respiratory failure

The ventilator mode used in this study was con­trolled mandatory ventilation. Although the ventilator rate was fixed, the use of the control mode does not necessarily assure complete absence of spontaneous respiratory efforts. In this context, controlled ventila­tion may be considered to be assisted ventilation. Other investigators have allowed the NPPV rate to be determined by patient triggering (assist control mode) with a set back-up rate. In fact, studies with conventional positive pressure ventilation in intubated patients have shown that once the inspiratory muscles have begun to contract, they continue to perform work even after the ventilator has been triggered to provide inspiratory flow. Since one of the goals of assisted ventilation is to provide rest by taking over the work of the inspiratory muscles, it would seem to be most efficacious to provide a controlled rate which is adequate to prevent the initiation of spontaneous inspiratory efforts as completely as pos­sible. Furthermore, previous work suggests that inhibition of spontaneous inspiratory efforts in normal subjects undergoing noninvasive mechanical ventila­tion occurs only in a setting of increased alveolar ventilation relative to spontaneous breathing.

When subjects were asked to actively open their mouths during NPPV, varying degrees of loss of phasic diaphragm suppression were observed (Fig 7). Presum­ably, a portion of the inspiratory volume generated by NPPV was diverted through the mouth, a lower resistance pathway than the respiratory system, and the minute ventilation provided became inadequate to suppress spontaneous inspiratory efforts. Despite this observation, the studies in which NPPV was applied during sleep, with apparent benefit, suggest that NPPV was able to assist ventilation, even if some degree of leakage through the mouth occurred.
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The successful application of assisted ventilation may be limited by a number of factors. Negative pressure ventilation can be provided without the need for an artificial airway. However, it has the disadvantages of inconvenience of application, poor access to patients for medical and nursing care, musculoskeletal discomfort, and upper airway obstruction. Alter­natively, conventional positive pressure ventilation requires an artificial airway with its associated com­plications. The technique of positive pressure ventilation through a nasal mask offers the advantages of being noninvasive and well-tolerated by patients. No untoward effects were noted in the present short- term study. Previous reports describing the long-term use of NPPV have reported minimal side effects, which included nasal abrasion, mask leaks and abdominal distension, none of which required dis­continuation of NPPV.

It is evident from this study that in order for NPPV to provide maximum reduction of inspiratory muscle activation, the ventilator settings must be individual­ized for each subject. Although it may not be practical to invasively determine indices of inspiratory effort in the routine clinical setting, our data demonstrate that the recording of diaphragm EMG activity from surface electrodes is, in fact, a simple, noninvasive method which can be used to ensure that NPPV is having the desired effect.

An effective, noninvasive method of assisted venti­lation may prove to be useful in situations other than respiratory failure where the suppression of sponta­neous inspiratory efforts is desirable. These include decreasing negative intrathoracic pressure swings in the treatment of flail chest and the reduction of respiratory muscle energy consumption in circulatory shock. The role of NPPV in these situations remains to be explored. levitra professional

In conclusion, this study demonstrates that NPPV can provide ventilation in a noninvasive fashion while effectively reducing inspiratory muscle energy expen­diture in acute and chronic hypercapnic respiratory failure. This effect was observed in patients with both restrictive and obstructive ventilatory disorders. Fur­thermore, these results indicate that nasal positive pressure ventilation may play an increasingly impor­tant role in obviating the need for more invasive modalities of assisted ventilation in a variety of clinical settings.