Image Gallery: Capnography

Figure 1 The four phases of capnography

Capnography has four phases as depicted on capnograms. Phase I is the inspiratory phase; during this time, the CO₂ level should remain at zero. Phase II is the ascending phase represented by emptying of the trachea and bronchi and beginning of alveolar gas. Phase III is referred to as the alveolar plateau, which may appear flat but should have a gentle upward slope. ETCO₂ is the peak at the end of the alveolar plateau. Phase IV is the descending phase represented by termination of expiration and beginning of inspiration as the CO₂ level rapidly falls to zero. The angle between phases II and III, called the alpha angle, should be 100° to 110°. The angle between phases III and IV, called the beta angle, is normally ≈90°. 

Figure 2 Appearance of normal capnogram

In this image, the red arrow points to the capnogram. Note that phase I falls all the way to 0 mm Hg on this normal capnogram. The alpha angle is normal, and the alveolar plateau is a gentle upward slope to the ETCO₂ prior to inspiration. The beta angle is slightly >90°. 

Figure 3 Hyperventilation (hypocapnia)

This capnogram shows the alveolar plateau and ETCO₂ below normal (35-45 mm Hg), which can occur in patients that are receiving positive-pressure ventilation or are breathing spontaneously. If a patient is receiving positive-pressure ventilation, the ventilator should be adjusted to decrease the rate or tidal volume needed for restoring normocapnia. If a patient is breathing spontaneously, the anesthetist should evaluate the animal’s status to determine the cause(s) of hyperventilation (eg, light plane of anesthesia, pain, hypoxemia).

Figure 4 Hypoventilation (hypercapnia)

Note that the alveolar plateau and ETCO₂ are above normal (35-45 mm Hg), which can occur in patients that are receiving positive-pressure ventilation delivering insufficient minute volume or in patients breathing spontaneously. Because of the respiratory-depressant effects of anesthetics, hypoventilation is commonly encountered in patients that are breathing spontaneously while under anesthesia. Depending on the severity and suspected cause of hypercapnia, affected patients should receive intermittent positive-pressure ventilation to restore normocapnia. In some cases, a mildly elevated ETCO₂ may be desirable. This permissive hypercapnia strategy causes sympathetic nervous system stimulation which supports the cardiovascular system.

Figure 5 Patient rebreathing CO2

Note the elevated baseline at phase I of this capnogram. The machine measured an inspired partial pressure (Fi) of 29 mm Hg as shown. Rebreathing CO₂ can be attributed to multiple problems associated with the anesthesia machine, including insufficient flow of fresh gas in nonrebreathing systems, faulty one-way valves (inspiratory or expiratory) in rebreathing systems, and exhausted soda lime. This rebreathing abnormality can be resolved by troubleshooting the equipment.
Image courtesy of Carrie Davis, DVM, University of Tennessee

Figure 6 Cardiogenic oscillations

During the alveolar plateau, the anesthetist may begin noting cardiogenic oscillations (ie, small waves) that are synchronized with the heartbeat. Having one or many cardiogenic oscillations with each expiratory cycle is normal and does not require treatment. Cardiogenic oscillations are believed to be associated with the motion of the heart against the lungs (ie, contact between heart and lungs). Although they are not completely understood, cardiogenic oscillations are more likely to occur in patients with negative intrathoracic pressure, low respiratory rate, low inspiratory:expiratory ratio, low tidal volume, and muscular relaxation.

Figure 7 Checking for leaks

As demonstrated in this capnogram, a leak is most likely occurring around the endotracheal tube cuff. Note the increased beta angle, indicating that a crisp conversion from expired to inspired gas is absent and alveolar gas is thus escaping via a route other than the breathing circuit. The anesthetist should check the circuit for leaks, starting with the endotracheal tube cuff. If no leaks in the circuit can be found, the capnograph sampling line should be inspected to ensure that all connections are tight and the tubing is free of cracks or holes. 

Figure 8 Acute drop in CO2 to zero

Acute drop in CO₂ can be caused by sudden apnea (ie, after patient receives bolus of drug or the ventilator is turned off), machine disconnection, esophageal intubation, or acute obstruction of the endotracheal tube. This is reflected on the capnogram when there is a sudden discontinuation of the capnogram, as seen in this image following the third waveform. The anesthetist should first confirm that the patient is alive and properly intubated and then troubleshoot the incident by ensuring all connections are intact and providing positive-pressure ventilation. Other parameters on a multiparameter monitor (eg, pulse oximetry, ECG, noninvasive blood pressure monitoring) can provide immediate information about the patient’s life status, thereby helping facilitate the anesthetist’s investigation.

Figure 9 Prolonged expiratory upstroke

Note the increased alpha angle associated with the expiratory upstroke, which is indicative of an obstruction. Prolonged expiration can be caused by endotracheal tube obstruction or kinking, positive end-expiratory pressure valves, or obstructive lung disease. If tube obstruction is suspected, the patient should be reintubated with a clean tube. Use of guarded endotracheal tubes can prevent kinking when an acute angle is applied to the cervical region. 

Figure 10 Exponential decrease in ETCO₂ 

An exponential decrease in ETCO₂ can be caused by sudden hypotension, a drop in cardiac output, massive blood loss, a pulmonary embolism, and of greatest concern, cardiopulmonary arrest. The patient should be rapidly assessed to rule out cardiopulmonary arrest before investigators pursue other potential causes.