basic pulmonary physiology - m5 board review

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11/12/15, 110asic Pulmonary Physiology - M5 Board Review

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This section complements the Respiratory Physiology section and the questions are more factual and less

clinical scenario based

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Which of the following muscles are not involved in inspiration:

At the end of a passive expiration, the lung volume is:

Airway resistance is primarily due to the:

In the large conducting airways of an individual with high airway resistance, which of the following

will decrease resistance:

An asthmatic patient has increased airway resistance, which of the following equations can explain

the mechanism for this:

At which lung volume or capacity would pleural pressure be most negative (with spontaneousbreathing):

Increasing altitude will result in which of the following:

BASIC PULMONARY PHYSIOLOGY


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Which of the following is not a cause of hyperventilation:

The following graph describes CO2 responsiveness, with A being normal. Which of the following

would explain line C:

Which of the following has the greatest dead space:


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The following was the ventilation to perfusion ratio plotted by rib number on the x axis. What is

most likely true regarding point A:

A patients end-tidal CO2 decrease while the paCO2 increase, which of the following is an explana-

tion:

After placing an otherwise healthy patient on mechanical ventilation, dead space:

A patient has a paCO2 of 30 mm Hg and an!TCO2 of 20 mm Hg, what is the percent dead space:

Which of the following venous systems DO NOT contribute to anatomic shunt:

Which of the following will result in the difference between the calculated A-a gradient utilizingthe alveolar gas equation and the actual post alveolar capillary (CcO2) to arterial PaO2 gradient to in-

crease:

A. High FiO2

B. Increased cardiac output

C. Very low mixed venous saturation

D. Mild anaemia

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The correct answer is:C: Very low mixed venous saturation

This was a tricky one. The first thing you need to figure out is what is this question asking! The A-a gradi-

ent is the difference between the calculated oxygen tension in the alveoli and the arterial blood. The CcO2

is the difference between the blood that immediately leaves the (ventilated) alveoli and the arterial blood.

The theoretical CcO2 should be the same as the pAO2 (alveolar O2 tension). The difference between thepAO2 or CcO2 and arterial O2 tension is due to shunt (as discussed in the next question). But this ques-

tion is bringing up that the assumption that pAO2 and CcO2 being identical is not always the case, even as-

suming a normal ventilated alveoli. Lets discuss this further:

Calculating the A-a gradient by using the alveolar gas equation (described in question 7) takes into account

the FiO2, barometric and water vapor pressure, the PaCO2, and the consumption of oxygen and formation

of CO2, but it doesnt tell you how desaturated the blood was as it approached the alveolus. You can imag-

ine that the more desaturated the pre-alveolar blood is, the less likely it will be completely saturated when

it leaves the alveoli (although in most cases, even very desaturated blood is fully saturated, but this is picky

theoretical minutiae that has almost no bearing on clinical practice, hence the stuff the boards seem to fo-

cus on). Therefore when you use the alveolar gas equation to calculate how saturated the blood leaving the

lungs are, it assumes that all of the blood is maximally saturated. The difference between this theoretical

value and the actual PaO2 (A-a gradient) is typically thought to be a measure of the patients lung disease

(understanding that there will always be some level of shunt as discussed in Q15). But, in the setting of a

very low mixed venous saturation, even perfectly functioning lungs can (theoretically) have post alveolar

capillary blood that has an oxygen tension well below that which was calculated. Since these values will be

lower, the actual CcO2-a gradient (difference) will be smaller than what was calculated (A-a gradient). How

does this matter? In practice youll probably not worry about it and increase the FiO2 or recruit more alve-

oli by increasing PEEP.

Increased cardiac output, to some degree, it would seem, could theoretically be so high that it also would

not be fully oxygenated after leaving the alveoli, but this doesnt actually happen. Perhaps this is because at

high cardiac outputs the mixed venous saturation will be increased (if this doesnt make sense, see the ICU

section). Severe anaemia can be associated with very low mixed venous saturations, but this is rarely true

with mild anaemia.

The A-a gradient is an imperfect measure for judging lung disease as it varies with FiO2. The higher theFiO2, the greater the normal gradient will be. In general, with plenty of exceptions, on room air a normal A-

a gradient will be about 20 (or less) and on 100% FiO2 it should be under 100. People also use A/a or a/A

ratios, as these are not affected by FiO2 changes (the relative ratio stays the same). A normal a/A ratio is

around 0.8. Since this all requires some math, we now commonly use the P/F ratio, which is the paO2/FiO2.

