Under Pressure. Dr. Robert Keyes CAEP -- Critical Care May 31, PDF Free Download

Under Pressure Dr. Robert Keyes CAEP -- Critical Care May 31, 2015

Disclosures 1. No disclosures, commercial supports, or conflicts of interest.

Learning Objectives 1. To understand the principles of pressure and its measurement. 2. To apply these principles to the clinical setting.

Do I really need to be here? There s a chance you don t. So we ll start with two skill-testing questions.

This is a set of equilibrium tubes. It is a series of vertical tubes of varying diameter. Connected at the bottom by a horizontal tube. This is a fairly simple one. Question #1

Ooooh. Question #1

But the principle is same. You pour water in the big tube. This fills the other tubes. Question #1

Question #1: If you pour until the big tube is exactly half full How full will the other tubes be? Question #1

Option A: The other tubes will also be half full. Question #1

Option A: The other tubes will also be half full. Option B: The other tubes will be fuller (to varying degrees). Question #1

Final answers? Question #1

Option A.

Pressure = Force / Area Wider columns of fluid apply more force.

Pressure = Force / Area But this force is dispersed over a larger area.

By way of analogy A large man who is barefoot and a small woman in heels are walking across a lawn. The man has no problem. What happens to the woman? Unless she can disperse her force over a larger area.

Pressure = Force / Area For any given fluid

Pressure = Force / Area For any given fluid the only thing that affects the amount of pressure below the surface

Pressure = Force / Area For any given fluid the only thing that affects the amount of pressure below the surface is the height below the surface.

This is why we can describe pressure using a. A measurement (eg. mm, cm) b. A liquid (eg. Hg, H2O)

Question #1 The equilibrium tube question was the easy one. Now for the hard one.

Question #2 A pressure transducer is set up as follows...

Question #2 A pressure transducer is set up as follows... Fluid-filled tubing. The transducer measures pressure and converts it into a number we can read. +/- Fluid-filled balloon

Question #2 First, our transducer is leveled. This means placing the transducer and the balloon at the same height.

Question #2 Then the transducer is zeroed Because unless we are in a vacuum, the balloon is already under pressure from billions of air molecules.

Question #2 Then the transducer is zeroed (not to scale) High Low Because unless we are in a vacuum, the balloon is already under pressure from billions of air molecules.

Question #2 Then the transducer is zeroed Zero Zeroing takes whatever pressure the balloon is currently under and calls it zero.

Question #2 Or 1 atm If the transducer was first zeroed in a vacuum, and then brought to sea level, what pressure would it read?

Question #2 Or 760 mm Hg If the transducer was first zeroed in a vacuum, and then brought to sea level, what pressure would it read?

Question #2 Or 76 cm Hg If the transducer was first zeroed in a vacuum, and then brought to sea level, what pressure would it read?

Question #2 Or 1033 cm H2O If the transducer was first zeroed in a vacuum, and then brought to sea level, what pressure would it read?

Question #2 For Question #2, assume we have normal water-filled tubing and a leveled, zeroed system.

Question #2 For Question #2, assume we have normal water-filled tubing and a leveled, zeroed system. Zero

Question #2 Then, without changing anything else Submerge the balloon in 10cm of water. Zero

Question #2 Then, without changing anything else Submerge the balloon in 10cm of water.? 10cm 1. Courtois et al, "Anatomically and Physiologically Based Reference Level for Measurement of Intracardiac Pressures," Circulation, 1995

Question #2 And since we are connecting two water-filled systems, we don t actually need a balloon.? 10cm So we ll get rid of it.

Question #2 Question 1.5 What pressure does the transducer read now?? 10cm

Question #2 Question 1.5 What pressure does the transducer read now? 10cmH20 10cm This may seem pretty intuitive.

Question #2 But now for the real question. The tubing is submerged another 20cm. 10cm

Question #2 But now for the real question. The tubing is submerged another 20cm. 10cmH20 10cm 20cm What does the transducer read now?

