A Tough Time Running Around the Block

A 61-year-old man with diabetes, hypertension, and sleep apnea presented with exertional dyspnea. One year prior cardiopulmonary exercise testing was performed showing peak oxygen consumption of 19.9 ml/min/kg (66% predicted) with peak heart rate 90 bpm, but no record of arrhythmia or ST segment deviation. Physical examination was normal, with body mass index of 30.5 kg/m². The resting electrocardiogram (ECG) showed right bundle branch block and first degree atrioventricular (AV) block. Plasma NTproBNP was 403 pg/ml. Echocardiography revealed a left ventricular ejection fraction of 66%, no valve disease, left atrial enlargement (39 ml/m²), estimated right ventricular systolic pressure 30 mmHg, and medial E/e’ ratio 9. The H₂FPEF score was 4, suggesting intermediate likelihood of HFpEF. The patient was referred for invasive hemodynamic cardiopulmonary exercise testing to determine the cause of exertional dyspnea.

Baseline right atrial pressure (RAP) and pulmonary capillary wedge pressure (PCWP) were normal (Figure 1A), with a resting heart rate (HR) of 59 bpm. With leg raise, the PCWP increased to 20 mmHg, with a prominent V wave (Figure 1B). As the patient became more dyspneic, grouped beating was noted on ECG, with PR interval prolongation followed by dropped beats (Figure 1C). Following conducted beats there was progressive PR prolongation, with P waves superimposed on the preceding T waves, such that atrial systole either preceded mitral valve opening or coincided with early rapid filling. This resulted in development of fused “cannon A-V waves” up to 50 mmHg in amplitude (Figure 1C). Peak oxygen consumption was severely depressed at 7.5 ml/min/kg, peak HR was 70 bpm, and peak cardiac output was 7.27 l/min (6% of the predicted increase based upon metabolic demand).

Figure 1.

Resting pulmonary capillary wedge pressure (PCWP, red) was normal with normal amplitude a and v waves. Lead II from the electrocardiogram is shown in green. With passive leg elevation prior to exercise there is an increase in PCWP, with a striking increase in the amplitude of the v wave. During exercise the p wave preceding the second QRS complex (*) is conducted with a PR interval of 1266 ms. The corresponding a wave in the PCWP tracing is normal in amplitude. Following this beat there is prolongation of the PR interval, such that the P wave is buried within the T wave of the second beat (†). Because ventricular systole had not yet ended, the left atrium contracted against a closed mitral valve during this beat, causing a “cannon a wave” that became superimposed with the V wave (a+v). In the next beat, the P wave (‡) is not conducted due to Mobitz Type I second degree AV block, resulting in another cannon a wave superimposed on the V wave in the PCWP tracing. This sequence of PR prolongation with dropped beats continued.

A 12-lead ECG was obtained immediately following exercise and confirmed the presence of Mobitz type I AV block, with 3:2 AV block alternating with 2:1 AV block. The patient underwent dual chamber permanent pacemaker implantation, resulting in substantial improvement of his dyspnea, which was noted immediately.

Discussion

Exertional dyspnea is a very common complaint encountered in practice, and diagnosis can be challenging.¹ In the case presentation, left ventricular EF was normal, and while there was no evidence of hypervolemia on physical examination, it is known that many patients with heart failure with preserved ejection fraction (HFpEF) display hemodynamic abnormalities exclusively during exercise.^{2, 3} This patient displayed an intermediate pretest probability that HFpEF was present based upon the calculated H₂FPEF score, which is a common clinical scenario where invasive hemodynamic exercise testing is most useful establish the diagnosis.^{1, 4}

Hemodynamics were normal at rest, but PCWP increased strikingly with passive leg raise. This indicates even prior to exercise that HFpEF is present, as the increase in venous return from the legs could not be accommodated without excessive increase in filling presssure.³ While the prominent V wave with leg elevation can be caused by dynamic mitral regurgitation, in most patients with HFpEF this simply reflects reduced left atrial compliance.⁵ Further assessments during exercise however indicated that this was not a case of ‘garden variety’ HFpEF alone, as the patient developed exercise-induced AV block during low-level exercise.

Atrioventricular block is commonly considered to contribute to exercise intolerance predominantly through chronotropic incompetence, which was evidenced in this case by the peak HR of only 70 bpm, resulting in severe cardiac output reserve limitation. However, this case also illustrates how significant diastolic abnormalities are observed related to the loss of AV synchrony. With prolongation of the PR interval, atrial contraction moves closer to the end of ventricular systole, resulting in a “P on T” phenomenon, where the atrium contracts against an incompletely relaxed ventricle, or even closed atrioventricular valve. In addition to causing superimposed “cannon a-v” waves in the atria (Figure 1C), this AV dyssynchrony compromises ventricular filling through loss of atrial booster function in late diastole, which is even more poorly tolerated in patients with HFpEF.

Exercise-induced AV block in patients with 1:1 AV conduction at rest is rare,^{6, 7} because conduction through the His-Purkinjie system is relatively unaffected by increased sympathetic discharge and may actually worsen during exercise as compared to conduction through the AV node. In our case, alternating Wenkebach periods were noted which are usually attributed to multilevel block due to transverse (horizontal) dissociation of one, or several, segments of the AV conduction system (atria, AV node, and His bundle).⁸ Pacemaker implantation substantially improved symptoms, likely related to both improvements in chronotropic reserve and restoration of AV synchrony.

In conclusion, the presented case emphasizes the extent to which patients with HFpEF in particular rely on optimal AV synchrony and chronotropic reserve to cope with the heightened metabolic demands of physical activity. The case also illustrates the value of hemodynamic exercise testing to identify the cause of symptoms among patients with unexplained dyspnea, allowing for actionable pathophysiologic insights that can guide treatment.