Ventilation is often completely controlled. In this circumstance the duration of the inspiratory cycle and the manipulation of pressure and flow rates are preset by the user. Sedation and the use of neuromuscular blocking drugs are often required.
Spontaneous breathing occurs in many ventilated patients and may form part of the ventilatory strategy. The use of inspiratory muscles has been highlighted during conventional ventilation, demonstrating that, far from being silent, respiratory muscle activity can continue throughout inspiration. When the respiratory muscles contract asynchronously, spontaneous work may be comparable with that measured in the non-ventilated state. A study of the timing and duration of inspiratory muscle activity during mechanical ventilation demonstrated the importance of inspiratory flow rate in turning respiratory muscles off. When inspiratory flow is low, respiratory muscle activity persists throughout a mechanical breath.
If patients can perform significant levels of work despite apparently adequate mechanical ventilation, how is rest achieved and which parameters should be adjusted to achieve rest? It may be possible to elicit a history of discomfort and discoordination. Clinical examination may yield several signs of inadequate ventilation. Inspiratory activity may be present (diaphragmatic, sternocleidomastoid, and alae nasa activity); this is particularly significant when it occurs without mechanical assistance. During constant-flow ventilation, inspiratory muscle activity can be detected from irregular asymmetric pressure tracings when these are displayed continuously. If intrathoracic pressure is not measured, an estimate can be made by observing the changes in central venous pressure. During a positive pressure breath, intrathoracic pressure should rise. Conversely, if the central venous pressure falls markedly during a mechanical breath, respiratory muscle activity is likely.
Although hypocapnia has traditionally been considered to be a mechanism underlying suppression of respiratory muscle activity in the mechanically ventilated, cessation of triggering can also be achieved by adjusting tidal ventilation in eucapnic patients. This is particularly evident in patients with airflow limitation ventilated with large-volume breaths.
When the interface between ventilator and patient is inefficient, asynchrony occurs. Triggering or detection of the onset of a spontaneous breath is generally achieved by detecting pressure or flow changes. This is generally measured within the ventilator, although sensitivity increases when the measurement is nearer to the patient. If the lung offers a high resistance to airflow, both pressure and flow signaling is delayed. If the trigger is moved even closer to the respiratory muscles, time delays and inspiratory work can be reduced considerably.
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