We are going to start posting questions or statements to start a significant discussion group.
Our goals are to :
- Start a wide berth of discussions.
- establish synergies of thought.
- improve patient care.
- improve interdisciplinary communications.
- introduce varied approaches to healthcare.
- decrease length of intubation.
- decrease length of time to liberation from mechanical ventilation.
- decrease length of hospital stay.
- decrease the cost of health care.
- introduce new or variant concepts of ICU care.
Here is the first 2 of many thought provoking statements.
Evoking Epipheny – 2019-05-001
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.
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.