A normal P/F ratio is above 300 (as youd expect at 100% FiO2 you should have a paO2 greater than 300).

Of all the indexes, this one has the most flaws, but its simple. ARDS is described in severity relative to P/F

ratios, for example.


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High FiO2

Increased cardiac output

Very low mixed venous saturation

Mild anaemia

A patient has an arterial haemoglobin saturation of 90% and a mixed venous saturation of 60%. Ap-

proximately, what is the shunt fraction:

A. 20%

B. 25%C. 30%

D. 40%


The correct answer is:B: 25%

Calculating shunt in clinical practice utilizing the physiologic shunt calculation is not easy. It utilizes the

content of oxygen in blood, which means you need to know the Hb, the saturation of Hb (Hbsat), and all the

variables of the alveolar gas equation:

Arterial content of blood (CaO2) = Hb X Hbsat X 1.34

Mixed venous content of blood (CmvO2)= Hb X Hbsat X 1.34

You also need to calculate the end capillary O2 content of blood (the blood just leaving the alveoli, which

uses the term CcO2.

CcO2 = Hb X Calculated Hbsat utilizing the alveolar gas equation X 1.34

In most cases the Hbsat utilizing the alveolar gas equation will be 100%. Above a PAO2 of, say 120, its safe


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to say that the Hb will have a near 100% saturation.

The final physiologic shunt equation is (Q = flow, CmvO2 = content of mixed venous blood):

Qshunt/Qtotal = (CcO2-CaO2)/(CcO2-CmvO2)

Because the pAO2 is almost always at least 120 and nobody likes equations that are complex, you can sub-

stitute the value 100% for CcO2. Furthermore, because the Hb will be the same on the arterial and venous

side, you can get rid of the content equation and just use the Hb saturations. What your left with is some-

thing easy to calculate and actually clinically useful, which is the ventilation-perfusion ratio (or VQI as it is

usually called). Its a very simple calculation:

VQI = (1-SaO2)/(1-SmvO2)

Using the values from the stem: (1-0.9)/(1-0.6) = 0.1/0.4 = 25%. Shunt in a healthy individual is typically less

than 5%. For example a normal person has a 99% arterial saturation and a 75% venous saturation, there-

fore: (1-0.99)/(1-0.75) = 0.01/.25 = 4%.

20%

25%

30%

40%

Above which shunt fraction would supplemental oxygen not expect to increase the PaO2 by more

than 10 mm Hg:

A. 10%


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B. 20%

C. 30%

D. 40%


The correct answer is:D: 40%

Its important to have a general idea at which degree of shunt will supplemental O2 not have an effect onincreasing the arterial pO2. Different people will have different answers on this, but what is established is

that above 30% shunt youll get very little increase in pO2 with increased FiO2, and at 40% shunt raising

the FiO2 from 21% to 100% will have almost no effect on arterial pO2. This is a nomogram that youll seein

a lot of textbooks:

10%

20%

30%

40%

Which of the following will increase venous admixture:

A. Increased inspiratory reserve volume

B. Decreased inspiratory reserve volume

C. Increased expiratory reserve volume

D. Decreased expiratory reserve volume


The correct answer is:D: Decreased expiratory reserve volume

Decreased expiratory reserve volume (ERV) will in turn decrease functional reserve capacity (FRC), as FRC= ERV + Residual volume (RV). FRC, as described in question 2, is the resting volume of the lung (post-exha-

lation), and at decreased volumes, less blood is being exposed to oxygenated alveoli, thus increasing the

shunt (also known as venous admixture, see question 15). Think of it this way, with reduced lung volumes

at end expiration it means that it will hold less oxygen for blood flow to pick upand that means the ration

of ventilation to perfusion will decrease (perfusion stays the same, ventilated alveoli decrease) and there-

fore shunt will worsen (increase). Decreasing inspiratory reserve volume (IRV) will decrease the largest

breath that one can take (total lung capacity) but will not affect the number of perfused but unventilated
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alveoli with normal tidal breathing. With normal tidal volumes, FRC is maintained and the ratio of ventila-

tion to perfusion is unchanged during both inspiration and expiration (during tidal breathing). Now that

weve tied in shunt to lung volumes, lets talk about PFTs next.

