The Patient
A Critical Care Transport team is called to small 10 bed emergency room for a female in her late teens who presented following an apparent suicide attempt. She ingested a large number of pills (exactly what was ingested and how much is unknown). Upon transport team arrival, it was observed that with every other breath, the ventilator alarms high pressure and low volume. The ventilator settings were volume / assist control, rate of 20 breaths per minute, tidal volume of 500 ml, PEEP of 10 cmH2O, and a fractured of inspired oxygen (FiO2) of 100%. Exhaled (measured) tidal volumes were reading between 0 and 950 ml.
An arterial blood gas (ABG) obtained shortly before arrival revealed a profound metabolic acidosis (pH 6.9, PaC02 32, PaO2 102, HCO3 12.3).
The Problem
What is going on here, and how can the team fix the problem?
Critical Care transport teams are often called to move intubated, mechanically ventilated patients. Regardless of the reason for mechanical ventilation, it is imperative that the patient breathe synchronously with the ventilator in order to achieve adequate oxygenation and ventilation. Patients will often demonstrate asynchrony if the ventilator settings are not meeting patient needs. The result can be triggering issues, mismatched patient inspiratory flow, and problems with cycling of ventilator. This discussion will highlight how a patient with metabolic acidosis who is attempting to compensate by blowing off CO2 can cause the mismatch of inspiratory flow and cycling issues, as a result of their attempts to compensate.
An acute state of acidosis activates the autonomic respiratory center in the brainstem resulting in increased rate and depth of respirations. If the patient is on a ventilator, a vigorous increase in the inspiratory flow rate is required. This flow rate may exceed what has been set on the ventilator, thus not meeting the patient’s demand. This is what is occurring in the described scenario.
The patient in this scenario was attempting to compensate for this with a deep, rapid respiratory rate (this breathing pattern is asynchronous with the ventilator and it appears as though the patient is struggling to breathe). This is shown graphically here:
So, what options do the crew have to fix the problem?
First, let’s consider flow rate. Flow rate, also known as peak inspiratory flow rate, is the maximum flow at which a set tidal volume breath is delivered by the ventilator. Flow rates should be titrated to meet the patient’s inspiratory demands. (For example, higher inspiratory flow rates would be useful is in the presence of obstructive airway disease, where the inspiratory volume will be delivered quickly, but will then allow extra time for prolonged expiration). Flow rates can be set explicitly; 60 L/min is typically used, but most modern ventilators can deliver flow rates between 60 and 120 L/min. Depending on the ventilator, inspiratory flow rates may also be controlled internally by the ventilator as a function of the TV, I/E ratio, and RR and via these other settings. (This is why you must understand the capabilities of your specific transport ventilator!).
A potential fix to this would be to increase the flow, or to change to a ventilation mode that allows for variable flow, such as Pressure Control or Pressure Support Ventilation. Because a patient’s inspiratory effort and flow demand often are variable (as demonstrated in the figure above), changing to pressure control for these patients may still cause asynchrony due if the flow rate is still too low to meet the patient’s peak demand. Unpredictable variable patient flow patterns that exceed what the ventilator is set to provide will still result in asynchrony.
Another possible but dangerous treatment option would be to sedate and/or paralyze the patient for transport. In patients with severe metabolic acidosis, this should be done with great caution due to the possibility of artificially decreasing minute ventilation by settings that do not meet the patient’s needs (i.e., the ventilator will be set to deliver less minute volume than the patient was getting on their own). This decrease in minute ventilation will cause increases in carbon dioxide retention and therefore further exacerbate acidosis. If patient condition warrants sedation and neuromuscular blockade, it is imperative to ensure that an appropriate adjustment in minute ventilation is made to mitigate the potential for a worsening derangement in acid-base balance.
Perhaps a more reasonable strategy would be to change the patient to a spontaneous breathing mode (e.g., CPAP with Pressure Support). Pressure support ventilation allows the patient to generate the breath, and flow is variable. Unlike a pressure control mode, times are not set. Instead of cycling from inspiration to expiration at a set time, the ventilator will cycle when it senses a decrease in flow, following a patient’s own breathing pattern. Typically, cycling occurs when flow drops by 25% of the maximum flow generated. Pressure support ventilation is flow cycled. As the lungs are filling, flow decreases, which may allow the patient to be more synchronous during variable respiratory patterns.
What the Transport Team Did
In the case presented above, the transport team placed the patient in a spontaneous breathing mode on the ventilator, essentially providing CPAP with 12 cmH2O of pressure support. After several minutes, the patient appeared to exhibit increased synchrony with the ventilator as evidenced by an easier respiratory pattern, fewer ventilator alarms, and waveforms that no longer exhibited the appearance of breath stacking. A follow up ABG showed improvement in pH and less CO2 retention.
“Take Home” Points
- Ventilator asynchrony is yet another “clinician induced” ventilator-associated issue that can cause a serious disparity between oxygen supply and demand.
- Completely taking over the patient’s ventilatory requirements provides a dangerous assumption that you as the clinician can fully meet the patient’s oxygen, ventilatory and metabolic demands. Miscalculating this can be fatal!
- A spontaneous mode of mechanical ventilation can provide airway protection while allowing for the patient to continue providing for his / her metabolic and ventilatory needs.
Final thoughts
Employing this aforementioned method of ventilatory support requires that the clinician understand his / her ventilator. A “non-invasive” mode of ventilation can disable vital alarms or may not provide an apnea back-up. A spontaneous mode that provides CPAP and pressure support designed for an “invasive” airway should be utilized if your ventilator is set up for this. Otherwise very close monitoring of patient status during all phases of transport is required.
References
Dres, M., Rittayamai, N., & Brochard, L. (2016). Monitoring patient–ventilator asynchrony. Current opinion in critical care, 22(3), 246-253.
Holanda, M. A., Vasconcelos, R. D. S., Ferreira, J. C., & Pinheiro, B. V. (2018). Patient-ventilator asynchrony. Jornal Brasileiro de Pneumologia, 44(4), 321-333.
Mellott, K. G., Grap, M. J., Munro, C. L., Sessler, C. N., & Wetzel, P. A. (2009). Patient-ventilator dyssynchrony: clinical significance and implications for practice. Critical care nurse, 29(6), 41–55. doi:10.4037/ccn2009612