Cardiac Function
title: Cardiac Function tags: #FFICM #cardiac #physiology notebook: ð-FFICM type: anki
category:
[[Acute Heart Failure]]
What is the ultimate point of Cardiac/circulatory Function? It is to ensure oxygen delivery to the tissues.
Assessing Ventricular Performance
The factors that define ventricle performance:
- Preload (Right and Left Atrial Pressures)
- Afterload (Mean Systemic and Pulmonary artery pressures)
- Heart Rate
- Systolic Function 1
Contractility - Amount of work heart can generate at set levels of preload and afterload. Defn would be maximum rate heart can generate pressure change over time. This is Inotropy.[^@cardiac_output]
| Flashcard | type:basic |
|---|---|
| What are the four factors that define ventricular performance? | Preload Afterload Heart Rate Systolic Function |
| What do we mean when we talk about contractility of a heart ventricle? | The amount of work a heart can generate at set levels of preload and afterload. |
Cardiac Output
Cardiac Output - Volume of blood ejected from each of ventricles per minute. Is product of HR * SV. Units are L/min[^@cardiac_output]
Cardiac Index - Is cardiac output referenced to body surface area. Units are L/min/m2[^@cardiac_output]
Ejection fraction - Fraction of total blood in ventricle that is ejected per beat - Normal is 55-65[^@cardiac_output]
| Flashcard | type:basic |
|---|---|
| What is the term that measures the volume of blood ejected from the ventricles per minute? | Cardiac Output |
| When we talk about an ejection fraction of a ventricle what do we mean? | The fraction of blood in the ventricle that is ejected per beat (Stroke Volume / End Diastolic Volume) |
| What is the formula to get the ejection fraction of a ventricle? | Stroke Volume / End Diastolic Volume |
| Define what cardiac output is measuring? | The amount of blood ejected from the ventricles each minute |
Stroke Volume
Stroke Volume - Blood ejected by each ventricle contraction, determined by preload, afterload, contractility. [^@cardiac_output]
Stroke Volume = CO / HR 1
Cardiac Output = HR * Stroke Volume
MAP = (CO Ã SVR) + CVP
So SV is dependent on preload, afterload, and myocardial contractility 1
You can also have the Stroke Volume Index (stroke volume adjusted by body surface area - SV/BSA)
| Flashcard | type:basic |
|---|---|
| What actually is the stroke volume of a ventricle talking about? | It's the blood ejected by a ventricle contraction |
| What is the formula to define stroke volume? | Cardiac Output / Heart Rate |
| What is the formula to define cardiac output? | Heart Rate * Stroke Volume |
| How can you use Cardiac Output, SVR, and CVP to get MAP? | MAP = (CO * SVR) + CVP |
| Who can you define Stroke Volume Index? | SVI = SV / Body Surface Area |
Ventricular Stroke Work (VSW)
Stroke work and stroke volume are different! The stroke volume is the amount of blood ejected with each beat but the stroke work is the work required to push that amount of blood. So RV may have same volume as LV but the work is smaller, cos PVR is way lower than SVR.
| Flashcard | type:basic |
|---|---|
| What is the difference between ventricular stroke work and ventricular stroke volume? | Stroke volume = Amount Blood Ejected Stroke Work = Work required to eject that amount |
| Why is RV stroke work usually smaller than LV stroke work, despite same stroke volume? | Much lower PVR than SVR |
| What is the formula for Ventricular Stroke Work? | VSW = SV * (Afterload - Preload) |
| What is the formula for Left Ventricular Stroke Work? | Left VSW = SV * (MAP - LAP) * 0.0136 g.m (Conversion for SI units) |
| What measurement do we use to estimate left ventricular afterload? | MAP! |
| What measurement do we use to estimate left ventricular preload? | Left Atrial Pressure (~Pulm Art Occlusion Pressure = ~Pulm Art Diastolic Pressure) |
| What measurement do we use to estimate systemic afterload? | MAP! |
| What cardiac load or pressure, does MAP estimate? | Systemic (Left Ventricular) Afterload. |
| What measurement do we use to estimate systemic preload? | Left Atrial Pressure (~Pulm Art Occlusion Pressure = ~Pulm Art Diastolic Pressure) |
| What is the formula for Right Ventricular Stroke Work? | Right VSW = SV * (mPAP - RAP) * 0.0136 g.m (Conversion for SI units) |
| What measurement do we use to estimate right ventricular afterload? | Mean Pulm Artery Pressure (mPAP) |
| What measurement do we use to estimate right ventricular preload? | Right Atrial Pressure |
| How do you convert Ventricular Stroke Work into Ventricular Stroke Work Index? | Divide by BSA |
| What is the average body surface area? | 1.65 |
| What is the conversion factor to get Ventricular Stroke Work into SI units? | 0.0136 |
VSW = SV * (Afterload - Preload)
Left VSW = SV * (MAP - LAP) * 0.0136 g.m
RVSW = SV * (mPAP - RAP) * 0.0136 g.m
0.0136 = the conversion factor needed for SI units
To get an LVSWI or an RVSWI, it's an index, so it's divided to adjust for body size, so divide by 1.65 (average surface area)
LV efficiency is then the ratio of work output to energy input. This can be less than 20% in AHF patients.