Increased inspiratory reserve volume

Decreased inspiratory reserve volume

Increased expiratory reserve volume

Decreased expiratory reserve volume

A patient has a forced vital capacity that is 90% predicted for his size, age, and gender. Which of the

following is true:

A. He does not have obstructive lung disease

B. He does not have restrictive lung disease

C. He does not have normal pulmonary function

D. None of the following conclusions can be made


The correct answer is:B: He does not have restrictive lung disease

Forced vital capacity (FVC) is one of the most important tests we look at with pulmonary function tests. Es-

sentially with an FVC, the patient takes a vital capacity breath and expires it as forcibly and rapidly as possi-

ble. By looking at the amount of air over time, one can look at the forced expiratory volume (FEV) at a given

time (most often we use the 1 second mark, or FEV1). The FVC to FEV1 ratio can be valuable in categoriz-

ing and grading lung disease. Additionally, we can look at different volume points during the FVC, such as

when 25% or 75% of the FVC volume has been exhaled (FEF25-75%, for example).

For normal individuals, a persons FVC should be within 20% of predicted, so this patient could be normal.

People with mild obstructive lung disease generally have a preserved FVC, but with severe disease the FVCcan decrease significantly, although less so than restrictive lung disease. Therefore this patient could have

mild (likely not very severe) COPD or asthma. Restrictive lung disease is associated with significantly de-

creased total lung capacities and FVCs. Even mild restrictive lung disease should have an FVC less than

80%, therefore it can be concluded that the patient does not have restrictive lung disease.

He does not have obstructive lung disease


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He does not have restrictive lung disease

He does not have normal pulmonary function

None of the following conclusions can be made

Which of the following graphs represent obstructive lung disease:

A. A

B. B

C. C


The correct answer is:B: B

With severe obstructive disease notice two things. First the FVC is moderately reduced. Second, and far

more importantly, the FEV1 is significantly reduced, meaning that they have a descent sized breath, but it

takes a very long time to completely exhale and after 1 second (FEV1) a very small proportion of the FVC is

exhaled. Therefore, the FEV1/FVC ratio will be reduced. The FEV1/FVC ratio is an important tool for grad-

ing COPD, the smaller the ratio, the worse the disease. A normal FEV1/FVC is about 80%. Severe COPD is

associated with FEV1/FVC ratios below 50%. See Q28 in the other respiratory physiology section for more

detail on COPD grading.


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Graph C has a severely reduced FVC and FEV1, but the FEV1/FVC ratio is preserved. Think of it this way,

with restrictive lung disease, the problem is more to do with inspiration (too small of breaths) and not expi-

ration.

A

B

C

Which of the following flow volume loops represent obstructive lung disease:

A. A

B. B

C. C


The correct answer is:C: C

Flow volume loops are generated by plotting gas flow during forced exhalation, followed by forced inhala-

tion (vital capacity). Restrictive and obstructive lung diseases have very characteristic patterns. Restrictive

lung disease maintains a general triangle shape during exhalation (remember restrictive disease has a rela-

tively normal exhalatory pattern) but has very reduced flow during inspiration. Also note that the volumes

are also very reduced.


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Obstructive disease has a scooped triangle for exhalation owing to the fact that there is an obstructive le-

sion during exhalation, whereas inhalation appears relatively normal. Graph A represents normal. In case it

ever comes up on an exam, a normal tidal volume will look like a circle, not a triangle with a rounded bottom

like the vital capacity breath does.

A

B

C

Regarding flow-volume loop C, which of the following is true regarding carbon monoxide diffusing

capacity (DLCO):

A. DLCO will most likely be increased

B. DLCO will most likely be normal

C. DLCO will most likely be decreased


The correct answer is:C: DLCO will most likely be decreased

The DLCO is a measurement of how well gas exchanges between the alveolus and capillary blood. Because

CO is has such a high affinity for Hb (200X more than O2) its a convenient test. Any disease process that

affects the alveolus will decrease the DLCO. Examples include disease states with junk that accumulates

inside the alveolus such as pulmonary oedema as well as diseases that affect the alveolus itself such as sar-


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coidosis, asbestosis, etc. In fact, most restrictive lung diseases (certainly all that are intrinsic to the lung)

will have a reduced DLCO. What you may not have known is that COPD can decrease DLCO as well, and

when it is as severe as in the above flow-volume loop, it certainly will. Recall that COPD will destroy the

alveolar-capillary interface, change alveolar geometry, and have associated loss of capillary beds and V/Q

mismatching. All of this will decrease DLCO. A couple other technical factors that will decrease DLCO test,

although not actually be due to lung disease is anaemia and elevated pCO2.

