Shabbir, Asma, et al. “FREQUENCY OF ECG ABNORMALITIES IN PATIENTS OF CHRONIC OBSTRUCTIVE PULMONARY DISEASE ACCORDING TO DISEASE SEVERITY: ECG Abnormalities in Chronic Obstructive Pulmonary Disease”. Pakistan Armed Forces Medical Journal, vol. 67, no. 4, Aug. 2017, pp. 569-74, https://pafmj.org/PAFMJ/article/view/689.