Preoxygenation – the administration of 100% oxygen to a patient before inducing anesthesia – is a common practice in human and veterinary anesthesia. The goal is to replace the air in the lungs with oxygen, creating a reserve that can delay the onset of hypoxemia (low blood oxygen) if the animal stops breathing or is difficult to intubate during induction. This article provides a detailed look at the pros and cons of preoxygenating dogs and cats, including the underlying lung physiology, the impact of certain respiratory diseases, the benefits and potential drawbacks of preoxygenation, and evidence-based recommendations for its optimal use. While preoxygenation can be life-saving in many situations, it is not always beneficial – a nuanced, case-by-case approach is essential.
Lung Physiology and Functional Residual Capacity (FRC) #
Oxygen Reserve and FRC: In normal lung function, there is an oxygen reserve within the lungs that helps maintain blood oxygenation during brief periods of apnea (no breathing). This reserve is largely determined by the Functional Residual Capacity (FRC), which is the volume of air remaining in the lungs at the end of a passive exhalation. At FRC, the lungs still contain oxygen (along with other gases like nitrogen) that can transfer into the bloodstream even if the animal isn’t actively breathing. In an average human adult, FRC is around 2–3 liters of air; animals have a proportional FRC based on their size. Importantly, FRC represents a balance point where the inward elastic recoil of the lungs is balanced by the outward recoil of the chest wall, meaning the lungs are partially inflated even at rest. This residual air contains oxygen that can sustain the body for a short time. However, during anesthesia induction, several factors can reduce FRC dramatically – sedative drugs, muscle relaxation, and recumbency (lying down) can all decrease thoracic volume. In fact, human studies have shown induction of anesthesia can cause about a 50% reduction in FRC (most humans are induced and intubated in “dorsal” recumbancy).
How Preoxygenation Affects FRC and Oxygen Reserves: Preoxygenation involves having the patient breathe 100% oxygen for a few minutes prior to induction. The aim is to replace the nitrogen in the lungs with oxygen, maximizing the O2 content of the FRC. With room air, the alveoli (air sacs) contain about 21% oxygen; with preoxygenation, the goal is to raise the alveolar oxygen concentration toward 90–100%. This process is sometimes called denitrogenation – essentially filling the lungs with oxygen instead of nitrogen. Hemoglobin in the blood becomes fully saturated, and additional oxygen dissolves in the plasma. The result is a greatly expanded oxygen reserve. For example, in human patients achieving an end-tidal oxygen concentration ~90% (meaning alveolar gas is mostly oxygen), the intrapulmonary oxygen store can reach ~2–2.5 liters. At a typical oxygen consumption of 250 mL/min, this could theoretically provide about 5 minutes of oxygen during apnea. In veterinary medicine, a study in dogs demonstrated this principle clearly: breathing 100% O2 for 3 minutes versus room air extended the time to desaturation (SpO2 dropping to 90%) from about 70 seconds to about 298 seconds (nearly 5 minutes) in healthy, sedated dogs. In other words, preoxygenation can delay the onset of hypoxemia by several minutes, giving the anesthetist a much larger safety margin to secure the airway. This delay is critical because it buys time to intubate the trachea and begin ventilation before the animal’s blood oxygen falls to dangerous levels. However, this maybe an buffer without need if intubation is fast (less than 70 sec) and a secure airway can be obtained reliably.
Mechanism of Delaying Hypoxemia: During apnea (which often occurs for a short period after induction before intubation and ventilation), the body continues to consume oxygen from the lungs. With a preoxygenation-enriched FRC, the animal has a higher reservoir of O2. The oxygen stored in the lungs diffuses into the blood, keeping hemoglobin saturation higher for longer. Essentially, preoxygenation “tanks up” the patient with oxygen. By contrast, without preoxygenation, the small amount of oxygen in the FRC is used up quickly and oxygen saturation plummets within seconds. It’s been reported that dogs who were not preoxygenated can become hypoxemic (SpO2 < 90%) in as little as 30 seconds after induction. With preoxygenation, that critical desaturation was significantly delayed. This benefit is particularly pronounced in animals that have any reduction in FRC or higher oxygen demand. For instance, obese or pregnant animals have reduced functional residual capacity (due to elevated pressure on the diaphragm and reduced thoracic compliance), so they have less reserve and will desaturate faster – preoxygenation is extremely valuable in such cases.
