title: Haemodynamic Monitoring tags: #FFICM #cardiac #shock #monitoring notebook: ð-FFICM type: anki

Haemodynamic monitoring

category: [[Shock]] [[cardiac]]

Introduction

Why monitor haemodynamics? To optimise delivery of oxygen to the tissues.
Styles of measurement are invasive or non-invasive.
You are attempting to get values for:
Preload
Contractility
Afterload
Arterial Blood Pressure
- Systemic
- Pulmonary
Cardiac Filling
Stroke Volume
Cardiac Output
Tissue Wellbeing ³

Pressure Based

MAP versus Sys/Dias

MAP is least altered by measurement damping
MAP is least dependent on invasive vs non invasive
MAP is least dependent on measurement site
It's MAP that determines tissue blood flow ³

Flashcard	type:basic
Give Four Reasons Why MAP is Better than Sys/Dias	1. It's MAP that determines tissue blood flow 2. MAP is less affected by invasive damping 3. MAP is less affected by invasive vs non invasive 4. MAP is less affected by site

Arterial Blood Pressure

Systolic, Diastolic, and MAP
The difference between Systolic and Diastolic does depend on where you measure them
MAP is less affected by where you measure it, and more resistant to a "damped trace" ³

Non Invasive

Most ICU cuffs are "automated intermittent oscillometric devices"
They inflate until all oscillations in cuff pressure stop
These can be bad for the limb if the limb has:
- Peripheral Vascular Disease
- Cannula
- AV Fistula
- Impaired Lymph Clearance (Like Lymph Node Clearance)
These non invasive cuffs overestimate low pressures, and underestimate high pressures.
These cuffs are less reliable in dysrhythmia, pregnancy, and childhood
Too small a cuff overestimates pressure, too big underestimates it ³

Flashcard	type:basic
What's the proper term for most non-invasive BP cuffs?	Automated Intermittent Oscillometric Devices
What do non-invasive BP cuffs do when measuring a low pressure?	Overestimate it
What do non-invasive BP cuffs do when measuring a high pressure?	Underestimate it
What blood pressures to non-invasive BP cuffs overestimate?	Low ones
What blood pressures to non-invasive BP cuffs underestimate?	High ones
Which BP measurement will be less accurate in dysrhythmia?	Non Invasive
What will too small a BP cuff do?	Overestimate pressure
What will too large a BP cuff do?	Underestimate pressure
What size BP cuff will underestimate pressure?	Too large
What size BP cuff will overestimate pressure?	Too small
How big is the right size for a non-invasive BP cuff?	40% of mid circumference of limb

A NIBP cuff width should be 40% of mid circumference of limb ³
Other non invasive methods are:
Volume-Clamp Method
- Bladder around the finger clamps the artery at a constant size
- A sensor is placed at heart level for reference
- The difference between the two sensors gives a pressure waveform
- It's no good if you've impaired perfusion or oedema in fingers (so it's no good in any critically ill patients basically) ³

Invasive BP Measurement

Cannulation of a systemic artery with continuous monitoring of:
Pressure waveform
HR
BP
Also allows blood testing
Can be sited:
Radial
Brachial
Dorsalis Pedis
(And any other artery basically)
Paeds put in Post Tib sometimes (not brachial)
Risk of permanent ischaemic complications in radial a-lines is less than 1 in 1000 (0.09%)
Test to see about collateral flow in hand is called the Allen's test
- It might not be any good, no evidence to say it minimises risks
Recommend smaller than a 20G cannula ³

Flashcard	type:basic
How likely is a radial art line to cause permanent ischaemic complications?	~1:1000
How likely is a radial art line to cause vascular thrombosis?	~7-30%

Then to stop clotting, in line, infuse with saline at 3ml/h (can be heparinised or not, generally not in UK setting)
Should check distal perfusion 8hrly ³
Indications for reemoval
Persistent blanching
Coolness
Loss of pulses
Raised compartment pressures ³

Complications

Vascular thrombosis - 7-30% risk in radial artery
Distal embolisation
Proximal embolisation of clot or air
Vascular spasm
Skin necrosis at site
Line disconnection and bleeding
Accidental drug injection
Infection
Damage to nearby structures (with direct or indirect haematoma)
AV fistula ³

Art Line System

Cannula is connected to a pressure transducer. ³

The pressure transducer is linearly responsive. ³

The connection is via a short, non-compliant, fluid filled, tubing. ³

Transducers need to be calibrated (static calibration), and zeroed (dynamic calibration.). Most transducers are precalibrated, and so that's not needed in unit. ²

