Attaining Low Tidal Volume Ventilation During Patient Triggered Ventilation in Sedated Subjects

BACKGROUND: Low tidal volume (V_T) ventilation has become the preferred approach in patients in the ICU. Sedation reduces V_T by attenuating respiratory drive. Even in deep sedation, some patients exhibit high V_T. We aimed to determine factors associated with low V_T ventilation in deeply sedated subjects who exhibited an inspiratory effort by examination of the acid/base balance using the Stewart model.

METHODS: The medical records of 630 consecutive subjects admitted to the ICU over 1 y were reviewed retrospectively, and daily data sets of patients with a persistent inspiratory effort, P_aO2/F_IO2 < 300 mm Hg, PEEP > 5 cm H₂O, and a Richmond Agitation Sedation Scale score of −4 or −5 who received assisted pressure-regulated ventilation were collected. The data sets were stratified into high V_T (≥ 8 mL/kg predicted body weight [PBW]) and low V_T (> 8 mL/kg PBW) groups.

RESULTS: Among 235 matched data sets from 100 subjects, 101 and 134 data sets were in the low V_T and high V_T groups, respectively. Set pressure was not different between the groups. PEEP was lower in the low V_T group, and opioids were more frequently used in the high V_T group. Strong ion difference (SID) was higher in the low V_T group. Multivariate analysis revealed that higher SID, lower total nonvolatile weak anion (A_TOT), and absence of opioid administration were associated with attaining low V_T ventilation. Furthermore, V_T/PBW and SID demonstrated a weak inverse correlation, whereas V_T/PBW and A_TOT exhibited a weak correlation. V_T/PBW was lower in the group with higher SID and lower A_TOT, indicating a tendency of metabolic alkalosis.

CONCLUSIONS: Despite weak effects of high SID and low A_TOT, efficient management of the buffering function might be a feasible strategy to achieve low V_T ventilation.