It’s important to note that even though preoxygenation increases the oxygen available in the lungs, it does not directly increase the amount of air (volume) in the lungs – it’s replacing one gas with another. So while FRC (volume) stays roughly the same until induction drugs cause it to drop, the oxygen fraction within the FRC increases. Thus, preoxygenation doesn’t “increase FRC” per se; rather, it maximizes the oxygen within the FRC. By doing so, it effectively utilizes the FRC as an oxygen reservoir to delay hypoxemia when breathing stops. This concept underpins why preoxygenation is a routine safety measure in anesthesia.
Common Pathologies Affecting Preoxygenation #
Not all patients respond to preoxygenation equally. Certain respiratory or cardiovascular conditions can alter lung function such that achieving or utilizing that oxygen reservoir is more challenging. Here we discuss how brachycephalic airway syndrome, pneumonia, and pulmonary hypertension affect lung physiology and the effectiveness of preoxygenation.
Brachycephalic Airway Syndrome: Brachycephalic breeds (such as Bulldogs, Pugs, and Persian cats) have congenital upper airway obstructions – narrowed nostrils, elongated soft palate, everted laryngeal saccules, etc., collectively known as brachycephalic obstructive airway syndrome (BOAS). These animals often have chronic airway compromise and work harder to breathe even when awake. The restricted airflow can lead to lower baseline oxygenation and higher CO2 (because they cannot ventilate efficiently). When inducing anesthesia, brachycephalic animals are at high risk of airway obstruction once they become relaxed, and intubation can be difficult due to anatomical challenges (excess soft tissue, smaller than expected trachea). This combination makes rapid desaturation very likely if any apnea occurs. Preoxygenation is therefore highly recommended in brachycephalic dogs and cats. By filling their lungs with oxygen before induction, we can delay hemoglobin desaturation in the event of apnea or a prolonged intubation attempt. In practice, that means giving several minutes of face-mask oxygen while the animal is still awake (often after a mild sedative to reduce stress). However, brachycephalic animals may not tolerate a tight mask if they are anxious (they already feel airway restriction). Care must be taken to gently restrain and calm them, because stress or struggling will increase oxygen consumption and can counteract the benefits of preoxygenation. Despite these challenges, effective preoxygenation in brachycephalics can be lifesaving. It provides precious extra time to secure their challenging airway, as evidenced by guidelines that stress preoxygenating brachycephalic patients prior to anesthesia to prevent rapid desaturation. In summary, brachycephalic airway syndrome doesn’t prevent achieving a high FiO2 in the lungs, but it makes the process more delicate – these patients benefit greatly from preoxygenation, if it can be performed without causing undue distress.
Pneumonia: Pneumonia (whether aspiration pneumonia, bacterial pneumonia, or viral) involves inflammation and often consolidation (fluid/exudate filling) of portions of the lungs. This leads to areas of lung that are poorly ventilated but may still be perfused with blood – a ventilation/perfusion (V/Q) mismatch. As a result, even breathing 100% oxygen might not effectively deliver oxygen to those diseased alveoli because they are not fully open or air-filled. Pneumonia often causes a right-to-left shunt effect: blood passes through the affected lung regions without picking up oxygen, leading to lowered arterial oxygen levels even if the patient is on O2. Thus, pneumonia can blunt the effectiveness of preoxygenation. The oxygen content in healthy lung regions will increase, but areas of consolidation will remain un-oxygenated. High-risk patients with severe pneumonia may therefore not get as long a safe apnea period from preoxygenation as a normal patient would – the advantage is “blunted in high-risk patients” with lung pathology. That said, these patients are also among those who need any possible oxygen reserve the most. Preoxygenation is still recommended for pulmonary parenchymal disease because it can markedly improve oxygenation of whatever functional lung remains. In conditions like pneumonia, one must also consider that the FRC may be reduced (due to collapsed alveoli and reduced compliance) and the patient’s oxygen consumption might be increased from the work of breathing and fever. All of this means less reserve and faster desaturation. Preoxygenation can partially compensate by maximizing O2 in the healthy parts of the lung. In practice, a patient with pneumonia should receive supplemental oxygen before induction, and induction should be performed rapidly and atraumatically to minimize apnea duration. Intubation and ventilation should then quickly ensure oxygen is delivered to the lungs. In summary, pneumonia alters lung function by creating low ventilation regions; preoxygenation still improves oxygenation, but its benefit may be limited by the diseased portions of lung. It remains a crucial step to “load up” any usable lung with oxygen before induction.
