Most important item(s) to evaluate patient’s pulmonary status : P/F ratio

Evaluate your patient’s P/F ratio :

P/F ratio = PaO2 / FiO2 ratio.

It is a great way to index your patient’s oxygenation status.

It is cheap and not complicated but tells a lot about your lung in 30 seconds.


Divide PaO2 by FiO2 (in decimal format).

Normal P/F ratio value : 500-600 (on any FiO2).

Normal patient : (breathing room air)

PaO2 = 100 ,  FiO2 = 21%  >>  PaO2 / FiO2 ratio = 100 / 0.21 = 500.

Normal patient : (breathing 100% FiO2 via NRB (non-breather)).

PaO2 =  600,  FiO2 = 100%  >>  PaO2 / FiO2  ratio = 500 / 1.00 = 500.


P/F ratio > 500-600  =  Normal.

P/F ratio < 300  =  ALI.

P/F ratio < 200  =  ARDS.

P/F ratio < 150  =  AHRF (acute hypoxemic respiratory failure).


P/F ratio on arrival from operating theatre / room :  PaO2 = 250,  FiO2 = 100%  >>  P/F ratio = 250s.

P/F ratio 4 hours after surgery :  PaO2 = 120,  FiO2 = 50% >> P/F ratio = 250s.

Pulmonary physiology – Alveolar-arterial gradient – depicted



Depiction of the alveolar-capillary unit representative of the alveolar-arterial gradient and easily and quickly

defined by the PaO2 / FiO2 ratio (P/F ratio).

As the distance between the alveolus and the capillary increases, it is referred to as an “widening ” of the A-a gradient.

Many things can occupy this potential space :  water, blood, infection (bacteria) as well as thickening membranes.

IABP – mechanism of action (MoA)

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The heart should have an ample supply of blood for proper functioning.

Normally, the heart receives its blood supply opposite to the body’s supply.

This interesting paradox occurs because during systole, the body cannot squeeze

blood into itself via the coronary arteries.

Systole : the heart squeezes, thereby allowing :

supply of 80% of all organ systems requisite blood supply.

supply of 20% of the heart’s requisite blood supply.

Diastole : the heart relaxes, thereby allowing :

supply of 80% of the heart’s requisite blood supply.

supply of 20% of all organ systems requisite blood supply.

The day that the cardiac blood supply is compromised, is the beginning of cardiac demise.

If the insult (AMI – acute myocardial infarction) is addressed early, an IABP may allow

for recovery of the insulted myocardium.

Red = rich blood supply to the myocardium.

Pink = weak blood supply to the myocardium.

Green = blockage in the coronary arteries compromising the myocardial blood supply.


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Once the IABP is inserted and situated in the proper position, during systole, the heart is provided

with some of its requisite (20%) blood supply.

coronary art iabp deflate 003

Upon balloon inflation, the blood is sent cephalad (towards the head) and because the Aortic valve is closed, the blood

is forcibly driven deep into the myocardium via the  coronary arteries.

The augmented coronary blood pressure is much higher than it is normally.