Factors associated with hypotension in dogs under general anesthesia

Inspired by: Dr. Natalie Chow, DVM, DACVAA, Lakeshore Animal Health Partners, Mississauga, Ontario, Canada

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In literature

Miller L, Duncan JC, Handel IG, Shaw DJ, McKenzie HE, Greenhalgh SN. Association between body mass and hypotension in dogs under general anesthesia. J Small Anim Pract. 2023;64(11):687–695. doi:10.1111/jsap.13671

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The study…

Hypotension is one of the most common complications during general anesthesia in dogs, along with hypoventilation and hypothermia. It occurs in up to 38% of cases and can lead to delayed recovery, neurological deficits, intestinal integrity disturbances (e.g., suture dehiscence), and renal ischemia and damage.

In this retrospective study, anesthesia records of 1,789 dogs at a referral clinic were examined. Hypotension was defined as at least two consecutive mean arterial pressure (MAP) measurements below 60 mmHg, spaced at least five minutes apart. Blood pressure measurement was performed invasively or non-invasively, depending on the anesthesiologist's assessment.

The following factors were significantly associated with an increased risk of hypotension:

• Brachycephalic breeds
• A higher ASA status (> III)
• Performing a surgical (as opposed to a diagnostic) procedure
• Bradycardia

In contrast, the following factors were associated with a lower risk of hypotension:

• Higher body weight
• Use of an alpha-2 agonist for premedication
• Higher body temperature during anesthesia

Hypotension occurred at least once in 32 cases (%), with a median time of 31 minutes after induction of anesthesia until the first occurrence. The duration of anesthesia had no significant influence on the risk of hypotension.

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🩺 What is a Hypotension in the dog?

📌 Definition

Hypotension describes a abnormally low blood pressure. This applies to dogs as well. mean arterial pressure (MAP) below 60 mmHg or a systolic pressure below 90 mmHg as critical, since the organs are then no longer adequately supplied with blood – and therefore with oxygen and nutrients.

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🧬 Physiology: Why is blood pressure important?

Blood pressure is the pressure the heart generates to pump blood through the circulatory system. Stable blood pressure is crucial for:

• Tissue and organ perfusion
• Maintaining the Kidney function
• Oxygen supply to the brain
• Metabolic stability

If the pressure is too low, these functions can be quickly impaired.

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⚠️ Causes of hypotension in dogs

🔹 During anesthesia:

• Vasodilation (Vascular dilation) by inhalation anesthetics such as isoflurane or sevoflurane
• Bradycardia (slowed heartbeat)
• Myocardial depression (Heart muscle activity is inhibited by medication)
• Volume deficiency (e.g., due to fasting, blood loss, or dehydration)
• Hypothermia (Loss of body temperature → slowed metabolism & reduced cardiac output)

🔹 General medicine:

• Heart disease (e.g. dilated cardiomyopathy)
• Sepsis or systemic inflammation
• Endocrine disorders (e.g. Addison's disease)
• Blood loss, shock, trauma

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🧩 Symptoms of hypotension

Often difficult to recognize, as symptoms are nonspecific:

• Weakness, apathy, lethargy
• Pale or cyanotic mucous membranes
• Prolonged capillary refill time (>2 seconds)
• Feeling cold, low body temperature
• Slowed pulse
• Tachycardia or bradycardia
• In severe cases: Collapse, unconsciousness, kidney failure

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🧪 Diagnosis

• Blood pressure measurement (non-invasive: Doppler, oscillometry / invasive: arterial catheter)
• Monitoring of accompanying vital parameters (Heart rate, EtCO₂, temperature)
• Blood tests to determine the cause (e.g., hematocrit, electrolytes, kidney function)

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🛠 Treatment and Management

Acute therapy:

• Volume replacement: Crystalloid liquids (e.g. Ringer's lactate, NaCl 0.9 %)
• Medication:
  • Atropine in bradycardia
  • Vasopressors (e.g. dopamine, dobutamine, norepinephrine)
• Heat retention: Heat mats, heat lamps
• Reduction of the depth of anesthesia, if possible

Prevention:

• Good premedication with a stable circulation profile
• Early monitoring measures, especially in high-risk patients
• Thermal management, Maintain stable fluid balance
• Regular blood pressure checks – every 5–10 minutes under anesthesia

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🐾 High-risk patients

Special caution is advised in the following cases:

• Small or geriatric dogs
• Brachycephalic races (higher vagal tone)
• Heart or kidney patients
• Dogs with high ASA status (>III)

Hypotension in dogs is a serious, potentially life-threatening complication, especially under anesthesia. A early detection and targeted intervention are crucial to Consequential damage to organs (especially kidney, brain, intestines) to avoid.

