Right Bundle Branch Block on ECG as a Predictor of Sudden Cardiac Arrest Due to Pulmonary Embolism

Introduction

High-risk pulmonary embolism (PE) is characterized by cardiac arrest or severe hemodynamic instability [1]. The mortality rate of cardiac arrest survivors is not significantly different between those with and without acute PE (68.3% vs 64%) [2]. Although PE-related sudden cardiac arrest (SCA) exhibits various features, including prolonged resuscitation duration, severe metabolic acidosis, a higher risk of bleeding complications, and a non-shockable initial rhythm [3], making a rapid diagnosis remains challenging. Electrocardiogram (ECG) analysis has superior diagnostic value for patients with high-risk PE, and right bundle branch block (RBBB) with a QR pattern in V1 not only offers a specific diagnosis but also predicts adverse outcomes [4]. Consequently, we present a case of SCA resulting from high-risk PE to illustrate the significance of ECG in diagnosis.

Case Report

A 44-year-old woman was found collapsed in the bathroom, gasping for breath and with clammy skin. The first responder reported that her blood pressure was 74/42 mmHg, pulse rate was 118 beats per minute (bpm), respiratory rate was 35 breaths per minute, and SpO2 was 72%. The initial ECG showed sinus tachycardia, an SIQIIITIII sign, and RBBB with a QR pattern in V1 (Figure 1A). Upon arrival at the emergency room, she lost consciousness, and ECG monitoring revealed a rapid decrease in heart rate to 0 bpm, indicating cardiac arrest. A diagnosis of SCA was made shortly after cardiopulmonary resuscitation and advanced life support were administered. She was also given multiple doses of epinephrine. Eventually, the patient returned to sinus rhythm with a heart rate of 116 bpm and blood pressure of 76/42 mmHg, which was maintained with norepinephrine (0.5 μg/kg/min) and dopamine (20 μg/kg/min). The patient was admitted to the intensive care unit for further treatment. Her family members denied any knowledge of her medical history or hereditary diseases. She had experienced a miscarriage at the age of 28 and has not been pregnant since. Her height is 166 cm, with a body weight of 71.5 kg, resulting in a body mass index (BMI) of 25.9.

Apart from stupor and distention of the jugular vein, no remarkable abnormalities were found during the physical examination. Laboratory examination results are shown in Table 1. The clinical features, especially the abnormal signs on the ECG, increased our suspicion of high-risk PE. However, echocardiography cannot be used immediately. As a result, urgent thrombolytic therapy with alteplase was administered to improve hemodynamics due to her critical condition. About half an hour later, transthoracic echocardiography (TTE) demonstrated right ventricular dilation with hypokinesis and flattening of the interventricular septum (IVS), and revealed a pulmonary artery systolic pressure of 42 mmHg (Figure 2). Two hours after thrombolysis, the patient’s blood pressure was 98/64 mmHg. Considering that the circulatory state remained stable, vasoactive agents were discontinued. Computed tomography pulmonary angiography revealed residual thrombus fragments in the distal branches of both the left and right pulmonary arteries (Figure 3A). However, the SIQIIITIII sign and RBBB with QR pattern in V1 returned to a normal QRS waveform (Figure 1B). After 5 days, a computed tomographic pulmonary angiogram (CTPA) showed that the thrombus had completely disappeared (Figure 3B). The patient was transferred to the general ward for rehabilitation therapy. After 6 days, tests for antiphospholipid antibody were positive. As a result, the patient was diagnosed with antiphospholipid syndrome (APS), and low molecular weight heparin was switched to warfarin. The patient followed up with the rheumatology department after discharge.