DLCO will most likely be increased

DLCO will most likely be normal

DLCO will most likely be decreased

Which of the following regarding the FEF is false:

A. It is effort independent

B. It is fairly variable within an individual

C. It is a late indicator of obstructive disease

D. Decreased values indicate medium-sized airway obstruction


The correct answer is:C: It is a late indicator of obstructive disease

The forced expiratory flow (FEF) is typically broken up into quarters: FEF , FEF , & FEF , which

means the exhaled volume at 25% of FVC, 50% of FVC, and 75% of FVC, respectively. Between the 25%

and 75% marks on the FVC curve is called the effort independent flow because it will theoretically not

change whether or not the patient is blowing at maximal effort. FVC and FEV1 will change significantly de-

pending on effort. Unfortunately, the FEF FEF is fairly variable when a patient blows for spirome-

tery (the standard is at least three forced exhalations). The FEF is an early indication of obstructive

disease and is often significantly reduced prior to the FEV1/FVC ratio decreasing. However, because of its

intra-individual variability and lack of significant evidence of being all that valuable clinically, its often not-

ed but ultimately ignored. Therefore, it is a boards favorite! So what is FEF actually telling us any-

way? A decreased value is indicative of medium (some say small) airway disease. File this under a sensitive,

but not specific test.

It is effort independent

It is fairly variable within an individual

It is a late indicator of obstructive disease

25%-75%

25% 50% 75%

25%-75%

25%-75%

25%-75%


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Decreased values indicate medium-sized airway obstruc-

tion

Which of the following is the best predictor of perioperative pulmonary complications:

A. FEF25%-75%

B. FVC

C. FEV1/FVC

D. Productive cough


The correct answer is:D: Productive cough

Despite so much emphasis of PFTs on the boards, there is little evidence any of it makes a big difference or

predicts perioperative pulmonary complications. Sorting through the data is a bit of challenge, so let me

put it this way: interpreting a PFT to predict perioperative pulmonary complications is of very low utility,

therefore there is little reason for an anesthesiologist to order one. The one exception is a pneumonecto-

my, which the reason for this is discussed in the question 5 of the cardiac respiratory & vascular surgery

section (advanced exam topic).

Patient factors which increase the incidence of perioperative pulmonary complications are COPD, asthma,

productive cough, smoking (especially >40 pack years), maybe obesity, exercise intolerance of less than one

flight of stairs, and age > 65 years. Surgical factors include upper abdominal surgery, thoracic surgery, and

length of surgery.

FEF25%-75%

FVC

FEV1/FVC

Productive cough

Which of the following patients would an ABG be most indicated preoperatively for a patient with

COPD?

A. Na 140, K 4.5, Cl 110, CO2 24, Cr 1.0, BUN 26

B. Na 120, K 4.5, Cl 85, CO2 25, Cr 1.5, BUN 26

C. Na 140, K 3.5, Cl 85, CO2 45, Cr 1.0, BUN 15


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D. Na 155, K 4.5, Cl 120, CO2 25, Cr 1.0, BUN 26


The correct answer is:C: Na 140, K 3.5, Cl 85, CO2 45, Cr 1.0, BUN 15

The worst of the worst of COPDers are the CO2 retainers, and these are people you will have a hard time

keeping extubated after surgery. CO2 retention classically has compensated elevated bicarb (CO2 on the

chem 7), and choice C describes such a patient.

Na 140, K 4.5, Cl 110, CO2 24, Cr 1.0, BUN 26

Na 120, K 4.5, Cl 85, CO2 25, Cr 1.5, BUN 26

Na 140, K 3.5, Cl 85, CO2 45, Cr 1.0, BUN 15

Na 155, K 4.5, Cl 120, CO2 25, Cr 1.0, BUN 26

Following an abdominal operation, at what time would the FRC be expected to be the lowest:

A. Immediately post-op

B. 12 hours post-op

C. 24 hours post-op

D. 48 hours post-op


The correct answer is:B: 12 hours post-op

FRC is affected by position, anesthesia, and operations. Following an operation FRC is classically the lowest

12 hours following an operation. This is due to splinting, atelectasis, residual affects of anesthesia (includ-

ing analgesics for post-op pain control), and post-operative positioning. Lets talk about this a bit more in

the next question.

