SAQ 2017.2 Question 6

Describe the effects of morbid obesity on the respiratory system.

The material to answer this is scattered through the recommended texts and most of it can be deduced if you have a reasonable general understanding of respiratory physiology. It’s also nicely summarised in Foundations on Anesthesia : Basic Sciences for Clinical Practice by Hemmings and Hopkins Chapter 71 if you can find a copy.

It’s Friday so instead of making this a TRUE/FALSE post I’ll talk about answering an SAQ using this question as the base.

One of the examiners gives the advice :


This is great advice. Unfortunately a lot of exam answers have step 2 omitted. Step 2 is very important, and in the heat of the exam it is easy to forget it. I have had a sneak preview of the exam report and for this question the marking examiner commented that ‘Notably there were no marks achieved for describing the metabolic, endocrine or cardiovascular effects of morbid obesity’.

I would build on his advice and say an even better answer would be created by :


For example with this question you could write : (note use of point form, common abbreviations and clear arrows showing direction of change – all acceptable and even encouraged by examiners)

  •  FRC ↓ or FRC ↓ so oxygen store ↓ esp with pre-oxygenation (does this decrease in FRC have other implications too?)
  • ↑ pulmonary blood volume or  ↑ pulmonary blood volume → ↓ compliance → ↑WOB   (this change in blood volume is also relevant to gas exchange, why?)
  • diaphragm displaced cephalad → why is this relevant to the preload of this muscle?





Aviation 5 – Time of Useful Consciousness – Part 2

BT_PO 1.31 Discuss the carriage of oxygen in blood, the oxyhaemoglobin dissociation curve, oxygen stores in the blood and their clinical significance and implications

The previous aviation post looked at how altitude affects the time it takes to for a person to become ineffective.  Today’s post is a series of questions and problems to solve, relating to the physiology underlying the rapid development of hypoxaemia.

The times of useful consciousness quoted previously were determined empirically (probably “volunteers” in an altitude chamber). See if you can come up with similar values as a desktop exercise.

  • Work out the total body stores of oxygen sitting in an airliner at a cabin altitude of 6000 feet.  (Remember to include FRC, arterial blood, venous blood, myoglobin and other tissues.)
  • What is a normal minutely consumption of oxygen for someone sitting in an airliner?
  • What happens to these stores if the cabin altitude suddenly changes to 35,000 feet (as might happen if a modern aircraft cabin is suddenly depressurised)?
  • Would the oxygen consumption increase?
  • Is it better to hold your breath or breath rapidly?
  • Oxygen usually moves from alveolus to blood.  Can the reverse occur?  What would this do to arterial oxygen tension?
  • How would you describe the shape of the graph of arterial oxygen tension versus time?
  • You will also need to consider regional blood flow and oxygen consumption as well as the volume of blood and oxygen content in various anatomical locations
  • Remember that this exercise involves dynamic not static situations

Feel free to post your calculations in the comments section

Aviation 4 – Time of Useful Consciousness – Part 1

Flying Officer Reynolds was flying combat air patrol just north of Cairo.

He sighted a Ju86P with two pilots sitting securely in a pressure cabin.  He then pursued the Ju86 north towards Alexandria at ever increasing height.

At 37,000 ft he closed in and began to zigzag to prevent overshooting past the Ju86, the Ju86 zigzagged in the same way trying to stay above the Spitfire and force it to lose height.

The duel continued to 40,000 ft where Reynolds realised he could not climb any higher until the aircraft weight was lightened.

Calculating how much fuel he could use and still reach home safely,  he threw out his life jacket and dingy, keeping only the parachute.

In the middle of these calculations he temporarily blacked out having only sufficient time to turn his oxygen supply to full emergency to bring himself around.

BT_PO 1.36  Discuss the physiological effects of hypoxaemia

BT_RT 1.11 Describe the physiological consequences of hypoxaemia 

The time of useful consciousness is the “time interval after loss of pressurisation or mask function during which the crew member is able to recognise and take action to correct the situation”

<18,000 ft = hours/days

18,000 ft = 20 – 30 min

25,000 ft = 3 – 5 min

30,000 ft = 1 – 2 min

35,000 ft = 30 – 60 sec

>43,000 ft = 9 – 12 sec

Crusing Altitude

As you can see from this photo, taken recently at airliner cruising altitude, if there is a sudden depressurisation, you will have less than 30 seconds to get to your oxygen mask.

TRUE/FALSE Hypoxaemia caused by ascending to altitude results in a left shift of the oxygen haemoglobin dissociation curve

TRUE/FALSE In cerebral hypoxia, acidosis causes more damage than depletion of high energy compounds

In an airliner:

TRUE/FALSE when the oxygen masks drop down from the ceiling put you child’s mask on first

TRUE/FALSE oxygen is already running when you put on the mask

Aviation 3 – Pressure Breathing

Helmet Shot

Today we continue the aviation theme.  The last aviation post asked what could be done to improve oxygenation when 100% oxygen was not enough.

Pilots flying at high altitude have access to “pressure breathing” or in medical terms: continuous positive airway pressure (CPAP).  An example of such a mask is seen in the photo above.