What would a graph of a bad ventricular function look like?
If you plotted stroke volume against filling pressure, a normal heart would have a nice straight line going diagonally up, a nice x=y line. Whereas when the ventricles get impaired you start to have that line flattening out, like you're going up and up on the x axis value, the filling pressure, but not increasing on the y axis, the stroke volume.
Ventricular Preload
Our measurement of preload is atrial filling pressures (Right - RAP, and left - LAP). 1
Preload - Tension in ventricular wall at end-diastole (maximum filling prior to contraction). End-diastolic pressure is used as a surrogate measure. Determined by venous return and gives indication of filling pressure of heart.[^@cardiac_output]
Preload will determine the end diastolic ventricular volume, which in turn will determine the stroke work generated by the ventricle in the next systole (the stroke volume itself will also depend on the resistance - afterload - that ithe ventricle is up against) 1
| Flashcard | type:basic |
|---|---|
| What measurement do Left Atrial and Right Atrial Pressures affect, when changing the preload? | Ventricular End Diastolic Volume |
| What formula is used to calculate ventricular wall tension? | Laplace's Law Wall Tension = (Intraventricular Pressure * Radius) / (Wall thickness * 2) |
| What factors affect ventricular wall tension in laplace's law? | Pressure of ventricular contents + Radius of ventricle |
| What actually is ventricular preload talking about? | The tension in ventricular wall at the end of diastole |
How do you calculate what the wall tension will be?
This is Laplace's Law: Wall Tension = (Intraventrixular Pressure * Radius) / (Wall thickness * 2)
Laplace's law: The tension in the walls of a container is dependent on both the pressure of the container's contents and it's radius. law
Measuring Vascular Pressures at mid-axillary line, in fifth intercostal space. 1
| Flashcard | type:basic |
|---|---|
| What is the main factor that affects ventricular preload? | Venous Return to Heart |
| What are the main factors that affects Venous Return to Heart? | Intravascular Volume and Venous Tone |
| What are the two general principles to increase venous return to heart? | Increase intravascular volume. Increase venous constriction |
| What are the general principles to decrease venous return to heart? | Decrease intravascular volume. Decrease venous constriction. |
Main thing affecting preload is venous return.
Main thing affecting venous return is intravascular volume and venous tone.
So if you have low preload, causing decreased BP or CO. You can increase this preload with volume (or venous constriction) 1
If you have a high preload:
- Too much volume
- Impaired myocardial contractility
- Increased afterload
So you can reduce it by:
- Remove volume (diuretics/filtration)
- Increase vascular bed by reducing tone (GTN/Morphine)
- Improve contractility (Inotropes)
When Estimating Preload with Pressures:
- Remember if intrathoracic pressures are high, then values of intravascular pressures will be falsely high.
