BT_PO 1.37 Describe the effect of morbid obesity on ventilation

Sticking with the topic of obesity.. The reference sources from  yesterday  will be valuable…

Look at that dessert (and the look of glee on my daughter’s face!). I can’t remember what it was called – obscene? -but just looking at it makes me put on weight 😊



BT_PO 1.37 Discuss the effect of the following on ventilation:

  • Changes in posture
  • Exercise
  • Altitude
  • Anaesthesia
  • Ageing
  • Morbid obesity

Morbid obesity is associated with decreased compliance of the respiratory system    TRUE/FALSE

FRC is 25% lower in a person with a BMI of 30kg/m2 compared with a person with a BMI of 20kg/m2    TRUE/FALSE

Resting  respiratory rate is increased by morbid obesity      TRUE/FALSE

The alveolar to arterial gradient (A-a gradient) of oxygen is increased with morbid obesity      TRUE/FASLE

The effects of obesity on the respiratory system are improved by lying down TRUE/FALSE

Patient of the week

Recently I was anaesthetising an adult with a congenital syndrome. I was quite worried about the airway—but in the end it wasn’t that which caught me out. She had no congenital heart disease, but had a pericardial effusion drained a few years previously. I was quite sparing with the induction agents as I wanted to maintain spontaneous respiration, but nonetheless…

About 5 minutes after induction, I noticed the blood pressure was 54/28.

BT_SQ 1.6

T/F At low levels of blood pressure, the NIBP tends to give spuriously low values.

T/F The most accurate component of the NIBP is the mean.

At the same time, her saturation dropped to 88, even though she was breathing 100% oxygen. The pleth had a good volume and looked normal.

BT_SQ 1.6BT_PO 1.29

T/F The fall in SpO2 was most likely to be artifactual.

I gave three doses of 1mg metaraminol, but, although the saturation improved, the blood pressure remained in the low 70s. Heart rate was in the 40s. Worried that I might see another fall in saturation I decided to run a noradrenaline infusion.

BT_PO 1.52

In such a situation, the most appropriate vasoactive agent would be:

a) Ephedrine

b) Metaraminol

c) Adrenaline

d) Noradrenaline

e) Isoprenaline

After a 20µg bolus dose of noradrenaline, the heart rate dropped to 28.

T/F The most likely cause of the fall in heart rate is alpha 1 receptor agonism in the SA node.

I found out, after the (otherwise uneventful) operation, that she normally has quite a low blood pressure. A good reminder that, when having trouble with anaesthesia, one should first look to the proximal end of the needle.

2017.1 : SAQ 11

Draw an expiratory flow volume curve obtained from a maximal expiratory effort after a vital capacity breath, for a person with:

A.  normal lungs

B.  restrictive lung disease

C.  obstructive lung disease

(10 marks)

Explain how and why these curves (and the derived parameters) are different in each disease state (15 marks)

We see a lot of patients with pulmonary function tests, their associated curves and parameters. Understanding the physiology, aids interpretation of the results.

BT_PO 1.20

A vital capacity breath is from expiratory reserve volume (ERV) to total lung capacity (TLC)  TRUE/FALSE

The effort independent component is due to dynamic airways closure TRUE/FALSE

The TLC is increased in obstructive lung disease TRUE/FALSE

The effort independent component is steeper in restrictive lung disease TRUE/FALSE

A normal peak expiratory flow rate would be 10 L/min TRUE/FALSE



BT_PO 1.12 Discuss ‘fast’ and ‘slow’ alveoli, including the concept of ‘time constants’

Theme for the day: mindfulness.

Q. Deflation and Inflation of alveoli can be considered an exponential process. TRUE/FALSE

Q. “Fast” and “Slow” alveoli refers to the compliance/resistance relationship between the alveoli and its supplying bronchiole. TRUE/FALSE

Q. A relatively non compliant alveolus paired with a patent wide bronchiole will comprise a slow lung unit TRUE/FALSE

Q. In the normal lung, alveoli in the apex are “slower” because compliance is less in the apex. TRUE/FALSE

Q. At fast respiratory rates, “slow” lung units may not have completed filling before the onset of expiration. TRUE/FALSE

BT_PO 1.11 Define compliance (static, dynamic and specific) and relate this to the elastic properties of the lung

Twice. So good, they named it twice.

Q. Most studies of lung compliance under anaesthesia indicate a decrease compared to the awake state T/F

Q. Age increases lung compliance T/F

Q. Dynamic compliance may be greater than static compliance T/F

Q. An elephant has greater compliance than a mouse T/F

Q. Stress relaxation is a component of dynamic compliance T/F


SS_PA 1.24 : paediatric physiology

Not a bad textbook, Evers & Maze… But rubbish for paediatric pharmacodynamics… So I’ve swapped over to some paediatric physiology instead. I used Miller as it was handy.

