Hypotension during anesthesia - Importance of measuring blood pressure during anesthesia

The first part of this article is aimed at patient owners and the second part at specialist audiences.

Importance of measuring blood pressure during anesthesia

Stable cardiovascular function is crucial to ensure adequate oxygen supply to the tissues. During anesthesia, measuring blood pressure is an important method to indirectly assess blood flow to the organs. Low blood pressure (hypotension) can have serious consequences, which is why regular monitoring and treatment is essential.

Hypotension during anesthesia
Hypotension during anesthesia 2

How do you recognize blood pressure values ​​that are too low?

Normal values ​​of arterial blood pressure in awake animals are:

  • Systolic blood pressure (SAP): 100-160 mmHg
  • Diastolic blood pressure (DAP): 60-100 mmHg
  • Mean blood pressure (MAP): 80-120 mmHg

These values ​​often decrease under anesthesia. In dogs and cats, a systolic pressure below 80-90 mmHg and a mean pressure below 60-70 mmHg are considered critical. Such levels can affect blood flow to vital organs.

Blood pressure monitoring options

Indirect methods:

  • Oscillometry or Doppler techniques measure blood pressure using an inflatable cuff. Accuracy depends on correct placement of the cuff and its width (approximately 40% of the circumference of the limb).

Direct methods:

  • A cannula is inserted into an artery to precisely measure blood pressure using special equipment. This method is recommended for seriously ill patients.

Treatment of hypotension during anesthesia

Treatment depends on the cause:

  • Volume depletion: Intravenous fluid therapy increases blood volume.
  • Weak heart muscle contraction: Drugs such as dopamine or dobutamine improve the strength of the heart to contract.
  • Vasodilation: Drugs such as phenylephrine or norepinephrine can narrow the vessels.

Reducing the anesthetic dose can also help stabilize blood pressure.


For patient owners: What does this mean for my animal?

During surgery, your pet will be closely monitored to ensure blood pressure, heart rate and oxygen levels remain stable. Low blood pressure (hypotension) is a common complication during anesthesia, but don't worry: veterinarians specialize in recognizing and treating such situations.

If your pet's blood pressure drops, several measures can be taken:

  • Fluid therapy: Your pet will receive fluids intravenously to stabilize circulation.
  • Adjustment of anesthesia: Anesthesia can be reduced to relieve pressure on the cardiovascular system.
  • Medication: Special medications help stabilize blood pressure.

It is important that every treatment is individually tailored to your animal’s health. Speak to your veterinarian if you have any questions or concerns - your animal's well-being always comes first!


The article is based on the publication by Khursheed Mama, DVM, DACVAA, Colorado State University
Last updated: November 2021.

Importance of measuring blood pressure during anesthesia - the technical part

Normal cardiovascular function is essential to ensure adequate oxygen supply to the tissues. Oxygen delivery is defined by the relationship between cardiac output and oxygen levels, but cannot be measured directly. Although cardiac output is not usually measured in animals under anesthesia, blood pressure can be used as an indirect parameter for oxygen supply and organ perfusion.

According to Ohm's Law, which relates pressure, flow and resistance, there are many potential causes of altered blood pressure. To manage hypotension, it is important to understand the influences of disease, anesthetics and other medications, supportive care, surgical procedures, and other factors.

Mean arterial pressure (MAP) as an application of Ohm's law:

  • MAP = cardiac output × systemic vascular resistance
  • Cardiac output = heart rate × stroke volume (preload, afterload, contractility)

How do you recognize blood pressure values ​​that are too low?

Normal arterial blood pressure values ​​in non-anesthetized patients are:

  • Systolic pressure (SAP): 100-160 mmHg
  • Diastolic pressure (DAP): 60-100 mmHg
  • Mean Pressure (MAP): 80-120 mmHg

Values ​​often drop in anesthetized patients. Hypotension is considered the most common complication of general anesthesia. For healthy adult dogs and cats, lower limits of 80-90 mmHg (SAP) and 60-70 mmHg (MAP) critical to maintain adequate organ perfusion.