Pulmonary Hypertension: Pulmonary hypertension (PH) is an increase in blood pressure within the pulmonary arteries, and it often occurs secondary to chronic heart or lung disease (for example, chronic mitral valve disease in dogs or chronic pulmonary disease). Pulmonary hypertension by itself does not directly impair ventilation of the lungs, but it indicates serious cardiopulmonary compromise. Animals with PH often have poor oxygenation at baseline due to underlying issues (like heart failure leading to pulmonary edema, or chronic pulmonary disease causing V/Q mismatch). Additionally, they have a limited cardiopulmonary reserve – meaning they may not tolerate even mild hypoxemia or apnea without serious consequences. Preoxygenation is advisable in patients with pulmonary hypertension to ensure their blood oxygen is as high as possible prior to induction. Doing so can help reduce reflex pulmonary vasoconstriction (the pulmonary arteries constrict in low-oxygen conditions, potentially worsening PH). By maximizing oxygen, we may avoid a spiral where hypoxemia exacerbates pulmonary artery pressure. However, similar to pneumonia, the effectiveness of preoxygenation might be limited if there is an intracardiac shunt or severe V/Q mismatch underlying the PH. For instance, some dogs with end-stage pulmonary hypertension develop small openings in the heart (patent foramen ovale) that allow unoxygenated venous blood to bypass the lungs; in such cases, no amount of preoxygenation will fully oxygenate blood that isn’t going through the lungs. Even so, oxygen therapy has been shown to reduce pulmonary vascular resistance in many patients with pulmonary hypertension. Therefore, preoxygenating before anesthesia in PH patients is generally beneficial to counteract any tendency toward hypoxemia. The main impact of pulmonary hypertension on preoxygenation is the high risk nature of these patients – they have zero tolerance for oxygen deprivation. The anesthetist must ensure a smooth, efficient induction with adequate preoxygenation, because even a brief hypoxic episode can precipitate a crisis (such as acute right heart failure or arrhythmias) in a patient with severe PH. In summary, pulmonary hypertension often coexists with impaired gas exchange; preoxygenation helps maximize oxygen delivery and mitigate hypoxia-induced complications, although it may not completely overcome the circulatory shunts or mismatches present. These patients illustrate how critical it is to use preoxygenation as one of many tools to ensure oxygen delivery during anesthesia.
Advantages and Disadvantages of Preoxygenation #
Preoxygenation offers clear benefits, but it is not without drawbacks. Anesthetists must weigh these advantages and disadvantages for each patient and situation. Below we outline the key pros and cons:
Advantages:
- Increased Oxygen Reserves and Delay of Hypoxemia: The primary advantage is a dramatic increase in the body’s oxygen stores. By replacing alveolar nitrogen with oxygen, preoxygenation creates an intrapulmonary reservoir that can supply oxygen to the blood during apnea. This significantly prolongs the safe period of apnea before desaturation. In dogs, as noted, 3 minutes of preoxygenation can delay SpO2 dropping to 90% by up to four- to five-fold (from ~1 minute to ~5 minutes). This extra time is invaluable if intubation is challenging or if there is an unexpected delay in establishing an airway. Essentially, preoxygenation buys time and provides a wider safety margin, especially for high-risk patients (obese, pregnant, brachycephalic, pulmonary disease, etc.).
- Maximizing Hemoglobin Saturation: Preoxygenation ensures that the hemoglobin in blood is fully loaded with oxygen (SaO2 ~100%). An animal breathing room air may have a normal saturation around 97-98%. Breathing 100% O2 for a few minutes will raise this to 100% and also greatly increase the dissolved oxygen in plasma. Although dissolved oxygen is a small component, at very high PaO2 (which can exceed 400 mmHg during preoxygenation), it becomes non-trivial. This means even before apnea, the blood contains somewhat more oxygen than usual, on top of the reserve in the lungs. In critical moments, every bit helps.
