Bovis ISSN 2398-2993

Hypotension

Contributor(s): Gayle Hallowell , Alex Dugdale

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Definitions and clinical significance

  • The main function of the cardiovascular system is to provide a blood flow 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 variable 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 upon 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 below, it can be appreciated that if systemic vascular resistance (SVR) increases, through vasoconstriction, measured ABP will also increase, without necessarily equating to improved tissue perfusio, ie a vasoconstricted tissue bed will lead to reduced flow. ABP only correlates with cardiac output when SVR is constant.
  • 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 ventricular 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; 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 low blood pressure: usually MAP <60 mmHg.
  • Hypotension is commonly observed in anesthetized cattle with volatile anesthetic agents and its prevalence appears to be greatly reduced with 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 to <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 <70 mmHg with an absolute minimum of 60 mmHg.

BP measurement

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Causes of hypotension during general anesthesia

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Treatment of hypotension

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Consequences of hypotension

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Further Reading

Publications

Refereed Papers

Other sources of information

  • Valverde A & Sinclair M (2015) Ruminant Anesthesia. In: Veterinary Anesthesia and Analgesia. Blackwell Publishing.
  • Clarke K & Trim C (2014) Patient Monitoring and Clinical Measurement and Anaesthesia of Cattle. Veterinary Anaesthesia. 11th Edition. Elsevier.


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