A New Computer-Controlled Pressure Support Mode
September 1, 2000 3 minutes read
A New Computer-Controlled Pressure Support Mode
Abstract & Commentary
This crossover trial enrolled 10 ventilator-dependent patients recovering from acute respiratory failure (ARF). The patients received either computer-controlled PSV (c-PSV) or physician-controlled (standard) PSV (s-PSV) in random order. The digital signals of respiratory rate (RR), tidal volume (VT) and end-tidal PaCO2 (PETCO2) were acquired from the ventilator, and the computer automatically adjusted the PSV level in targeting RR between 12 and 28 breaths/min, VT above 300 mL, and PETCO2 below 55 mm Hg. Airway occlusion pressure (P0.1), a measure of central respiratory drive, was also measured.
The average time spent with acceptable ventilatory support was 66% ± 24% of the total ventilation time with s-PSV vs. 93% ± 8% with c-PSV. The mean PSV level was not different between the two PSV modes. The time with P0.1 above 4 cm H2O (presumably indicating increased drive associated with air-hunger) was significantly shorter in c-PSV than s-PSV. Dojat and colleagues conclude that c-PSV provided ventilator-dependent ARF patients with less WOB, and speculated that this mode might improve the outcome of this patient population (Dojat M, et al. Am J Respir Crit Care Med 2000;161:1161-1166).
COMMENT BY JUN TAKEZAWA, MDM
Closed-loop adjustment of the PSV level was first offered commercially in the Siemens Servo 300 ventilator, called volume support ventilation (VSV). In this mode, the minute volume (MV) the given patient may require is set first, and then the RR the patient may require is set. As a result, initial target VT is automatically set. The ventilator guarantees both the set MV and set VT by adjusting the PSV level. The advantage of this mode is preservation of MV, even though the patient’s respiratory drive and mechanics may be unstable. The disadvantages of this mode are: 1) when the patient’s RR is decreased, the PSV level increases to an unexpectedly high level in targeting set MV; 2) when patient RR is increased, the PSV level may increase or decrease, depending on the MV the patient-ventilator interaction accomplishes; and 3) when the patient-required MV increases or decreases, due to the change of respiratory drive and mechanics, the ventilator attempts to maintain the set MV by adjusting the PSV level, resulting in insufficient unloading of the patient WOB or hyperventilating the patient, respectively. Therefore, as far as both the patient’s required MV and his/her respiratory drive are stable, VSV may become similar to c-PSV. However, when patient-required MV fluctuates, ventilatory support by VSV becomes more or less.
The c-PSV designed by Dojat et al and described in this paper targets RR and VT instead of both MV and VT. The VT always fluctuates in healthy normal people (Jammes RT, et al. Bull Eur Physiopathol Resp 1979;15:527), and this fluctuation can become greater in the patient with a critical illness because of agitation, an increase in metabolic demand, level of sedation, and/or deterioration of respiratory mechanics. Therefore, it is unknown whether RR is an ideal target in terms of unloading patient WOB in any type of respiratory disease. The patient attempts to provide respiratory pump performance in the most efficient way, depending on the deterioration of his/her respiratory mechanics; when resistance is high, low RR and high VT are preferable, while when compliance is low, high RR and low VT are most efficient. Therefore, RR could conceivably be a good target (by adjusting VT) in patients with ARF where resistance is normal and only compliance is reduced. In case of the patient with high airway resistance, RR may not be a good target, where the patient attempts to breathe with a large VT and low RR.
Alternatively, proportional assist ventilation (PAV) can be used as an ideal partial ventilatory assist in patients with unstable respiratory mechanics and stable respiratory drive. Additional study is required to confirm which mode is superior in providing better respiratory muscle unloading during long-term ventilatory support. However, it also should be taken into account that frequent measurements of elastance and resistance are required during PAV.
In summary, c-PSV provides the ventilator-dependent patient who has normal resistance and low compliance with better ventilatory support than standard, clinician-adjusted PSV. Thus, this new mode may reduce the workload and need for frequent adjustment of the PSV level by the clinician and may also increase patient comfort. However, a comparison between c-PSV and PAV would be interesting in terms of long-term ventilatory support and patient outcome.
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