Special patient groups:

  • Chronic diseases (e.g. renal insufficiency): Blood pressure should be maintained in higher ranges during anesthesia.
  • Young animals (puppies, kittens): Since these have physiologically lower blood pressure values, slightly lower values ​​can be tolerated as long as other cardiovascular parameters are within the normal range.

Important: Increased pressure does not necessarily mean improved perfusion, as vasoconstrictive medications may increase blood pressure but blood flow may still be impaired. Primarily, attempts should be made to improve flow (cardiac output) before increasing vascular resistance.


Blood pressure monitoring options

Indirect methods:

  • Pulse palpation: Provides qualitative information about stroke volume (difference between SAP and DAP), but is not reliable for accurately estimating blood pressure values.
  • Oscillometry and Doppler technology:
    • Oscillometry: Provides SAP, DAP and MAP values.
    • Doppler: Particularly useful for SAP trend monitoring.

The accuracy of indirect measurement is influenced by factors such as cuff size (approximately 40% of the circumference of the limb), measurement location and position in relation to the heart.

Specific challenges:

  • Katzen: Oscillometric methods are often unreliable, and Doppler can underestimate SAP values.
  • Dogs: Vasoconstriction and bradycardia often influence measurement results.

Direct methods:

  • Aneroid pressure gauges and strain gauges: These methods are more precise and require cannulation of a peripheral artery. They are particularly suitable for patients with physiological limitations as they enable rapid assessment and intervention.
    • Aneroid manometer: Measures MAP and is inexpensive.
    • Strain gauges: Records SAP, DAP and MAP and also provides a pressure curve that can reveal causes such as lack of contractility.

Management of perianesthetic hypotension

Treatment should focus on the cause:

  • Lack of volume:
    • IV fluids or colloids to increase preload (for dehydration, blood loss, vasodilation from medications).
    • Caution should be exercised in patients with anemia or cardiovascular disease, as overloading of the circulatory system must be avoided.
  • Depression of myocardial contractility:
    • Commonly caused by inhalation anesthetics, some injection anesthetics or cardiovascular diseases (e.g. dilated cardiomyopathy).
    • Therapy: Positive inotropes (e.g. dopamine, dobutamine) and lowering the anesthetic dose.
  • Vasodilation:
    • Induced by medication (e.g. acepromazine, propofol) or caused by toxins.
    • Therapy: Vasoconstrictors such as phenylephrine or norepinephrine.
  • Heart rate abnormalities:
    • Bradycardia caused by medications: Use of anticholinergics (e.g. atropine, glycopyrrolate) or antagonists.
    • Tachycardia: Treatment with specific medications (e.g. lidocaine for ventricular tachycardia).

Additional measures include correcting acid-base or electrolyte imbalances (e.g., hyperkalemia) and using balanced anesthesia techniques with cardiovascular-sparing medications.

Dosages for Cardiovascular Support

Dosages for Cardiovascular Support

The following table shows the dosages of medications that can be used for cardiovascular support during anesthesia:

drugdosageindication
Atropine0.01-0.02 mg/kg IV
0.02-0.04 mg/kg IM/SC
Bradycardia
Glycopyrrolate0.005-0.01 mg/kg IV
0.01-0.02 mg/kg IM/SC
Bradycardia
Ephedrine0.05-0.1 mg/kg IM/IVHypotension
Dobutamine2-10 mcg/kg/min IV CRIHypotension
Dopamine2-7.5 mcg/kg/min IV
Higher doses for vasoconstriction
Hypotension
Phenylephrine0.5-1 mcg/kg/min IVVasodilation-related hypotension

FAQ: Hypotension during anesthesia in pets

Why is regular blood pressure monitoring during anesthesia so important?

