CPAP as a LOW TIDAL VOLUME VENTILATION STRATEGY

Image 1: Ventilator Screen Shot of Continuous Positive Airway Pressure. 

In October’s issue of the Chest Journal, a few colleagues of mine authored the abstract “The Use of Invasive Continuous Airway Pressure for Low Tidal Volume Ventilation”.

 This abstract was very intriguing to me for various reasons, one this goes against many traditional methods of providing ventilatory support for the patient with Acute Lung Injury and/ or ARDS.

Zealots of Airway Pressure Release Ventilation would argue that using pure CPAP would lead to extreme work of breathing and not off load the respiratory muscles sufficiently.

I myself would be concerned about using CPAP only, especially in extra-pulmonary ARDS primarily sepsis and septic shock where patients metabolic demand is high, which leads to a vigorous inspiratory demand. Off-loading these patients is very difficult and even using APRV or traditional methods leads to severe hypercapnea.  

However, I was baffled so I questioned my friend Troy Whitacre, a coauthor of the abstract to find more details and to share his experience.

Commentary from a Q & A with Troy Whitacre RRT, University of Missouri

Scott: When were patients place on CPAP? As soon as possible? Even, during their most acute and severe phase of ALI or ARDS?

Troy: Some were quite severe and placed on CPAP initially. That is, if tidal volume goal of 6ml/kg was being met or exceeded as inspiratory pressure was titrated toward zero.

Scott:  Did you apply it to patients with extra-pulmonary ARDs (e.g. Sepsis related)?

Troy: Yes. Anyone who responded to or required higher level PEEP, weren’t agitated, tachypnic, flow starved or asynchronous.

Scott:  Any issues with hypercapnea and hypoventilation? Especially in the patients who were septic?

Troy: No more than what we would normally be dealing with in a more conventional ventilator mode. Average spontaneous frequency in the group wasn’t excessive. High PaCo2 was 55torr/Low pH 7.24.

Scott:  What levels of CPAP used? Highs & lows?

Troy:  Average CPAP level 14-23cmH2O.

Scott:  What was CPAP adjusted by? Pao2, tidal volume, work of breathing, esophageal balloon?

Troy:  I wish we had used an EBC to measure pleural pressure swings as huge pressure swings would probably not be helpful. Most of the patients had refractory hypoxemia that only responded to higher level PEEP. Also, this group would not have had acceptable tidal volume with 5cm CPAP but as CPAP was titrated upward, tidal volumes normalized and often exceeded ARDSnet goals.

Scott:  What type of patient failed miserably?

Troy: Patients that was agitated/delirious. Patients with severe acidosis (metabolic & respiratory).

Scott:  How did you wean? Directly from CPAP and how low CPAP? Wean criteria/ extubation?

Troy: Weaned directly from CPAP. If patients tolerated a 30 minute Spontaneous Breathing Trial they were extubated. Patients that were eventually trached, trials were started when CPAP <=10cm.

Scott: Do I think this is for everyone?

Troy: No, although it may be helpful with a subset of patients with ALI/ARDS. I think of it as APRV without the release or HFO without the wiggle. More needs to be done to fine tune who, if any may benefit from this. An EBC may be helpful to determine pleural pressure gradient when using stand-alone CPAP.

After Thought

Utilization of CPAP as a low tidal volume strategy is very appealing to me. This technique could be used on any ICU or sophisticated transport ventilator. This is an easy concept to comprehend unlike APRV. APRV has been utilized for over 20 years and currently is still presented during educational conferences. I do not know the last time I have seen a presentation on invasive CPAP?

As mentioned this technique is not for everyone, I believe the most important thing is continuous bedside monitoring of the patient. I have had the great pleasure to work with Respiratory Care Practitioners at the University of Missouri Medical Center and have experienced their extraordinary ventilator management.    

I believe this technique could be coupled with Mid-Frequency Ventilation and provide a safe, low cost, efficient lung protective strategy for a wider range of patients.

Thanks, Troy.

Read another interview with Troy on the topic of esophageal balloon monitoring for PEEP titration in the following post: “A Problem with Plateau Pressures”.

REALTED POST

Application of Mid-Frequency Ventilation