Question #2 If you knew the answer was 10cmH2O, you can go. 10cmH20 10cm 20cm

Explanation If you didn t get this answer, let s go back to our original zeroed and leveled transducer.

Explanation If you didn t get this answer, let s go back to our original zeroed and leveled transducer. 0 cmh20 Start by focusing on the water-filled tubing.

Explanation Assume the tubing is about 30cm tall. 30cm 0 cmh20

Explanation Now divide this tubing into two seperate columns of water--each 30cm tall. 30cm 0 cmh20 WTF? - These two columns aren t the same shape or length! - It doesn t matter.

Explanation From a pressure standpoint, only the height of a fluid column matters. 30cm 0 cmh20

Explanation The left column of fluid is applying 30cmH20 pressure towards the transducer. 30cm 0 cmh20 This is read as positive pressure.

Explanation The right column is applying 30cmH20 pressure away from the transducer. 30cm 0 cmh20 This is read as negative pressure.

Explanation For a net pressure to the transducer of zero. 30cm 0 cmh20 Follow-up question Would things change if the tubing were twice as tall? Or more tangled up?

Explanation Like this? No. 60cm

Explanation Like this? No. 60cm 0 cmh20 60 cmh20 pressure pushing left. 60 cmh20 pressure pushing right. Net pressure zero.

Explanation What about something crazy?

Explanation What about something crazy? Net pressure still zero.

Explanation If the transducer and the end of the tubing are level Then all the other opposing columns of fluid must balance out.

Explanation If the transducer and the end of the tubing are level Then all the other opposing columns of fluid must balance out. 0 cmh20

In contrast Explanation If the transducer and the end of the tubing are not level, things get trickier.

Explanation For your consideration

Explanation Assuming that the tubing is water-filled

Explanation And the vertical grey ruler is marked in 5cm gradients...

Explanation What will each of these transducers read? 5cmH20 10cmH20-5cmH20-10cmH20

Explanation The height of the end of tubing (relative to the transducer) is all that matters. 5cmH20 10cmH20-5cmH20-10cmH20

Explanation All the other tubing cancels out. 5cmH20 10cmH20-5cmH20-10cmH20

Explanation Which helps to explain Question #2.

Explanation Which helps to explain Question #2. 10cm 10cm 10cmH20 10cmH20 20cm Why do both transducers read 10cmH20?

Explanation 10cm 10cm 10cmH20 10cmH20 20cm If you take away the glasses of water, the question is actually quite easy.

Explanation What do the transducers read here? 0 cmh20-20 cmh20 20cm It s just like the previous questions.

Explanation Then, without changing anything else 20cm Submerge the tubing on the left into 10cm of water.

Explanation Then, without changing anything else. 10cm 20cm Submerge the tubing on the left into 10cm of water.

Explanation Then, without changing anything else. 10cm 20cm This applies an additional 10cm of H20-pressure to the end of the tubing.

Explanation Then, without changing anything else. 10cm 10 cmh20 20cm 0cmH20 + 10cmH20 = 10cmH20.

Now for the right. Explanation 10cm 10 cmh20-20 cmh20 20cm Without the glass of water, the transducer reads 20cmH2O.

Now for the right. Explanation 10cm 10 cmh20 20cm Then the tubing is submerged into 30cm of water.

Now for the right. Explanation 10cm 10cm 10 cmh20 20cm Then the tubing is submerged into 30cm of water.

Now for the right. Explanation 10cm 10cm 10 cmh20 20cm This applies an additional 30cm of H20-pressure to the end of the tubing.

Now for the right. Explanation 10cm 10cm 10 cmh20 10 cmh20 20cm -20cmH2O + 30cmH20 = 10 cmh20

Explanation In fact, with this set-up... 10cm 10cm 10 cmh20 10 cmh20 20cm No matter how deep you submerge the tubing

Explanation In fact, with this set-up... 10cm 10 cmh20 10 cmh20 The transducer will always read 10cmH20.