System is zeroed at the mid-axillary line of fourth intercostal space. ²

Transducer lower than the zero will over-estimate pressure. Transducer higher than the zero will under-estimate pressure. A 10 cm change in the direction = ~7mmHg change in pressure ²

Flashcard	type:basic
What are the two forms of calibration that arterial lines need?	Static Calibration Dynamic Calibration (Zeroing)
In intensive care arterial lines, do you perform static or dynamic calibration?	Dynamic
Where on the outside of a patient do you zero an arterial line?	Mid Axillary Line - Fourth Intercostal Space
If the transducer on an arterial line is placed too low, what does that do to blood pressure figures?	Over-estimate them
If the transducer on an arterial line is placed too high, what does that do to blood pressure figures?	Under-estimate them
How much of a BP change in an art line will you get by moving the transducer 10cm?	~7mmHG
How much do you need to move an art line transducer to get ~7mmHg under/overestimate?	10cm

Art Line gives you

Systolic BP
Diastolic BP
HR
MAP ²

It also gives you:

The pulse pressure (Difference between diastolic and systolic)
Respiratory Changes over time (Pulse Pressure Variation) ²

Flashcard	type:basic
On an invasive BP measurement, what is the pulse pressure?	Systolic Minus Diastolic
What is the term for the difference between the Systolic and Diastolic Blood Pressures?	Pulse Pressure
On an invasive BP measurement, what is the Pulse Pressure Variation?	How Much the Pulse Pressure changes over respiratory cycle
What is the term for how the Arterial Pulse Pressures change over the respiratory cycle?	Pulse Pressure
On an invasive BP measurement, what is the pulse pressure?	Systolic Minus Diastolic
What is the term for the difference between the Systolic and Diastolic Blood Pressures?	Pulse Pressure Variation
On an arterial line, what can you use as a surrogate measure of contractility?	The angle of the upstroke on systolic part of curve That's pressure change over time!
On an arterial line, what can you use as a surrogate measure of stroke volume?	The area under the curve on the systolic part of waveform (the first peak)
On an arterial line, what can you use as a surrogate measure of diastolic resistance?	The angle of the downstroke on diastolic part of curve That's pressure change over time!
On an arterial line, what does the angle of the upstroke on systolic part of curve give you?	Surrogate measure of contractility
On an arterial line, what does the area under the curve on the systolic peak on the art waveform give you?	Surrogate measure of stroke volume
On an arterial line, what does the angle of the downstroke on diastolic part of curve give you?	Surrogate measure of diastolic function/resistance

You could also measure:

The rate of the upstroke on systolic part of the curve
As a surrogate measure of contractility (dP/dt)
The area under the curve on the systolic part (the first peak)
As a surrogate measure of stroke volume
The rate of the downstroke on the diastolic part of curve
As a surrogate marker of diastolic resistance ²

Physics of an art line

²

All monitoring needs 4 main components:

Biological Variable
Sensor
Integrator
Output ²

Flashcard	type:basic
What are the 4 main components that every medical monitor system needs?	1. Biological Variable To Measure 2. Sensor to Measure It Integrator to Interpret it Output to display it
What is the point of a arterial line transducer?	To convert energy in presssure form into electrical signal
What is the part of an arterial line system that converts pressure energy into electrical signal?	The transducer
What is the part of an arterial line transducer that gets pushed up and down with pressure exerted by the pulse?	The diaphragm
What does a diaphragm in an arterial line transducer do?	It gets pushed up and down by the pressure exerted by the pulse
What does a strain gauge in an arterial line transducer do, when the diaphragm moves?	It gets streched and thinner, increasing electrical resistance in it.
What is the name for the part of an arterial line transducer that gets stretched by diaphragm movement, getting thinner and increasing electrical resistance?	The strain gauge
What is the name of a circuit of strain gauges in an arterial line transducer?	A wheatstone bridge
What is a wheatstone bridge made up of?	A circuit of strain gauges
What is the point of a wheatstone bridge in an arterial line transducer?	To use the changes of resistance in the strain gauges, to convert movement of the diaphgragm into electrical signals.
Which part of a traditional arterial line transducer isn't really needed anymore?	A wheatstone bridge
Where does the electrical signal from a wheatstone bridge get sent off to in an arterial line?	The integrator
What form of machine is an integrator in an arterial line system?	It's a little computer
What does an integrator do in an arterial line system?	It takes electrical signal from the transducer, and adjusts it to take into account static and dynamic calibration
What is the part of an arterial line that takes signal from the transducer, and then adjusts it to take into account static and dynamic calibration?	The integrator

