Does Longer Anesthesia Increase Risk in Dogs? What the veterinary evidence actually shows

By Steve Mehler, DVM, DACVS

The short answer: anesthesia should be performed efficiently, but the evidence does not support a universal time limit or a predictable amount of danger added by every additional minute. Patient health, urgency, procedural complexity, physiologic stability, monitoring, and recovery care are more informative than the stopwatch alone.

Why the stopwatch is an imperfect measure

It seems intuitive that a longer anesthetic must be more dangerous. Anesthetized dogs can develop hypotension, hypoventilation, hypoxemia, hypothermia, reflux, or airway complications. A longer procedure provides more time for these problems to occur or persist.

But duration is not an isolated exposure. A two-hour orthopedic procedure is fundamentally different from a ten-minute diagnostic procedure. Dogs undergoing longer procedures may be sicker, have more invasive disease, experience greater blood loss, or require more complex surgery. Conversely, very short anesthetics may be performed urgently or with less preparation because the procedure is perceived as minor.

That makes anesthesia time both an exposure variable and a marker for many other variables. Observational studies can identify associations, but they cannot ethically randomize comparable dogs to unnecessarily short or long anesthesia. Therefore, they cannot determine a causal risk per minute.

What the major mortality studies found

The CEPSAF studies

The Confidential Enquiry into Perioperative Small Animal Fatalities was a landmark prospective investigation conducted across 117 veterinary practices in the United Kingdom. Brodbelt and colleagues collected approximately 98,000 canine anesthetic and sedation events and reported an overall anesthesia-related mortality near 0.18 percent. Risk was much lower in healthy dogs than in sick dogs, and a substantial proportion of deaths occurred after the procedure rather than during maintenance.

A companion case-control analysis compared 148 dogs that died with 487 dogs that survived. Increasing ASA physical status, greater urgency, older age, lower bodyweight, major procedures, and greater intended procedure duration were associated with higher odds of anesthesia-related death.

The wording matters: the variable was intended procedure duration. It may have represented anticipated procedural difficulty as much as actual anesthetic exposure. This study supports treating duration as a risk marker, but it does not prove that each additional minute independently caused mortality.

France and Spain

Bille and colleagues evaluated 3,546 dogs and cats in a French referral setting. Higher ASA status was again central to mortality risk. Their findings reinforced the importance of underlying disease and perioperative management, but did not establish a universal duration threshold.

Gil and Redondo subsequently followed 2,012 canine anesthetics across 39 Spanish clinics. Mortality was strongly influenced by patient health. The use of opioids together with NSAIDs was associated with lower mortality, suggesting that balanced analgesia and anesthetic management can matter as much as crude elapsed time. Duration was not established as the dominant independent predictor.

Japan and United States primary care

Itami and colleagues investigated preoperative characteristics in dogs anesthetized at Japanese referral hospitals. Higher ASA status and deficiencies in preanesthetic assessment were associated with death. This study addressed the condition in which a dog entered anesthesia rather than proving a causal effect of anesthetic duration.

Matthews and colleagues examined anesthetic-related deaths across 822 primary-care hospitals in the United States. Older age and nonelective procedures increased risk in dogs, while underweight dogs had markedly greater odds of death than dogs that were not underweight. Anesthetic duration was not among the principal independently reported predictors.

The United Kingdom VetCompass study

Shoop-Worrall and colleagues evaluated 157,318 dogs undergoing general anesthesia or sedation in UK primary-care practices. The study again emphasized age, health status, and procedural urgency and type. Differences in electronic-record detail limited the ability to isolate duration as a causal exposure.

The worldwide study of more than 55,000 anesthetics

The most provocative duration finding came from Redondo and colleagues. This prospective worldwide study included 55,022 canine anesthetics from 405 veterinary centers. The investigators grouped anesthetic duration as short, less than 15 minutes; medium, 15 to 60 minutes; or long, more than 60 minutes.

Unadjusted mortality was 1.41 percent in the short group, 0.55 percent in the medium group, and 0.78 percent in the long group. In the multivariable analysis, short procedures remained associated with greater mortality than more prolonged procedures.

The authors proposed that clinicians may be overconfident during very short or unscheduled procedures. Some patients may receive less comprehensive preanesthetic assessment, monitoring, intravenous access, intubation, or optimized premedication because the procedure is expected to finish quickly. This explanation is plausible but remains a hypothesis. The study does not prove that short anesthesia itself causes death, nor does it prove that long anesthesia is protective.

Time still matters physiologically

Rejecting a simple stopwatch rule does not mean that duration is irrelevant. Longer exposure can increase the opportunity for physiologic abnormalities to develop and persist. The important distinction is between total anesthetic time and time spent in an abnormal physiologic state.

Hypotension and respiratory abnormalities

In a retrospective review of 1,281 canine anesthetics, Redondo and colleagues reported hypotension in 37.9 percent, bradycardia in 36.3 percent, hypoventilation in 63.4 percent, and hypoxemia in 16.4 percent of cases, using the study's definitions. These abnormalities were common, but the study did not show that total minutes under anesthesia independently produced them.

From a biologic standpoint, a dog that remains hypotensive for 45 minutes is likely exposed to a different risk than a dog whose blood pressure drops briefly and is corrected. The same principle applies to excessive anesthetic depth, hypoventilation, and hypoxemia. A single total-time number conceals these clinically important differences.