Art Lines Arterial lines are really just a transducer and water-filled tubing.

Art Lines They still need to be zeroed and leveled before we use them. But where should we level them?

Art Lines By convention, art lines are leveled at the heart. This provides a stable reference for blood pressure measurements made over time, and between different patients.

Art Lines The external landmark for the heart is the phlebostatic axis.

Clinical Scenario #1 You insert a radial art line and hook it up to a transducer. The transducer is leveled at the heart The patient holds his arm exactly at heart level.

Clinical Scenario #1 Then the bed is raised 20cm.

Clinical Scenario #1 What effect will this have on the BP read by the transducer? It will increase by 20cmH2O.

Clinical Scenario #1 To convert cmh2o to mmhg, divide by 1.36. So the BP read by the transducer will increase by ~15mmHg.

Clinical Scenario #1 The opposite occurs if the bed drops 20cm.

Clinical Scenario #1 The opposite occurs if the bed drops 20cm. Now the BP read by the transducer drops by 20cmH2O Obviously, the true BP has not changed at all

Clinical Scenario #1 Take home message If your transducer isn t leveled, things get weird.

Clinical Scenario #2 Second scenario similar set-up. A radial art line is hooked-up to a zeroed and leveled transducer.

Clinical Scenario #2 Then the patient raises his arm 20cm.

What effect will this have on the BP read by the transducer? None. Why? Clinical Scenario #2

Clinical Scenario #2 The blood vessels in the patient s arm act as just more fluid-filled tubing.

Clinical Scenario #2 You have an extra 20cm column of water pushing towards the transducer

Clinical Scenario #2 Balanced by an extra 20cm column of blood pulling away from the transducer. For a net change in pressure of zero

Clinical Scenario #2 Now technically The density of water and blood are not identical, so they won t balance out precisely. But this difference is clinically negligible.

Clinical Scenario #2 Take home message Arterial lines give the pressure of the arterial system at the level of the transducer.

Clinical Scenario #2 Which may help us to understand the glass of water from earlier.

Clinical Scenario #2 Which may help us to understand the glass of water from earlier. 10 cmh20

Clinical Scenario #2 Arterial lines give the pressure of the arterial system at the level of the transducer. Pressure lines give the pressure of the measured system at the level of the transducer. 10 cmh20

Clinical Scenario #3 Next scenario You need to perform a lumbar puncture and opening pressure. The spinal needle is placed while the patient is sitting. Question: Why should you not measure the opening pressure in this position?

Clinical Scenario #3 Backing up a bit The opening pressure is designed to give a measurement of brain (intracranial) pressure. ICP is best represented by the pressure in the lateral ventricles.

Clinical Scenario #3 The lateral ventricles are attached to the spinal subarachnoid space by a continuous column of CSF. If the patient is lying down, the cerebral ventricles and the spine are approximately at the same level.

Clinical Scenario #3 After placing the spinal needle Intracranial pressure forces CSF out the needle and up the glass tubing. This continues until the column of CSF in the glass tubing balances out the intracranial pressure.

Clinical Scenario #3 The ICP is then read directly from the height of the CSF in the glass tubing. Technically, the pressure is in cmcsf, not cmh20. But CSF and H2O have such a similar density, the difference is negligble.

In contrast Clinical Scenario #3 If the patient is sitting up, things are trickier.

Clinical Scenario #3 The glass tubing is now measuring the pressure of both: a. The brain. b. The column of CSF sitting in the spine above the spinal needle.

Clinical Scenario #3 Theoretically, you could correct for this: 1. Measure the vertical distance from the meatus of the ear (cerebral venticles) to the spinal needle. This gives the height (and pressure) of the CSF column in the spine 2. Subtract that number from the height of the CSF in the glass tubing to isolate the brain pressure.

Clinical Scenario #3 Technically, this should work In practice, the CSF may overflow the glass tubing.