Transducer

What is a transducer? It's an electrical component to convert one form of energy into another. So an art line transducer converts pressure into electrical signal ²

The transducer has a diaphragm that moves up and down with the pressure exerted by the pulse (the pressure from the cardiac cycle) ²

The diaphragm has a "strain gauge" above and below it. These are electrically conductive wires that zig zag. When they are stretched (by the diapragm moving away and pulling on them), they get thinner, and electrical resistance increases. So this change in resistance can then be used to know what the change in pressure is. ²

The circuit these strain gauges were used in, the original version was called a "Wheatstone bridge" ²

These strain gauges were put in a circuit called a wheatstone bridge, that allowed us to measure the resistance passing through the strain gauge (and so know how much it was being streched by the diaphragm, and how much the diaphragm was being pushed by the pressure) ²

Modern arterial line transducers don't need a Wheatstone bridge to work out the resistance in the strain gauge any more. However they still use them as a supplementary thing, to improve the resistance to other factors messing up the signal. ²

This electrical signal then goes to an integrator (a computer). This takes the signal and takes into account static calibration (making sure theres that linear relationship between pressure and resistance) and dynamic calibration (zeroing) ²

Zeroing an Art Line (Physics)

Resonance

Flashcard	type:basic
What are the two types of wave frequencies seen in an arterial line system?	The frequency of the heart rate The "natural frequency" of the arterial line system itself
What is the problem with resonance in an arterial line system?	If the natural frequency of the arterial line system, and the frequency of the heart beat overlap, you will get resonance, which will overestimate the amplitude of those overlapping beats
What is it called if the frequency of a heart rate, and the "natural frequency" of the arterial line system itself overlap?	Increased resonance
What actually defines the "natural frequency" of an arterial line system?	It's how frequently the pressure in it oscillates without any damping
What is the term for how frequently the pressure in an arterial line system oscillates without interference from damping?	The "natural frequency"
How do we prevent the natural frequency of an arterial line and the frequency of the heart rate itself from overlapping and causing resonance problems?	We keep the natural frequency of an arterial line much higher (~10x higher) than the frequency of a heart rate.
How do you keep the natural frequncy of an arterial line system really high?	Short Art Line Catheter Stiff Art Line Catheter Skinny Art Line Radius
How high do you want the natural frequency of an arterial line system?	~10x more than the heart rate frequency. So 200+ Hz

What is resonance? It's how oscillations in a wave (a vibrating system) increase in amplitude as energy is applied to them. ²

How much these oscillations increase is in harmonic proportion to the natural frequency. ²

The natural frequency of the vibrating system is the frequency of its oscillations in the abscense of damping. ²

What this means is we don't want our art line measurement system to naturally vibrate at a frequency that is close to the heart rate. ²

So if HR is ~60 - 180bpm, then it is ~ 1-3Hz (Hertz) ²

So to keep our art line uninterfered with by resonance caused by the HR, we keep the frequency of our system 8x higher than the frequency of the HR (this increase in frequency by 8x is also called an increase by 8 harmonics) ²

A Modern art line system has a natural frequency of 200+ Hz ²

What affects the natural frequency of an arterial line system. ²

Catheter length (short cannulae increases the natural frequency)
Catheter elasticity (stiff cannulae increase the natural frequency)
Catheter radius
Density of blood ²

Damping

What is damping? It's the loss of energy in a system. This loss of energy will reduce the amplitude of oscillations. ²

Some damping will occur in an art line due to friction of fluid within the tubing. ²

We want some damping, so we can look at the amplitude of the next heart beat without being interfered with by the previous one. ²

There is a damping coefficient. This is a way of stating mathematically how much damping is in a system. It uses the amplitude ratio to work out the coefficient. ²

The amplitude ratio. The ratio of the difference in size between a oscillation (wave), and the one before it. Amplitude Ratio = D2/D1. D2 is the amplitude of one wave. D1 is the wave that was before it. ²

The best damping for our purposes has about a damping coefficient of ~ 0.64 ²

Too much damping (a coeffecient of > 0.7) would mean that the systolic and diastolic pressures will be falsely close together. The MAP will still be correct though. This will happen when there is air in the line, or when there is too much, or too compliant (stretchy) tubing used. ²