Immediately post-op

12 hours post-op

24 hours post-op

48 hours post-op


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Which of the following will decrease the FRC the least:

A. Changing from upright to supine position

B. Volatile anesthesia

C. Changing from supine to prone position

D. Changing from upright to prone position


The correct answer is:C: Changing from supine to prone position

This one is a bit of a poorly worded question, as if you realize that changing from a supine to prone position

will actually increase FRC in many cases, can you still say that it will decrease the FRC the least? I did not

revise this questions stem to illustrate the ambiguity and poor wording of questions youll come across.

Now with the basic exam at the end of the CA-1 year, you can see exactly what I mean!

Heres the deal with FRC and positioning: FRC will decrease in a healthy adult by 15% just by changing from

an upright to supine position (from a loss of ERVbut you already knew thatright?). Changing from an

upright to prone position will also drop the FRC quite a bit, but less so. Changing from supine to prone posi-tion has pretty mild effects, unless youre paralyzed, mechanically ventilated, and obese, in which case it

improves FRC significantly.

General anesthesia will decrease FRC by about 10-20%* (above that caused by position differences) and

the greatest decrease is 10 minutes after induction.

What can you do to increase FRC. Hmmm, lets seeyou can raise the head of the bed, you can get them

out of bed early, and you can give them PEEP (or CPAP post-operatively).

*Closer to 50% reduction in very obese individuals!

Changing from upright to supine position

Volatile anesthesia

Changing from supine to prone position

Changing from upright to prone position

Regarding smoking cessation:


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A. It will take 72 hours for carboxyhaemoglobin levels to fall to normal levels

B. Ciliary function does not improve until 8 weeks after smoking cessation

C. Sputum production decreases daily for the first two weeks

D. Pulmonary complications decrease if cessation occurs 8 weeks prior to surgery but not 24 hours


The correct answer is:D: Pulmonary complications decrease if cessation occurs 8 weeks prior to surgery but not

24 hours

Smoking cessation is actually a mildly controversial subject for anesthesiologists in regard to its effects on

perioperative respiratory function and complications. Smoking is associated with decreased ciliary func-

tion, increased sputum, up-regulation of destructive enzymes and reactive metabolites that lead to devel-

opment of COPD, immunologic dysregulation , changed surfactant, small airway reactivity, and epithelial

permeability. After a list like that, what could the controversy be?!?

Following cessation of smoking it takes about 2-4 weeks for ciliary function to return to normal. Immedi-

ately following cessation of smoking, sputum production actually increases, if the patient stops smoking

just before surgery, they will still have impaired ciliary function and even more increased sputum produc-

tion. Even a day of not smoking will significantly decrease levels of carboxyhaemoglobin back to normal val-

ues, which will increase oxygen carrying capacity (DO2). Because there is an increased risk with quitting

smoking just before surgery and no significant benefit if they smoked within the past few weeks, some

anesthesiologists argue that patients who cannot quit at least a month or two prior to surgery should be

told to smoke up until the day of surgery as the only benefit is a decreased carboxyhaemoglobin. This of

course, in my humble opinion, is asinine, as other effects of cigarette smoking include wound healing and

infection. Maximal benefit from smoking cessation is realized when smoking has been stopped for 8 weeks

or more.

It will take 72 hours for carboxyhaemoglobin levels to fall

to normal levels

Ciliary function does not improve until 8 weeks after

smoking cessation

Sputum production decreases daily for the first two

weeks

Pulmonary complications decrease if cessation occurs 8

weeks prior to surgery but not 24 hours

A patient with COPD underwent a preoperative ECG. Which of the following is most likely to be

present:

A. Inverted t-waves in a coronary specific pattern


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B. Left bundle branch block

C. Poor R-wave progression

D. Accelerated junctional rhythm


The correct answer is:C: Poor R-wave progression

COPD puts strain on the right heart and can lead to right heart failure. Classic findings on the ECG aresigns of right heart strain, such as poor R-wave progression, enlarged P-waves (P pulmonale), R waves

greater than S waves in V1, RBBB, and right axis deviation. Also present, on exams, are low voltage ECGs

because of dynamic lung hyperinflation. Likewise, classic for exams is increased incidence of multifocal atri-

al tachycardia (MAT), which will present with at least three different distinct wandering P waves and an ir-

regular rhythm.

