Control of Respiration

Physiol-18A11 Describe the respiratory response to hypoxaemia in both the awake and anaesthetised patient.

Physiol-15A2 Outline the role of carbon dioxide in the maintenance of ventilation.

Physiol-06A10 List the physiological factors which increase respiratory rate. Include a brief explanation of the mechanism by which each achieves this increase. (also Physiol-02B10, Physiol-95B9)

Mechanics of Breathing

Physiol-18A13 Define and describe lung compliance. Discuss the difference between static and dynamic compliance.

Physiol-13A9 Describe the cough reflex.

Physiol-11A13 Describe the determinants of work of breathing in an adult human at rest. (also Physiol-06B10, Physiol-01B2, Physiol-00A5)

Physiol-10B12 Describe the function of the muscles involved in ventilation. 39%

Physiol-10B9 Describe the factors that affect respiratory system compliance. (also Physiol-10A14, Physiol-03A14)

Physiol-08B13 Explain the concept of time constants and relate these to “fast” and “slow” alveoli.

Physiol-03B13 Describe the factors that affect airways resistance. (also Physiol-99B3, Physiol-98B8)

Pulmonary Gas Volumes and Ventilation

Physiol-15A9 What are the physiological consequences of decreasing functional residual capacity by one litre in an adult? (also Physiol-04A12, Physiol-01A8)

Physiol-05B16 Explain the changes on Functional Residual Capacity (FRC) that take place with the administration of anaesthesia.

Physiol-03B11 Briefly describe the potential causes of a difference between measured end-tidal and arterial partial pressure of carbon dioxide. (also Physiol-96B7)

Physiol-00B3 Draw an expiratory flow volume curve for a forced expiration from total lung capacity. Describe its characteristics in people with normal lungs, as well as those with obstructive and restrictive lung disease.

Physiol-98A2 Draw a flow/volume curve for a maximum forced expiration in a person with healthy lungs from: (a) Total lung capacity; (b) Function Residual capacity. Explain your curves.

Physiol-97B7 Explain the factors influencing the distribution of ventilation during the inhalation of 500ml of air from Functional Residual Capacity in the erect posture.

Physiol-96B6 Draw a respiratory flow/volume loop and outline how it is obtained. Briefly explain the physiological mechanisms involved in the concept of flow limitation.

Physiol-MAKEUP Describe the single breath nitrogen washout technique

Ventilation Perfusion Inequalities

Physiol-15B3 What affect does placing a patient in the right lateral position have on lung ventilation and perfusion?

Physiol-07A9 Define ‘Venous Admixture’. Briefly explain how venous admixture influences arterial 02 tension and how an increase in inspired 02 concentration may affect this. (also Physiol-02A3, Physiol-95B1 )

Physiol-97A7 Compare the effect on arterial blood C02 and 02 levels of ventilation/perfusion inequalities.

Gas transport in the Blood

Physiol-18B2 Discuss factors that affect oxygen transport from the alveoli to the tissues.

Physiol-14B9 What lower limit of SpO2 would you accept in an ASA 1 young male under general anaesthesia?

Physiol-13A8 Explain how oxygen supply of organs is maintained during isovolaemic haemodilution. (also Physiol-02A1, Physiol-96B5)

Physiol-11B12 Outline the similarities and differences between myoglobin and adult haemoglobin, explaining the physiological relevance of the differences. 24%

Physiol-11A9 Describe the ways in which CO2 is carried in blood. (also Physiol-04B9, Physiol-99B5, Physiol-97A3)

Physiol-10A9 Explain the physiological factors that may lead to a decrease in mixed venous blood oxygen saturation. (also Physiol-00A2, Physiol-96A1)

Physiol-09A16 Outline the effects of acute exposure to air at an altitude where barometric pressure is 347mmHg. What compensatory mechanisms occur with gradual exposure to increasing altitude?