Too little damping (coefficient of less than 0.6) would mean the pressures will overshoot. You will get a higher systolic pressure and a lower systolic pressure. The MAP will still be correct though. ²

Flashcard	type:basic
What is damping in an arterial line system?	It's loss of energy, resulting in smaller amplitude oscillations
Why do we want damping in an arterial line system?	It prevents each heartbeat's blood pressure being affected by the previous one (resonance?)
What is it called in an arterial line when energy is lost, resulting in smaller amplitude pressure waves?	Damping
How do we prevent each arterial beat in an art line affecting the results of the next heart beat?	Through damping
What is the term for looking at the ratio between the size of one arterial beat (oscillation) and the one before it, when looking at the physics of arterial lines?	The Amplitude Ratio
When looking at the physics of arterial lines, what is the Amplitude Ratio?	It's the ratio between the size (amplitude) of one arterial beat, and the one before it.
What can we use the amplitude ratio in arterial lines to work out?	The "damping coefficient"
How can we measure the damping coefficient in an arterial line system?	We use the amplitude ratio
In arterial line, what would be the best damping coefficient?	Around 0.65
In arterial lines what damping coefficient would be too much damping?	Greater than 0.7
In arterial lines what damping coefficient would be too little damping?	Less than 0.6
What would be the result of an overdamped arterial line trace?	The MAP would be fine, but the systolic and diastolics will be falsely close together
What would be the result of an underdamped arterial line trace?	The MAP would be fine, but the systolic and diastolics will be falsely far apart
What physical problems can cause an overdamped arterial line system?	Air in arterial line Too much tubing Too stretchy (compliant) tubing

CVC Monitoring

Allows measurement of CVP
CVC is needed for some CO monitoring
For ScvO2 monitoring
Or transpulmonary indicator dilution systems ³

CVC uses same pressure transduction system for art lines. ³

Normal numbers are in [[cardiac-function-normal-figures]]

When healthy CVP is a good estimate of Pulmonary Artery Occlusion Pressures PAOPs (also called wedge pressures) ³

When unhealthy CVP and wedge pressure drift apart. ³

CVP does not differentiate patients likely to respond to fluids (unless extreme numbers) Dynamic changes have been used (in an ignore the number focus on the trend sense), but that may not be useful either. ³

Some ways that CVP could be useful:

If CVP goes steeply up following fluid challenge, the heart may be on plateau portion (as in more fluid is not going to help cardiac output)
If severe hypotensive with low/normal cvp, it's probably not going to be things that should increase cvp (PE, Cardiac Tamponade, Tension Pneumothorax)

Cardiac Output Monitoring

Defns

[[cardiac-function]]

Clinical Indicators

HR
BP (Helpful but not the mosthelpful)
Pulse Strength
Colour
RR
Core To Peripheral Temp Gradient
Urine Output
CRT
Cognitive Function
Response to Straight Leg Raise
Lactate
Base Deficit
Central Venous Sats¹

Overview

To be able to use starling's law, to know how much preload stretching would be optimal for a patient, we need to be able to measure preload/measulre left ventricular end diastolic volume.¹

Best surrogate for that is using pulmonary artery catheter for pulmonary occlusion pressure (estimates left atrial pressure, which is an estimate of lvedp, which is surrogate for lvedv). Problems with it is that positive pressure ventilation, PEEP, and catheter tip placement can also all affect it.¹

Thermodilution catheters may give more accurate data but little evidence to say that data is helpful in improving survival.¹

Now we want to get less invasive so we can be less risky.¹

There are two main approaches:

Doppler analysis of aortic blood velocity (from oesophagus)
Analysis of shape of arterial waveform¹

The arterial waveform ones use some modified arterial catheter and thermo-dilution to estimate it.¹

A problem may be that they rely on population data to calibrate. This may not be too accurate considering it's healthy volunteers.¹

Another approach is stroke volume variation or pulse pressure variation from transduced arterial waveform. This is a numerical value for the swing in an art line.¹

Flashcard	type:basic
Give me four main methods to measure or estimate cardiac output of a patient	Direct measurement using PA Catheters Measure Aortic Blood Velocity (Oesophageal Doppler) Measure shape of arterial waveform Echocardiography
What does an oesophageal doppler do in cardiac output measuements?	Looks at blood velocity in descending aorta to estimate cardiac output.
What are the two main forms of dilution measurement of cardiac output?	Lithium Dilution Thermodilution

Ultrasound/echo

Oesophageal Doppler

This measures blood flow in desceding aorta, from viewpoint of distal oesophagus.¹

