I got a capnograph - now what?

When I graduated from veterinary school, lo these many years ago, safety in anesthesia was maximized by choosing only healthy patients and working quickly. Monitoring of the patient's physiologic state - if conducted at all - was often limited to placement of a pulse oximeter. Over the past couple of decades, however, the types of patients and procedures we encounter on a routine basis have become more complex, necessitating that veterinarians up their monitoring game. It's one thing to anesthetize a 4 month old healthy dog for a 15 minute spay - and quite another to keep a 15 year old cat with a heart murmur and a creatinine of 1.8 mg/dL safe during a 3 hour dental procedure.

That ETCO2 is a bit high, indicating hypoventilation. You may want to consider manually or mechanically ventilating your patient when ETCO2 is above about 55 mmHg. (Ok the BP is on the high side too - yikes! And why is the temperature probe registering room temp? Let's make use of our equipment options!).

Many practices now have multiparameter physiologic monitors that include blood pressure, electrocardiography, pulse oximetry, temperature, and capnography. All this information is great, but in order to make the most of it, you have to be able to analyze and apply the data that you are collecting. This means that the most important monitor available to veterinarians is a highly-trained anesthetist (usually a technician, but sometimes another veterinarian) who can interpret the physiologic data and make adjustments in supportive care to maintain a fairly normal physiologic state for the patient.

Monitoring of carbon dioxide in expired gases during human anesthesia has been the standard of care in high income countries since the late 1980s, when it was found that the combined use of pulse oximetry and capnography reduced the risk of airway and ventilation accidents leading to potentially fatal hypoxemia (see ASA Closed Claims Analysis https://pubmed.ncbi.nlm.nih.gov/2339799/). In July 2011, the American Society of Anesthesiologists expanded that standard to include capnography for sedated, non-intubated patients, due to the risk of respiratory compromise in these patients.

What is capnography and what does it tell us?

Capnography is the continuous measurement of carbon dioxide concentrations in the patient's airway over the respiratory cycle. Monitors that detect CO­­2 and display a waveform are called capnographs. The capnogram is the graphical waveform of expired CO2 concentrations vs time. Capnometry is numerical measurement of expired CO2 without an associated waveform. End tidal CO2 is the measurement of carbon dioxide at the end of exhalation, is abbreviated ETCO2, and is a noninvasive estimate of CO2 concentrations in the arterial blood.

Capnography provides information about four aspects of the anesthetized state:

1. Ventilatory adequacy Hypoventilation is one of the most common anesthetic complications. A little CO2 buildup is ok in healthy patients but a lot of CO2 in the blood can cause acidemia that may effect cellular function.
   

2. Pulmonary perfusion/cardiac output If the lungs aren’t perfused (poor cardiac output or massive pulmonary thromboembolism), no gas exchange occurs. In fact, because capnography is a noninvasive reflection of cardiac output, it is the single most useful monitoring tool during CPR efforts, indicating the quality of compressions and predicting return of spontaneous circulation.
   

3. Breathing circuit and endotracheal tube (ETT) patency Detection of CO2 in expired gas is the gold standard for assessment of correct placement of the ETT. If CO2 is present in the expired gas, the ETT is in the trachea and is not obstructed. Additionally, expired CO2 indicates that the breathing circuit/capnograph adaptor are also connected to the ETT.

4. Metabolism – under- or over-production of CO2 at the tissue level will affect expired CO2 concentrations.

The Normal Capnogram

Phase I - inspiratory baseline. Inspired carbon dioxide should always be zero. Phase II - begins at 0 as dead space gas moves out of the respiratory tract and then rapidly increases as alveolar gas empties. Phase III is the alveolar plateau. Phase 0 is the beginning of inspiration and CO2 rapidly returns 0. Occasionally, you will see this labeled as phase IV.

There is much information that can be gleaned by looking at the shape of the capnogram. For example, an obtuse alpha angle indicates uneven emptying of alveoli, usually due to bronchospasm or partial airway obstruction. An obtuse beta angle may mean a leak around the ETT cuff. Lack of a return to baseline means that rebreathing of carbon dioxide is occurring. In an excellent NAVAS-VIN rounds session in May of 2021, Dr. Teela Jones DVM DACVAA covered abnormal capnograms and what to do in response. Below are a few commonly seen waveforms.

Mild but progressive hypoventilation. Due to the effects of anesthetics and analgesics on the brainstem, mild hypoventilation is expected during sedation and anesthesia. Supplementing oxygen prevents hypoventilation from leading to hypoxemia but ETCO2 concentrations over about 55 mmHg should be addressed though manual or mechanical ventilation.

The X axis (time) is compressed on this graph so you can see the trend. Sudden loss of the capnogram is a potentially emergent situation as it can mean any of the following: apnea, disconnection of the ETT from the circuit, complete airway obstruction, extubation, or cardiac arrest. The first response to this is to check your patient, ensuring that an arrest has not occurred, and then investigate the airway integrity and manually ventilate.

Rebreathing of CO2 indicated by a lack of return to baseline and inspired CO2 > 0 mmHg. Equipment issues are typically the underlying cause and can include too much mechanical dead space for the patient's tidal volume, exhausted CO2 absorbent, or too low of a fresh gas flow on a nonrebreathing circuit. Inspect your equipment to ensure proper function.

A sloping inspiratory downstroke is usually indicative of a leak around the ETT which is entraining room air and diluting out ETCO2 readings. Check the cuff on the ETT and inflate gently to secure the airway & avoid contamination of your breathing space with waste gas.

Decreasing ETCO2 can be indicative of overzealous ventilation. However, if ventilation is reasonable and unchanged, it could be reflective of decreased cardiac output and pulmonary perfusion. In addition, if the decrease is sudden, it could indicate that a pulmonary thromboembolism has occurred.

This is my favorite capnogram because it looks like a shark swimming to the left! The obtuse alpha angle indicates uneven emptying of alveoli, usually due to bronchospasm causing obstruction to expiration. This is a classic waveform for a cat having an asthmatic crisis during anesthesia. Bronchodilators such as terbutaline, albuterol, or even epinephrine may be indicated.

Capnography gives you so much information about your patient's physiology and it really shines because it can be an early warning of things that can lead to hypoxemia. How do you use capnography during anesthesia?