Discussion

High-risk PE, characterized by cardiac arrest or severe hemodynamic instability, accounts for 9.1% of all out-of-hospital cardiac arrests (OHCA) with non-cardiac origin [5]. A retrospective study on cardiac arrests related to PE found that the initial rhythm presented as pulseless electrical activity (63%), followed by asystole (19%), and then ventricular fibrillation (3%) [6]. Post hoc analysis of an OHCA study in Prague revealed that patients with high-risk PE on admission experienced prolonged resuscitation, severe metabolic acidosis, and elevated lactate levels [3]. While our patient displayed many clinical features of high-risk PE, these features were not sufficient to distinguish her case from cases with other causes of cardiac arrest. The guidelines recommend using TTE or emergency CTPA to diagnose PE in patients with hemodynamic instability [7]. However, these tests may delay cardiopulmonary resuscitation and require the presence of trained medical personnel for patients experiencing cardiac arrest.

The ECG tends to be the initial objective test in the emergency room and can offer potential diagnostic clues. This patient’s ECG showed RBBB with a QR pattern in V1, which raised a high suspicion of PE. The Italian Pulmonary Embolism Registry (IPER) study found that PE patients in a hemodynamically unstable condition had at least 1 ECG abnormality, such as RBBB, QR in V1, S1Q3 pattern, T-wave inversion in V1-V3, or ST elevation in inferior leads [8]. Another study found that the QR pattern in V1 had a specificity and sensitivity of 100% and 55%, respectively, for diagnosing PE [4]. The ECG changes mentioned above in PE originate from right ventricular strain. When the pulmonary artery is occluded by a massive thrombus, pulmonary vascular resistance increases significantly. This leads to significant dilation of the right ventricle (RV), while the volume of the left ventricle (LV) is compressed. Lead V1 mainly records the electrical activity of the RV. Therefore, RV dilation and changes in the RV/LV ratio can affect the QRS waveform in lead V1 (Figure 4). Due to RV dilation and an increase in the RV/LV ratio, the clockwise rotation of the QRS axis in the horizontal plane results in RBBB. When RV dilation worsens significantly and the IVS flattens noticeably, the initial QRS vector deviates from the position of lead V1. As a result, the first R wave of the RBBB is no longer visible in V1. Therefore, V1 presents a QR pattern, while other precordial leads manifest RBBB. We believe that the QR pattern in V1 can be considered an indication of further deterioration of the RBBB in cases of high-risk PE. Some studies also suggest that the progression from RBBB to a QR pattern in V1 indicates worsening right ventricular strain [9,10]. Furthermore, the QR pattern in V1 generates a positive predictive value of 81% and a negative predictive value of 58% for diagnosing cardiac arrest [9]. The QR pattern in V1 is an independent predictor for patients with high-risk PE who require escalation of therapy, such as mechanical ventilation, and thrombolysis [4].

Less than half of the sudden deaths attributed to coronary artery disease exhibit evidence of acute plaque complications [11]. Differentiating between PE and acute coronary syndromes (ACS) using ECG can be challenging. A retrospective study revealed that only 1 patient diagnosed with ACS exhibited a right ventricular strain pattern on ECG [9]. The negative T waves in PE are primarily seen in leads III and V1, whereas in ACS, negative T waves are frequently observed in leads V2 to V4, as well as in lead I and aVL [12]. Although PE and ACS can coexist, this condition is rare. We found no evidence of ACS in this patient, thus coronary angiography was not performed during hospitalization. Adequate anticoagulation is crucial for the treatment of APS, with commonly prescribed anticoagulants including warfarin, heparin, or low molecular weight heparin.

In cases of catastrophic APS or when conventional antithrombotic therapy is ineffective, glucocorticoids and immunosuppressants may be used in combination. For refractory patients, intravenous immunoglobulin, plasma exchange, and rituximab can also be considered as treatment options [13].

Conclusions

An ECG showing RBBB with a QR pattern in V1 has a high positive predictive value for high-risk PE. The QR pattern in V1 is an independent predictor for patients with high-risk PE who require advanced treatment, such as systemic thrombolysis. If clinical features and ECG strongly suggest high-risk PE, prioritizing thrombolysis before confirming with CTPA may be considered.