It calculates output by measuring descending aorta diameter. It will measure ventricular ejection time (to measure preload) and peak flow velocity for contractility of ventricle.¹

You can also do some wave form interpretation.¹

The advantages of this are minimally invasive, quick to do and quick to analyse, invasive enough to avoid interference from bone and soft tissue. But the disadvantages are it requires sedation, is user dependent, is contraindicated in oesophageal varices, may be inaccurate if measures the wrong vessel, or if some cerebral blood flow being prioritised over aorta.¹

echocardiography

Directly visualise the contracting heart. Gives qualiative and quantitative values.¹

But it's more expensive than oesophageal, and has greater requirement for examiner skill, there is risk of trauma in oesophageal.¹

Dilution

These use a central vein to inject a minimally metabolised marker substance, and a central vein to cannulate to measure the arterial content.¹

If blood flow between injection and measuring sites is constant then flow can be calculated. Using concentration versus time.¹

These are less invasive than pulmonary arter catheters. But can only be used in ventilated patients in sinus rhythm. They are still invasive and user dependent and can underestimate output in low output states. ¹

Lithium Dilution

Flashcard	type:basic
Which (non PA) cardiac output monitors use lithium dilution plus pulse contour analysis?	Lidco PulseCo
Which (non PA) cardiac output monitors use thermodilution plus pulse contour analysis?	PICCOplus
Which (non PA) cardiac output monitors use just pulse contour analysis?	Vigileo
What type of technique does the Lidco cardiac output monitor use?	Lithium dilution + Pulse Contour Analysis
What type of technique does the PulseCo cardiac output monitor use?	Lithium dilution + Pulse Contour Analysis
What type of technique does the PICCOplus cardiac output monitor use?	Thermodilution + Pulse Contour Analysis
What type of technique does the Vigileo cardiac output monitor use?	Pulse Contour Analysis
What measurements does a Pulse Contour Analysis cardiac output monitor give you?	Cardiac Output Stroke Volume Stroke Volume Variation Pulse Pressure Variation

LidCo
PulseCo
Lidcoplus¹

These use lithium dilution and pulse contour analysis. ¹

Theres an ion selective electrode in the artline to detect lithium. ¹

It acnt be used in patients who are on lithium or those whove received vecuronium or atracurium. It's not useful in non sinus rhthems. ¹

Thermodilution

PICCOplus ¹

This also uses dilution and pulse contour analysis. ¹

Inject cold fluid into central venous catheter that will go through pulmonary circultion. This temp change is measured in systemic artery and wil derive cardiac output, plusextravascular lung water. ¹

This does need a specialised arterial catheter in the brachial or femoral. Plus a thoracic/femoral central line. ¹

This can be used for bp monitoring ans sampling. It can estimate preload as well as things above. ¹

But the art line is large and expensive. You need to recalibrate every 12 hours. Recalibration can be affected by speed of injection. ¹

Pulse Contour Analysis

ProAQT
Vigileo
LIDCOrapid ¹

These can work on any arterial line and use pulse contour analysis. ¹

They estimate continuous cardiac output, stroke volume, stroke volume variation, pulse pressure variation. To get PPV/SVV you need ventilation with fixed TVs. ¹

Pulmonary Artery Flotation Catheters

[[pa_catheter]]

Mixed Venous Oxygen Saturations - SvO2

[[Acute Heart failure]]

Central Venous Oxygen Saturations - ScvO2

[[Acute Heart failure]]

There is a CeVox system. ¹

Thoracic Bioimpedance

This is a measure of resistance to alternative current, which reflects throracic fluid volume. It is the least invasive methofd and was conceived for space flight, but less clear how it correlates with pulmonary artery catheters in ill patients. ¹

Sources

[Acute Heart failure]: "Acute Heart Failure" [Acute Heart failure]: "Acute Heart Failure"

Cardiac output monitoring. Update in Anaesthesia. Monitoring in the ICU. Thomas Lawson and Andrew Hutton. ↩↩↩↩↩↩↩↩↩↩↩↩↩↩↩↩↩↩↩↩↩↩↩↩↩↩↩↩↩↩↩↩↩
Invasive Arterial Blood Presssure Measurement ↩↩↩↩↩↩↩↩↩↩↩↩↩↩↩↩↩↩↩↩↩↩↩↩↩↩↩↩↩↩↩↩
Oh's Intensive Care Manual - Haemodynamic Monitoring ↩↩↩↩↩↩↩↩↩↩↩↩↩↩↩↩↩↩