Vol. 22 • Issue 8 • Page 14
A resident once told me mechanical ventilation was sexy. This was his way of trying to weasel out of a discussion on the proper method to wean an asthma patient-the direct result of some potentially dangerous weaning orders written on the chart.
The respiratory resident didn’t understand the difference between weaning a cardiac post-op patient per protocol and weaning an asthmatic.
The first issue I had to address: Mechanical ventilation is a bridge to recovery, not a destination. The idea is to provide support when the patients need it and to withdraw it safely and effectively while minimizing negative outcomes.
Or put more clinically: We are temporarily restoring gas exchange while minimizing their work of breathing until they can resume these functions on their own.
For asthma patients, gas exchange usually is not the problem. We could sedate them, crank up the positive end-expiratory pressure, boost the tidal volume, and achieve excellent gas exchange and blood gas results suitable for textbook weaning protocols (and per our written orders in this case). We also will never get them off the ventilator this way.
The asthmatic’s problem is an increased work of breathing caused by high airway resistance from the accumulation of airway secretions, constriction of airway smooth muscle and inflammation of the airway mucosal lining-aka the asthma attack.
In direct contrast to the aforementioned weaning orders for this case, most studies suggest that a controlled hypoventilation strategy with permissive hypercapnia is the preferred way to wean an asthmatic patient. Monitoring of the airway plateau pressure is essential to achieving this safely.
Plateau pressures in the target range of 20-30 cm H2 are usually recommended as a guide for adjustment of a minute volume, which should probably start in the 90-130 mL/kg ideal body weight range.
It is not uncommon for asthma patients to have intrinsic auto-PEEP caused by their disease process, so the importance of monitoring airway pressures to optimally ventilate and prevent barotrauma cannot be stressed enough.
As their airflow obstruction (airway inflammation and secretions) subside, their pressures will change and their ventilator settings should be adjusted to facilitate weaning. From a clinical perspective, the airway pressures are more important indicators and a better guide for weaning these patients than the Qam blood gas.
Some data suggest helium-oxygen (heliox) mixtures can benefit asthma patients. In theory, because the heliox has a lower density than oxygen and standard room air gas mixtures, the turbulent flow at the site of the airway obstruction is converted to a more laminar or smoother flow. The ventilation becomes easier as airway resistance and work of breathing decrease.
This smoother flow also facilitates the use of a smaller pressure gradient to deliver the same flow rate with a heliox mixture than the flow rate necessary in an oxygen/room air mixture.
Keep in mind: Delivery of an FIO2higher than 40 percent is difficult with heliox; however, some anecdotal reports describe its success. Practitioners also should note specific monitoring requirements exist when using heliox, and not all mechanical ventilators are equipped to deliver the mixture.
Some are heliox-ready; others can be adapted. It depends on the type of ventilator being used and the software package installed on the unit.
Other limited data show the use of extracorporeal life support (ECLS) in the management of asthma patients. ECLS alleviates the occurrence of ventilator-induced lung injury because the patients aren’t intubated, or they are just minimally supported.
ECLS takes on the role of balancing the physiologic gas exchange needs, so airway resistance and work of breathing aren’t an issue. Most of the data for ECLS use in asthma patients are limited to case reports originating in the emergency department.
In fairness, a large controlled trial of ECLS in asthmatics would be almost impossible to conduct due to the potential risk to the patients when other proven treatment modalities are available.
In the ED setting, much data show the effective use of noninvasive ventilation for asthmatics. This modality is usually a safer, cheaper option.
With our particular case, the patient weaned from the ventilator and did fine. The resident never agreed with our hypoventilation strategy.
Mixed kudos should go to the pulmonary fellow for re-writing the weaning orders, though he could have jumped into the conversation at any time. Hiding behind the monitor at the desk and snickering does not constitute helping respiratory.
The take-home lessons from this case are:
1. Mechanical ventilation is a bridge, not a destination.
2. It is important to understand the underlying physiology of the disease state when weaning.
3. Residents: Please don’t branch out and write orders that you don’t understand. You’re here to learn, not to teach us “your method.” (And no, we won’t try it just once and call it a study.)
Margaret Clark is a Georgia practitioner.
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2. Tuxen DV, Andersen MB, Scheinkestel CD. Mechanical ventilation for severe asthma. In: Hall JB, Corbridge TC, Rodrigo C, Rodrigo GV, editors. Acute asthma: assessment and management. New York: McGraw Hill; 2000. pp. 228-9.
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