You have successfully recruited your patient’s pulmonary alveoli. Now, will the alveoli STAY patent so that you can extubate your patient with great success? Go to minimal pulmonary support and continue to assess … if your patient’s tidal volume trend stays neutral or increases, you will likely have a successful extinction.
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.
You are called for a patient who is agitated & having a desaturative event > you see the following as you enter the room :
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?
The patient was on the following ventilator parameters in the CTICU : SIMV-VCV, RR(set)=8, Vt(set)=700, FiO2=50%, PEEP=5.
The patient will be in the MRI suite for the next 75 minutes.
a) what will the settings be on this ventilator ?
b) how do you set patient trigger ?
c) how do you set Vt(set) on this ventilator (the “cheat sheet” was ripped off of the side of this ventilator).
d) if you use the vent with the settings that are currently dialed in, what will the ventilators translate to in standard ventilator parameters.
This “patient” is getting ready to go into MRI suite in the next minute.
a) how will the patient fare in the next 10 minutes.
b) defend your answer.
c) in 5 minutes, what will your ABG results be (give best educated guess).
If the patient at this bedside deteriorates to requiring CPR,
a) who will ventilate the patient ?
b) how will they ventilated the patient ?
c) what tool will you need (hopefully in your lab jacket pocket) ?
d) what does ambu stand for ?
set-up : clock, coke bottle, ETT, ballard suction device, suction source.
BEFORE : coke bottle (=2L of volume) – approximate the size of your lung.
AFTER : how long to suction 2L of air out of a closed system (totally FLATTENED)? 15 seconds.
In a nutshell :
Benefit : rids the lung of secretions.
Risk : may cause atelectasis to varied extents depending on the patient’s pulmonary status.
The risk : benefit ratio approaches unacceptable level of risk when the lung has a propensity to want to collapse.
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The long winded version :
A long time ago, I read that when suction is initially applied on a closed system that has been connected since the last time the patient was suctioned, the initial insult of the “built-up suction back-pressure” (BUSBP) might be excessively high.
If the pressure had been set at -100cmH2O, the “built-up suction back-pressure” may be in the range of -250-300cmH2O.
Therefore, it was important to break the suction circuit before apply suction to the patient.
That article suggested / demonstrated that excessively negative pressures might collapse a lung.
In this case, the demo shows that a 2L coke bottle can be flattened in approximately 15seconds of normal continuous suctioning.
Most people I see suctioning are quick to suction the trachea but leave continuous suctioning on (~15 seconds) while they clear the line with NSS (normal saline solution).
THE SCENARIO :
Situation : Patient sounds junky.
Background : Patient has been in the hospital for 2days post-op. HD=2 POD=2
Assessment : Patient suctioned but “did not get anything.”
Recommendation : Start mucomyst.
THE LESSON :
Be wary of frivolous suctioning.
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 !
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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.
