Equis ISSN 2398-2977

Anesthesia: peri-operative complications - hypotension

Contributor(s): Adam Auckburally, David Bardell, Louise Clarke, Mark Senior, Kate Thompson

Definitions and clinical significance

  • The main function of the cardiovascular system is to provide a flow of blood to tissues, such that oxygen and nutrients are delivered and waste products produced by cellular metabolic processes are removed.
  • Blood pressure (BP) is a fundamental cardiovascular parameter that describes the force driving tissue perfusion, where BP is the pressure exerted by blood on the walls of the arteries and arterioles of the systemic circulation. It also determines the workload of the myocardium (afterload).
  • BP is commonly measured in millimeters of mercury (mmHg).
  • Systolic arterial blood pressure (SAP) is the pressure exerted on the arterial walls during left ventricular contraction.
  • Diastolic arterial blood pressure (DAP) is the pressure exerted on the arterial walls during ventricular relaxation due to the Windkessel effect (elastic recoil of arteries and arterioles).
  • Mean pressure = diastolic pressure + 1/3 (systolic pressure - diastolic pressure).
  • Arterial blood pressure (ABP) is dependent on a number of factors: equations 1 and 2 (see below).
  • The number the clinician sees on the screen of the monitor does not necessarily equate to good tissue perfusion. Using the equations it can be appreciated that if systemic vascular resistance (SVR) increases, through vasoconstriction, measured ABP will also increase, without necessarily equating to improved tissue perfusion, ie a vasoconstricted tissue bed will lead to reduced flow.
  • ABP is also dependent on cardiac output, which is significantly more difficult and expensive to measure.
  • Despite its limitations, ABP measurement is still used to assess the cardiovascular system in anesthetized patients due to its relative simplicity.

Equation 1

  • BP = cardiac output x SVR.

Equation 2

  • Cardiac output = heart rate x stroke volume.

Equation 1+2

  • ABP = heart rate x stroke volume x SVR.

Cardiac output = the amount of blood pumped out of the left ventricle per minute.

Stroke volume = the amount of blood ejected from the left ventricle during each systole. Stroke volume is dependent on preload (the amount of blood returning to the right atrium), afterload (the resistance to blood ejection from the left ventricle), and contractility (the force of contraction).

SVR = sum of the resistance in the systemic vasculature (synonymous with total peripheral resistance).

  • ABP can be represented as a waveform over time   Cardiovascular: arterial blood pressure - waveform  ; the maximum point being the systolic pressure and the minimum point of diastolic pressure.
  • Integration of the area under the waveform over consecutive beats, and then averaged, gives the average or mean ABP (MAP).
  • Hypotension is defined as an abnormally low blood pressure: usually MAP <60 mmHg.
  • Hypotension is common in horses anesthetized with volatile anesthetic agents and its incidence appears to be greatly reduced in horses anesthetized using total intravenous anesthesia (TIVA) or by using supplemental intravenous anesthetic or analgesic agents (SIVA), to reduce the amount of volatile agent necessary to maintain unresponsiveness to surgery.
  • Cerebral, coronary and renal blood flow becomes dependent on systemic blood pressure when MAP falls <50-60 mmHg. Above these pressures, flow to these organs is autoregulated. Flow to skeletal muscle is not autoregulated.
  • Common anesthetic practice would be to treat BP when the MAP is decreasing to <60-70 mmHg with an absolute minimum of 60 mmHg.

Neonates generally have a lower MAP than an adult, so 40-60 mmHg would be an acceptable goal in foals under general anesthesia.

BP measurement

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Causes of hypotension during general anesthesia

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Treatment of hypotension in healthy horses

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Treatment of hypotension in horses with surgical colic

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Consequences of hypotension

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Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Schauvliege S & Gasthuys F (2013) Drugs for cardiovascular support in anesthetized horses. Vet Clin North Am Equine Pract 29 (1), 19-49 PubMed.
  • Young S S & Taylor P M (2010) Factors influencing the outcome of equine anaesthesia: a review of 1,314 cases. EVJ 25 (2), 147-151 PubMed.
  • Wagner A E (2008) Complications in equine anesthesia. Vet Clin North Am Equine Pract 24 (3), 735-752 VetMedResource.
  • Duke T, Filzek U, Read M R, Read E K & Ferguson J G (2006) Clinical observations surrounding an increased incidence of postanesthetic myopathy in halothane-anesthetized horses. Vet Anaes Analg 33 (2), 122-127 PubMed.
  • Raisis A L (2005) Skeletal muscle blood flow in anaesthetized horses Part II: effects of anaesthetics and vasoactive agents. Vet Anaes Analg 32(6), 331-337 PubMed.
  • Johnston G M et al (2002) The confidential enquiry into perioperative equine fatalities (CEPEF): mortality results of phases 1 and 2. Vet Anaes Analg 29, 159-170.
  • Richey M T, Holland M S, McGrath C J et al (1990) Equine post-anesthetic lameness: A retrospective study. Vet Surg 19 (5), 392-397 PubMed.
  • Donaldson L (1988) Retrospective assessment of dobutamine therapy for hypotension in anesthetized horses. Vet Surg 17 (1), 53-57 PubMed.
  • Grandy J L, Steffey E P, Hodgson D S & Woliner M J (1987) Arterial hypotension and the development of postanesthetic myopathy in halothane-anesthetized horses. AJVR 48 (2), 192-197 PubMed.

Other sources of information

  • Muir W W & Hubbell J A E (2009) Equine Anesthesia: Monitoring & Emergency Therapy.2nd edn. Elsevier, USA.
  • Doherty T & Valverde A (2006) Manual of Equine Anesthesia & Analgesia. Blackwell Publishing Ltd, UK.


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