pulmonary angiography; the ECG plays a supportive role.
- The ECG patterns of acute overload and chronic right ventricular hypertrophy are similar.