When choosing anesthesia monitoring equipment, it’s tempting to ask whether a less expensive device can accomplish the same job as a more sophisticated one. Monitoring respiratory function is essential for patient safety during anesthesia, and many commercial monitoring devices are available at different price points. Some veterinary practices elect to use simpler devices, such as apnea alarms, believing they provide a comparable level of respiratory monitoring at a fraction of the cost.
But how much useful information does a patient’s respiratory rate actually provide? Is a respiratory rate of 12 breaths per minute an indicator of adequate ventilation and oxygenation? And when anesthetic drugs depress respiratory function, can a device that crudely detects breaths provide the information needed to recognize a developing problem?
What Does the apAlert Actually Measure?
Respiratory depression is very common during general anesthesia and may result from the effects of anesthetic and analgesic drugs, patient positioning, obesity, muscle weakness, or pre-existing respiratory disease. These factors can significantly impair ventilation, even when spontaneous respiratory movements continue. If unrecognized and uncorrected, inadequate ventilation can lead to hypercapnia, respiratory acidosis, and avoidable anesthetic complications. But recognizing these problems requires more than simply confirming that the patient is breathing. It requires monitoring whether ventilation is actually adequate.
The apAlert is one of the most widely used apnea alarms on the veterinary market, with more than 20,000 units reportedly sold worldwide. The device uses a temperature-sensitive sensor positioned within the breathing circuit to detect changes in gas temperature as the patient inhales and exhales. These temperature fluctuations are used to identify respiratory cycles, allowing the device to display respiratory rate and sound an alarm when breathing is no longer detected for a predetermined interval.
The appeal is understandable: the apAlert is relatively inexpensive, simple to operate, and provides an audible indication of respiratory activity. However, the information it provides is fundamentally limited and often misleading.
The apAlert does not directly measure airflow, tidal volume, minute ventilation, alveolar ventilation, carbon dioxide elimination, or oxygenation. Instead, it measures temperature changes and uses these as a surrogate for respiratory activity. It therefore cannot determine how much gas is moving through the respiratory system or whether sufficient alveolar ventilation is occurring to maintain adequate gas exchange.
A patient can continue breathing at an apparently normal rate while developing severe hypoventilation and hypercapnia, without triggering an apnea alarm.
A Misleading Marketing Claim: Does the apAlert Measure Oxygen Supply?
One particularly concerning statement published in promotional material for the apAlert claims that the device measures oxygen supply to the patient and effectively performs the function of a pulse oximeter, “without the fuss, bother, or cost.“
This comparison is physiologically incorrect and misleading. To understand why, let’s distinguish three separate physiological concepts:
Respiratory gas movement is the movement of gas in and out of the respiratory system. This is what the apAlert attempts to detect by using temperature changes as a surrogate for air movement.
Oxygenation refers to the oxygen content or partial pressure of oxygen in blood. Pulse oximetry estimates arterial hemoglobin oxygen saturation (SpO₂), providing information about arterial oxygenation. Oxygenation depends not only on the movement of gas through the airway, but also on the inspired oxygen concentration, delivery of oxygen to the alveoli, diffusion across the alveolar-capillary membrane, and adequate pulmonary perfusion—factors that can, and do, change during anesthesia.
Oxygen delivery refers to the amount of oxygen transported to the tissues per unit of time. It depends primarily on cardiac output and arterial oxygen content. The movement of gas through the airway provides virtually no information about either of these variables, making the respiratory temperature fluctuations detected by the apAlert almost entirely unrelated to the actual delivery of oxygen to the tissues.
These are not interchangeable measurements. The apAlert provides no information on oxygen concentration, arterial oxygen saturation, arterial oxygen content, or oxygen delivery. It cannot determine whether the patient is hypoxemic or whether adequate oxygen is reaching the tissues.
Life-threatening hypoxemia can occur in patients that are generating respiratory movements. Small tidal volumes insufficient to ventilate the alveoli can still produce enough air movement to prevent an apnea alarm from activating.
Claiming that a respiratory movement detector performs essentially the same function as a pulse oximeter is physiologically false. The two devices measure fundamentally different variables, and the apAlert provides none of the information necessary to assess arterial oxygenation. Presenting them as interchangeable risks misleading veterinary professionals into believing they are providing monitoring that the device is incapable of performing.