- Beneficial for At-Risk Patients: Patients with known risk of rapid desaturation benefit the most. Preoxygenation is routinely recommended in obese animals, pregnant animals, neonates, brachycephalics, animals with heart failure or lung disease, etc., where even a brief apnea could be disastrous. In these cases, the advantage of preoxygenation is magnified because their baseline reserve is poor. For example, brachycephalic dogs often maintain better oxygenation during induction if preoxygenated, as it mitigates the effects of their small airway and any intubation delays. In animals with pulmonary disease, providing 100% O2 can raise arterial oxygen tension and help compensate for areas of the lung that are not oxygenating well.
- Simple and Non-Invasive: Preoxygenation is easy to perform – typically using a snug face mask with an oxygen flow (usually around 100 mL/kg/min) from an anesthesia machine or oxygen source. It does not require advanced equipment or invasive procedures (if the animal tolerates a mask or flow-by).
- Reduces Hypoxemia on Induction and Recovery: While usually discussed for induction, preoxygenation (and a period of 100% O2 breathing) can also be used at extubation (recovery) to buffer against hypoventilation when waking from anesthesia. Additionally, maintaining oxygenation can reduce the reflex pulmonary vasoconstriction in diseases like pulmonary hypertension, as noted, which is another indirect advantage.
Disadvantages:
- Stress and Patient Tolerance Issues: One of the biggest practical drawbacks is that some animals do not tolerate the face mask or nasal insufflation required for preoxygenation. A conscious (or lightly sedated) dog or cat may become frightened or agitated by having a mask on its face or being restrained. Struggling, panting, or fighting the mask can be counterproductive – not only is it stressful for the patient, but it dramatically increases oxygen demand and can cause them to use up oxygen faster. In an extreme case, a stressed patient might even reach exhaustion or start to hyperventilate, neither of which is good before induction. Cats in particular can get very anxious with masks; a frantic cat will have elevated catecholamines and oxygen consumption, potentially nullifying the benefits of preoxygenation. For this reason, if preoxygenation causes significant stress, its benefits may be outweighed and it should be avoided or modified and focus should be on rapid intubation and airway management. It is better to induce anesthesia promptly in a calm manner than to prolong a struggling attempt at preoxygenation. Patient compliance can often be improved with judicious sedation (a calm, sedated animal is more likely to accept the mask). In summary, stress and poor tolerance are major limitations – the animal that needs preoxygenation the most might also be the one least able to tolerate the procedure without agitation.
- Airway Irritation and Discomfort: The flow of dry cold oxygen and the presence of a mask can cause airway irritation in some animals. High oxygen flow rates (necessary to flush out CO2 and nitrogen effectively) blasting into a mask can dry the mucous membranes and may cause coughing or discomfort. Some patients might start to cough or hold their breath if the flow rate is too high or if the mask fit is uncomfortable. Airway irritation could potentially trigger bronchospasm in a reactive airway (for example, a cat with asthma). Overall, while mask preoxygenation is generally safe, a small number of patients might experience minor airway irritation or anxiety from the apparatus.
- Absorption Atelectasis: Breathing 100% oxygen for even a few minutes can lead to absorption atelectasis in the lungs. This occurs because oxygen in alveoli (especially alveoli that are not being well-ventilated or are at the lower lobes with less movement) gets absorbed into the blood faster than it is replaced, since there’s no nitrogen to keep those alveoli inflated. Alveoli that have a low ventilation-perfusion ratio can collapse when filled with 100% O2. Essentially, oxygen is taken up and the alveolus shrinks and collapses (since nitrogen, which normally acts as an inert filler gas, is absent). Atelectasis from preoxygenation has been well documented in human anesthesia – it can often be seen on postoperative lung scans. The clinical impact of these small collapsed segments is usually minor in healthy patients, but in those with marginal lung function it could contribute to reduced oxygenation post-induction. It’s worth noting that despite this known effect, the practice of preoxygenating with 100% O2 remains in human anesthesia because the benefit in preventing hypoxemia outweighs the downside. To counteract absorption atelectasis, one can use techniques like applying a brief positive end-expiratory pressure (PEEP) or a recruitment breath after intubation to re-expand the alveoli, or use a slightly lower FiO2 (like 80%) for preoxygenation in prolonged cases – but using less than 100% O2 will reduce the oxygen reserve gained. In short, absorption atelectasis is a real but manageable drawback: it’s something to be aware of, especially if the patient has lung disease.
