- Hypotension during anesthesia – the importance of blood pressure measurement during anesthesia
- Importance of blood pressure measurement during anesthesia
- Treatment of hypotension during anesthesia
- For pet owners: What does this mean for my animal?
- Importance of blood pressure measurement during anesthesia – the technical aspect
- FAQ: Hypotension during anesthesia in pets
- Comprehensive summary: Hypotension during anesthesia
Hypotension during anesthesia – the importance of blood pressure measurement during anesthesia
The first part of this article is aimed at pet owners, and the second part at a professional audience.
Importance of blood pressure measurement during anesthesia
Stable cardiovascular function is crucial to ensure adequate tissue oxygenation. During anesthesia, blood pressure measurement is an important method for indirectly assessing organ perfusion. Low blood pressure (hypotension) can have serious consequences, making regular monitoring and treatment essential.
How can you recognize low blood pressure?
Normal values for arterial blood pressure in awake animals are:
- Systolic blood pressure (SAP): 100–160 mmHg
- Diastolic blood pressure (DBP): 60–100 mmHg
- Mean arterial 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 values can impair blood flow to vital organs.
Options for blood pressure monitoring
Indirect methods:
- Oscillometry or Doppler method Blood pressure is measured using an inflatable cuff. Accuracy depends on correct cuff placement and its width (approximately 40 times 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
The treatment depends on the cause:
- Volume deficiency: Intravenous fluid therapy increases blood volume.
- Weak heart muscle contraction: Medications such as dopamine or dobutamine improve the contractility of the heart.
- Vasodilation (widening of blood vessels): Medications such as phenylephrine or norepinephrine can constrict blood vessels.
Reducing the dose of anesthetic can also help stabilize blood pressure.
For pet owners: What does this mean for my animal?
During surgery, your pet will be closely monitored to ensure that blood pressure, heart rate, and oxygen levels remain stable. Low blood pressure (hypotension) is a common complication during anesthesia, but don't worry: veterinarians are specialized in recognizing and treating such situations.
If your pet's blood pressure drops, several measures can be taken:
- Fluid therapy: Your pet is receiving intravenous fluids to stabilize its circulation.
- Adjustment of the anesthesia: The level of anesthesia can be reduced to relieve the cardiovascular system.
- Medication: Special medications help to stabilize blood pressure.
It is important that each treatment is individually tailored to your pet's health condition. Talk to your veterinarian. Veterinarian, If you have any questions or concerns – your pet's well-being is always our top priority!
The article is based on the publication of Khursheed Mama, DVM, DACVAA, Colorado State University
Last updated: November 2021.
Importance of blood pressure measurement during anesthesia – the technical part
Normal cardiovascular function is essential to ensure adequate tissue oxygenation. Oxygenation is defined by the relationship between cardiac output and oxygen content, but it 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 oxygenation and organ perfusion.
According to Ohm's law, which relates pressure, flow, and resistance, there are many potential causes for altered blood pressure. Managing hypotension requires understanding the influence of diseases, anesthetics and other medications, supportive measures, surgical interventions, 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 can you recognize low blood pressure?
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
In anesthetized patients, blood pressure values often drop. Hypotension is considered the most common complication under general anesthesia. For healthy adult dogs and cats, lower limits of 80–90 mmHg (SAP) and 60–70 mmHg (MAP) as critical to maintain adequate organ perfusion.
Special patient groups:
- Chronic diseases (e.g., renal insufficiency): Blood pressure should be kept in higher ranges during anesthesia.
- Young animals (puppies, kittens): Since these individuals 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 blood pressure does not necessarily mean improved perfusion, as vasoconstrictive medications may raise blood pressure but still impair blood flow. The primary goal should be to improve blood flow (cardiac output) before increasing vascular resistance.
Options for blood pressure monitoring
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 technique:
- Oscillometry: Provides SAP, DAP, and MAP values.
- Doppler: Particularly useful for monitoring SAP trends.
The accuracy of indirect measurement is influenced by factors such as cuff size (approx. 40 % of the limb circumference), measurement location and position relative to the heart.
Specific challenges:
- Cats: Oscillometric methods are often unreliable, and Doppler can underestimate SAP values.
- Dogs: Vasoconstriction and bradycardia often affect the measurement results.
Direct methods:
- Aneroid manometer 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 allow for rapid assessment and intervention.