Why Respiratory Rate Is Not Enough
Consider two anesthetized patients, both breathing at a rate of 12 breaths per minute. The first patient has an appropriate tidal volume and adequate alveolar ventilation. The second patient’s respiratory effort is poor, and the tidal volume is severely decreased. Although the respiratory rate is identical, most of each breath the second patient takes only moves air back and forth in the trachea and bronchi (anatomical dead space). Ventilation of the lungs themselves is inadequate, and carbon dioxide starts to accumulate in the blood, causing a dangerous drop in pH.
An apnea alarm may display the same respiratory rate for both patients. Capnography, however, reveals an increasing end-tidal carbon dioxide concentration (ETCO₂) in the second patient, prompting assessment and intervention.
This illustrates an important physiological principle: alveolar ventilation depends on both respiratory rate and the volume of gas reaching the alveoli. A normal respiratory rate does not prove that ventilation is adequate.
What Does Capnography Tell Us?
Capnography continuously measures carbon dioxide in respiratory gases, displaying both a numerical ETCO₂ value and a waveform representing changes in carbon dioxide concentration throughout the respiratory cycle.
It provides several important types of information.
1. Adequacy of Ventilation
In addition to respiratory rate, ETCO₂ provides valuable information about alveolar ventilation.
In patients with relatively stable carbon dioxide production and pulmonary perfusion, increasing ETCO₂ commonly indicates decreasing alveolar ventilation. This allows the anesthetist to identify hypoventilation and intervene before severe hypercapnia and respiratory acidosis develop.
Conversely, a decrease in ETCO₂ may indicate increased alveolar ventilation, reduced pulmonary perfusion, or another physiological or equipment-related change.
ETCO₂ is not always equivalent to arterial carbon dioxide tension (PaCO₂), particularly in patients with ventilation-perfusion abnormalities. Nevertheless, continuous capnography provides substantially more useful information about ventilation than respiratory rate alone.
2. Airway Patency and Breathing Circuit Integrity
The presence and morphology of a capnogram provide valuable information about respiratory gas movement through the airway and breathing circuit.
Changes in waveform shape may indicate airway obstruction, bronchospasm, or other abnormalities.
An abrupt disappearance of the capnogram may indicate apnea, accidental extubation, breathing circuit disconnection, or a sampling-system problem.
Importantly, the absence of a capnogram is not itself diagnostic of any one complication. It is a warning that requires immediate assessment of the patient, airway, and equipment.
3. Rebreathing and Anesthesia Machine Problems
Capnography can identify inspired carbon dioxide, which should be minimal or absent in a correctly functioning breathing system. An elevated inspired carbon dioxide concentration may indicate:
- Exhausted carbon dioxide absorbent
- Malfunctioning unidirectional valves
- Inadequate fresh gas flow
- Excessive apparatus dead space or other breathing system problems
These abnormalities may be clinically important even when respiratory rate and movement appear normal. Apnea alarms provide no information about inspired carbon dioxide and cannot identify these problems.
4. Cardiovascular Function and Pulmonary Perfusion
Carbon dioxide must be transported from the tissues to the lungs by the circulation before it can be eliminated through ventilation. Therefore, ETCO₂ is influenced not only by ventilation but also by carbon dioxide production and pulmonary blood flow. A sudden drop in ETCO₂, particularly when ventilation remains unchanged, can be a red flag for a substantial reduction in lung perfusion or cardiac output.
During cardiopulmonary resuscitation (CPR), capnography provide useful information about pulmonary blood flow generated by chest compressions and may help identify return of spontaneous circulation.
Thus, capnography provides information extending well beyond respiratory rate.
Can an Apnea Alarm Be Used to Assess Anesthetic Depth?
Another problematic suggestion in apnea alarm marketing is that respiratory depression is an indicator of anesthetic depth. Although anesthetic drugs frequently depress ventilation, the relationship between respiratory function and anesthetic depth is neither sufficiently consistent nor specific enough to support this approach.
Respiratory depression is influenced by numerous factors, including:
- The anesthetic and analgesic drugs administered
- Drug combinations and their interactions
- Patient temperature
- Metabolic and acid-base disturbances
- Respiratory and neurological disease
- Individual patient variability
Severe respiratory depression can occur even without excessive anesthetic depth. Conversely, even excessively anesthetized patients often continue breathing at an apparently acceptable respiratory rate. Respiratory rate alone is not a reliable indicator of anesthetic depth.
Assessment of anesthetic depth requires integration of appropriate clinical signs, including muscle tone, eye position, reflexes, responses to stimulation, and other physiological observations.
