Effects of prolonged working hours on heart rate variability in internal medicine physicians

Prior studies have utilized heart rate variability (HRV) as the assessment tools for psychological and physiological stress during 24-h shift. However, data regarding effects of prolonged working hours > 24 h on HRV are limited. We aimed to compare between pre- and post-call HRV among physicians who worked 24 plus 8 h. The study included 60 physicians in the internal medicine training. All subjects underwent Holter ECG monitoring for HRV assessment. We compared between HRV of an 8-h regular workday (8am to 4 pm) before on-call duty (pre-call HRV) and an 8-h workday after 24-h on-call duty (post-call HRV). The mean age was 26 ± 2.5 years. Mean total sleep time during on-call duty was 238.9 ± 88.3 min. In overall population, the time-domain and frequency-domain HRV parameters were not different between pre- and post-call day. However, the physicians reported their sleep time in the 1st quartile (< 180 min) had significant increase in SDNN, pNN50, high frequency (HF), and decrease in low/high frequency ratio (LF/HF). In contrast, the physicians reported their sleep time in the 4th quartile (> 307.5 min) had significant decrease in pNN50, LF, HF, and increase in heart rate. Multiple linear regression revealed total sleep time as an independent factor associated with pre- and post-call HRV alterations. More sleep during on call (> 5 h) was associated with HRV pattern suggesting both increased sympathetic activity and reduced parasympathetic activity, while less sleep (< 3 h) during on call was associated with post-call parasympathetic rebound HRV pattern.