Clinical Scenario #4 Final scenario This scenario is about Cerebral Perfusion Pressure (CPP). It is more relevant to ICU than Emerg. But if you understand this, you understand pressure better than many Intensivists.

Clinical Scenario #4 CPP = MAP - ICP The CPP is the pressure needed to perfuse the brain. The MAP is the arterial pressure pushing forward driving blood into the brain. The ICP is the brain pressure pushing back.

Clinical Scenario #4 Grossly elevated ICP can be thought of as compartment syndrome of the brain. What does a cerebral perfusion scan look like in patients with grossly elevated ICP?

Clinical Scenario #4 Different sources argue about how much CPP is necessary to perfuse the brain. A CPP >60-70mmHg is commonly quoted. I don t find this debate very interesting. I prefer discussing how to measure CPP.

Clinical Scenario #4 Here is your brain-injured patient.

Clinical Scenario #4 Here is your brain-injured patient. Approach the problem in steps...

Clinical Scenario #4 First, be aware that we are going to measure the pressure of two different systems: 1. The brain (ICP) 2. The arteries (MAP) So we need two different pressure lines.

Clinical Scenario #4 For ICP, we need a pressure line to the brain. This requires placing a catheter (EVD) into the cerebral ventricles.

Clinical Scenario #4 The transducer for the ICP must be leveled where we want to know the pressure. At the cerebral venticles (ie. external meatus)

Clinical Scenario #4 The transducer for the ICP must be leveled where we want to know the pressure. At the cerebral venticles (ie. external meatus) Now we know the ICP

Clinical Scenario #4 For the MAP, we need an arterial line. So a radial art line is placed, and attached to a regularly zeroed and leveled transducer.

Clinical Scenario #4 For the MAP, we need an arterial line. So a radial art line is placed, and attached to a regularly zeroed and leveled transducer. Now we know the MAP.

Clinical Scenario #4 So we re done, right? CPP = MAP ICP Or is there a problem?

Clinical Scenario #4 Arterial lines--by convention--are typically leveled at the heart. This provides a stable reference point for BP measurements over time. But the CPP is an exception to this guideline.

Clinical Scenario #4 For CPP, we need to know who s winning: 1. The MAP (pushing forward) 2. The ICP (pushing back) The MAP needs to be winning by >60mmHg.

Clinical Scenario #4 But the MAP vs. ICP battle is not taking place at the level of the heart. It s taking place at the level of the brain So we need to know both pressures at that level

Look at the patient below The arterial line is giving the MAP at heart level. Will the MAP at brain level be higher or lower? Lower. Clinical Scenario #4

Clinical Scenario #4 You could calculate how much lower By measuring the vertical height from the external meatus to the phlebostatic axis. Remembering your units... cmblood, not mmhg.

Clinical Scenario #4 But it might be easier to: a. Raise the arterial line transducer to brain level.

Clinical Scenario #4 But it might be easier to: a. Raise the arterial line transducer to brain level. b. Lie the patient flat (so brain and heart are level).

Clinical Scenario #4 Whichever technique you use MAP is measured in mmhg, but ICP is often measured in cmh20. So convert your units before calculating CPP.

That s it.

Under Pressure some conclusions 1. Pressure can be described using any given unit of measurement and any given fluid. 2. Pressure lines (arterial or intracranial) give the pressure of the measured system at the level of the transducer. 3. Non-leveled transducers make it weird.

Under Pressure Any questions?

References 1. Courtois et al, "Anatomically and Physiologically Based Reference Level for Measurement of Intracardiac Pressures," Circulation, 1995 2. Guyton et al, "Textbook of Medical Physiology," 9 th edition, Saunders, 1996 3. Smith et al, Evaluation and Measurement of Elevated Intracranial Pressure in Adults, UpToDate, last updated July, 2013; current as of April, 2015

Under Pressure. Dr. Robert Keyes CAEP -- Critical Care May 31, 2015