Blood pressure monitoring during anesthesia is critical to ensure the animal's safety. Blood pressure indirectly reflects the blood flow and oxygen supply to the organs. Too low blood pressure (hypotension) can lead to insufficient supply to vital organs such as the heart, brain and kidneys. This can have serious consequences, particularly in patients with existing illnesses (e.g. renal insufficiency).
Many factors can affect blood pressure during anesthesia, including:
Anesthetics: Many anesthetics, such as isoflurane or propofol, lower blood pressure by dilating blood vessels or impairing heart function.
Volume loss: Dehydration or blood loss during surgery can cause blood pressure to drop.
Specific patient risks: Older animals, young animals and animals with chronic diseases are more susceptible to hypotension.
Continuous monitoring makes it possible to identify and resolve problems early, e.g. B. through fluid therapy, adjusting the anesthetic dose or the use of medication. This can significantly reduce the risk of complications during and after the operation.

Which blood pressure values ​​are considered critical in dogs and cats?

In non-anesthetized animals, the following blood pressure values ​​are considered normal:
Systolic blood pressure (SAP): 100-160 mmHg
Diastolic blood pressure (DAP): 60-100 mmHg
Mean arterial pressure (MAP): 80-120 mmHg
These values ​​often decrease under anesthesia, and There are defined lower limits that should not be fallen below:
Dogs and cats: SAP: Below 80-90 mmHg critical
MAP: Below 60-70 mmHg critical
A MAP below 60 mmHg can impair organ perfusion, which can lead to damage to sensitive organs such as the kidneys. In animals with chronic diseases, particularly hypertension, blood pressure levels should be maintained in higher ranges during anesthesia.
Young animals, on the other hand, can temporarily tolerate lower blood pressure values ​​because their cardiovascular system is even more adaptable. Nevertheless, it is important to keep an eye on all other cardiovascular parameters to ensure that blood flow remains adequate.

How is blood pressure monitored during anesthesia?

There are two main methods of measuring blood pressure: indirect and direct methods.
Indirect methods:
Oscillometric devices: These automatic devices measure SAP, DAP and MAP using an inflatable cuff. They are user-friendly, but their accuracy may vary for small patients or cats.
Doppler technique: This uses a Doppler ultrasound signal to measure systolic blood pressure. This method is particularly useful for cats and small dogs because it reliably shows trends. However, it requires manual inflation of the cuff.
Direct procedures:
Invasive blood pressure measurement: Here an artery is catheterized with a cannula that is connected to a pressure gauge or an electronic sensor. This method provides more precise values ​​and allows continuous monitoring. However, it is more complex and is primarily used for critically ill patients.
The choice of method depends on the individual needs of the patient, the availability of equipment and the experience of the anesthesia team.

How is hypotension treated during anesthesia?

Treatment depends on the underlying cause of hypotension. The most common approaches include:
Fluid therapy:
Crystalloids: Standard solutions such as Ringer's lactate increase intravascular volume and therefore blood pressure.
Colloids: These solutions (e.g., hydroxyethyl starch) are useful in cases of major volume loss or hypoproteinemia.
Drug treatment:
Positive inotropes (e.g. dopamine, dobutamine) strengthen the contractility of the heart and thus increase cardiac output.
Vasoconstrictors (e.g. phenylephrine, norepinephrine) constrict blood vessels and increase vascular resistance, which increases blood pressure.
Anesthesia adjustment:
Reducing the dose of inhalation anesthetics such as isoflurane.
Use of anesthesia-sparing techniques, e.g. B. by combining it with analgesics such as opioids or low-dose ketamine.
Treatment of bradycardia:
administration of anticholinergics such as atropine or glycopyrrolate.
A multidisciplinary approach is crucial to ensure the best possible care for the patient.

What are the most common mistakes in monitoring and treating hypotension?