Physiol-07B10 Discuss factors which affect the partial pressure of carbon dioxide in mixed venous blood. (also Physiol-05A12)

Physiol-04A11 What is 2,3-DPG? How is it produced in the red blood cell and how does it interact with haemoglobin? What is its relevance in altitude exposure, anaemia and stored blood? (also Physiol-95A1)

Physiol-03B12 Explain the difference between perfusion limitation and diffusion limitation in the transfer of gas between alveolus and pulmonary capillary. Outline the factors that determine whether gas transfer is perfusion or diffusion limited.

Physiol-99A3 Describe the factors that affect the transport of oxygen and carbon dioxide from the alveolus to the blood.

Physiol-96B8 Briefly explain how an oxygen debt arises and how the body deals with it

Pulmonary Circulation

Physiol-15B09 Briefly outline the differences between the pulmonary circulation and the systemic circulation. (also Physiol-12B16, Physiol-04A9)

Physiol-05B9 Describe the gravity dependent processes which affect pulmonary blood flow. What changes take place when the pressure increases in the pulmonary vessels? (also Physiol-97A2)

Physiol-02A4 Outline the physiological factors that influence pulmonary vascular resistance. (also Physiol-00A3)

Other Respiratory Physiology

Physiol-16B1 Describe the respiratory effects of adding positive end expiratory pressure (PEEP) to intermittent positive pressure ventilation (PPV). (also Physiol-16A11)

Physiol-14A5 Explain the effects of intermittent positive pressure ventilation on left ventricular output. (also Physiol-01B1, Physiol-98A1, Physiol-96B3)

Physiol-14A4 Describe the physiological basis of methods used to prevent hypoxaemia prior to intubation in a rapid sequence induction. Include any adverse effects of these methods.

Physiol-12B12 Discuss the physiological causes of early post-operative hypoxaemia.

Physiol-11B14 Describe the changes in respiratory function tests that occur with long term increases in small airways resistance.

Physiol-08B15 Describe the changes that occur with ageing that can affect oxygen delivery to the tissue during moderate exercise.

Physiol-08A16 Discuss the physiological causes of early post-operative hypoxaemia.

Physiol-05A13 Describe the non-respiratory functions of the lung.

Physiol-MAKEUP Describe the beneficial and adverse effects of CPAP/PEEP, and the mechanisms by which these effects occur


3 thoughts on “Respiratory

  1. Hi
    Regarding Physiol03B11 Briefly describe the potential causes of a difference between measured end-tidal and arterial partial pressure of carbon dioxide.
    In the model answer with manipulating the Bohr Eq (Enghoff mod):
    VDAlv/VT≈ (PaCO2 − PETCO2)/PaCO2
    PaCO2 is the arterial CO2 partial pressure
    PETCO2 is the end-tidal CO2 partial pressure
    VDAlv is the alveolar dead space

    I’m trying to get my facts straight about the use of end-tidal PCO2 in the equation.
    At first approximation it seems reasonable seeing as it is convention to say that end-tidal PCO2 is most reflective of alveolar gas
    But the examiners report when this was asked in 1996 (Brandis’s website) stated:
    Inappropriate and/or incorrect use of the Bohr equation, with substitution of end tidal for mixed expiratory partial pressure of carbon dioxide. This was used to quantify alveolar dead space. Nunn has modified the equation to obtain alveolar dead space/alveolar volume ratio by using end tidal carbon dioxide. When used, this was usually done incorrectly.

    The discussion in Nunn’s says
    ‘end-expiratory gas is a small capital E with a prime (E’) and mixed expired gas a small capital E with a bar.
    When he later discusses the Bohr equation he uses the E with a bar (i.e. mixed expired) and says its physiological dead space.
    Later in the Measurement of Dead-space section he writes:
    arterial/end-expiratory PCO2 difference is a convenient and relatively simple method of assessing the magnitude of the alveolar dead space….end-expiratory gas is shown to consist of a mixture of ‘ideal’ alveolar gas and alveolar dead space gas…if, for example, ‘ideal’ alveolar gas has a PCO2 of 40mmHg and the end-expiratory PCO2 is 20mmHg it follows that the end-expiratory gas consists of equal parts of ‘ideal’ alveolar gas and alveolar dead space. Thus if the tidal volume is 500ml and the anatomical dead space is 100ml the components of the tidal volume are 100ml anatomical deadspace + 200ml alveolar deadspace + 200ml ‘ideal’ alveolar gas.