- Delayed Detection of Intubation Problems: Another subtle risk of successful preoxygenation is that it can mask signs of an airway management problem. If an animal has been well preoxygenated, their blood oxygen may remain near-normal for a couple of minutes even if the endotracheal tube is accidentally placed in the esophagus or if it is not placed at all. This means the pulse oximeter might not immediately drop, potentially giving a false sense of security that the intubation is correct. In human anesthesia literature, a delayed detection of esophageal intubation is cited as a risk of preoxygenation. In veterinary cases, we rely on multiple confirmation methods (visualizing the tube placement, end-tidal CO2 monitor, chest excursion), but if those are not immediately used, one might not realize the animal isn’t actually being ventilated properly until the oxygen reserve starts to deplete. By that time, the patient could suddenly crash. So, while preoxygenation buys time, it also means you must remain vigilant – do not assume things are fine just because the SpO2 is okay in the first minute or two. Always verify intubation quickly and start ventilation to avoid a hidden disaster. The prolonged saturation is a double-edged sword: good if used wisely, dangerous if it leads to complacency.
- Hemodynamic Effects: Breathing high concentrations of oxygen can cause changes in cardiovascular function. In some patients, especially if they are compromised, hyperoxia may lead to reduced cardiac output or changes in blood flow distribution. For example, oxygen can cause systemic vasoconstriction and a decrease in heart rate (via baroreceptor reflex due to increased O2). Usually, these hemodynamic effects are minimal during a short preoxygenation period. However, if an animal has very fragile cardiovascular status, one could consider that a slight drop in cardiac output combined with sedation could affect oxygen delivery to tissues (even if blood O2 content is high). Overall this is a minor concern – studies have noted that the short duration of preoxygenation is insufficient to cause significant detrimental hemodynamic effects. Still, anesthetists should be mindful in patients with, say, critical anemia or shock that oxygen content in blood is only one part of the equation – maintaining circulation is also key. Preoxygenation does not replace the need for supporting blood pressure and cardiac output.
In summary, the disadvantages of preoxygenation center around practical challenges (patient struggling or not tolerating it) and physiological side effects (atelectasis, delayed signs of hypoxia, minor oxidative and cardiovascular effects). None of these contraindications are absolute – they simply mean that preoxygenation should be applied thoughtfully. If an animal is calm enough to tolerate it (or can be made so with sedation), the benefits usually outweigh the risks. On the other hand, if attempting to preoxygenate will cause a fight-or-flight response, it may be wiser to skip it or use an alternative method. Next, we’ll discuss how to maximize the benefits and minimize the downsides with some best practice recommendations.
Evidence-Based Recommendations for Effective Preoxygenation #
Given the above advantages and drawbacks, how can veterinarians optimally use preoxygenation in dogs and cats? The following evidence-based recommendations can help ensure preoxygenation is performed effectively and safely, providing the most benefit:
- Use an Appropriate Technique (Mask vs. Flow-By): To achieve meaningful preoxygenation, a high concentration of oxygen must reach the patient’s lungs. A tight-fitting face mask attached to an oxygen source or anesthesia circuit is most effective. When done properly, a mask can raise the fraction of inspired oxygen (FiO2) in the lungs close to 1.0 (100%). If a mask is well-sealed, FiO2 of 0.7 (70%) might be attained, which is much higher than room air (21%). In contrast, a flow-by method (holding oxygen tubing or a mask near the animal’s face without a seal) provides only a modest increase – often FiO2 less than 0.4 (40%). Flow-by is less effective in denitrogenating the lungs, but it may be better tolerated by anxious animals since nothing encloses their face. Recommendation: Use a snug mask whenever possible for maximal preoxygenation. If the patient absolutely will not accept a mask, use flow-by oxygen (hold the tube close to the nose/mouth) as an alternative, recognizing it will offer only partial benefit. In some cases (especially in cats), placing the patient in an oxygen enclosure or chamber for a few minutes can increase FiO2 somewhat, but this is usually less efficient and harder to time with induction.