BT_PO 1.35 Discuss the physiological consequences of intermittent positive pressure ventilation and positive end-expiratory pressure

As you know, CPAP is commonly used for treatment of sleep apnoea, patients with pulmonary oedema and other conditions.

While these conditions are not examinable in the Primary Exam, the physiological effects of CPAP are important to understand.

TRUE/FALSE CPAP increases respiratory compliance

TRUE/FALSE CPAP reduces venous return

TRUE/FALSE Gastric dilation is a common problem during CPAP

TRUE/FALSE CPAP has no effect on cardiac output

TRUE/FALSE The limit to the level of CPAP that can be tolerated by an awake patient is 15 mmHg


Aviation 2 – Duel in the Stratosphere

To add to the interest, we will be following the fortunes of two remarkable pilots from World War Two.

Flying Officer’s Emmanuel Galitzine and G.W.H. Reynolds were Spitfire pilots who, in 1942, came up against German Junkers 86 (Ju86) high altitude bombers in two different theatres of war: Galitzine over England and Reynolds over North Africa.

To understand the incredible feats of these pilots it is important to know that some high altitude duels happened at over 40,000 ft and that the Ju-86’s had pressurised cabins but the Spitfire’s cockpit did not.

BT_PO 1.22 Describe the composition of ideal alveolar and mixed expired gases

Using the alveolar gas equation, calculate the alveolar oxygen tension at each of the following altitudes.

Altitude      Atmospheric pressure

18,000 ft     380 mmHg (Mt Everest Base camp)

33,000 ft     190 mmHg

40,000 ft     142 mmHg

45,000 ft     111 mmHg

50,000 ft     87 mmHg

Did you allow for changes in carbon dioxide tension caused by the response to hypoxaemia?

Our Spitfire pilots had face masks and regulators that allowed delivery of up to 100% oxygen.

Re-calculate the alveolar oxygen tensions at the altitudes above assuming 100% oxygen was being administered.

Do you think 100% oxygen would have allowed our pilots to function at these altitudes?

What else could be done to improve oxygenation in this situation?

How might the effective atmospheric pressure be increased?

BT_RT 1.39 Interpret blood gas analysis in respiratory failure

TRUE/FALSE  When breathing room air, an arterial PO2 of less than 20 mmHg is not compatible with life.

TRUE/FALSE  Respiratory failure is ruled out if blood gas analysis shows an arterial PCO2 of 27 mmHg.

TRUE/FALSE  Generally, arterial PO2 gives a better indication of respiratory failure in comparison to arterial PCO2.

TRUE/FALSE  Arterial PCO2 in excess of 85 mmHg is usually never due to iatrogenic causes.

TRUE/FALSE  It takes only 30 seconds to develop hypoxaemia with acute hypoventilation in room air.


BT_PO 1.38 Discuss the cardiovascular responses to:

· Exercise


TRUE/FALSE It is only possible to perform mild exercise whilst taking Beta Blocking drugs.

TRUE/FALSE Systolic blood pressure increases by 15-20% with intensive exercise.

TRUE/FALSE Cardiac output shows similar rises with overdrive pacing and moderate exercise.

TRUE/FALSE The cardiac output increases because of an increased tissue demand for oxygen.

TRUE/FALSE Muscle blood flow increases to a similar degree as cardiac output.

BT_PO 1.38 : Define humidity and outline the importance of humidification

Nunn won’t be so much help today, it’s more application of some basic physics.

Turbulent flow in the upper airway is better than laminar flow for humidification TRUE/FALSE

High minute volumes will move the isothermic saturation boundary distally TRUE/FALSE

With an unchanged absolute humidity, relative humidity will decrease as temperature increases  TRUE/FALSE

Gas in an oxygen cylinder has a relative humidity of 50%  TRUE/FALSE

Physiological humidification of inhaled gas requires more energy than warming it  TRUE/FALSE



BT_PO 1.39 : Outline the non-ventilatory functions of the lungs

I was still reading Nunn’s Applied Respiratory Physiology today.

The lung is particularly poor at clearing thrombo-emboli  TRUE/FALSE

With quiet nasal breathing the isothermic saturation boundary is around the 1st to 2nd airway generation   TRUE/FALSE

Surfactant contains bactericidal proteins TRUE/FALSE

The optimal size for drug particle deposition in the lungs is 20 microns  TRUE/FALSE

There is an active cytochrome P-450 system in alveoli and airways  TRUE/FALSE

BT_PO 1.31 : Discuss the carriage of oxygen in blood, the oxyhaemoglobin dissociation curve, oxygen stores in the blood and their clinical significance and implications

Today I was reading Nunn’s Applied Respiratory Physiology, some of these statements are more inspired by my reading than directly in the text, but you should be able to figure out the answers if you understand the text.

It is valid to put the points (40mmHg, 75%) and (26.3mmHg, 50%) on the same oxyhaemoglobin dissociation curve (ODC)  TRUE/FALSE

The ODC flattens at higher pO2s as at these levels binding of oxygen is inhibited  TRUE/FALSE

A shift of the ODC to the right will impair oxygenation in the lungs  TRUE/FALSE

Foetal haemoglobin is left shifted with respect to adult haemoglobin  TRUE/FALSE

The P50 of myoglobin is less than the P50 of adult haemoglobin  TRUE/FALSE