BT_PO 1.21 Discuss dead space, its measurement and apply the Bohr equation and alveolar gas equation

Core stuff and you should know the answers to all these questions. If you don’t then any physiology text will edify you.

The Bohr equation is used to calculate physiological dead space  T/F

End-tidal and mixed alveolar CO2 are very similar in the healthy subject  T/F

The Enghoff modification refers to substituting arterial CO2 for alveolar CO2  T/F

Anatomical dead space extends down as far as the fifteenth generation of airways  T/F

Intubation per se increases dead space  T/F

The PAO2 coming from the CGO of an anaesthetic machine with an FI02 of 40% can be calculated by the formula* = 0.4(760-47)- PaCO2/0.8  T/F

The calculated alveolar oxygen tension is altered depending on what you eat  T/F

ET CO2 decreases in hypotension because of increased anatomical dead space  T/F

*little tricky but nothing you can’t handle

2018.1 SAQ 13 Lung Compliance

Define and describe LUNG compliance. Discuss the difference between static and dynamic compliance.

BT_PO 1.11  Define compliance (static, dynamic, specific) and relate this to the elastic properties of the lung
BT_PO 1.12  Discuss ‘fast’ and ‘slow’ alveoli, including the concept of time constants
there are other related LO’s as well

I often observe that clinicians tend not to think much about physiology, until they have a really sick patient, or things aren’t going well. In order to be capable of intelligently managing a patient with very low respiratory compliance (an obese patient having laparoscopic surgery in the head-down position, for example), it is necessary to have a really solid grasp of this area of respiratory physiology.

* although some of the statements below overlap with total respiratory compliance, the SAQ was specifically asking about LUNG compliance only.

T / F  Lung compliance is the change in alveolar pressure for a given change in lung volume.

T / F  Your ventilator screen displays a pressure-volume loop. It tells you the compliance is 50 mL/cmH20. This is LUNG compliance, and is normal for a healthy intubated patient.

T / F  Deriving the compliance from a P-V loop during IPPV is an example of dynamic compliance.

T / F  Static compliance is always higher (better) than dynamic compliance due to the variations in alveolar time constants.

T / F  Increasing PEEP will always improve lung compliance.

T / F  Increasing inspiratory time on the ventilator can improve ventilation of areas of lung with poor compliance, because their time constant will be slower

“Pressure Control-Volume Guarantee” (PC-VG) might be helpful in patients with poor respiratory compliance. Can you explain why? If you need a hint, look at the pressure and flow graphics for this ventilation mode, and try to link this to the concept of time constants.

2018.1 SAQ 11


Describe the respiratory response to hypoxaemia in both the awake and anaesthetised patient.

You could give a very brief but pertinent answer to this question by stating that the responses are very different!  Being reliant on oxygen for sustaining life we have evolved powerful mechanisms to combat hypoxia/ hypoxaemia (not the same thing but usually co-exist) but these same mechanisms are mostly ablated when we anaesthetize our patients.

T/F  The peripheral chemoreceptors are located in the carotid sinus

T/F  Hypercarbia impairs the ventilatory response to hypoxaemia

T/F  Volatile agents will mostly ablate the ventilatory response to hypoxaemia at low MAC values

T/F  If you underwent bilateral carotid endarterectomy you would lose your ability to respond to hypoxaemia

T/F  Sustained hypoxaemia causes a triphasic response in the awake subject

T/F  Hypoxic pulmonary vasoconstriction is markedly impaired by 1 MAC volatile

All the answers are in Nunn of course. Possibly the only physiology text you should contemplate reading from cover to cover.

BT_AM 1.4 Describe the physiological consequences of anaesthesia and patient positioning on the respiratory system and their management

T/F Functional residual capacity is reduced during anaesthesia

T/F Total respiratory system compliance is unchanged during anaesthesia

T/F Airway resistance during anaesthesia is unchanged compared to a supine awake patient

T/F Airway resistance during anaesthesia is unchanged compared to an upright awake patient

T/F Arterial pO2 is lower during anaesthesia in the lateral position than when supine

T/F During anaesthesia, FRC is higher in the prone position than when supine

T/F Atelectasis occurs in appromimately 40% of patients undergoing anaesthesia with muscle paralysis

BT_AM 1.19 Describe different modes of ventilation available on modern ventilators and their physiological consequences

T/F During pressure-cycled ventilation, inspiratory flow is constant

T/F PEEP can decrease left ventricular afterload

T/F PEEP decreases total lung water

T/F CPAP can be achieved by partially closing the APL valve on a circle circuit

T/F PEEP or CPAP can increase left ventricular transmural pressure

T/F PEEP can increase right ventricular volume

T/F During pressure support ventilation, cycling into expiration occurs when the inspiratory flow rate decreases to a pre-set level

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 base of the upright lung is about 0.6, because there is more perfusion than ventilation

T / F   a V/Q ratio of infinity is alveolar shunt

T / F   in a conscious patient with left lower lobe collapse, hypoxaemia would be WORSE when lying on their left side

T / F   in an intubated ventilated patient with left lower lobe collapse, hypoxaemia would be WORSE when lying on their left side

T / F   hypoxic pulmonary vasoconstriction can reduce the degree of hypoxaemia caused by V/Q mismatch – HPV is mediated by BOTH alveolar and mixed venous PO2


Some patients have very little room to move with gas exchange! Here is the CXR of a very sick patient with LLL collapse and L pleural effusion. If lateral positioning was required, you should be able to anticipate the effect that this would have on V/Q mismatch.

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?