By the end of this USMLE Step 1 lesson, you will be able to answer the following questions:
What pressure can you estimate in the left ventricle by using wedge pressure? Why?
Could you diagnose mitral stenosis by only using an increase in wedge pressure? Why or why not?
In what condition is wedge pressure a a poor estimate of LVEDP? Why?
What would be the PA pressures and wedge pressure be in primary pulmonary hypertension? Why?
What would be the PA pressures and wedge pressure be in left-sided heart failure? Why?
What would be the PA pressures and wedge pressure be in severe mitral stenosis? Why?
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Ready for the recall-type USMLE Step 1 questions?
What is the equation for flow across a heart valve?
Flow = (P1 – P2)/Resistance
You may have seen this as:
P1-P2 = Flow x Resistance
If we were looking at flow across the mitral valve, P1 and P2 would refer to the pressures in which chambers?
P1 = pressure in the higher pressure system (in this case, the left atrium)
P2 = pressure in lower pressure system (left ventricle)
If there is an increase in resistance, what would have to change in order to maintain flow? What common scenario would involve an increase in resistance across a valve?
An increase in valvular resistance occurs in any sort of stenosis. To maintain flow, the pressure difference (P1 – P2) must increase, meaning that theoretically, either P1 would have to ↑ and/or P2 must be ↓.
How physiologically would it make sense that the body would ↑ P1?
Resistance ↑ → flow ↓ → more blood would accumulate upstream → pressure ↑
In mitral stenosis, what is the primary parameter affected – pressure, flow, or resistance?
Resistance
Note: the pressure increase is a RESULT of having resistance ↑, NOT the underlying cause.
In order to maintain flow across the mitral valve in mitral stenosis, what would have to increase to do so? Why?
Recall: P1 – P2 = Flow x Resistance, where P1 = L atrial pressure, and P2 = L ventricular pressure
Mitral stenosis: Resistance ↑
There, to maintain flow, we must increase the pressure difference between left atrium and left ventricle
Thus, the ↑ in LA pressure is an EFFECT of the increased resistance in mitral stenosis (many students are confused by the difference between pressure and resistance)
What is a Swan Ganz catheter? What are the components relevant for measuring wedge pressure?
Swan Ganz catheter is a pulmonary artery catheter.
It is balloon-tipped, w/ a pressure sensor at tip beyond the balloon
What is the purpose of the balloon? Why is it important to have a pressure sensor distal to the balloon?
The catheter is typically inserted into a vein, then threaded through the RA and RV, into the pulmonary artery, until it reaches one of the smaller branches of the pulmonary artery.
The balloon is meant to occlude blood flow through the small branch of the pulmonary artery, so that the distal pressure sensor will only measure the pressure DISTAL to the balloon occluding the artery
What is the basis for wedge pressure? What is the process by which this is measured, and why is this logical?
Balloon-tipped catheter advanced through venous system, through RA/RV, then into pulmonary artery.
When balloon is DEFLATED, the pressure measured by the catheter is the pulmonary artery pressure.
When the balloon is INFLATED, the pressure measured by the catheter is equivalent to the pulmonary capillary pressure, which is roughly equivalent to the pressure in the pulmonary vein, which is roughly equivalent to the pressure in the left atrium.
Would the wedge pressure be pulsatile or nonpulsatile? Why?
Nonpulsatile
The balloon has occluded the small branch of the pulmonary artery, so none of the blood from the RV contraction (i.e. the “pulse”) should be reaching the catheter tip.
When the balloon is DEFLATED, every time the RV contracts, blood will flow through the pulmonary arteries, and will ↑ the pressure in the pulmonary arteries. However, if we occluded the blood flow to the small pulmonary artery where the catheter is by inflating the balloon, the pressure measured distal to the occlusion would be less, and would NOT be exposed to the pulsatile flow of blood.
Wait!
I strongly recommend that you attempt to answer the final questions by yourself first, before looking at the answers. Remember, the USMLE Step 1 exam will test your ability to make connections on the spot. The more practice you have, the higher your score! Then, when you think you might know the answer (or are completely stumped), look at the answers below!
Ready to apply and integrate your knowledge like Step 1 will require you?
What pressure can you estimate in the left ventricle by using wedge pressure? Why?
LVEDP
Recall: Flow = (P1 – P2)/Resistance
Under normal circumstances, resistance across the valve is negligible, so there is only needs to be a minor pressure difference between LA and LV pressure to accomplish flow during diastolic filling.
In what condition is wedge pressure a a poor estimate of LVEDP? Why?
Wedge pressure = poor estimate of LVEDP during mitral stenosis
Recall: Flow = (P1 – P2)/Resistance
In mitral stenosis, the resistance will be ↑ → the pressure in the LA must be ↑ to maintain flow across the valve.
Could you diagnose mitral stenosis by only using an increase in wedge pressure? Why or why not?
No, you cannot.
All you could measure would be an increase in LA pressure. However, there are OTHER causes of ↑ LA pressure
What would be the PA pressures and wedge pressure be in primary pulmonary hypertension? Why?
PA pressure ↑
Wedge pressure normal
Wedge pressure normal
The primary problem in pulmonary HTN is an increase in pulmonary artery RESISTANCE. Thus, the pressure generated by the RV must be higher to maintain pulmonary blood flow.
However, the amount of blood getting to the pulmonary veins would be largely unchanged, or even ↓ → wedge pressure NOT elevated
What would be the PA pressures and wedge pressure be in left-sided heart failure? Why?
PA pressure ↑
Wedge pressure ↑
Wedge pressure ↑
Left heart failure = Lack of forward flow of blood through LV → back-up of blood in LA → back-up of blood in pulmonary vein, then pulmonary capillary, then pulmonary artery, etc.
What would be the PA pressures and wedge pressure be in severe mitral stenosis? Why?
PA pressure ↑
Wedge pressure ↑
Wedge pressure ↑
Similar reasoning to left heart failure = Lack of forward flow of blood into LV → back-up of blood in LA → back-up of blood in pulmonary vein, then pulmonary capillary, then pulmonary artery, etc.
What should you do next?
- Turn the narrative, “Pathogenesis to Presentation” questions into Anki cards by copy/pasting the question/answer into the “Front” and “Back” fields in Anki. Do the same for the fundamental facts that you were unfamiliar with, to maximize your chances of USMLE Step 1 success! Remember: the USMLE is a test of understanding, so the better you can understand these questions, the better your score!
- Add reverse cards when appropriate to your Anki cards
- Re-word the questions/explanations as desired, and BOLD the important text to make it easier to review in the future
- Learn something new? Something unclear? Comment below!
- If you liked this post, please consider sharing it on Facebook/Twitter! I judge the utility of these posts by the number of comments / shares they receive, so if you’d like more, or would like a particular topic addressed, please let us know!