Inverted t-waves in a coronary specific pattern

Left bundle branch block

Poor R-wave progression

Accelerated junctional rhythm

Following upper abdominal surgery:

A. Residual volume (RV) increases by 10%

B. Expiratory reserve volume (ERV) increases by 25%

C. Vital capacity (VC) is typically preserved

D. Tidal volume (TV) decreases by 75%


The correct answer is:A: Residual volume (RV) increases by 10%

Abdominal surgery is very injurious to the respiratory system, and if youre asked about this, good chance

youll be asked in relation to lung volumes. The main take away clinically is that abdominal surgery decreas-

es FRC, which decreases lung compliance and increases shunting and hypoxia. The decrease in FRC is due

to a 25% loss in ERV. Interestingly, following abdominal surgery RV increases by a little over 10%. TV de-creases moderately (~20%) as does total lung volume, following surgery. These effects are usually worst

12 to 24 hours after surgery* and are back to baseline in 2 weeks.


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*It would appear that upper abdominal surgery may decrease FRC more than most other types of surgery,

have its maximal decrease later, and recover slower.

Residual volume (RV) increases by 10%

Expiratory reserve volume (ERV) increases by 25%

Vital capacity (VC) is typically preserved

Tidal volume (TV) decreases by 75%

Which of the following is the best anatomic estimate for the level of the carina:

A. 3rd thoracic vertebrae

B. Sternal angle

C. Nipple line

D. One finger breadth below the sternal notch


The correct answer is:B: Sternal angle

This is an important reference point.

3rd thoracic vertebrae

Sternal angle

Nipple line

One finger breadth below the sternal notch

Which of the following is the best marker for dynamic compliance:

A. Plateau pressure

B. Peak pressure

C. Peak pressure plateau pressure


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D. Transpulmonary pressure


The correct answer is:B: Peak pressure

Understanding peak and plateau pressure is important and it is discussed ad nauseam in the advanced car-

diopulmonary ICU section. In this section, lets just concentrate on the basics. Plateau pressure is measured

with an inspiratory hold so that there is time for airflow from the ventilator to the alveoli to equilibrate (inother words the net movement from the higher pressure ETT to the lower pressure alveoli equilibrate and

all airflow stops). This means the pressure you are measuring is the equivalent pressure that the alveoli are

experiencing. Furthermore, this is the pressure that is needed to keep the lungs inflated at the given vol-

ume. This is what is called static compliance. It is measured by the following simple equation:

Static compliance = Volume/ Pressure = Tidal volume delivered/ (Plateau pressure PEEP).

The next subject is dynamic compliance. Dynamic compliance is the compliance of the respiratory system

during inspiration. This means that the pressure is taking into account not only the static compliance (the

pressure needed to keep the lungs inflated, but also the pressure needed to overcome the intrinsic airway

resistance to deliver the air to the alveoli). Its equation is also simple:

Dynamic compliance = Volume/ Pressure = Tidal volume delivered/ (Peak pressure PEEP).

The difference between the peak and plateau represents the resistance flow. This means that it is the

pressure needed to overcome the resistance to flow within the airways (remember from question 3 that

most of this occurs in the conducting airways). Other terms you will come across is calling the peak pres-

sure the airway pressure and the plateau pressure the alveolar pressure.

Transpulmonary pressure is another important concept. It is the alveolar (plateau pressure) minus the

pleural (esophageal) pressure. It represents the actual pressure across the lungs. This is also discussed inthe advanced ICU section, but it has a lot of relevance to daily anesthetic delivery as well so check out

question 36.

Plateau pressure

Peak pressure


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Peak pressure plateau pressure

Transpulmonary pressure

Which of the following will most likely increase peak pressure without an increase in plateau pres-

sure:

A. Kinked ETT

B. ARDS

C. Pleural effusion

D. Trendelenburg position


The correct answer is:A: Kinked ETT

This is about as classic of a simple boards question as it gets. I supplied you with easy choices. There are

some similar questions to this throughout the M5, and the point is not to just retell the same question over

and over, but emphasize this point, which is so high yield, its just shy of guaranteed. Remember that peak

pressure measures airway compliance (pressure) and plateau pressure measures alveolar pressure (respi-

ratory system compliance). With a kinked ETT there will be increased resistance to deliver the air to the

alveoli (peak pressure high) but the actual alveoli themselves will not be affected (plateau normal). With

