Joseph L. Rau

A comparison of the effects of assist-control, SIMV, and SIMV with pressure support on ventilation, oxygen consumption, and ventilatory equivalent

OBJECTIVE To quantify the ventilatory efficiency of different modes of mechanical ventilation used to achieve full ventilatory support in normal subjects. Modes compared were assist-control, synchronized intermittent mandatory ventilation (SIMV), and SIMV with 10 cm H2O (0.98 kPA) of pressure support. DESIGN Prospective, randomized blocks repeated measures design. Subjects served as their own controls. SETTING A university affiliated pulmonary laboratory. SUBJECTS Ten healthy volunteers, aged 31-54 years. OUTCOME MEASURES Minute volume, respiratory rate, average tidal volume, oxygen consumption, and ventilatory equivalent. INTERVENTION Baseline spontaneous ventilation data collection was followed by mechanical ventilation by mouthpiece in each of three modes in a random sequence. All modes used a machine set rate of 12 breaths per minute, VT of 10 cc/kg of ideal body weight, inspiratory time of 1 second, square wave flow pattern and a sensitivity of -1 cm H2O (-0.09806 kPa) to achieve full ventilatory support. Data were collected continuously for 5 minutes and the mean values were reported. Ventilatory equivalent for oxygen is a measure of the efficiency of the ventilatory pump at various work loads and was calculated by dividing VE (BTPS) by the VO2 (STPD). RESULTS There were significant differences by mode of mechanical ventilation in average tidal volume (p = 0.02), minute volume (p = 0.02), oxygen consumption (p = 0.04), and ventilatory equivalent (p = 0.01) using ANOVA. There was no significant difference (p = 0.66) by mode of ventilation in respiratory rate. Pairwise follow-up comparisons for these variables found that SIMV with pressure support produced a significantly greater average tidal volume, minute volume, oxygen consumption, and ventilatory equivalent than SIMV alone. SIMV with pressure support also produced a significantly greater minute volume and ventilatory equivalent than assist-control. There were no significant differences between assist-control and SIMV. All three modes produced a lower ventilatory equivalent and higher oxygen consumption than spontaneous breathing. CONCLUSIONS SIMV with pressure support significantly increased minute volume and ventilatory equivalent when compared with assist-control or SIMV alone, and thus was the most efficient mode of full ventilatory support for our subjects. We found no difference in ventilatory efficiency between assist-control and SIMV. All three mechanical modes were less efficient for our subjects than spontaneous breathing. The inspiratory time of 1 second used in this study, although common in clinical practice, may be inadequate for some patients.

Recent developmentsin respiratory care pharmacology

Bronchoactive inhaled aerosol drugs target the respiratory tract directly and seek to minimize systemic exposure and reduce side effects. Common delivery devices such as the metered dose inhaler, the small volume nebulizer, or the dry powder inhaler each deliver approximately the same fraction of dose (10%) to the lungs, although their dose amounts are not equivalent. Major respiratory drug groups are reviewed, and include the beta-adrenergic and anticholinergic bronchodilators, mucolytic agents, corticosteroids, mediator antagonists, anti-infective agents, and exogenous surfactants. New agents in each group are identified and briefly described, along with the clinical use and most commonly observed side effects for each class of drugs.