    If I do the equation with say end-tidal CO2 15mmHg and the PaCO2 40mmHg then the result is 0.625 (? a percent, as in 62.5%) so if I use the approach as illustrated in Nunn’s –> I think then you are supposed to conclude that of the end-expiratory gas 34.5% (i.e. the part you actually got back) is ‘ideal’ alveolar gas and 62.5% is alveolar dead space and so then you must treat the tidal volume accordingly including minus the Anatomical dead space and distribute the proportions correctly – as before 500 VT – 100VDanat = 400ml x 0.625 = 250ml of alveolar dead-space and 150ml of ‘ideal’ alveolar gas

    So I’m not sure whether that equation is right or has, as the examiners previously stated, a terminal error in it due to an erroneous conclusion? Maybe it’s partly right and needed the additional steps?


  2. Actually I’d conclude the equation is problematic and would garner the comment from examiners: Inappropriate and/or incorrect use of the Bohr equation – when used this was done incorrectly:

    PaO2 – PETCO2: The notion started here
    Nunn JF, Hill DW. Respiratory dead space and arterial to end-tidal CO2 tension difference in anaesthetized man. J Appl Physiol 1960;15:383–9.
    Hence the 1996 examiners comment about the ‘Nunn modification’ and how that discussion found its way into his book.

    Hardman and Aitkenhead (2003) Anesthesia and Analgesia:
    Nunn and Hill have suggested that there is a relationship between the arterial to end-tidal CO2 tension gradient (Pa-e’co2) and the Vdalv fraction, but they did not investigate this relationship.

    Hardman and Aitkenhead had to do COMPUTER MODELLING in 2 separate papers:
    Hardman JG, Aitkenhead AR. Estimation of alveolar deadspace fraction using arterial and end-tidal CO2: a factor analysis using a physiology simulation. Anaesth Intensive Care 1999;27:452–8.

    We concluded in that investigation arterial to end-tidal CO2 tension gradient (Pa-e’co2/Paco2) had a roughly constant, linear relationship with (VDalv/VTalv)BohrFowler as follows: (VDalv/VTalv)Bohr-Fowler = 1.14 x Pa-e’co2/Paco2 – 0.005, and that it could be substituted acceptably for the conventional calculation

    Estimating Alveolar Dead Space from the Arterial to End-Tidal CO2 Gradient: A Modeling Analysis
    Anesthesia and Analgesia 97: 1846 – 1851
    Pa-e’co2/Paco2 was approximately 59.5% of (VDalv/VTalv)Bohr-Fowler


  3. Hi jjverden, your reading into this topic far exceeds my very superficial understanding of the topic.

    Here’s a summary of my basic understanding:
    (1) Bohr equation calculates physiological deadspace based on conservation of mass and thus it is “exact” (assuming no V/Q mismatch)

    (2) The alveolar partial pressures of CO2 that appear in Bohr equation is cumbersome/difficult to measure in practice – this motivated the development of modifications/approximations

    (3) Enghoff modification approximates alveolar partial pressure of CO2 with arterial partial pressure of CO2 – this tends to overestimate physiological deadspace (esp. in presence of shunt or low V/Q)

    (4) Another modification has been proposed whereby alveolar partial pressure of CO2 is approximated by end tidal CO2 – this tends to reduce the overestimation of physiological deadspace by Enghoff as well as avoiding repeated arterial blood gases.

    For a simple summary including clear diagrams, see M. Siobal, Monitoring Exhaled Carbon Dioxide, Respir Care. 2016 Oct;61(10):1397-416. doi: 10.4187/respcare.04919. (free full text is available online)

    Hope this helps + happy propofol dreaming!!

    Liked by 1 person

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