44. Respiratory Failure

Question 1

What percentage of the air is O₂?

What are the 4 levels of the O₂ cascade?

What happens to pO₂ at each level?

What is the kPa of O₂ by the time the air enters the traches?

What are the only 2 factors affecting alveolar pO₂?

Answer 1

21% (21.2kPa at sea level) NB: partial pressure allows O₂ to transfer, not conc

Inspired gas -> alveolar gas -> arterial blood -> cell

It drops

19.9kPa

Ventilation, O₂ consumption

Question 2

What is the usual range for PaCO₂ and how is it measured?

And PaO₂?

And alveolar (PACO₂)?

What determines the alveolar to arterial PO₂ difference?

Answer 2

4.7 - 6kPa, blood gas

12.7kPa

13.7kPa

Shunting

Question 3

What is shunting?

List 3 things that influence the delivery of O₂ to tissues.

What is the single most important sign in a patient with (impending) respiratory failure.

What are some late signs of respiratory compensation and increased sympathetic tone?

Answer 3

An area of the lung that is perfused but not ventilated. 3 - 5% of C.O. undergoes anatomical shunt (small degree = normal): mixed venous blood goes to arterial blood -> desaturates it -> can get hypoxia (commonest cause).

O₂ saturation, Hb concentration, C.O.

Tachypnoea. Any pt with resp rate >25 be suspicious of.

Use of accessory muscles, nasal flaring, intercostal/suprasternal recession, tachycardia, hypertension, sweating. And much later: cyanosis, altered mental state, flap, bounding pulse.

Question 4

What is respiratory failure?

What are the 2 types?

Answer 4

Respiratory system fails in one/both of its GE functions (ocygenation and CO2 elimination). May be acute (life-threatening derangements in arterial blood gas and acid-base status) or chronic (less dramatic).

Type 1: hypoxaemia only, PaO₂ <8kPa. Caused by damage to lung tissue prevening adequate oxygenation of blood, but remaining normal lung ok to excrete CO2 being produced in tissue metabolism. E.g. pulm. oedema, asthma, pneumothorax etc. If leave them they get tired and start to retain CO₂ -> go to type 2 resp failure

Type 2: hypoxaemia and hypercapnia. PaO₂ <8kPa, PaCO₂ >7kPa. Alveolar ventilation insufficient to excrete the CO2 produced. Affects lung as whole. E.g. hypoventilation, stroke, head trauma, phrenic nerve damage

Question 5

What is the number 1 indication for O₂ therapy?

What are the 2 different types of respiratory support techniques?

What sort of pt do you use the following masks for:

a) O₂ mask, nasal canulae
b) Face mask with reservior bag
c) Venturi mask

Answer 5

Tachypnoea. If high rep rate and not sure, give anyway while you do ABG. Also resp failure, cyanosis, hypotension, MI, metabolic acidosis.

Non-invasive for conscious pts via face mask. Invasive for unconscious pts via ET tube.

a) Pt with normal vital signs (post op)
b) Higher O₂ conc needed (asthma attack, pneumonia, sepsis)
c) COPD - controlled treatment

Question 6

What are some limitations of pulse oximetry?

What would be more accurate?

What are the differnces in treating a pt with hypoxia vs hypercapnia?

Answer 6

Only tells us about oxygenation, not ventilation. Error sources include poor peripheral perfusion, dark skin, false nails/varnish, bright light, motion.

ABG - keeps fraction of inspired O₂ to minimum required to acheive adequate ventilation.

Hypoxia: give oxygen Hypercapnia: ventilate (= hypoventilation)