Discussion
Cannabis consumption is mostly associated with chronic sinus tachycardia and other tachyarrhythmia like atrial fibrillation and ventricular tachycardia. A scoping study of 27 cases of arrhythmia associated with marijuana reported that most cases were young males and the mortality rate was high with Marijuana associated arrhythmia (11%). Atrial fibrillation (26%) and ventricular fibrillation (22%) were the most common arrhythmias. Brady arrhythmia were not common; first and second degree AV block and sinus arrest (3.7 % each) were the common findings among bradyarrythmia group14 In another extensive review about the association of cannabis use and cardiac dysrhythmias, out of 42 subjects, third degree AV block was found in 12% of cases while symptomatic bradycardia was present in 10 % of the cases .15
Table 1 depicts reported cases of severe bradyarrythmia due to marijuana toxicity. Till date about eleven cases of severe bradyarrythmia are reported, out of which three cases were third degree heart block, two cases were second degree heart block and six cases were of sinus arrest. Pacemaker implantation was done in two cases of complete heart block and 3 cases of sinus arrest and in rest of the five cases conduction improved after abstinence from marijuana while one case was lost to follow-up. In none of the reported cases electrophysiological study was done to determine the site of blockade.
There are several proposed mechanisms for tachyarrhythmias caused due to cannabis like altered conduction property of myocardial tissue; cardiac ion channel modulation; autonomic dysfunction due to imbalance between sympathetic and parasympathetic outflow; type 2 myocardial infarction due to tachycardia, elevated carboxyhaemoglobin , slow coronary flow due to endothelial dysfunction, coronary vasospasm, increased platelet aggregation leading to ischaemic injuries/ scar creating a milieu for arrhythmia. However the exact mechanism by which it causes bradyarrythmia is not well known .1,2 Agonistic action of THC on CB1R causes sympathetic inhibition and increased cardiac vagal tone leading to bradycardia, lower doses cause sympathetic stimulation while higher doses is associated with parasympathetic action. The electrophysiological effects of intravenous THC in human experimental studies on cardiac conduction includes a change in P wave morphology, decrease in sinoatrial (SA) conduction, delay in A-H (atrium to his bundle) interval and decrease in atrioventricular (A-V) node refractory period. However, In this case the HV interval was mildly prolonged , which is not explainable completely by previously described mechanisms and it is possible that Δ9-tetrahydrocannabinol (THC) has some direct toxic effects on cardiac conduction systems through cannabinoid receptors 1. Figure 3 demonstrates the possible mechanism of bradycardia caused by cannabis. Our Patient, who was young medical professional consumed marijuana for about 4 years and his previous medical record were not suggestive of any risk factor for complete heart block except chronic marijuana use. After initial evaluation he was implanted permanent pacemaker. During follow-up patient claimed complete abstinence from marijuana use. Patient atrio-ventricular conduction also improved in about 3 months duration.
Marijuana use is increasing world over for both medicinal and recreational purposes as states are legalizing cannabis and physicians can expect to encounter more patients who use or abuse marijuana .It can cause severe bradyarrythmia, possibly mediated by enhanced vagal tone and might be reversible in case of withdrawal of usage. Awareness regarding such cardiovascular side effects of marijuana is warranted. Screening for marijuana use should be encouraged, especially in young patients presenting with cardiovascular disease.
Contributorship : All authors were equally involved in patient care , drafting and editing of the manuscript