Hypothermia

Hypothermia is one of the clearest cumulative concerns. In the large retrospective canine study by Redondo and colleagues, postanesthetic hypothermia was extremely common. Longer preparation and anesthetic exposure contribute to heat loss because anesthetic drugs impair thermoregulation, cause peripheral vasodilation, and reduce heat production. Small dogs and procedures involving large exposed body surfaces are especially vulnerable.

This supports active warming and temperature monitoring. It does not establish a single safe maximum anesthetic duration. A longer procedure with aggressive temperature management may produce less hypothermia than a shorter procedure in a small dog receiving no warming.

What the 2026 scoping review concluded

Rose, Wobeser, and Pang reviewed the broader literature on perianesthetic death in dogs and cats. They found major differences among studies in case definitions, follow-up periods, practice types, patient populations, and methods used to attribute death to anesthesia. These differences make direct numerical comparisons difficult.

Despite that heterogeneity, several findings recur: increasing ASA status, emergency or nonelective procedures, extremes of age or body condition, major procedures, and inadequate monitoring are consistently important. The review also highlights a change in when deaths occur. In modern studies, the early postanesthetic period is often more dangerous than induction or maintenance.

Recovery may matter more than the final surgical minute

In the worldwide cohort, 81 percent of anesthesia-related deaths occurred postoperatively. During recovery, oxygen demand rises, anesthetic support is withdrawn, the endotracheal tube is removed, and patients may receive less intensive monitoring. Brachycephalic dogs face particular risks from upper-airway obstruction, and every patient can develop hypoventilation, hypoxemia, dysphoria, hypothermia, or cardiovascular deterioration after extubation.

A narrow effort to save a few intraoperative minutes can therefore miss the period in which many deaths actually occur. Safe anesthesia includes continued observation, temperature management, oxygenation and ventilation assessment, appropriate analgesia, and readiness to intervene throughout recovery.

The evidence based conclusion

It is inaccurate to say that anesthetic time never matters. Longer exposure can contribute to heat loss and provides more opportunity for hypotension, hypoventilation, hypoxemia, or other complications. Older mortality data also associated greater intended duration with death.

It is equally inaccurate to claim that each additional minute adds a known amount of risk or that a specific time threshold makes anesthesia unsafe. The newest large worldwide study found higher mortality in procedures lasting less than 15 minutes, illustrating how preparation, monitoring, patient selection, and procedural context can overwhelm a simple relationship with time.

A carefully planned and continuously monitored two-hour anesthetic may be safer than a rushed 20-minute anesthetic. The goal is not to ignore time. It is to manage the patient rather than the stopwatch.

Practical implications

• Assign and document ASA physical status before anesthesia.

• Stabilize correctable abnormalities before elective procedures.

• Treat short procedures as real anesthetics, with appropriate preparation, airway planning, monitoring, and recovery care.

• Monitor trends in blood pressure, ventilation, oxygenation, heart rate, temperature, and anesthetic depth rather than recording isolated numbers.

• Minimize avoidable delays, but never rush positioning, aseptic preparation, analgesia, hemostasis, surgical technique, or recovery.

• Track the duration and severity of physiologic abnormalities, not only total anesthetic time.

• Continue active monitoring through extubation and early recovery.

References

Brodbelt DC, Blissitt KJ, Hammond RA, et al. The risk of death: the Confidential Enquiry into Perioperative Small Animal Fatalities. Vet Anaesth Analg. 2008;35:365-373.

Brodbelt DC, Pfeiffer DU, Young LE, Wood JLN. Results of the Confidential Enquiry into Perioperative Small Animal Fatalities regarding risk factors for anesthetic-related death in dogs. J Am Vet Med Assoc. 2008.

Bille C, Auvigne V, Libermann S, Bomassi E, Durieux P, Rattez E. Risk of anaesthetic mortality in dogs and cats: an observational cohort study of 3546 cases. Vet Anaesth Analg. 2012;39:59-68.

Gil L, Redondo JI. Canine anaesthetic death in Spain: a multicentre prospective cohort study of 2012 cases. Vet Anaesth Analg. 2013.

Itami T, Aida H, Asakawa M, et al. Association between preoperative characteristics and risk of anaesthesia-related death in dogs in small-animal referral hospitals in Japan. Vet Anaesth Analg. 2017;44:461-472.

Matthews NS, Mohn TJ, Yang M, et al. Factors associated with anesthetic-related death in dogs and cats in primary care veterinary hospitals. J Am Vet Med Assoc. 2017.

Shoop-Worrall SJW, O'Neill DG, Viscasillas J, Brodbelt DC. Mortality related to general anaesthesia and sedation in dogs under UK primary veterinary care. Vet Anaesth Analg. 2022;49:433-442.

Redondo JI, Otero PE, Martinez-Taboada F, Domenech L, Hernandez-Magana EZ, Viscasillas J. Anaesthetic mortality in dogs: a worldwide analysis and risk assessment. Vet Rec. 2024.

Redondo JI, Rubio M, Soler G, Serra I, Soler C, Gomez-Villamandos RJ. Normal values and incidence of cardiorespiratory complications in dogs during general anaesthesia: a review of 1281 cases. J Vet Med A Physiol Pathol Clin Med. 2007;54:470-477.

Redondo JI, Suesta P, Serra I, et al. Retrospective study of the prevalence of postanaesthetic hypothermia in dogs. Vet Rec. 2012;171:374. doi:10.1136/vr.100476.

Rose N, Wobeser B, Pang DJ. Perianesthetic death in dogs and cats: a scoping review. J Vet Diagn Invest. 2026.