Using respiratory depression as a surrogate for anesthetic depth risks confusing a potentially harmful drug effect with the intended anesthetic state.
What Do Current Veterinary Anesthesia Guidelines Recommend?
The 2025 American College of Veterinary Anesthesia and Analgesia (ACVAA) Small Animal Anesthesia and Sedation Monitoring Guidelines identify capnography as a minimum recommended monitoring modality during general anesthesia in dogs and cats. The guidelines specifically recommend continuous time-based capnography because it provides essential information about ventilation, airway and breathing circuit integrity, and pulmonary perfusion.
Although the guidelines acknowledge apnea monitors as an alternative when capnography or capnometry is unavailable, they explicitly recognize that these devices cannot assess the adequacy of ventilation. This acknowledgment should not be interpreted as an endorsement of apnea alarms as an equivalent or acceptable long-term substitute for capnography. The fact that a device may be used when recommended equipment is unavailable does not make it an acceptable substitute for that equipment.
There is an important distinction between managing anesthesia with limited resources and deliberately choosing not to invest in recommended monitoring equipment. Capnography is an established component of modern anesthesia monitoring, and for veterinary practices that routinely perform general anesthesia, it should be considered essential.
The guidelines also emphasize the importance of a dedicated anesthetist who remains with the patient throughout anesthesia. Electronic monitoring equipment supplements continuous clinical observation; it does not replace appropriately trained personnel.
Capnography and pulse oximetry provide different, complementary information, and neither can replace the other. An apnea alarm cannot replace either.
Is the apAlert a Reasonable Alternative to Capnography?
Apnea alarms provide an indirect indication of respiratory activity. But respiratory movement by itself is not particularly useful information for monitoring anesthetized patients and ensuring their safety.
An apnea alarm cannot detect hypoventilation, measure carbon dioxide elimination, assess oxygenation, identify rebreathing, or provide the additional diagnostic information available from a capnogram.
Furthermore, apnea alarms provide little additional information to an attentive anesthetist, who should already be continuously assessing respiratory movements, breathing circuit function, and other clinical signs throughout anesthesia. In this context, an apnea alarm is largely redundant: it provides an audible indication of respiratory activity that should already be apparent through direct observation, without offering meaningful information about the adequacy of ventilation.
More concerning is the use of apnea alarms as a substitute for continuous patient observation, particularly in practices where a dedicated anesthetist is not assigned to each patient. Rather than improving patient safety, relying on an apnea alarm in these circumstances may create a false sense of security, encouraging staff to divide their attention between anesthesia and other responsibilities. This is contrary to the principles of the ACVAA monitoring guidelines, which emphasize the importance of a dedicated individual responsible for monitoring the anesthetized patient.
Capnography, by contrast, provides clinically actionable information that cannot be obtained through direct observation alone. It complements the work of an attentive anesthetist rather than attempting to replace it. For practices deciding how to allocate resources toward anesthesia monitoring, capnography is the accepted standard—and the clear winner against apnea alarms.
The Bottom Line
Veterinary anesthesia monitoring should do more than confirm that a patient appears to be breathing. It should provide the information necessary to recognize physiological abnormalities, identify equipment problems, and intervene before preventable complications result in patient harm.
Apnea alarms detect respiratory gas movement, but provide little useful physiologic information. They do not assess the adequacy of ventilation, measure oxygenation, or provide actionable information about oxygen delivery.
Capnography provides substantially more valuable information about ventilation, airway and breathing circuit integrity, and pulmonary perfusion.
For veterinary practices routinely performing general anesthesia, capnography should be considered essential monitoring equipment, not an optional luxury.
Patients deserve monitoring that identifies clinically important problems, not merely a device that counts breaths.
References
- Bailey K, et al. The American College of Veterinary Anesthesia and Analgesia Small Animal Anesthesia and Sedation Monitoring Guidelines 2025. Veterinary Anaesthesia and Analgesia. 2025;52(4):377–385. https://doi.org/10.1016/j.vaa.2025.03.015
- Grubb T, et al. 2020 AAHA Anesthesia and Monitoring Guidelines for Dogs and Cats. Journal of the American Animal Hospital Association. 2020;56(2):59–82. https://doi.org/10.5326/JAAHA-MS-7055
- apAlert distributor promotional material. Product claims regarding oxygen supply and comparison with pulse oximetry. https://www.apalert.net/details.htm