A Guide to Capnography in Small Animal Anesthesia

The term capnography is derived from the ancient Greek word kapnós, which means “smoke,” and in medical terminology, the prefix capno- refers to carbon dioxide. Capnography is the measurement and monitoring of carbon dioxide (a product of cellular metabolism) in expired air, which provides useful information about the ventilation, respiratory, and cardiovascular status of anesthetized or critically ill patients.

In practice, the veterinary nurse anesthetist is integral to the effective use of capnography as a monitoring modality. Responsibilities include appropriate equipment selection and setup, continuous monitoring/interpretation of values and waveforms, and systematic troubleshooting of abnormalities. These actions support early detection of equipment- or patient-associated complications and contribute directly to patient safety and management.

This article reviews fundamental pulmonary physiology; describes equipment and waveform interpretation; and provides practical clinical applications, troubleshooting strategies, and step-by-step instructions for effective use of capnography with intubated patients (VIDEO PLAYLIST). Many types of capnography monitoring systems are available. The author uses a multiparameter veterinary patient monitor (LifeWindow LW9x, Digicare Animal Health) equipped with Masimo SET pulse oximetry and a capnography module. Capnography technology is modular and may vary between systems; selected images in this article include monitors utilizing Masimo-based capnography.

Pulmonary Physiology

Relevant to capnography is gas exchange in the respiratory system,^1,4 which facilitates oxygen delivery to vital organs and tissues and removal of carbon dioxide. During inhalation, oxygen diffuses into arterial blood; during exhalation, carbon dioxide from venous blood (diffused into alveoli) is eliminated (FIGURE 1). The efficiency of this process depends on ventilation, respiration, diffusion, and perfusion.¹ Capnography records the process as end-tidal carbon dioxide (ETco₂), the concentration or partial pressure of carbon dioxide at the end of exhaled breaths; ETco₂ is measured in millimeters of mercury (mm Hg).¹

Figure 1. Gas (oxygen and carbon dioxide) exchange in the alveoli. Twinkle picture/shutterstock.

Capnographs

A capnograph consists of a sensor that measures the carbon dioxide concentration in the sampling line or adapter and a monitor that displays the results as numeric values and graphs of the waveform.¹ The choice between mainstream and sidestream capnography depends on patient size, clinical context, and available equipment.^1,3

Mainstream capnography measures carbon dioxide directly at the airway (an infrared sensor is in the adapter) and provides immediate, real-time monitoring of intubated patients. However, mainstream adapters are bulky and increase the weight at the endotracheal tube. Within the adapters are electrical components that may overheat if placed near warming devices.³

The following are clinical considerations for use of mainstream capnography adapters:

• Because mainstream adapters add additional weight to the breathing apparatus, when they are used on smaller patients, care should be taken to not inadvertently disrupt endotracheal tube placement when connecting.
• Caution is warranted when positioning thermoregulation devices; mainstream adapters are prone to overheating if under warm air blankets or in contact with heating pads.³

Sidestream capnography aspirates a small volume of gas through a sampling line to a sensor. A water trap is often included and is located before the sensor to help prevent moisture from contaminating the gas sample. Sidestream adapters, which attach the endotracheal tube to the sampling line, are lighter and usually less expensive to replace than mainstream adapters. However, because of the more remote location of the sensors, sidestream sampling introduces a slight delay and is susceptible to moisture buildup (FIGURE 2) or damage (cracks in the line).³

Figure 2. Moisture buildup in sidestream adapter.

The following are clinical considerations for use of sidestream adapters:

• Although quite versatile, care and attention when using these adapters is paramount. Moisture buildup or damage to the sampling line contribute to erroneous readings; therefore, close inspection before use is imperative to ensure that all components are intact and are free from fluid.
• It is helpful to keep additional adapters on hand for easy replacement as needed.
• Rotating adapters between procedures help reduce condensation buildup.

Capnograph Adapters

Capnograph adapters connect the patient’s airway (via endotracheal tube) to the capnography monitoring system (FIGURES 3 AND 4). Adapters should be selected according to patient size to minimize mechanical dead space (FIGURE 5) and respiratory workload, particularly in small patients.^1,3 Always refer to the manufacturer’s guidelines to verify endotracheal tube size limitations for pediatric capnograph adapters. The inner diameter of the adapter should either match or be larger than the size of the endotracheal tube. If the diameter of the adapter is smaller than that of the endotracheal tube, air flow will be restricted or reduced, which increases resistance and respiratory effort for the patient. If the adapter is too large, excessive dead space can affect ventilation in small or compromised patients and distort ETco₂ readings.^1,3

Capnography Waveforms

The capnogram is classically divided into 4 waveform phases (FIGURE 6), which are displayed as graphs on the capnograph monitor.¹

Phase I: Inspiratory baseline.