- Aneroid manometer: It measures the MAP and is cost-effective.
- Strain gauges: It records SAP, DAP and MAP and also offers a pressure curve that can reveal causes such as lack of contractility.
Management of perianesthetic hypotension
Treatment should focus on the underlying cause:
- Volume deficiency:
- IV fluids or colloids to increase preload (in cases of dehydration, blood loss, or vasodilation due to medication).
- Caution is advised in patients with anemia or cardiovascular disease, as circulatory overload must be avoided.
- Depression of myocardial contractility:
- Often caused by inhalation anesthetics, some injectable anesthetics, or cardiovascular diseases (e.g., dilated cardiomyopathy).
- Therapy: Positive inotropic agents (e.g., dopamine, dobutamine) and reduction of the anesthetic dose.
- Vasodilation:
- Drug-induced (e.g., acepromazine, propofol) or toxin-induced.
- Therapy: Vasoconstrictors such as phenylephrine or norepinephrine.
- Heart rate abnormalities:
- Drug-induced bradycardia: use of anticholinergics (e.g., atropine, glycopyrrolate) or antagonists.
- Tachycardia: Treatment with specific medications (e.g., lidocaine for ventricular tachycardia).
Further measures include the correction of acid-base or electrolyte disturbances (e.g. hyperkalemia) and the use of balanced anesthesia techniques with cardiovascular-friendly drugs.
Dosages for cardiovascular support
The following table shows the dosages of medications that can be used for cardiovascular support during anesthesia:
| drug | dosage | indication |
|---|---|---|
| Atropine | 0.01–0.02 mg/kg IV 0.02–0.04 mg/kg IM/SC |
Bradycardia |
| Glycopyrrolate | 0.005–0.01 mg/kg IV 0.01–0.02 mg/kg IM/SC |
Bradycardia |
| Ephedrine | 0.05–0.1 mg/kg IM/IV | Hypotension |
| Dobutamine | 2–10 µg/kg/min IV CRI | Hypotension |
| Dopamine | 2–7.5 µg/kg/min IV Higher doses for vasoconstriction |
Hypotension |
| Phenylephrine | 0.5–1 µg/kg/min IV | Vasodilation-induced hypotension |
FAQ: Hypotension during anesthesia in pets
Why is regular blood pressure monitoring so important during anesthesia?
Blood pressure monitoring during anesthesia is crucial to ensure the animal's safety. Blood pressure indirectly reflects the blood flow and oxygen supply to the organs. Low blood pressure (hypotension) can lead to an insufficient supply of blood to vital organs such as the heart, brain, and kidneys. This can have serious consequences, especially in patients with pre-existing conditions (e.g., kidney failure).
Many factors can affect blood pressure during anesthesia, including:
Anesthetics: Many anesthetics, such as isoflurane or propofol, lower blood pressure by widening blood vessels or impairing heart function.
Volume loss: Blood pressure can drop due to dehydration or blood loss during surgery.
Specific patient risks: Older animals, young animals and animals with chronic illnesses are more susceptible to hypotension.
Continuous monitoring allows problems to be detected and resolved early, for example through fluid therapy, adjustment of the anesthetic dose, or the use of medication. This can significantly reduce the risk of complications during and after the operation.
What blood pressure values are considered critical in dogs and cats?
The following blood pressure values are considered normal in non-anesthetized animals:
Systolic blood pressure (SAP): 100–160 mmHg
Diastolic blood pressure (DBP): 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 undercut:
Dogs and cats:SAP: Critical below 80–90 mmHg
MAP: Critical below 60–70 mmHg
A mean arterial pressure (MAP) below 60 mmHg can impair organ perfusion, potentially leading to damage to sensitive organs such as the kidneys. In animals with chronic conditions, particularly hypertension, blood pressure should be maintained at higher levels during anesthesia.
Young animals, on the other hand, can temporarily tolerate lower blood pressure levels because their cardiovascular system is still more adaptable. Nevertheless, it is important to monitor all other cardiovascular parameters to ensure adequate blood flow.
How is blood pressure monitored during anesthesia?
There are two main methods for measuring blood pressure: indirect and direct Procedure.
Indirect methods:
Oscillometric instruments: These automatic devices measure SAP, DAP, and MAP using an inflatable cuff. They are user-friendly, but their accuracy can vary in small patients or cats.
