John is a 23-year-old man admitted to the hospital with shortness of breath, fever, and cough. His oxygen saturation is low and he is using accessory muscles to breathe. Blood gas results show hypoxemia and respiratory acidosis. He has wheezing and crackles in his lungs. The document discusses the pathophysiology of John's acute respiratory failure, including ventilation-perfusion mismatching, increased dead space, and the effects of mechanical ventilation.

1. (Patho-)Physiology of
Acute Respiratory Failure
CT1 Education Series (Intro)

2. Respiratory Physiology Curriculum
Gas exchange - O2 & CO2 transport, hypoxia & hypercapnia,
hyper- and hypo-baric pressures
Functions of Hb in O2 carriage & acid-base equilibrium
Pulmonary ventilation: volumes, flows, dead space
Effects of IPPV on lungs
Mechanics of ventilation: V/Q abnormalities
Control of breathing; acute & chronic ventilatory failure;
effects of O2 therapy
Non-respiratory functions of the lungs

3. Respiratory Failure
pO2 < 8 kPa (60 mmHg) and/or
pCO2 > 6 kPa (45 mmHg)
Type I = hypoxia
Type II = hypercapnia
Type III = Perioperative (atelectasis)
Type IV = Shock (Hypoperfusion)

4. Case scenario
John is a (slightly weird) 23 year old with mild
asthma. He is admitted to hospital with 3 day
history of myalgia and fever, and increasing
shortness of breath and cough over the last 36
hours.
His room air O2 sats are 84% and his
respiratory rate is 37/minute and shallow. He is
using accessory muscles and there is active
exhalation
ABG: pH 7.34, pCO2 6.1 pO2 7.8 on air
He is sweaty and tiring rapidly. You can detect
creps at his right base and widespread
wheeze.

5. Key Issues
Mechanisms of hypoxia
Work of breathing
Dead space and alveolar ventilation
Hypoxic pulmonary vasoconstriction
Alveolar gas composition & A-aDO2
Shunt
Effects of anaesthesia on all of these
Effects of mechanical ventilation

6. Why is John hypoxic?

7. Why is John hypoxic?
Mechanisms:
Ventilation/perfusion (V/Q) mismatch
Shunt
Diffusion impairment
Alveolar hypoventilation (normal A-aDO2)

8. What about John’s breathing pattern?

9. What about John’s breathing pattern?
Increased work = increased O2 consumption
Rapid shallow breathing = increased VD/VT ratio
Rapid shallow breathing = turbulent flow
Turbulent flow = increased work (vicious O)
Bronchospasm = gas trapping

10. Work of breathing - f/VT

11. Work of breathing

12. Dead-space
Alveolar minute ventilation = MVA
MVA = (tidal volume - deadspace) x RR
Anatomical = volume of air in conducting
airways
Alveolar = gas volume in unperfused
alveoli
Physiological = anatomical + alveolar

13. Bohr Equation
Volume of CO2 removed from ideal alveolar
gas = volume exhaled in mixed expired gas
%CO2Alv [=PaCO2] x VAlv [=VT-VD] =%CO2Exp x VExp [=VT]
VD/VT = (PaCO2-PECO2)/PaCO2
Normal VD/VT = 0.2-0.3
Higher deadspace = bigger gap between
PaCO2 and ETCO2

14. John’s still alive....just!
You got worried and intubated John before he
collapsed. After 15 minutes on the ventilator
(PEEP 5, VT 550, rate 14, FiO2 80%) his pH is
7.34 pCO2 5.8 pO2 8.6

15. What is John’s A-aDO2?
What does that mean?

16. Alveolar gas composition
PAO2 = PIO2 - PaCO2/R
PAO2 =Alveolar partial pressure of O2
PIO2 =FiO2 x (PB -PH2O)
R = resp. quotient = 0.8 (ish)
A-aDO2 = PAO2 - PaO2

17. Shunt & venous admixture
Deoxygenated blood returning to left heart
e.g. Bronchial veins (< 1% of CO)
Thebesian veins from LV walls (0.3% of CO)
Congenital heart disease
Alveolar collapse
Venous admixture = amount of mixed venous
blood required to mix with pulmonary capillary
blood to produce observed A-aDO2

18. Shunt equation (!)
Total O2 delivery = cardiac output O2 content = Shunt O2
content + Pulmonary blood flow O2 content
Cardiac output = total flow = Qt
Shunt flow = Qs, Pulmonary flow = Qt-Qs
CaO2 = arterial O2 content
CvO2 = mixed venous O2 content
CcO2 = pulmonary end capillary O2 content (calculated from
alveolar gas equation & O2 dissociation curve)
Qs/Qt = (CcO2 - CaO2)/(CcO2-CvO2)

20. How are you going to fix John?
What is wrong with his lung compliance?
How can the ventilator help?
How will you know it is helping?
How is it going to help his oxygenation?

21. Compliance

22. Lung Volumes

23. Closing capacity & volume
Dependent airways close during expiration
This occurs at closing capacity (CC)
The closing volume = CC - residual volume
CC < FRC in young adults
CC is independent of position but FRC isn’t!
CC = FRC at 44 years in supine position
CC = FRC at 66 years when upright