Phase II: Expiratory upstroke (mixing of dead space and alveolar gas) immediately after the end of inspiration, indicating rapid washing out of the fresh gas mixture in the anatomic dead space. This phase should be steep.

Phase III: Expiratory plateau (alveolar gas). ETco₂ is measured at the end of phase III.

Phase 0: Expiratory downstroke (introduction of fresh gas), occurring immediately after the end of expiration and brought about by the rapid washing out of carbon dioxide as the fresh gas mixture enters the airway.

Figure 6. Waveform phases.

Setting Up and Using the Capnograph

Accurate readings depend on proper setup, zeroing of the sensor, correct positioning of sampling lines, and moisture management. The sensor should never be zeroed while connected to oxygen or to the patient.^1,2

Step 1: Turn on the multiparameter monitor or capnograph monitor (practice dependent).

Step 2: If using a multiparameter monitor, turn on the capnograph module (if not automatically on). Note: If the capnograph module is not on when the machine is first turned on, or if the sampling lines/adapter have been changed, zero the sensor while it is exposed to room air (refer to the manufacturer’s guidelines for your specific machine).

Step 3: Select the appropriate adapter size (adult versus pediatric) for the patient.

Step 4: Inspect the adapter, sampling line, and water trap (sidestream) or electrical connection cord (mainstream) for damage and/or moisture buildup.

Step 5: Connect the adapter between the patient’s endotracheal tube and the breathing circuit. Upright positioning of the sampling line of a sidestream adapter can reduce moisture buildup.

Interpreting Waveform Abnormalities

Capnography requires interpretation within clinical context, providing information used in conjunction with parameters derived from other monitoring modalities to evaluate the clinical status of the patient and guide decision-making during the anesthesia event. Observing waveform shape (FIGURE 7) and correlating numeric ETco₂ can help with troubleshooting. A systematic approach to abnormalities, first evaluating patient physiology and then evaluating equipment and connections, improves resolution of the problem and patient safety.¹

Figure 7. The various types of waveform abnormalities that can be observed during capnography.

Equipment-Associated Abnormalities

Capnography values can be distorted by equipment-associated abnormalities, such as a leaky endotracheal tube cuff or a kinked or blocked endotracheal tube.

Step 1: Verify that there are no urgent issues affecting the patient (e.g., closed pop-off valve, incorrect intubation, cardiopulmonary arrest, apnea secondary to induction agents).

Step 2: Check the endotracheal tube for leaks, mucus plugs, accidental extubation, or disconnection from the circuit.

Step 3: Confirm anesthesia equipment integrity by checking for leaks, cracks, or kinks that can distort values. Confirm circuit limb integrity and look for inner tube damage if using a coaxial circuit (i.e., Universal F or Bain).

Step 4: Replace water trap or occluded sampling lines as condensation can impede accurate readings. It is helpful to have extra sidestream adapters on hand to swap out between patients in order to mitigate moisture buildup or replace them if damaged.

Step 5: Check for exhaustion of carbon dioxide absorbent and properly functioning 1-way valves.

Patient-Associated Abnormalities

Patient-associated waveform abnormalities can indicate hypercapnia, hypocapnia, or elevated inspiratory carbon dioxide; other abnormalities, some of which may also be equipment-associated, can be indicated by absent waveform, obtuse β angle, obtuse α angle, and curare cleft.^1-3

Hypercapnia

ETco₂ greater than 55 mm Hg may indicate hypoventilation (decreased alveolar ventilation secondary to shallow breathing or low respiratory rate), increased dead space, or increased metabolic carbon dioxide production. Hypercapnia (FIGURE 7, TABLE 1) is commonly associated with anesthesia-induced respiratory depression or inadequate ventilatory support. Severe hypercapnia may result in respiratory acidosis, vasodilation, dysrhythmias, and carbon dioxide narcosis. If capnography indicates hypercapnia, follow the steps in BOX 1.^1,2