- Adjust for this by thinking about the transmural pressure (Vascular Pressure - Thoracic Pressure)
-
This is in cases like
- Gas Trapping
- High PEEP when ventilating
- Inverse I:E ratio when ventilating 1
-
Remember in diastolic dysfunction, you have a rubbish compliance of heart. So a certain pressure doesn't mean a certain volume of stretching. True preload is the stretch, it's the volume. So diastolic dysfunction again may over estimate preload. 1
| Flashcard | type:basic |
|---|---|
| What can diastolic dysfunction do to your estimates of preload? | Result in over estimation |
| What can a high intrathoracic pressure do to your measurements of intravascular pressures? | Falsely over estimate them |
| What is the name of a pressure where you take into account the intrathoracic pressures? | Transmural pressures |
Plus some additional questions from [[mcqs-and-sbas-in-intensive-care-medicine-oxford-higher-special-training]]
| Flashcard | type:basic |
|---|---|
| If intrathoracic pressure goes up, does transmural pressure across left ventricle go up or down? | Down |
| If intrathoracic pressure goes up, does venous return to heart go up or down? | Down |
| If intrathoracic pressure goes up, does left ventricular afterload go up or down? | Down |
| If intrathoracic pressure goes up, does right ventricular output go up or down? | Down |
| If intrathoracic pressure goes down, does transmural pressure across left ventricle go up or down? | up |
| If intrathoracic pressure goes down, does venous return to heart go up or down? | up |
| If intrathoracic pressure goes down, does left ventricular afterload go up or down? | up |
| If intrathoracic pressure goes down, does right ventricular output go up or down? | up |
Afterload
Afterload - Tension in ventricular wall needed to eject blood into arterial system in systole. This is largely determined by SVR.[^@cardiac_output]
SVR
$SVR = \frac{(MAP - CVP) * 80}{CO}$ 2
| Flashcard | type:basic |
|---|---|
| What actaully is ventricular afterload talking about? | The tension in ventricular wall that is needed to eject blood into arterial system |
| What is the formula to estimate systemic vascular resistance? | (MAP in mmHg - CVP in mmHG) * 80 / Cardiac Output in L/min = SVR |
| What units can we measure SVR in? | Dyn.sec/cm^5 OR Wood Units (mmHg.min/L) |
| How do you convert Dyn.sec/cm^5 into Wood Units? | Times Dyn units by 80 |
SVR units from above equation = $\frac{dyn*sec}{cm^{5}}$ 2
- MAP as mmHg
- CVP as mRAP (Preload) as mmHG
- CO = HR (L/beat) * HR (beat/min) as L/min 2
But really we want to look at SVR units as Wood Units = $\frac{mmHG*min}{L}$ 2
You get these Wood units by: $80\frac{dynsec}{cm^{5}}$ 2 1
SVRI
The SVRI (Systemic Vascular Resistance Index), is the SVR adjusted for Body Surface Area
So with an average body surface area as 1.65$m^2$ you times by 1.65
Physics behind SVR 2
This is the hydraulic version of Ohm's Law ($R = V/I$, Resistance = Voltage / Current) 2
| Flashcard | type:basic |
|---|---|
| What is the physics formula that you care about to get Vascular Resistance? | Ohm's Law (Resistance = Voltage / Current) |
| How can you adapt Ohm's Law for Systemic Vascular Resistance? | Resistance = Delta P / Q Resistance = Change in Pressure from Start to End of Loop (LVOT to RA) / Cardiac Output |
Flow = Pressure / Flow Rate 2
$R = \Delta P / Q$ 2
$R$ = Resistance 2
$\Delta P$ = Change in Pressure from Start to End of Loop = LVOT to RA 2
$Q$ = Flow = Cardiac Output 2
Pulmonary Vascular Resistance
Pulmonary Vascular Resistance is going to be the difference between the start and the end of the circuit. So that is RVOT pressure (Pulmonary Artery Pressure) - LA Pressure. Divide that by cardiac output. Remember cardiac output has to be the same both in left and right as it's a joined system..
$PVR = \frac{(PAP - LAP)*80}{CO}$
Units are still this $\frac{dyn*sec}{cm^{5}}$
So again you can times by 80 to get to Woods units
Venous Return
Main things affecting this are: 1
Vascular Tone
Main things affecting venous tone is:
- Autonomic Nervous System
- Circulating Catecholamines
- Local Factors
- pO2
- pCO2
- pH 1
Intravascular tone can act as a resevoir for circulation. It can stretch and contract, useful for dealing with volume loss. Compliance can go from 30mlL/mmHg to 300 ml/mmHg 1
| Flashcard | type:basic |
|---|---|
| What is the range of vascular compliance(stretch)? | 30ml/mmHg to 300 ml/mmHG |
The reason that you need this increase in tone in volume loss is else preload would drop, causing decreased cardiac output, and decreased global O2 delivery. 1
Vascular tone can increase a lot faster than it can decrease. If you refill someone too quickly, they may not relax fast enough (mainly controlled by sympathetic tone), and the LAP can rise to a point where you have back pressure on pulmonary vasculature and venous congestion. 1
Normal Figures
[[cardiac-function-normal-figures]]
[Acute Heart Failure]: