Ventilation / Perfusion (V/Q) Relationships

BT_PO 1.26 Discuss normal ventilation-perfusion matching

BT_PO 1.29 Discuss ventilation-perfusion inequalities, venous admixture and the effect on oxygenation and carbon dioxide elimination


T / F   the V/Q ratio at the apex of the upright lung is 3.3, because the apex receives most of the alveolar ventilation

T / F   in a conscious person lying on their left side, the left lung will receive more ventilation AND perfusion than the right lung

T / F   in an anaesthetised ventilated patient lying on their left side, the left lung will receive more ventilation AND perfusion than the right lung

T / F   atelectasis results in an increase in alveolar dead space, which can cause hypercapnoea

T / F   a decrease in cardiac output can decrease mixed venous PO2 – this will magnify the hypoxaemia produced by any alveolar shunt


BT_SQ 1.5 Describe basic physics applicable to anaesthesia, in particular:
…. principles of humidification and use of humidifiers ….


T / F   during quiet breathing, air reaching the carina is close to 37 degrees C and 100% relative humidity

T / F   at 37 degrees C, air can hold a maximum of 44 mg/L of water vapour

T / F   during expiration, water vapour condenses onto the airway mucosa

T / F   absolute humidity depends upon both the temperature and the atmospheric pressure

T / F   a HME can warm inspired gases to about 30 degrees C, but this takes about 20 minutes

Carbon dioxide carriage in the blood

BT_PO 1.32   Discuss the carriage of carbon dioxide in blood, the carbon dioxide dissociation curve and their clinical significance and implications

Most of the dissolved carbon dioxide in the blood is in the erythrocytes     TRUE/FALSE

Carbonic anhydrase is found in erythrocytes   TRUE/FALSE

Carbonic anhydrase is found in pulmonary capillary endothelium   TRUE/FALSE

As temperature decreases, there is a lower pCO2 for a given mass of CO2 in the blood   TRUE/FALSE

Reduced Hb has a tenfold ability to carry CO2 over oxyhaemoglobin   TRUE/FALSE

Work of Breathing

BT_PO 1.10 Describe the Work of Breathing

1 Joule of work is done when 1 litre of gas moves in response to a pressure gradient of 1 kilopascal     TRUE/FALSE                                    This uses the SI unit of kPa, how many cm of water is that? So how many joules per breath? How many joules per minute? What is the efficiency of breathing? How many joules/calories are you expending on breathing? What % of your daily calorie use is that?

Breathing at rest is responsible for approximately 0.5% of the body’s oxygen consumption     TRUE/FALSE

Work is calculated by integrating a pressure volume curve     TRUE/FALSE

1/2 of the energy created in inspiration is stored as heat to be used for expiration      TRUE/FALSE

Bonus unexaminable question : Earth’s gravity accelerates a falling body at approximately 10 m/s/s and 1 Newton is defined as the force moving 1 kg at 1 m/s/s. Assuming that a 17th century apple had a mass of 100g, with what force would it have hit a man’s head if it fell out of a tree?


Pulmonary circulation

BT_PO 1.33 Discuss the difference between the pulmonary and systemic circulations 

The short length of the pulmonary vasculature contributes to its low resistance     TRUE/FALSE

Pulmonary endothelium synthesises prostaglandins     TRUE/FALSE

Parasympathetic stimulation constricts the pulmonary circulation     TRUE/FALSE

The pulmonary capillary bed is pulsatile     TRUE/FALSE

20% of foetal cardiac output passes through the pulmonary circulation     TRUE/FALSE

BT_PO 1.9 Hypoxia

Hypoxia is a strong driver to increase respiratory rate. TRUE/FALSE.

The aortic body peripheral chemoreceptors are the most important peripheral chemoreceptors in humans. TRUE/FALSE.

The central chemoreceptors respond directly to changes in the H+ concentration in the CSF. TRUE/FALSE.

Inflation of the lungs that is detected by pulmonary stretch receptors increases respiratory rate. TRUE/FALSE.

Rapid breathing in left heart failure is potentially due to stimulation of the junta-capillary receptors. TRUE/FALSE.

Pulmonary Circulation

An overall increase in vascular tone reduces blood volume within the pulmonary circulation. TRUE/FALSE

Pulmonary arterial pressure is much less then the systemic arterial pressure, although the capillary and venous pressure are not greatly different in the two circulations. TRUE/FALSE

Pulmonary vascular resistance tends to fall as flow increases. TRUE/FALSE

The arterioles are the main point providing resistance in the pulmonary vasculature. TRUE/FALSE

The greatest drive for hypoxic pulmonary vasoconstriction is the pulmonary arterial Po2. TRUE/FALSE.

Ventilation and Perfusion

Since the pulmonary circulation operates at low pressure, the distribution of blood is similar to the distribution of ventilation. TRUE/FALSE

Alveoli with no ventilation will have PO2 and PCO2 values that are the same as mixed venous blood. TRUE/FALSE.

A pulmonary embolism is a shunt. TRUE/FALSE

Pulmonary capillary blood flow + Venous admixture = Cardiac Output. TRUE/FALSE

Venous admixture increases arterial blood carbon dioxide content above that of pulmonary end-capillary blood. TRUE/FALSE

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?