Despite good intentions, errors can occur in practice that endanger patient safety. Some of the most common mistakes include:
Improper blood pressure cuff placement:
A cuff that is too small or too large will result in inaccurate readings.
The cuff must be positioned at heart level, as differences in height distort the measurement (± 0.7 mmHg per cm).
Overlooking trends:
Individual blood pressure values ​​are less meaningful than trends. It is important to measure repeatedly in order to detect deviations at an early stage.
Inadequate fluid therapy:
Too little fluid leads to persistent hypotension, while too much fluid can lead to volume overload in sensitive patients (e.g., those with heart failure).
Incorrect use of medication:
Vasoconstrictors should not be used as a first measure as they can further restrict perfusion.
Lack of cause analysis:
Hypotension is often multifactorial. A comprehensive approach that considers anesthetics, fluid status, and surgical influences is essential.
Thorough preparation, continuous monitoring and rapid adjustment of therapy are the key to successful management.

Comprehensive summary: Hypotension during anesthesia

Hypotension during anesthesia is one of the most common complications occurring in animals under general anesthesia. It describes a drop in blood pressure below critical values, which can endanger blood flow and oxygen supply to vital organs. Hypotension during anesthesia requires careful monitoring and timely intervention to prevent serious consequences.

The importance of blood pressure measurement cannot be overemphasized, as hypotension during anesthesia is a common risk for dogs, cats, and other animals. Normal blood pressure values ​​in nonanesthetized animals vary but often decrease during anesthesia. Systolic values ​​below 80-90 mmHg and mean arterial values ​​below 60-70 mmHg are considered critical. Hypotension during anesthesia is particularly dangerous in animals with chronic diseases such as renal insufficiency or heart problems, as it can further impair organ blood flow.

Hypotension during anesthesia has a variety of causes, including the effects of anesthetics such as isoflurane or propofol, which can lower blood pressure through vasodilation or reduced cardiac output. Dehydration due to dehydration, blood loss, or fasting also often contributes to hypotension during anesthesia. Inadequate preparation or monitoring may increase the likelihood that hypotension will go undetected during anesthesia.

There are two main methods of monitoring: indirect measurements such as oscillometry and Doppler and direct arterial measurements. Indirect techniques are less invasive but often less accurate, especially in cats. Direct measurement methods, on the other hand, enable more precise monitoring, but are technically more demanding and are usually used on seriously ill patients. Both methods are crucial for early detection of hypotension during anesthesia.

Management of hypotension during anesthesia requires a targeted approach. First, the cause should be identified. If volume status is reduced, intravenous fluids such as crystalloids or colloids can help increase blood volume and improve preload. If myocardial contractility is reduced, positive inotropes such as dopamine or dobutamine are used. Vasoconstrictors such as phenylephrine may be helpful in drug-induced vasodilation. It is crucial that the treatment of hypotension during anesthesia is individually tailored to the patient's condition.

Hypotension during anesthesia poses a particular challenge in young animals and geriatric patients. Young animals can better tolerate lower blood pressure values ​​as long as cardiovascular function remains stable. Geriatric patients, on the other hand, are more sensitive to hypotension during anesthesia, especially if they suffer from chronic diseases.

Hypotension during anesthesia can also be exacerbated by external factors such as surgical procedures or positive pressure ventilation. It is therefore important to take these influences into account when planning and implementing anesthesia. Careful adjustment of anesthesia dose and use of balanced anesthesia techniques incorporating opioids or low-dose ketamine can help minimize hypotension during anesthesia.

A common mistake in treating hypotension during anesthesia is focusing exclusively on blood pressure without considering perfusion. Improvement in blood pressure with vasoconstrictors can worsen organ perfusion if the underlying cause is not addressed. It is therefore essential that measures such as optimizing cardiac output are prioritized.

In practice, hypotension during anesthesia remains a challenge that requires an interdisciplinary approach. However, the risks can be significantly reduced through early diagnosis, adapted therapy and the use of modern monitoring technologies. Hypotension during anesthesia always requires an individual and patient-specific approach to achieve optimal results.

In summary, hypotension during anesthesia is a common but manageable complication that may not cause permanent damage with careful monitoring and targeted management. Proper preparation and understanding of the underlying mechanisms help to effectively control hypotension during anesthesia and ensure patient safety

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