Rationale:Altered autonomic responses can be observed in patients in the intensive care unit (ICU), and these changes in abnormal autonomic responses are known to be associated with patient prognosis. Therefore, it is important to monitor autonomic nervous system activity in these critically ill patients. While the utility of monitoring critically ill patients using heart rate (HR) variability measurements has been reported, portable automated pupillometers are small, lightweight, and easy-to-operate medical devices that may be more easily evaluated for autonomic nervous system function.Patient concerns:An unconscious 80-year-old female patient with chronic obstructive pulmonary disease was brought to the medical emergency department after a call from her caregiver.Diagnosis:On arrival, the patient’s Glasgow coma scale score was 7, her blood pressure was 140/80 mm Hg, her HR was 114 bpm, and her respiratory rate was 27 breaths/minutes with increased breathing effort. Oxygen saturation was 90% on a venturi mask (3 L of supplemental oxygen). The arterial blood gas analysis showed a pH of 7.196, a partial pressure of carbon dioxide (CO2) of 89.6 mm Hg, a partial pressure of oxygen of 87.5 mm Hg, and a bicarbonate level of 29.4 mmol/L. Other than CO2narcosis, there were no abnormal findings to induce impaired consciousness. The patient did not respond to support with a bag-valve mask and was intubated. One hour after intubation, her impaired consciousness improved. The patient was extubated 20 hours later and discharged on Day 3.Interventions:The patient was admitted to the ICU after being intubated, where vital signs and blood gas analysis were monitored every 2 hours, and consciousness was assessed using the Glasgow coma scale. Using a portable automated pupillometer (NeurOptics NPi™-200, Neuroptics Inc., Irvine, CA), pupillary responses, including pupil size or light reflex, were measured every 2 hours during ICU stay.Outcomes:Changes in respiratory rate and partial pressure of CO2values correlated with pupil size and constriction velocity, but HR changes were contrary.Lessons:Pupillary responses exhibited by automated pupillometers observed in patients with CO2narcosis may be linked to vital signs and allow for autonomic evaluation.