Regular monitoring, risk assessment and good anesthesia management are the best measures for prevention and control.

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🩺 What is the ASA status?

📌 Definition

The ASA status is a classification of General condition of a patient before anesthesia and was originally by the American Society of Anesthesiologists (ASA) developed.

It serves to... anesthesiological risk to assess and document in a standardized way – both in human and veterinary medicine.

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🎯 Goal of ASA status:

• Risk assessment before anesthesia or surgery
• Communication aid in the medical team
• Basis for the selection of:
  • Anesthesia records
  • Monitoring intensity
  • Postoperative care

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📊 ASA classification (1–5) in veterinary medicine

ASA status	Description	Example with dogs
ASA I	Healthy patient without illness	Young, healthy dog for castration
ASA II	Mild systemic illness without functional impairment	Mild skin infection, stable heartworm disease without symptoms
ASA III	Severe systemic disease with impairment of bodily function	Grade II heart failure, chronic kidney failure
ASA IV	Severe, life-threatening systemic disease	Shock, decompensated heart failure
ASA V	Moribund patient, survival without surgery not expected.	Gastric torsion, severe trauma, terminal illness

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⚠️ Additional classification: „E“ for „Emergency“

If an operation emergency needs to be done „E“ attached (e.g. ASA IV-E).

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🧠 Practical significance

• The higher the ASA status, the more The risk of anesthesia is higher.
• Patients with ASA III or higher need:
  • More intensive monitoring (e.g., invasive blood pressure monitoring)
  • Special anesthetics (less cardiodepressant)
  • Often pre- and post-anesthetic management (infusion, pain therapy)
• The ASA status is not dependent on the type of surgery, rather solely from the state of health of the animal.

The ASA status is an important tool, in order to systematically assess a dog's health status and anesthesia risk.
He helps veterinarians to, safe decisions to make decisions during the planning and administration of anesthesia – and especially To specifically protect at-risk patients.

…and the most important findings the study

Key practical insights:

• Most episodes of hypotension occurred within the first 30 minutes after induction of anesthesia.
• Small patients have a lower ratio of body mass to body surface area, which makes them more susceptible to hypothermia – a possible risk factor for hypotension.
• Brachycephalic dogs exhibit increased vagal tone, which can lead to bradycardia – this reduces cardiac output and increases the risk of hypotension.
• Non-invasive blood pressure measurement (e.g. Doppler, oscillometry) is technically more difficult and often less reliable in small or brachycephalic dogs.
• Although invasive measurement provides more precise data, observing trends with the non-invasive method (e.g., continuously decreasing end-tidal CO₂ level, drop in heart rate and blood pressure) can indicate hypotension early and allow for intervention.

🩺 Practical tips for anesthesia monitoring

1. Early and continuous monitoring

• Start monitoring immediately after premedication and put them continuously through all phases (Induction, maintenance, recovery phase) continues.
• Use Multi-parameter monitors, to simultaneously record heart rate, blood pressure, temperature, end-tidal CO₂ (EtCO₂), SpO₂ and ECG.

2. Use blood pressure monitoring in a targeted manner

• Invasive (arterial catheter) in high-risk patients (e.g., small breeds, brachycephalic dogs, ASA > III, long surgery duration).
• Non-invasive (Doppler or oscillometry) with a suitable cuff size (width = approx. 40 % of the leg or tail circumference).
• Pay attention to Trend observation, not just individual values – a steady decline is a warning sign.

3. Watch for signs of hypotension

• MAP < 60 mmHg = critical
• Warning signs:
  • Decrease in heart rate (bradycardia)
  • Decreasing EtCO₂ levels (indication of reduced perfusion)
  • Pale mucous membranes, prolonged capillary refill time

4. Keep body temperature stable

• Small dogs cool down faster → use heat pads, heat lamps and pre-warmed IV fluids.
• Hypothermia promotes bradycardia and further lowers blood pressure.

5. Choose premedication consciously

• Alpha-2 agonists (e.g. dexmedetomidine) can reduce hypotensive risk, but must be used with caution in patients with heart disease.
• Have anticholinergics (e.g., atropine) on hand, especially for bradycardic patients.

6. Individually adjust fluid therapy

• Standard: 5–10 ml/kg/h crystalloid infusions
• In case of hypotension, possibly... Bolus (e.g. 3–5 ml/kg over 5–10 minutes), then check the effect
• Too much volume → risk of volume overload, especially in cardiac patients

7. Team communication and documentation

• Clear communication within the operating room team regarding changes in vital signs.
• Complete documentation of every change and reaction to it (e.g., bolus administration, temperature adjustments, etc.)

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📌 Key principle for hypotension prevention:

„"Small, cold, short of breath – especially vigilant!"“
→ Small, hypothermic, brachycephalic patients have an increased risk of hypotension!

