Vasoactive Drugs

Objectives

  • Understand the principles of Ohm’s Law in relation to Cardiac Output, Systemic Vascular Resistance and Blood Pressure
  • Describe how Preload, Contractility Afterload affect Stroke Volume and how these can be optimised clinically
  • Recall the classification of adrenoceptors and their physiological action, especially in relation to Inotropy, Chronotropy and Vasoconstriction
  • Recall the pertinent pharmacokinetic and pharmacological properties of Metaraminol and Ephedrine
  • Compare and contrast the properties of these drugs and their clinical uses in anaesthetic practice
  • Use this knowledge and understanding to select an appropriate Vasopressor for use perioperatively to optimise organ perfusion pressure in various clinical scenarios

Perfusion

Major body systems rely on adequate blood supply to function. 

During an anaesthetic, there are many reasons why this blood supply to vital organs may becomes transiently (or permanently) disrupted and so it is essential you have a good understanding of circulatory physiology and how to optimise Cardiac Output in order to use vasoactive drugs safely. 

In order to ensure an adequate blood flow to the vital organs, an adequate perfusion pressure is vital. 

For example, 

Cerebral Perfusion Pressure (CPP) = Mean Arterial Pressure  (MAP) – Intracranial Pressure (ICP)

There are various factors that influence a patient’s optimal MAP, but generally, 65-75mmHg is sufficient to meet the metabolic requirements of vital organs such as the brain and kidneys. 

As we have discussed, there are many reasons why a patient undergoing an operation/anaesthetic may become hypotensive. It is essential to consider the cause of the hypotension to be able to address it. In order to do so, you also need to be aware of Ohm’s Law.

Ohm’s Law

Ohm’s Law governs electrical series circuits. 

Potential Difference (V) = Current (I) × Resistance (R)

Potential Difference, Current and Resistance all have analogous physiological variables in the circulatory system and so the equation as applied to the body is as follows:

MAP – Central Venous Pressure = Cardiac Output × Systemic Vascular Resistance

This equation now dictates that hypotension can be caused by either:

  • reduced Cardiac Output
  • or a reduced Systemic Vascular Resistance

Let’s look at each of these in a little more detail. 

As we know, Cardiac Output is a product of Stroke Volume and Heart Rate.

There are 2 variables that increase Stroke Volume:

  • Preload (End-Diastolic Volume)
  • Contractility

There is 1 variable that reduces Stroke Volume:

  • Afterload

Vasodilatation reduces the SVR. 

Vasoconstriction increases the SVR. 

While we can use vasopressors to induce systemic vasoconstriction, it is important not to ‘over-squeeze’ the patient with excess vasopressor – as this can actually lead to a reduction in end-organ perfusion, even though you may be normalising the blood pressure. If the cause of the hypotension is a reduction in Cardiac Output rather than a reduction in SVR, it is best to correct the Cardiac Output issue (e.g. with fluids / inotropes) rather than using vasopressors. 

The α1 adrenoceptor is present in vasculature – agonists at this receptor induce vasoconstriction

Metaraminol

Ephedrine

Quiz

3
Created on By Anaestheasier

Vasoactive Agents

1 / 15

A 56-year-old lady is undergoing an emergency laparotomy for bowel obstruction. You used intra-operative cardiac output monitoring to guide your management of the case. You have been using goal-directed fluid therapy and you are currently running a metaraminol infusion at 5ml/hr. The monitor is currently showing: HR 73 (SR), BP 76/50 (59), SpO2 98% (on FiO2 46%), etCO2 5.3. The Cardiac Output Monitor is currently showing (including normal values):

CI 3.6 (2.5 to 4 L/min/m2)

SVRI 1629 (1970 – 2390 dynes · sec/cm5/m2)

SVV 5% (<10%)

Based on these values alone, which of the following would be the best intervention to reverse her hypotension?

2 / 15

Which of the following agents is a sympathomimetic with predominantly α1 effects?

3 / 15

The clinical effects of which of the following agents are primarily α1-related?

4 / 15

Metaraminol should be prepared in a syringe to what concentration?

5 / 15

A 62-year-old male is undergoing a circumcision under general anaesthetic. He had a heart transplant back in 1998 and has T2DM and hypertension. Propofol + Fentanyl were used for induction of anaesthesia and Sevoflurane is being used as maintenance. The patient is positioned on the table ready for the start of the procedure and you are waiting for the surgeon to scrub still, when you notice the patient’s observations:

etCO2 4.2, RR 12, SpO2 96%, HR 35, BP 91/45 (60).

Which drug is most appropriate to correct the abnormality?

6 / 15

A 55-year-old gentleman with HTN and Ischaemic Heart Disease is undergoing a shoulder replacement under a General Anaesthetic with an Interscalene Block. The monitor is currently showing: HR 93 (SR), BP 83/35 (51), SpO2 99% (on FiO2 36%), etCO2 4.8. Giving an agonist to which of the following receptors would be most appropriate?

7 / 15

The activation of which of the following receptors would result primarily in vasoconstriction?

8 / 15

A 73-year-old lady is having a Hemiarthroplasty for a Fractured Neck of Femur on the Trauma List.

She has a background of AF, hypertension, T2DM, obstructive sleep apnoea, COPD and asthma.

Unfortunately, several attempts at a spinal anaesthetic failed and so she has just been put to sleep with 120mg Propofol, 100mg Fentanyl + 40mg Atracurium. Her observations are as follows:

etCO2 6.1, RR 14, SpO2 98%, HR 50, BP 74/48 (56)

What would be most appropriate choice of drug to administer now?

9 / 15

Which of the following statements regarding metaraminol is incorrect?

10 / 15

Look at the following equation:

Mean Arterial Pressure – Central Venous Pressure = Cardiac Output × Systemic Vascular Resistance

It is analogous to this one:

V = I × R

(Potential Difference in Volts = Current × Resistance)

What is this latter equation called?

11 / 15

Giving an agonist at which of the following receptors would increase heart rate and contractility?

3mg/ml

12 / 15

Your consultant asks you to give 6mg Ephedrine to a hypotensive patient intra-operatively. How many millilitres would this be?

13 / 15

A 63-year-old lady who weighs 55kg is undergoing a hysteroscopy +/- biopsy on your list. She has a background of hypertension for which she usually takes 10mg amlodipine OD. You induce anaesthesia with 200mg Propofol + 100mcg Fentanyl and then insert a size 3 iGel and start IPPV.

The monitor reads the following: etCO2 5.3, RR 12, SpO2 97%, HR 109 (SR), BP 73/46 (55).

Which drug would be most appropriate to administer now?

14 / 15

Which of the following is incorrect regarding Ephedrine?

15 / 15

Which of the following statements regarding ephedrine is incorrect?

Your score is

The average score is 80%

0%

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