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

 

SHORT VERSION OF EXPLANATION :

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.

 

LONG WINDED VERSION OF EXPLANATION :

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.

HOW  TO  CALCULATE  P/F  RATIO :

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.

WHAT  ARE  DIAGNOSTIC  VALUES :

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).

WHAT  IS  THE  AVERAGE  SURGICAL  PATIENT’S  P/F  VALUE :

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.

eLibrary – high value pix – alveolus ARDS (normal vs. injured)

hi-value-pix-ards-nl-vs-injured-alveolus-nejm-2000

This picture printed in NEJM 2000 is a great picture to learn from.

I will expand on content at a later date.

Simply GOOGLE search for images with the following “nejm 2000 ards alveolus”.

Pick the image.

Then choose “view image” for a picture of moderate resolution.

 

eLibrary – high value pix – alveolus ARDS (injured vs. repaired)

hi-value-pix-ards-injured-vs-repaired-nejm-2000

This picture printed in NEJM 2000 is a great picture to learn from.

I will expand on content at a later date.

Simply GOOGLE search for images with the following “nejm 2000 ards alveolus”.

Pick the image.

And then choose “View image” for a picture of moderate resolution.

 

QoW – 2016 – Q4 – 001 – The Consummate Respiratory Therapist – Thinking Outside the Box

You are called for a patient who is agitated & having a desaturative event  >  you see the following as you enter the room :

monitor-spo2-69pct

pvroute006

uryll

avsv

chtb

The Intensivist meets you at the patient bedside to discuss the patient.  His SBAR report :

S – Situation :  Patient agitated & demonstrating oxymetric desaturation to 69%.

B – Background :  Cardiogenic shock  > Post-op day #3 ; Hospital day #4.

A – Assessment :  Patient agitated but looks comfortable.

P – Plan :  Increase FiO2 from 35% to 50%.

Do you concur?