Ritonavir has been shown to cause modest asymptomatic prolongation of the PR interval in some healthy adult subjects. Rare reports of 2nd or 3rd degree atrioventricular block in patients with underlying structural heart disease and pre-existing conduction system abnormalities or in patients receiving medicinal products known to prolong the PR interval (such as verapamil) have been reported in patients receiving ritonavir. Norvir should be used with caution in such patients.
Norvir Summary of Product Characteristics, AbbVie Ltd, September 2016.

When ritonavir is coadministered with verapamil, plasma concentrations of verapamil may be increased and a decrease in dose may be needed. The impact on the PR interval of co-administration of ritonavir with other drugs that prolong the PR interval (including calcium channel blockers) has not been evaluated. As a result, co-administration of ritonavir with these drugs should be undertaken with caution, particularly with those drugs metabolized by CYP3A. Patients should consult their physician if they experience symptoms such as dizziness, lightheadedness, abnormal heart rhythm, or loss of consciousness. Caution is warranted and clinical monitoring of patients is recommended.
Norvir Prescribing Information, AbbVie Inc, December 2016.