Pressure and Flow Limitations of Anesthesia Ventilators

Effect of inspiratory time on tidal volume delivery in anesthesia and intensive care unit ventilators operating in pressure control mode

“…Unlike modern ICU ventilators, which vent exhaled gases to the atmosphere and directly release gas from the wall outlet into the circuit, anesthesia ventilators recirculate exhaled gases into the inspiratory limb. This feature mandates a large internal volume, which reduces inspiratory flow capacity when airway pressures are high [9]. During volume control ventilation, where inspiratory flow rates normally range between 40 and 60 L/min [10], such limitations rarely affect clinical care.…”

“…When airway pressures are high, however, most anesthesia ventilators deliver significantly less flow than do their ICU counterparts [9,10]. We hypothesized that an anesthesia ventilator operating in an environment with short inspiratory times and high airway pressures would produce smaller Vts than would an ICU ventilator with identical settings.…”

New Developments in Anesthesia Ventilators

“…Most anesthesia ventilators, however, deliver progressively less flow as airway pressures increase. 9 This reduction in gas flow rates leads to decreased tidal volumes with increased airway pressures, regardless of changes in the respiratory rate or inspiratory time. Figure 2 illustrates how ventilators can differ in these 2 important respects.…”

“…These differences can be clearly seen when comparing the performance of the Siemens 900D ICU ventilator with several older anesthesia machines (Fig 3). 9 The large internal volume characteristic of anesthesia ventilators is very likely the most important reason why ICU ventilators outperform anesthesia machines when airway pressures are high. 9 Unlike ICU ventilators, which vent exhaled gases into the atmosphere, anesthesia machines deliver precise gas mixtures while conserving the use of environmentally damaging volatile agents.…”

“…9 The large internal volume characteristic of anesthesia ventilators is very likely the most important reason why ICU ventilators outperform anesthesia machines when airway pressures are high. 9 Unlike ICU ventilators, which vent exhaled gases into the atmosphere, anesthesia machines deliver precise gas mixtures while conserving the use of environmentally damaging volatile agents. Thus, anesthesia ventilators typically include a CO 2 absorber to condition the exhaled gas mixture and a bellows containing the mixture of fresh and recirculated gases that will be delivered on inspiration.…”