- Adequate Duration – At Least 3 Minutes: The classic regimen is to administer 100% oxygen for 3 minutes prior to induction. This is based on both human and veterinary studies showing that 3 minutes of normal tidal breathing on 100% O2 achieves a high fraction of alveolar oxygen and significantly prolongs time to desaturation. Some protocols extend this to 5 minutes for an extra margin, especially in larger patients or those with lung disease. Remember that when starting the preoxygenation process most anesthesia machines using a rebreathing (circle) system will contain mostly room air. Exchange of that gas with oxygen will occur simultaneously as you are preoxygenating making the process longer than if you were to fill the system with 100% oxygen before beginning to apply the mask to the patient. Realistically, oxygen washin to the FRC will be a time/breath dependent half-life and 3 min may be more than needed when patients are breathing deeply. In human anesthesia, there are rapid techniques (e.g., 8 deep breaths in 60 seconds) for emergency preoxygenation; in an awake animal, however, coaching them to take deep breaths is not feasible. If time allows, a full 3 minutes of preoxygenation is ideal. If an emergency requires rapid induction, even 30-60 seconds of high-flow oxygen is better than nothing – but understand it won’t provide as long a buffer. Evidence: One veterinary study noted that 3 minutes at 100 mL/kg/min flow was effective in dogs, and clinical practice has adopted the “3 minute rule” commonly. Using a pulse oximeter, one can actually observe the SpO2 often climbing to 99-100% during this period, indicating successful preoxygenation (though keep in mind SpO2 won’t tell you how much reserve is in the lungs, just that the blood is saturated).
- Sedation and Calm Environment: To address the issue of patient stress, it is strongly recommended to premedicate or sedate the animal before attempting preoxygenation. A lightly sedated dog or cat may be more tolerant of the mask and handling. For example, administering a dose of a tranquilizer (like acepromazine) and/or an opioid (like morphine, methadone, or buprenorphine) can calm the patient and even reduce oxygen consumption by relieving anxiety. In brachycephalic breeds, sedation is often a crucial step to allow preoxygenation to be performed without the dog panicking. Always ensure the sedation is not so deep that the animal loses their airway – it’s a balance. The sedated animal should still be breathing adequately. Additionally, the environment should be as quiet and stress-free as possible: minimize loud noises, handle the patient gently, and consider having the owner present to calm the pet during preoxygenation (if the pet is comforted by their presence). In one study in dogs, modifying the mask with a soft material (memory foam) made it more tolerable, and all dogs in that study accepted the face mask calmly. Key point: A calm patient will preoxygenate more effectively. If the animal struggles, stop and rethink your approach (e.g., deepen sedation or switch to flow-by).
- Optimal Positioning: Position can affect FRC. In humans, a head-elevated or semi-upright position increases FRC and improves preoxygenation efficacy. In veterinary practice, preoxygenation is often done with the animal in sternal recumbency (upright on chest) if possible, rather than lying flat on the side or back. A sternal position allows better diaphragm movement and may reduce compression of the lungs (especially in obese or pregnant animals). If the animal is very calm or small, one can even hold them in a sitting position with neck extended comfortably to open the airway. Brachycephalic dogs often breathe better in sternal (like the “tripod stance”), so preoxygenating them in that position with their chin lifted slightly can help maximize airflow. Avoid positions that kink the airway or compress the abdomen. While position changes during preoxygenation are a subtle factor, every little improvement in FRC can translate to a longer safe apnea time.
- Avoid Delays After Preoxygenation: Once you’ve spent several minutes preoxygenating, it’s best to induce anesthesia promptly. The benefit of preoxygenation begins to decay as soon as the patient goes back to breathing room air. So, make sure everything is ready for induction before finishing preoxygenation. This means having the induction drugs drawn up, the endotracheal tubes and laryngoscope prepared (multiple sizes especially for brachycephalics) and the personnel organized. When you’re ready, some clinicians will increase oxygen flow to flush the circuit, then give induction drugs while keeping the mask on delivering oxygen until the moment of intubation. In brachycephalic dogs, one technique is to continue delivering oxygen via mask or even a nasal catheter up until the intubation is done, to eke out every second of oxygenation. In any case, do not preoxygenate and then remove the mask and spend minutes fiddling with other tasks – you will lose the gained advantage.
