To date there has been little evidence to guide clinicians in their choice of ventilator settings for non-ARDS patients. Approximately 40 years ago, Mead et al, developed an equation that calculates an optimal respiratory rate (ORR) to minimize work of breathing. We attempt to verify this equation in a mechanically ventilated mixed medical/surgical ICU patient population.
All patients admitted to a single mixed medical/surgical ICU who underwent mechanical ventilation with a Drager, Evita 4, ventilator were included. Parameters to satisfy the Mead equation–minute ventilation, tidal volume, peak expiratory flow, and measured respiratory rate (MRR) were measured daily and during spontaneous breathing trials when possible. For patients being ventilated with combined SIMV and PS modes, calculations were made only for the spontaneous (PS) parameters. ORR was then calculated using the Mead equation. Weaning and ventilator settings were determined by individual practitioners. All measurements were made by a single observer. Data obtained from post CABG patients was analyzed separately.
A total of 73 measurements, 16 on PS and 57 on AC, were completed on 17 patients. The mean difference between MRR and ORR was 3.1 breaths/min (25.4 vs 22.3 breaths/min), p=0.02, r=0.78. When analyzed separately for mode, the mean difference between MRR and ORR during PS mode was 6.0 breaths/min (22.9 vs 16.9 breaths/min), r=0.70. The mean difference between MRR and ORR with AC mode was 2.5 breaths/min (26.4 vs 23.9 breaths/min), p=0.08, r=0.84. Analysis of post CABG data showed a mean difference between MRR and ORR to be 6.6 breaths/min (17.7 vs 11.1 breaths/min), p=0.01, r=0.76.
The Mead equation for predicting ORR correlates with MRR in a mixed medical/surgical mechanically ventilated population. MRR during AC mode ventilation correlated best with the Mead predicted ORR.
AC mode ventilation may provide superior patient-ventilator interactions allowing for decreased work of breathing. Further quantification of modifiable factors that may contribute to deviation from the ORR maybe be helpful to guide clinicians in the choice of ventilator parameters and is warranted.
A. Bass, None.