Respiratory physiology

Question 1

SVP of water vapour at 37’C (oxygen cascade)

Answer 1

6.3kPa

Question 2

Recite the alveolar gas equation

Answer 2

PAO2 = FiO2 (Patm - PH2O) - PACO2/0.8

Question 3

Stages of o2 cascade (from air to veins -> 7)

Answer 3

Dry air, Humifidied air (SVP water vapour), Ventilation (Alveolar gas equation), Diffusion to capillary (negligible drop in O2), Artery (shunt, V/Q mismatch), Capillary beds and mitochondria (PO2 ~1.5kpa), venous system (6.3kpa)

Question 4

What is the pasteur point

Answer 4

The partial pressure of oxygen at which oxidative phosphorylation in the mitochondria can no longer occur (~1mmHg / 0.13kPa)

Question 5

3 main causes of A-a gradient, normal gradient in healthy individual

Answer 5

Shunt, V/Q mismatch, venous admixture, <2kPA

Question 6

Normal O2 difference (ml O2 /dL) between arterial and venous blood

Answer 6

~5ml O2 / dL

Question 7

Oxygen content equation

Answer 7

(Hb x 1.34 x SO2) + PO2 x 0.0225

Question 8

Ways to improve oxygen content and delivery

Answer 8

Content - supplemental oxygen, blood transfusion (if anaemic), hyperbaric oxygen (partial pressure o2 dissolved)

Delivery (CO = SV * HR)
optimise HR - 60-90 bpm, sinus
optimise stroke volume - preload, contractility
perfusion pressure - afterload/systemic vascular resistance
(FLUIDS INOTROPES)

Question 9

Draw supply demand curve DO2 / VO2, what is the critical DO2.
What explains the changes on this graph

Answer 9

Critical DO2 ~ 400ml/min
Increased oxygen extraction ratio by tissue until the critical DO2 where VO2 can no longer be maintained.
Critical DO2 - the point at which oxygen supply can no longer meet demand.

Question 10

Normal O2 extraction ratio

Answer 10

20-30% (capacity for DO2 to drop and maintain aerobic respiration)

Question 11

Definition of shunt

Answer 11

Blood which enters the arterial system without participating in gas exchange.

Question 12

What is PiO2 and why is it different to FiO2

Answer 12

PiO2 accounts for dried and humidified air, resultant from FiO2 (PATM-PH2O) -> pH2O ~ 6.3kpa

Question 13

What is deadspace and what are its components, what volume does it take?

Answer 13

Volume of inspired gas that does not partake in gas exhcange, comprised of anatomical deadspace (1-16, nasal cavity -> terminal bronchioles) and alveolar deadspace (ventilated but not perfused)

2ml/kg in adults.

Question 14

Factors that decrease anatomical deadspace

Answer 14

Intubation, tracheostomy, supine, bronchoconstrictors

Question 15

Factors that increase anatomical deadspace

Answer 15

Increased size of transporting airway (neck extension jaw protrusion, bronchodilators, standing, pregnancy (progesterone), face mask.