ARDS, or any type of pulmonary oedema, the alveoli are filled with fluid (as well as a surfactant deficiency,

etc, etc, etc) and it takes much more pressure to open those alveoli, therefore plateau pressure will in-

crease. Now, when plateau pressure increases, peak pressure will also increase, but the difference between

peak and plateau will remain about the same (if peak pressure was 8 cm H20 above plateau without a dis-ease that only affects the alveoli, lets imagine that it will stay the same after the alveoli are affected). In re-

ality ARDS will increase both (plateau more, but lets keep in basic, right!). With a pleural effusion, the lung

will have a decreased space in which to inflate, which means that plateau will be increased but the peak-

plateau difference will remain the same (in theory, for illustrationagain nothing is actually that simple).

Obesity and trendelenburg will have the same effect.

Kinked ETT

ARDS

Pleural effusion

Trendelenburg position

Which of the following will increase the plateau pressure without increasing the peak-plateau dif-


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ference:

A. Negative pressure pulmonary oedema

B. Pneumothorax

C. Asthma exacerbation

D. Foreign body aspiration


The correct answer is:A: Negative pressure pulmonary oedema

All pulmonary oedema will increase plateau pressures, and in most cases, the peak-plateau difference will

be less affected. Cardiogenic pulmonary oedema and ARDS are diseases which have many affects and can

lead to swelling of airways, vascular congestion causing obstruction of conducting airways (cardiac asth-

ma), plugging, etc. Negative pressure pulmonary oedema, on the other hand is about as clean as pul-

monary oedema gets, as it occurs typically in strong, young people inspiring forcefully against a closed glot-

tis (laryngospasm, etc) with a net movement of water from the pulmonary circulation into the alveoli and

little affect on the conducting airways. Therefore, youd expect an increase in plateau pressure and an un-

changed peak to plateau pressure difference. Pneumothorax will increase both peak and plateau, but ingeneral one would expect the peak to be increased more. Asthma will primarily increase peak pressures (in-

creased airway resistance), but can raise plateau pressures in severe disease through dynamic hyperinfla-

tion (se the advanced ICU section). Foreign body aspiration may have localized hyperinflation distal to the

foreign body or lung collapse. Whatever the case, the most noticeable change will be an increased airway

resistance and increased peak pressure.

Negative pressure pulmonary oedema

Pneumothorax

Asthma exacerbation

Foreign body aspiration

A morbidly obese patient is anesthetized and mechanically ventilated. The plateau pressure is mea-

sured at 35 cm H20 and has an esophageal pressure of 20 cm H20. Which of the following are true:

A. The patient is at very high risk of barotrauma

B. The patient has an acceptable transpulmonary pressure

C. The tidal volume should be reduced

D. A chest tube should be placed


The correct answer is:B: The patient has an acceptable transpulmonary pressure


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Again, this subject is discussed in more detail in the ICU sections, but lets introduce it here in case you do

not complete the ICU advanced cardiopulmonary section in preparation for the basic exam (which I think

would not be a bad idea, but the ICU principles section is an absolute must, as oxygen utilization and trans-

port are huge topics on the boards as well as the basics of mechanical ventilation strategies. Above a

plateau pressure of 30 cm H20, the risk of barotrauma (air dissecting out of the lungs into places it should

not be such as pneumothorax, pneumomediastinum, pneumopericardium, etc) increases greatly, therefore

we want to keep the plateau pressures less than that, even if we significantly limit tidal volumes. In obese

people, because of large abdomens and noncompliant chests, their static compliance of the respiratory sys-tem is very low, meaning that a plateau pressure above 30 cm H20 is very likelyBUT, this doesnt mean

they are at risk for barotrauma. Why? Barotrauma really occurs when the transpulmonary pressure (re-

member it is alveolar pressure minus pleural pressure) is high. Patients with low compliant thoracic cavities

will have a high pleural pressure, and we measure that with the esophageal pressure. Therefore, this pa-

tients transpulmonary pressure is only 35 20 = 15! Thats nothing.

The patient is at very high risk of barotrauma

The patient has an acceptable transpulmonary pressure

The tidal volume should be reduced

A chest tube should be placed

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basic pulmonary physiology - m5 board review

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