Hypocapnia

ETco₂ lower than 30 mm Hg may indicate hyperventilation (increased alveolar ventilation secondary to increased respiratory rate) or decreased pulmonary blood flow and cardiac output, limiting carbon dioxide delivery to the lungs. Hypocapnia (FIGURE 7, TABLE 2) can lead to respiratory alkalosis and vasoconstriction. If capnography indicates hypocapnia, follow the steps in BOX 1.^1,5

Elevated Inspiratory Carbon Dioxide

Inspiratory carbon dioxide is elevated if the waveform baseline is greater than zero (FIGURES 7 AND 8). Failure of the baseline to return to zero indicates rebreathing, most commonly resulting from exhausted carbon dioxide absorbent, malfunctioning unidirectional valves, or inadequate fresh gas flow (TABLE 3). If capnography indicates elevated inspiratory carbon dioxide, follow the steps in BOX 1.^1,2

Absent Waveform

An absent waveform (FIGURE 7) may indicate esophageal intubation, apnea, circuit disconnection, capnograph adapter disconnection, sampling line damage/occlusion, endotracheal tube occlusion, equipment failure, or cardiopulmonary arrest and must be treated as an emergency until proven otherwise.¹

Step 1: Alert the veterinarian.

Step 2: Check patient status, vital signs, and anesthesia depth to be sure that the patient is not apneic. Rule out cardiopulmonary arrest.

Step 3: Check the endotracheal tube for correct placement, mucus plugs, or disconnection from the circuit.

Step 4: Check the anesthesia machine and circuit. Be sure that the pop-off valve is open if the patient is not being mechanically ventilated, that there is no damage to the circuit limbs, and that all connections are intact.

Step 5: Check to see if the modality is working properly, and verify sampling line integrity.

Obtuse β Angle Waveform

A leaky cuff or leak in the system/circuit generally causes changes to the expiratory downstroke on the waveform (FIGURE 7), secondary to an inadequate seal potentially causing reduced ability to properly inhale fresh gases primarily via the anesthesia machine and breathing circuit. In such instances, atmospheric gases (room air) may be inhaled, leading to inadequate anesthesia depth. A leaky endotracheal tube cuff could potentially lead to aspiration or difficulty maintaining an appropriate plane of anesthesia.¹

Step 1: Check patient status, vital signs, and anesthesia depth.

Step 2: Check the endotracheal tube; correct any leak by filling the pilot balloon with an appropriate amount of air, if needed.

Step 3: Inspect the circuit and apparatus for damage or disconnection.

Obtuse α Angle Waveform

Often referred to as a shark fin, this waveform (FIGURE 7) is displayed as a sharp incline to the expiratory plateau and is secondary to partial endotracheal tube blockage, kinks, or bronchospasm (e.g., in feline patients with asthma).¹

Step 1: Alert the veterinarian.

Step 2: Check patient status, vital signs, and anesthesia depth. Auscultate the lungs bilaterally to confirm that breath sounds are audible.

Step 3: Check the endotracheal tube, and apply suction or reintubate with a different tube as needed.

Step 4: Treat bronchospasm according to veterinarian orders (bronchodilators may be indicated).

Curare Cleft Waveform

Curare cleft is a waveform represented by small indentations (FIGURE 7) and may be seen with patients that are undergoing mechanical ventilation, indicating spontaneous respiratory effort or ventilator–patient desynchronization. This waveform may also appear if there is increased pressure on the patient’s thorax (e.g., surgical equipment or personnel leaning/pressing on the thorax). Other potential reasons a patient may be breathing against mechanical ventilation are light plane of anesthesia, pain, hyperthermia, hypercapnia, or hypoxemia.^1,5

Step 1: Alert the veterinarian.

Step 2: Check patient status, vital signs, and anesthesia depth. Treat pain or anesthesia depth accordingly. Be sure that oxygenation and ETco₂ levels are normal. Check the patient’s temperature.

Step 3: Check for pressure on the patient’s thorax.

Summary

Capnography is a noninvasive and highly informative monitoring modality that integrates pulmonary physiology, technology, and clinical decision-making, significantly enhancing patient safety in veterinary anesthesia and critical care. Veterinary nurses play a central role in using this modality through accurate setup, continuous monitoring, interpretation, and systematic troubleshooting of abnormalities. As veterinary practices continue to adopt higher monitoring standards, capnography remains a cornerstone of anesthesia safety and is a core recommendation in resuscitation guidelines.⁵