- Special Techniques for Difficult Airways: If an animal is expected to have a very difficult airway (like a dog with a tumor obstructing the larynx, or a cat with laryngospasm prone to obstruction), consider additional methods to maintain oxygenation. One such method is apneic oxygenation – after inducing and during laryngoscopy, you can leave a small catheter delivering O2 into the mouth or nasal passage. This relies on the principle that oxygen will diffuse and some level of gas exchange can occur even without chest movement, as long as there’s an open airway and O2 supply. Another method post-intubation is to use PEEP (positive end-expiratory pressure) immediately after intubation to help re-expand any atelectatic alveoli from preoxygenation. These are more advanced maneuvers but evidence from human medicine suggests they can further extend safe apnea time and mitigate downsides of preoxygenation. For example, in critical cases, providing a few cmH2O of CPAP during preoxygenation (if the animal tolerates it) can increase alveolar recruitment, but this usually requires specialized breathing circuits and a cooperative patient, so it’s rarely done outside of controlled settings.
- When to Avoid or Limit Preoxygenation: As emphasized, if an awake animal is extremely uncooperative or distressed by preoxygenation, it may be better not to attempt it in the usual way. For instance, a fractious cat that cannot be handled may need to be induced quickly with an injectable agent without prior mask oxygen to avoid injuring itself or staff – in such a case, one might try to minimize the risk by pre-placing an IV catheter (for rapid induction) and perhaps using a towel to funnel some oxygen near the cat briefly (but not a full mask). If an animal has a condition where oxygen could be contraindicated (very rare – one example might be a patient on certain chemotherapy drugs like Bleomycin can have lung toxicity exacerbated by high FiO2, or an animal with chronic hypercapnia where hypoxic drive is keeping them breathing – though in induction scenarios this is moot because we control breathing after), those are special considerations. Generally, almost all anesthesia candidates can benefit from preoxygenation except when the process of doing so is more dangerous than not doing it. Use your clinical judgment: if struggling with a mask will likely cause more harm (trauma, extreme stress) than the anticipated benefit, adjust your plan.
- Train and Practice: Effective preoxygenation is a skill. Practice achieving a good mask seal, try different mask sizes and shapes (for example, brachycephalic breeds might do better with a mask that has an oblong shape or even using a clean self-inflating bag mask held gently over the face). Ensure all staff understand the importance so that the extra 3 minutes of time is always incorporated into anesthesia planning for critical cases. By making it a routine part of anesthesia setup (like pre-oxygenating while clipping for a catheter), it becomes second nature and doesn’t feel like it’s slowing down the procedure. Also, always remember to turn off oxygen or remove the mask if you must delay induction for any reason – leaving an animal on 100% O2 for a very prolonged time (tens of minutes) might dry their airway or cause more atelectasis. Thus, incorporate it wisely and monitor the patient throughout.
In conclusion, preoxygenation in dogs and cats is a simple technique with potentially life-saving benefits, but it must be tailored to the individual. Healthy young animals with quick intubation times may not need preoxygenation as critically (they have more tolerance for apnea), whereas brachycephalics, obese, and sick patients almost always should be preoxygenated if possible. The nuanced stance is that preoxygenation is usually beneficial and should be performed in most anesthetic inductions except when it unduly stresses the patient or in certain very mild cases where the risk of hypoxemia is truly negligible.
By understanding lung physiology (FRC and oxygen stores) and how disease conditions affect gas exchange, veterinarians can predict which patients need preoxygenation the most. By weighing the advantages (increased oxygen reserve, prevention of hypoxemia) against disadvantages (stress, atelectasis, etc.), one can make an informed decision for each case. Finally, by following best practices – proper technique, adequate sedation, and preparation – the benefits of preoxygenation can be maximized while mitigating its risks. In practice, preoxygenation has become a valuable component of anesthetic safety in veterinary medicine, but it is not a one-size-fits-all mandate. The optimal use of preoxygenation is to deploy it thoughtfully, ensuring our canine and feline patients get the oxygen support they need, when they need it, without causing harm.
References #
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