The Effect Of Assisted Mechanical Ventilation On Patient-Ventilator Asynchrony

Abstract

Mechanical ventilation is the critical event of intensive care unit (ICU) management. It is a lifesaving intervention in patients with acute respiratory failure .The main objectives of mechanical ventilation are to reverse hypoxemia and acidosis and to relieve respiratory distress and elevated work of breathing. However ,if mechanical ventilation is applied without coordination to inspiratory muscle activity (asynchronous), ventilation can reduce respiratory drive primarily via a chemoreceptor response. Patient-ventilator interaction is a complex topic, where interaction between patient effort, triggering, assist levels, off-cycling. In the conventional modes of MV, one or more of the followingventilation variables are controlled: pressure, flow, volume or time.The greater the number of these variables controlled by the ventilator, the greater the likelihood that asynchrony will develop unless the causes of asynchrony are carefully assessed and properly adjusted to the needs of the patient . Thus, asynchrony can be categorized into four general types: flow asynchrony, trigger asynchrony, cycle asynchrony and mode asynchrony. In Flow Asynchrony: The gas delivery pattern from the ventilator does not match inspiratory pattern of the patient.Trigger asynchrony occurs when the initiation of the inspiratory phase does not occur with the onset of the patient's inspiratory effort.Its forms are : Trigger delay, Missed triggering and Auto-triggering.Cycle Asynchrony: may be premature cycling (termination of assisted breathing despite the patient’s continued effort) or delayed cycling (an assisted breath and resultant inspiratory flow may continue after a patient’s expiratory effort stop).Mode Asynchrony:occurs when the selected mode results in a significant level of asynchrony. New modalities of ventilation using more complex closed loops and integrating physiological principles to overcome this problem have been developed such as proportional assist ventilation (PAV) and NAVA. These modes do not control ventilatory pattern of the patient. The patient is allowed to select the pattern that his respiratory center considers appropriate. Neither pressure, volume, flow, nor time is set by the ventilator but all are controlled by the patient. Vt remaine stable with NAVA and PAV, despite increasing the assist levels so these modes protect against overdistention. During NAVA, the increase in breath-to-breath variability leads to a direct improvement of neuro mechanical coupling. PAV also improves neuro-mechanical coupling in similar ways. NAVA improves the expiratory cycling by suppression of delayed and premature cycling, which is expected because of the EAdi-based termination of pressurization . Limitations of NAVA: it does not suppress double triggering during invasive ventilation.Also,very high levels of NAVA might result in unstable periodic