A DOPE approach to capnography

Tim Nowak // September 25, 2020

Sponsored by Bound Tree Medical

Use this strategy when applying capnography to measure the efficacy of oxygen therapy, especially when troubleshooting

Whenever something goes wrong with regards to an intubated patient, the acronym DOPE – for displacement, obstruction, pneumothorax and equipment failure – should pop into your mind.

But how, if your tube is already set in place, will you know that something went wrong? After all, you saw the tube pass through the vocal cords, secured it appropriately and even applied a cervical collar to the patient to help restrict movement of their head.

The answer: Start with capnography.

Well before you notice the patient’s belly getting bigger (due to gastric inflation) or see their trachea deviate (which you’ll likely never actually see just looking at the patient), your capnograph should be your first line of notification that something’s wrong. That’s how you’ll know that your ventilations are no longer effective and your patient is no longer being oxygenated.

Even though capnography is a measure of the patient’s perfusion and ventilation – not necessarily oxygenation – there still is a correlation. When your capnograph waveform doesn’t seem to make sense, or your EtCO2 value seems to be off – along with the patient’s SpO2 value – it’s time to start troubleshooting to find out why.

D is for Displacement

An intubated patient without ventilations being delivered will surely lead to hypoxia (apnea will do that to anyone). If your endotracheal tube becomes displaced, biologic apnea occurs despite the fact that you’re still ventilating the patient, because those breaths aren’t going to the right place. Thus the patient – the most important person in this equation – develops apnea. So, how do we detect this?

If you saw your tube go past the vocal cords, you’ve confirmed its initial placement by auscultation and visualization, and you’ve even applied a colorimetric verification device, there may be no doubt that your tube was placed correctly – but there’s no proof that it stayed in place. In-line capnography measuring accomplishes this because it shows you real-time data regarding your patient’s status. It shows you real-time proof that ventilations are being provided to the patient’s lungs and that oxygen is being delivered (even at an FiO2 of 21%).

Whenever your EtCO2 value rapidly drops or disappears or your waveform goes flat, think about tube displacement. Check to make sure your 8.0 endotracheal tube is still properly secured at 24 centimeters at the teeth. Insert a bougie to show the pathway of your tube’s placement, deflate your tube’s cuff and re-visualize where your tube actually is resting. Verify where you’re actually ventilating and oxygenating.

O is for Obstruction

Even outside the context of an intubated patient, the O in DOPE can be applied to other respiratory emergency patients, such as those experiencing a COPD exacerbation.

If your patient’s lungs are obstructed – either due to fluid/mucus buildup or restricted flow pathways – your capnograph may provide you with the evidence you need to seek better options to provide ventilatory and oxygenation therapy. Not all respiratory problems result in “shark fin” presentations – and not all shark fins are fixed by albuterol and ipratropium, alone.

If your patient’s oxygenation level is below the normal baseline or is acutely low due to an acute exacerbation of a respiratory disease process, it may also be reflected in their capnograph and EtCO2 presentations. “Air trapping” can lead to carbon dioxide buildup, despite rapid breathing. This often represents respiratory acidosis.

How can we fix it? With high-powered oxygen to remove the obstruction (referencing CPAP – continuous positive airway pressure). More surface area to touch oxygen molecules leads to better oxygenation, and this can first be noticed through abnormalities presented on your capnograph.

P is for Pneumothorax

Check for tracheal deviation, jugular vein distention and absent breath sounds – do you really ever notice all three? What would be great here is to notice a “barcode” of defined black, white and gray lines while looking at your patient’s lungs (referencing the use of ultrasound), but turning to your capnograph to determine this form of obstruction may be one of your best early indicators.

Unlike the O for obstruction consideration outlined prior, the P for obstruction consideration is more of a perfusion measurement (and a good play on O for oxygen obstruction versus P for perfusion obstruction). When someone suffers a pneumothorax – and an ultimate build-up of tension – it results in decreased cardiac output. Decreased cardiac output results in decreased oxygenation and decreased perfusion, which manifests itself as decreased carbon dioxide (which isn’t always a good thing).

Hypocapnea makes sense in the context of a patient that is rapidly breathing, but it doesn’t make sense when the patient is not rapidly breathing or is apneic altogether. This presents a ventilation/perfusion mismatch, and a (tension) pneumothorax may be its culprit.

In the context of your ventilated patient – or even your awake patient that has a nasal EtCO2 cannula in place – any time that your patient’s SpO2, EtCO2, respiratory rate and blood pressure don’t seem to make sense when compared with one another, your mind should gravitate toward a perfusion (V/Q) problem, and a cardiac output obstruction (e.g., pneumothorax, pulmonary embolism) just might be the culprit.

E is for Equipment

If something isn’t working correctly, it’s always fair to rule out operator error or equipment error. After checking your monitors to get somewhat of a status update, double-checking your connections may be the next logical step to take.

Constricted intake lines on your EtCO2 adapter may make your monitor alarm as "no" CO2 becomes detected, or an SpO2 sensor clip that was inadvertently removed as your patient was lifted from the floor and placed onto your cot could also lead to beeping and flashing lights.

Just like ACLS courses of the past have taught us in asystolic rhythms, always remain wary of those disconnected limb leads. Without a sensor connected or intact to detect exhaled CO2, you’re sure to receive faulty readings respective to your ventilations (and ultimately, oxygenation).

On top of all of these factors is the need to document your findings. Whenever your monitor senses an issue, it will likely track and notate it as it interfaces with your patient care reporting software. Instances of inaccurate, false or largely disparate readings need to be either deleted from the record or documented accordingly. Follow your protocols and policies regarding these instances.

The old adage, "If you didn’t document it, it didn’t happen," always remains true, but so does its cousin stating that, "If you did document it, you’re now liable to account for it."


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