Doppler technique: This method uses a Doppler ultrasound signal to measure systolic blood pressure. It 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 connected to a manometer or an electronic sensor. This method provides more precise readings and allows for 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 the hypotension. Common approaches include:
Fluid therapy:
Crystalloids: Standard solutions such as Ringer's lactate increase intravascular volume and thus blood pressure.
Colloids: These solutions (e.g., hydroxyethyl starch) are useful in cases of significant volume loss or hypoproteinemia.
Drug treatment:
Positive inotropics (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, thereby raising blood pressure.
Adjustment of the anesthesia:
Reduction of the dose of inhalation anesthetics such as isoflurane.
Use of anesthesia-saving techniques, e.g. 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. The most common errors include:
Incorrect placement of the blood pressure cuff:
A cuff that is too small or too large will lead to inaccurate measurements.
The cuff must be positioned at heart level, as differences in height will distort the measurement (± 0.7 mmHg per cm).
Missed out on trends:
Individual blood pressure readings are less informative than trends. It is important to measure repeatedly to detect deviations early.
Inadequate fluid therapy:
Too little fluid leads to persistent hypotension, while too much fluid can lead to volume overload in sensitive patients (e.g., with heart failure).
Incorrect use of medications:
Vasoconstrictors should not be used as a first-line treatment, as they can further restrict perfusion.
Lack of root cause analysis:
Hypotension is often multifactorial. A comprehensive approach that considers anesthetics, fluid status, and surgical factors is essential.
Thorough preparation, continuous monitoring, and rapid adjustment of therapy are 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 levels, which can compromise the blood flow and oxygenation of vital organs. Hypotension during anesthesia requires careful monitoring and timely intervention to prevent serious consequences.
The importance of blood pressure monitoring cannot be overstated, as hypotension during anesthesia is a common risk for dogs, cats, and other animals. Normal blood pressure values in non-anesthetized 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 conditions such as renal insufficiency or heart problems, as it can further impair organ perfusion.
Hypotension during anesthesia has various causes, including the effects of anesthetics such as isoflurane or propofol, which can lower blood pressure through vasodilation or reduced cardiac output. Fluid depletion due to dehydration, blood loss, or fasting also frequently contributes to hypotension during anesthesia. Inadequate preparation or monitoring can increase the likelihood that hypotension during anesthesia will go undetected.
There are two main methods for monitoring blood pressure: indirect methods such as oscillometry and Doppler ultrasound, and direct arterial measurements. Indirect techniques are less invasive but often less accurate, especially in cats. Direct methods, on the other hand, allow for more precise monitoring but are technically more demanding and are usually used in critically ill patients. Both methods are crucial for the timely detection of hypotension during anesthesia.
Managing hypotension during anesthesia requires a targeted approach. First, the underlying 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 decreased, positive inotropic agents such as dopamine or dobutamine are used. Vasoconstrictors such as phenylephrine can be helpful in cases of drug-induced vasodilation. Crucially, the treatment of hypotension during anesthesia must be tailored to the individual patient's condition.
Hypotension during anesthesia presents a particular challenge in young and geriatric animals. Young animals can tolerate lower blood pressure levels better, provided cardiovascular function remains stable. Geriatric patients, however, 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 interventions or positive pressure ventilation. Therefore, it is important to consider these influences when planning and administering anesthesia. Careful adjustment of the anesthetic dose and the use of balanced anesthetic techniques that incorporate opioids or low-dose ketamine can help minimize hypotension during anesthesia.
A common mistake in managing hypotension during anesthesia is focusing solely on blood pressure without considering perfusion. Improving blood pressure with vasoconstrictors can worsen organ perfusion if the underlying cause is not addressed. Therefore, it is essential to prioritize measures such as optimizing cardiac output.
In practice, hypotension during anesthesia remains a challenge requiring an interdisciplinary approach. However, early diagnosis, tailored therapy, and the use of modern monitoring technologies can significantly reduce the risks. Hypotension during anesthesia always necessitates an individualized and patient-specific approach to achieve optimal results.
In summary, hypotension during anesthesia is a common but manageable complication that, with careful monitoring and targeted management, need not cause lasting damage. Proper preparation and an understanding of the underlying mechanisms help to effectively control hypotension during anesthesia and ensure patient safety.