Veterinary analgesia

Importance of monitoring blood pressure and management of hypotension during anaesthesia

// Published June 20, 2016 by Admin

Why monitoring blood pressure during anaesthesia?
Hypotension is the most common complication associated with anaesthesia. Most importantly, severe hypotension is an anaesthetic emergency that requires immediate attention and intervention. Monitoring blood pressure helps guiding therapeutic interventions.
Furthermore, good survival is associated with good tissue perfusion and oxygenation. Tissue oxygen delivery depends primarily upon cardiac output and arterial oxygen blood content. However, those three parameters are clinically difficult to measure. The “next best thing” to assess tissue perfusion and oxygen delivery is mean arterial pressure, which is clinically easily measured. Mean arterial pressure above 60 -70 mmHg is necessary to maintain auto-regulation of blood flow to vital organs.

monitoring blood pressure

What is mean arterial pressure?
Mean arterial pressure reflects the ability of blood to perfuse tissues. It can be thought as the result of the interaction of a pump – the heart represented by cardiac output, and pipes – the blood vessels represented by systemic vascular resistance. The following equation illustrates this interaction:

MAP= CO x SVR = SV x HR x SVR where

MAP is mean arterial pressure,
CO is cardiac output,
SVR is systemic vascular resistance,
SV is stroke volume, and
HR is heart rate.

Stroke volume itself depends on preload (diastolic filling volume), afterload (arterial pressure and systemic vascular resistance), and intrinsic contractility of the heart. Systemic vascular resistance depends on vasomotor tone, i.e. the degree of vasodilation or vasoconstriction of the blood vessels. Anything that will affect any of these factors will in turn affect blood pressure.
It is important to remember that blood pressure is not synonym of tissue perfusion, but is the driving pressure for blood flow. Blood pressure alone does not correlate with tissue perfusion. However tissue perfusion is more likely to be compromised when hypotension is associated with a reduced cardiac output accompanied by vasoconstriction. When mean arterial pressure is low, it is difficult to know with certainty whether it is related to changes in stroke volume or systemic vascular resistance. Monitoring systolic and diastolic arterial pressure values can be of assistance in determining the culprit, and guiding therapeutic interventions (see later). Low systolic arterial pressure (SAP) measurement is more likely to be associated with heart contractility issues, whereas low diastolic arterial pressure (DAP) measurement is more likely to be associated with systemic vascular resistance (absolute or relative) issues.

What is hypotension?
Hypotension is defined as mean arterial pressure (MAP) below 60 mmHg.

SAP MAP
Mild hypotension < 90 mmHg < 60 mmHg
Moderate hypotension < 80 mmHg < 50 mmHg
Severe hypotension < 60 mmHg < 40 mmHg

Be prepared: Are there risk factors?
Pre-operative assessment is paramount to identify patients at increased risk of intra-operative hypotension. Risk factors include:
– Co-morbidities such as hypertension, Addison’s disease, hypothyroidism, treatment with anti-hypertensive drugs, etc.
– Hypovolaemia (relative or absolute) such as haemorrhage, vomiting, diarrhoea, sepsis, obstruction to venous return, etc.
Anaesthesia and surgery related causes of hypotension should also be identified:
– Direct action of anaesthetic agents: All anaesthetics agents (intravenous or inhalational) cause dose-dependent cardiovascular depression
– Anaesthetic over-dosage
– Intermittent positive pressure ventilation (IPPV)
– Anaesthesia technique: Use of local anaesthetics in epidural or spinal anaesthesia
– Blood loss during surgery
– Adverse patient position

How to prevent hypotension during anaesthesia?
Although hypotension cannot always be avoided during anaesthesia, some simple and easily achievable steps can help prevent its occurrence or lessen its severity and duration:
– Minimise pre-operative dehydration/hypovolaemia:
o Assess the degree of dehydration
o Replace losses pre-operatively if appropriate
– Obtain venous access that is reliable, with the largest bore possible
– Provide fluid therapy intra-operatively as well pre- and post-operatively if indicated
– Monitor blood pressure

Veterinary analgesia

How to manage hypotension?
Using a step-by-step procedure to try and eliminate known causes for hypotension is a simple and reliable way to manage hypotension.
1. Can you reduce the isoflurane/sevoflurane? Can you safely lighten the plan of anaesthesia? Check eye position and jaw tone.
2. Can you reduce IPPV (pressure or rate)?
3. Can you administer a bolus of fluids (caution if risk of fluid overload)?
Administer a bolus of crystalloids with a total volume of 10 mL/kg given over 10 – 15 min.
4. Remember MAP = HR x SV x SVR, so guide your decision based on
– HR:
1. Is the patient bradycardic?  Administer anticholinergics such as glycopyrrolate or atropine? Atipamezole if (dex)medetomidine was administered?
2. Are there significant other arrhythmias present? Anti-arrhythmics? Atipamezole if (dex)medetomidine was administered?
– SAP: Is SAP low? Is heart contractility significantly decreased?  Administer positive inotropes such as ephedrine, dobutamine or dopamine?
– DAP: Is DAP low? Are blood vessels too vasodilated?  Administer vasopressors such as ephedrine, phenylephrine, noradrenaline and vasopressin?

veterinary anaethesia

Drug Effect Agonist  Main effect Side effect Dose
Ephedrine Ino-constrictor – Mainly β (β1>β2) at lower doses

– Mainly α at higher doses

– Increase contractility

– Peripheral vasoconstriction

Tachycardia 0.01-0.2 mg/kg
Dobutamine Positive inotrope – Mainly β1 at low to moderate rates, but also β2 and α1 – Increase contractility Reflex bradycardia at low doses

Tachycardia

2-10 μg/kg/min
Dopamine Positive inotrope – Mainly DA1 and DA2 at low doses

– Mainly β1 at moderate doses

– α1 and α2 at high doses

– Vasodilation

 

– Increase contractility

– Peripheral vasoconstriction

Up to 2.5 μg/kg/min

2.5-5 μg/kg/min

>5-10 μg/kg/min

Phenylephrine Vasopressor – α1 – Peripheral vasoconstriction Reflex bradycardia 0.1-3 μg/kg/min
Noradrenaline Vasopressor (for refractory hypotension) – Mainly α1 at low doses

 

 

– Some β effects at higher doses

– Peripheral vasoconstriction

 

-Increased contractility

Reflex bradycardia 0.01-1 μg/kg/min
Vasopressin Vasopressor (for refractory hypotension) – V1 – Peripheral vasoconstriction Reflex bradycardia 0.01-0.5 UI/kg/h

5. Check for electrolytes and acid-base disturbances
6. Check patient position for compression of vena cava and/or aorta

 

Perrine Benmansour DVM, MVetSc, Dip ECVAA, MRCVS. RCVS & European Veterinary Specialist in Anaesthesia and Analgesia

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