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July 1, 2011
Capnography and CPAP: Making the best better
What can be said about the single most important intervention to enter the pre-hospital scope of practice since CPR? Continuous Positive Airway Pressure (CPAP) reduces the need for intubation by more than 70 percent, and reduces the mortality rate from 25 percent without it to 8 percent with it. It is simple to apply and patients generally tolerate it well.
Capnography. A lot can and should be said about capnography because, despite the overwhelming amount of literature to support its use, very few services use it beyond verification of tube placement. The American Heart Association (AHA) recognizes that capnography is not just for ventilation, but also provides vital information regarding perfusion. The AHA recommends its use both during and after cardiac arrest resuscitation.
So how can the two be used together and significantly help the patient? The simple answer is that, for patients with bronchospasm, the capnography waveform provides you with information about the severity of the bronchospasm and the effects of your therapies such as nebulized albuterol.
In Congestive Heart Failure (CHF), patients are usually tachypneic and may or may not be retaining CO2. As they improve with CPAP and nitrates, their work of breathing improves which, will be reflected in a slower rate with normal waveform and level of CO2.
The more clinically significant answer is that capnography can help identify those patients who are failing to improve with CPAP and may require urgent advance airway interventions such as Rapid Sequence Intubation (RSI) . Without capnography, the most reliable indicator of respiratory failure is level of consciousness. If the patient is alert enough to assist you in holding the CPAP mask to their face, then they probably have sufficient drive to breathe.
However, if you wait until they lapse into unconsciousness before addressing their respiratory arrest, their mortality rate increases dramatically. As the work of breathing increases, end tidal CO2 rises. In fact, in a preliminary study I conducted, I found that if the CO2 level rose more than 10 points after CPAP initiation, 100 percent of patients ultimately failed and required intubation either in the ambulance or the ER.
I have found that, initially, CO2 levels rise in most CPAP cases. This occurs while the patient is adjusting to the pressure and accepting the coaching required to gain the most benefit from CPAP. Therefore, if you set the initial CO2 level and monitor it closely, you can predict who is going to ultimately require advanced airway measures. If you don’t have RSI , you can prepare to bag the patient and notify the ER of the potential need for intubation upon arrival. If you do have RSI, then your medical director may provide protocols that permit intubation or advanced airway placement prior to respiratory arrest.
Now that you see the benefit of capnography and CPAP, the question becomes how to perform it.
The type of CPAP device and capnography sensor you use will affect your ability to obtain an accurate reading.
In a study I conducted last year, I found that the following is required:
1. First, you must use a closed circuit CPAP device. Open circuit devices cause significant wash out as the high flow of oxygen and air passing the sensor cannula dilutes the patient's expired carbon dioxide.
2. Secondly, a device should provide intermittent CPAP flow only during inhalation and exhalation, and provide the most accurate waveforms as continuous flow devices blunt the physiological capnograph waveform.
3. Thirdly, where you place and what type of sensor you use significantly affects the reading. Despite claims by CPAP manufacturers, placing a capnography sensor anywhere other than a nasal or oral cannula results in artificially low CO2 readings. You cannot sample CO2 anywhere but at the patient's mouth. Additionally, you must use a cannula that samples both from the nasal and oral passages, because patients in respiratory distress will breathe primarily through their mouth and then start breathing through their noses as they improve.
Together, CPAP monitored with capnography will improve your care of these critically ill patients. You don’t have to be a pulmonary physiologist to understand it. In fact, capnography provides you more information about your patient's ventilation and perfusion status than pulse oximetry, which has become as commonplace as oxygen.
Now is the time to incorporate the added benefit of capnography into your scope of practice, and CPAP is a great place to start.