Background: Maintaining respiratory sufficiency is a vital component of postoperative care, yet clinicians often rely on subjective clinical assessments or secondary indicators of respiratory status. Patients with apneic episodes are often considered to have “respiratory insufficiency” regardless of whether these apneic episodes impact overall respiratory status or are compensated by large recovery breaths. We used a non-invasive respiratory volume monitor (RVM) to quantitate minute ventilation (MV) to assess ventilation in patients experiencing apnea in the PACU and on the general floor.

Methods: We used an RVM (ExSpiron1Xi, Respiratory Motion, Waltham, MA) to continuously monitor MV for 48 hr following elective abdominal surgery. MV was expressed as percent of MV predicted (MVPRED) based on body surface area and sex; Low MV (hypoventilation) was defined in previous work as MV<40% MVPRED. For each apnea, defined as a pause >10 s, we calculated the patient’s MV over 30, 60, 90, and 120 second windows following the start of the apnea.

Results: 216 patients (110 males, BMI: 26.7 (15-41) kg/m2) were monitored for 42.0±0.9 hr. We recorded a total of 49,985 apneas ranging in length from 10-117s (Fig 1A). Apnea >10sec was observed in 99% of patients, suggesting that apnea per se is a poor indicator of respiratory insufficiency. The average MV was 73±2.4% MVPRED, as patients were often sleeping or mildly sedated. We assessed the effects of each apnea on the temporally associated MV (Fig 1B). While apneas lasting 10-18 s decrease instantaneous MV by as much as 30%, their effect over the course of 1 minute is less than 10%. On a 2-minute time scale, even 60 s apneas lead to Low MV just 20% of the time (Fig 1C).

Conclusion: While apneas were ubiquitous, they seldom led to sustained Low MV over clinically relevant time scales. Compensatory breaths following an apnea generally restored MV to near pre-apnea levels. Nonetheless, some apneas can become dangerous when ignored, as when sedation decreases compensatory breath size. RVM data provides a better overall metric of respiratory competence, driving better assessment of patient risk and individualization of care.

08 / Assessing the effects of duration of apnea on adequacy of ventilation across a broad cohort of surgical patients in the post-operative environment