Most important item(s) to evaluate patient’s pulmonary status : pulmonary compliance



Pulmonary compliance = how easy it is to move volumes tidally in and out of the lung.

Pulmonary compliance formula = dV / dP = 350 / 35 = 10 mls / cmH2O) in this image.

Relevance :  To move 300 mls of spontaneous tidal volume, a patient has to generate -10cmH2O of negative pressure while a normal person would generate -3cmH2O of negative pressure.  The patient spends 3x more energy than you trying to breathe the same sized breath.  That will translate to increased WOB and increased likelihood of respiratory failure and an increased need for proper caloric intake.



Evaluate your patient’s pulmonary compliance.

Some quick definitions :

PIP = peak inspiratory pressure.

PEEP = positive end expiratory pressure.

dP (delta P) = change in pressure, during tidal breathing.

dV (delta V) = change in volume during tidal breathing.

Compliance = ease of lung distensibility.

Compliance = change in volume (tidal volume) / change in pressure = delta V / delta P = dV / dP.

If you are in any Pressure mode (ex : PCV, PSV) , this is very simple :

delta Volume = 350 mls (see above image).

delta Pressure = 35 cmH2O (see above image :  PIP=49 ; PEEP=14 >> dP=35).

Compliance = 350 / 35 = 10 mls / cmH2O.

What is the value of  normal pulmonary compliance?

100 mls / cmH2O.

What is the average pulmonary compliance of a “normal”, intubated ICU patient ?

30 mls / cmH2O.

What exactly does pulmonary compliance mean in practical terms ?

This means for every 1 cmH2O, 100 mls of volume will be moved (the normal non-intubated patient).

This means for every 1cmH2O, 30 mls of volume will be moved (the average ICU patient).

This means the patient works 3x as hard as a non-intubate patient.

For better understanding, the statement can be rephrased as :

(With the caveat statement that we all move approximately 200-300 mls of spontaneous tidal volume).

How much pressure must be generated by a diaphragm to move 300 mls of spontaneous tidal volume in a normal patient?

If the pulmonary compliance = 100 mls / cmH2O, then only 3 cmH2O of negative inspiratory pressure has to be generated.

How much pressure must be generated by a diaphragm to move 300 mls of spontaneous tidal volume in an an average ICU patient?

If the pulmonary compliance = 30 mls / cmH2O, then 10 cmH2O of negative inspiratory pressure has to be generated.

This difference can translate into work of breathing (WOB) and if the patient is underfed, this can start to become a significant impediment to quick weaning and liberation from mechanical ventilatory support.




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.

The dynamic renal system – the “poor man’s Swan”


In a nutshell : urine is formed in proportion to the cardiac output exerted on the nephron.

If the clinician is improving cardiac output, the urine output should improve.

Ex :

give Epinephrine  >> cardiac output increases >> urine output increases !

give 500cc NSS in a dehydrated patient >> cardiac output increases >> urine output increases !

apply Alveolar Recruitment Techniques (ART)  >> cardiac output increases >> urine output increases !

decrease PEEP (in a stellar pulmonary status patient)  >>  cardiac output increases >> urine output increases !


The “long winded story” :

The renal system in all of its dynamic glory is a beautiful passive organ system.

The dynamic renal system is referred to as the “poor man’s Swan”.

It’s state of flux demonstrates the strength of the cardiodynamic system.

When cardiac output is strong, urine is formed quickly and flows to the urimeter quickly.

When cardiac output is weak, urine is formed slowly and flows to the urimeter slowly.

For doctors / hospitals / states / countries that cannot afford the luxury of a Swan-Ganz catheter, the urine collection system is thus referred to as the “poor mans Swan”.

As long as there is integrity in the system from the nephron (beginning)  to the end (the urimeter), it is a great way to tell of the cardiodynamic status.

This attached movie clip (p1090771 – activate the “replay” feature on your PC’s movie player app) will allow you to envision the urine formation.

The process :   cardiodynamics will bring blood from  :    LV  >> LVOT >> Ao  >> descend Ao >> renal artery >> nephron  >> renal collecting tubules >> bladder >> urimeter.

In this movie clip, the patient had excellent cardiodyanmics and was given Lasix (forced diuresis) so the patient was literally “pouring out” urine.

Traumatic brain injury (TBI) / nueuro-injury may result in sequelae to include SIADH (syndrome of inappropriate anti-diuretic hormone) which allows urine to “pour out” as well.

The beauty of this movie clip is to envision for “x” amount of heart beats, a drop of urine is formed.  When you see urine forming at the rate you see in this movie clip, one has to stand in awe of a passive physiologic process.


2 min Evaluation – Urimeter = clear – 10 seconds to reach a conclusion


The quick explanation :

Situation :    urine = clear.

Background :    fluid overloaded.

Action :    forced diuresis.

Recommendation :     Alveolar recruitment technique (ART).



The “long winded” explanation :

S-B-A-R format reporting – quick reporting format for handoff from one healthcare professional to another.

A lot of this presentation is conjecture … but time and experience has proven correct 99 out of 100 times.

Clear – forced diuresis (Lasix) – fluid overloaded – increased CVP .

Fluid overloaded due to a transient hypotensive crisis.

The fluid is coursing its way thru the vasculature and some of that fluid is finding its way into the pulmonary parenchyma.

Use Alveolar Recruitment Technique (ART) to mitigate the effects of rogue intravascular fluid … finding its way to the extravascular space (pulmonary interstitium.

QoW 2016 – 005

A 70kg patient who had an emergent appendectomy 2 hours ago and is currently in PACU is on the following

ventilator settings :  SIMV-PRVC,  RR(set)=15,  Vt(set)=400,  FiO2=100%,  PEEP=8,  PS=10 with

resultant PIP=45cmH2O.

ABG values as follows :  pH 7.40    PaCO2=41,    PaO2=259,    HCO3=26,    SpO2=100%.

Given the above, please answer the following :

A)   What is the P/F ratio of this patient ?

B)   What is the lung compliance of this patient ?

C)  How are the patient’s lungs ?

QoW 2016 – 004

Who is doing better?    (Assumptions :  same H&P, medical and surgical history, same admitting diagnosis, same lab and test values).

The patient who is on PSV :  FiO2=40%,  PEEP=14,  PS=10.


The patient who is on PCV :   SIMV-PCV,  RRset=8,  dP=10  x  ti=2sec,  FiO2=40%,  PEEP=14,  PS=10.

Pulmonary physiology – P/F ratio defining your patient’s pulmonary status


PaO2 / FiO2 = P/F ratio.

This simple calculation helps define your patient’s lung status :

P/F ratio                                    pulmonary status

.>600                                              normal

.<300                                               ALI (acute lung injury)

.<200                                                ARDS

.<150                                                AHRF (acute hypoxemic respiratory failure)


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.

Tricks of the trade : the 4-5-6 ; 7-8-9 rule


This is a “cute” way to remember SpO2 vs. PaO2.

When the spo2=40%, the pao2=70.

When the spo2=50%, the pao2=80.

When the spo2=60%, the pao2=90.

SpO2                 PaO2

40%                    70

50%                    80

60%                    90