SS_PA 1.24 Describe the physiology of the cardiovascular, respiratory, renal and neurological systems in the neonate and the changes that occur with growth and development and the implications of this for anaesthetic care

During the first 2 weeks of age a neonate can flip back into a foetal circulation   TRUE/FALSE

The neonate has more compliant ventricles than an adult   TRUE/FALSE

Infants have more type I muscle fibres in their diaphragm   TRUE/FALSE

Neonates have decreased intracardiac calcium stores   TRUE/FALSE

Oxygen consumption in infants is higher than in adults   TRUE/FALSE

BT_PO 1.37 The stressed respiratory system

BT_PO 1.37 Discuss the effect of the following on ventilation:

· Changes in posture

· Exercise

· Altitude

· Anaesthesia

· Ageing

· Morbid obesity


Peru. 4200m. #lifeafterexam


TRUE/FALSE Periodic breathing while asleep leading to oxygen saturations of 50% is common when first ascending over 4000 m

TRUE/FALSE  Minute ventilation is proportional to oxygen consumption at all levels of exercise

TRUE/FALSE  Response to hypoxaemia and hypercapnia are usually unaffected by obesity

TRUE/FALSE  FRC is reduced to a greater extent during anaesthesia, when a muscle relaxant is used than when one is not used

TRUE/FALSE  1 MAC of anaesthesia preserves diaphragmatic function but can abolish EMG activity of other inspiratory muscles (If this is true how would this affect your anaesthesia plan for renal and ureteral lithotripsy?)




BT_PO 1.37 The stressed respiratory system

BT_PO 1.37 Discuss the effect of the following on ventilation:

· Changes in posture

· Exercise

· Altitude

· Anaesthesia

· Ageing

· Morbid obesity

TRUE/FALSE  FRC in healthy adult males, is reduced by approximately 500ml when supine

TRUE/FALSE  Ventilatory adaptation to high altitude takes approximately one week

TRUE/FALSE  During bag/mask ventilation, total dead space (apparatus and physiological) comprises approximately half the tidal volume

TRUE/FALSE  FRC reduces with age

TRUE/FALSE  Increased respiratory resistance in obesity is mostly due to increased airways resistance

BT_AM 1.2 Physiology of the airway 

Whilst in Tasmania recently, I visited an excellent exhibition at MONA – on the origin of art. I would definitely recommend seeing it if you are in Hobart before April 17 ( even enough time for a quick post exam trip). There were a series of photos of beautiful “flowers”. The one below features a chicken’s vocal cords

Since there has already been a post on airway anatomy, I thought I would look at airway reflexes. Do chickens cough?

BT_AM 1.2  Describe the physiology of the airway including airway reflexes

Nasal breathing provides better humidification than mouth breathing TRUE/FALSE

The afferent impulses for lung reflexes are mediated via the vagus nerves TRUE/FALSE

Pharyngeal dilator muscles contract reflexively during normal inspiration to prevent pharyngeal obstruction TRUE/FALSE

The expiration reflex may be stimulated at the larynx and sites lower in the airway TRUE/FALSE

Pharyngeal reflexes are maintained, unchanged, during sleep TRUE/FALSE

I’ll show you the flower composed of duck tongues tomorrow….

Dynamic airway closure

It seems as though nerve conduction is not a very popular topic – it is a bit dry.

Perhaps today’s topic, dynamic airway closure, will be of more interest as it is one that candidates really struggle with in vivas.

BT_SQ1.6 Describe the methods of measurement applicable to anaesthesia, including clinical utility, complications and sources of error in particular:
· Methods used to measure respiratory function, including:
– Forced expiratory volume
– Peak expiratory flow rate
– Vital capacity
– Flow-volume loops
– Functional residual capacity and residual volume

West describes the topic well in his book, where the following diagram is taken from. The red circle shows the net pressure gradient between the intrapleural space and the airway.


Dynamic airways closure may occur during normal tidal breathing TRUE/FALSE

Dynamic airway closure accounts for the effort dependent portion of the expiratory limb of the flow-volume loop TRUE/FALSE

During forced expiration, positive pressure generated will be transmitted equally across the respiratory system TRUE/FALSE

The trachea is never subject to dynamic airway closure TRUE/FALSE

During the effort independent part of an expiratory flow volume loop, maximum air flow rate is determined by lung volume TRUE/FALSE