5 in-depth questions on the subject „"Hypotonia in dogs under general anesthesia"„

Comprehensive summary on the topic: Hypotension in dogs under general anesthesia

The Hypotension in dogs under general anaesthesia Hypotension is one of the most frequent and potentially most dangerous complications during surgical or diagnostic procedures. Studies show that up to 38% of all anesthetized dogs experience at least one episode of hypotension. Effective prevention, early detection, and treatment of hypotension are crucial. Hypotension in dogs under general anaesthesia is therefore a central goal of every anesthesiological management in veterinary medicine.

The Hypotension in dogs under general anaesthesia This arises from the complex interplay of various factors. The most common causes include drug-induced vasodilation, myocardial depression, bradycardia, hypothermia, and hypovolemia. Inhalation anesthetics such as isoflurane or sevoflurane systemically reduce vascular resistance, leading to a drop in blood pressure. If the cardiovascular system does not adequately counteract this, the typical [symptoms] result. Hypotension in dogs under general anaesthesia.

high-risk patients for Hypotension in dogs under general anaesthesia These include, in particular, small dogs, brachycephalic breeds, geriatric patients, animals with pre-existing cardiovascular conditions, and patients with a high ASA (American Society of Anesthesiologists) status. Special care is required with these animals because the Hypotension in dogs under general anaesthesia occurs more quickly and can have more serious consequences.

Monitoring vital signs is essential to detect an emerging condition. Hypotension in dogs under general anaesthesia Early detection is crucial. In particular, continuous measurement of mean arterial pressure (MAP), end-tidal CO₂ (EtCO₂), heart rate, oxygen saturation, and body temperature provides information about the circulatory status. A MAP below 60 mmHg is considered clinically relevant. Hypotension in dogs under general anaesthesia.

Non-invasive blood pressure measurement methods such as Doppler or oscillometry are helpful, although inaccurate in very small or restless patients. Invasive arterial measurement is the gold standard, especially for quantitative assessment and continuous monitoring of blood pressure trends. Hypotension in dogs under general anaesthesia.

If left untreated, the Hypotension in dogs under general anaesthesia This can have serious consequences: It leads to reduced organ perfusion, which can particularly damage the kidneys, the central nervous system, and the intestinal mucosa. Wound healing can also be impaired by insufficient blood flow. Postoperative morbidity and mortality increase significantly when the Hypotension in dogs under general anaesthesia is not recognized and treated in time.

The therapy of Hypotension in dogs under general anaesthesia Treatment depends on the cause and severity. Key measures include reducing the depth of anesthesia, administering fluid boluses with crystalloid solutions, giving atropine in cases of bradycardia, and the targeted use of inotropes and vasopressors such as dobutamine, dopamine, or norepinephrine. Normothermia should be maintained through active heat management measures to prevent temperature-related complications. Hypotension in dogs under general anaesthesia to avoid.

An important aspect in the prevention of Hypotension in dogs under general anaesthesia The individual anesthesia protocol is crucial. Choosing appropriate premedication, especially alpha-2 agonists in stable patients, can improve circulatory stability. However, their use must always be carefully considered on an individual basis, as these medications can also lead to bradycardia – an additional risk factor for… Hypotension in dogs under general anaesthesia.

Documentation and trend monitoring are essential: A single measurement can... Hypotension in dogs under general anaesthesia They cannot be reliably detected. Only close monitoring every 5 minutes allows for the timely detection of dangerous drops in blood pressure. Accompanying parameters, such as a decrease in EtCO₂, can also indicate reduced perfusion – often an indirect sign of... Hypotension in dogs under general anaesthesia.

Thermoregulation plays a key role. Many small dogs quickly develop hypothermia due to their high surface area to volume ratio, which can lead to serious health problems. Hypotension in dogs under general anaesthesia can exacerbate the situation. The use of heating mats, heat lamps, and pre-warmed infusion solutions is a simple but effective protective mechanism.

In summary, the Hypotension in dogs under general anaesthesia A multifactorial, frequent, and clinically relevant complication. It requires a high degree of vigilance, experience, and a well-thought-out, patient-specific approach. Only through targeted preparation, continuous monitoring, differentiated treatment options, and structured communication within the surgical team can the risk of [this complication] be minimized. Hypotension in dogs under general anaesthesia successfully minimize.

A holistic approach to anesthesia management, which also pays attention to postoperative aftercare and fluid balance, can help mitigate the consequences of the Hypotension in dogs under general anaesthesia to further reduce the risk. The goal must be to ensure not only the patient's survival, but also their complete recovery – without the long-term consequences of an undetected or inadequately treated condition. Hypotension in dogs under general anaesthesia.