How does morbid obesity affect washout of volatile anaesthetic?

This is one of the more complex questions in anaesthesia! 

Here's the actual WHO definition that doesn't use the term morbid obesity but rather obesity class 1, 2 and 3.

Obesity is frequently subdivided into categories:

• Class 1: BMI of 30 to < 35
• Class 2: BMI of 35 to < 40
• Class 3: BMI of 40 or higher. Class 3 obesity is sometimes categorized as “extreme” or “severe” obesity.

NB LISTEN TO NEXT EPISODE FOR DEEP DIVE INTO  THE EFFECT OF CARDIAC OUTPUT ON WASHOUT!!!

Millers Chapter 20 Inhaled Anesthetic Uptake, Distribution, Metabolism, and Toxicity 
Section: Recovery from Anesthesia and figures 20.4 and 20.5 show FASTER washout with LOW cardiacs output.
This is also referenced by the ex chair of first part exam at the primary LO website. 

This chapter also has good graphs to replicate for the exam.



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Transcript

This is 2019 Q.7 august. Describe how morbid obesity might affect washout after inhalation anaesthesia 

Morbid obesity = BMI > 35. Excess bodyweight is:

-      2/3 fat mass (poorly vascularised)

-      1/3 organ and muscle mass (highly vascularised)

Washout is dependent on:

1. Partial pressure in muscle and fat compartments at end of anaesthesia. Affected by:

a)     Time constants based on:

-      Solubility of volatile agent 

-      Size of muscle and fat compartments and its corresponding blood flow

TIME CONSTANT | Muscle | Fat
- Desflurane | 50 min | 1,350 min
- Isoflurane | 80 min | 2,110 min

b)    Amount delivered based on:

-      Fraction inspired (concentration)

-      Duration of administration (context sensitive half-time) 

The time constants, means that choice of volatile agent affecting washout, will become significant if cases exceed 1 hour (i.e. time to emergence quicker with desflurane). Even then, no significant difference between sevo and des was shown when titrating to BIS (i.e. controlling concentration) during these long cases.

2. Routes of excretion

a)     Hepatic

-      Obesity does increase metabolism of volatile agents, but des (0.02%) and sevo (2-5%) are minimally metabolised so not clinically significant.

b)    Lung

-      Alveolar ventilation

Incr BMI --> basal atelectasis --> incr ­ shunt fraction --> decr Transfer of volatile from arterial blood to alveoli

-      Degree of rebreathing affected by fresh gas flow and absorption into anaesthetic circuit

-      Dilution from the second gas effect (N2O)

 To ­ increase washout the anaesthetist can ­ increase alveolar ventilation and FGF, as well as use N2O.

63.8% of candidates achieved a pass in this question 

The main domains assessed in this question were 

• ×  Definition of morbid obesity 
• ×  Pharmacokinetics associated with morbid obesity 
• ×  Effect of specific inhalational agent solubility on washout 
• ×  Inhalational washout curve 
• ×  Effect of anaesthetic duration 
• ×  Respiratory problems of morbid obesity during recovery affecting washout 

Credit was given for other relevant correct material such as context sensitive recovery from inhalational anaesthesia, tissue equilibration times, decrement times of inhalational agents and the effect of controlled ventilation by the anaesthetist during emergence. 

Common problems were 

• Poor knowledge of respiratory changes of morbid obesity in the awake state and 

upon emergence of anaesthesia 

• Misconception about the effect of cardiac output on inhalational washout 
• Washout curves with wrongly labelled y-axis 

Better answers incorporated the effects of morbid obesity and specific inhalational agents into the washout curve.