Tuesday, August 31, 2010

Flow Mismatch: Patient Ventilator Asynchrony Associated With Volume Ventilation

From post surgical patients to patients with ARDS volume ventilation (VC-CMV or VC-IMV) remains a very popular modality. Traditional volume ventilation is easy to use and comprehend, extensively available on numerous ventilators, and is able to provide adequate gas exchange by presetting minute ventilation. Additionally, VC-CMV is the most common mode used to ventilate patients with ARDS[i], due to fact that the operator may limit the delivered tidal volume thus insuring a low tidal volume strategy. 

Even though, volume ventilation optimizes the ventilation/perfusion relationship by guarantying minimum ventilation, it cannot provide patient ventilator synchrony during spontaneous inspiratory efforts. A common flow asynchrony associated with volume ventilation is “Flow Mismatch”. Flow mismatch results when the patient’s respiratory drive increases and the fixed/set flow rate cannot provide enough assist to meet the patient’s demands. This can be present in patients with a high metabolic demand (e.g. burns, sepsis, fever), patients ventilated with a low tidal volume strategy (4-6 ml/kg), agitation; due to pain, substance abuse withdraw, ICU psychosis, sleep deprivation or anxiety, and during sleep wake cycles.

Flow mismatch is easy to identify from ventilator waveform analysis.
When utilizing volume ventilation evaluate the pressure waveform first, this will provide the most information in regards to changes in lung mechanics, appropriate flow setting, patient effort, and synchrony. Flow mismatch is identified by ‘pressure scooping’ in the pressure waveform.

Flow mismatch as evidence by the scooping in the pressure (blue) waveform, an indication of inappropriate flow to meet patient demands.

Flow mismatch may be present during any part of the inspiratory phase, the above image demonstrates flow mismatch during the middle of the inspiratory flow phase or mid-inspiratory flow. Additionally, the operator can monitor the “Airway Occlusion Pressure” at 0.1 Second (P0.1) to quantify excessive effort.

To correct flow mismatch titrate the flow rate to match the patient’s inspiratory demands. Another corrective action is switching from a constant flow pattern to a decelerating flow pattern this provides a high initial peak flow. One must consider that changes in ventilatory demand may result in unnecessary higher than average assist resulting in ventilator induced diaphragm dysfunction[ii], a lower PaCO2 set-point, and delay in liberation.

To prevent flow mismatch consider using pressure control based modalities (e.g. PC-CMV, PS-CSV, Adaptive Pressure Control) this allows for the inspiratory flow to increase automatically during changing ventilatory demands. Basic pressure modes do allow for the patient to receive inspiratory flow based on their demand however, these modes may not unload ventilatory muscles sufficiently in patients with compromised lung mechanics (e.g. ARDS). Kallet[iii], when comparing VC-CMV to PC-CMV and Adaptive Pressure control the pressure modalities provided no advantage over VC-CMV with a high peak flow rate.

Another consideration is utilizing advance pressure based modes (PAV, NAVA) which provide both unlimited inspiratory flow and decrease work of breathing by providing assistance proportional to the patients demand (PAV) or relative assistance to the demand detected by a neural signal (NAVA) . Adaptive Support Ventilation may also be considered, this advance pressure based mode utilizes the least work of breathing equation to determine frequency and tidal volume. Further more the operator can adjust the percent minute ventilation setting to allow for additional ventilator assistance during periods of increased respiratory drive.

Lastly, consider increasing sedation if the patient’s ventilatory demand and/or tidal volume exceed clinical goals.

Flow mismatch is a common patient ventilatory asynchrony associated with volume ventilation, which may lead to cardiovascular instability, increased oxygen consumption, increased carbon dioxide production, increased patient discomfort and prolonged mechanical ventilation[iv]. Fortunately, flow mismatch can be simply identified with the proper assessment of the pressure waveform, and can be prevented by utilizing pressure based ventilatory modalities.

[i] Esteban et. al. American Journal of Respiratory Care Medicine. 2008;177:170-177
[ii] Levine, S. et. al. Rapid Disuse Atrophy of Diaphragm Fibers in Mechanically Ventilated Humans.
New England Journal of Medicine. 2008;358 (13): 1327-1225.
[iii] Kallet, R. et. Al. Work of Breathing During Lung-Protective Ventilation in Patients with Acute
Lung Injury and Acute Respiratory Distress Syndrome: A Comparison Between Volume and
Pressure Regulated Breathing Modes. Respiratory Care. 2005; 50 (12): 1622-1631.
[iv] Steinburg, K. & Kacmarek, R. Should Tidal Volume be 6 ml/kg Predicted Body Weight in Virtually all Patients with Acute Respiratory Failure? Respiratory Care. 2007; 52 (5): 556.
5 Nilsestuen, J. & Hargett, K. Using Ventilator Graphics to Identify Patient-Ventilator Asynchrony. Respiratory Care. 2005; 50 (2): 202-231.