5 - Measurement of Cardiac Output Flashcards
What is cardiac output?
The cardiac output (in volume blood per unit time, L/min) is the blood flow required to match the metabolic demands of the tissue, especially for oxygen delivery and is equal to the stroke volume multiplied by the heart rate
How can you determine cardiac output?
- the sum total of the blood flows to individual tissues OR…
- venous return
What is the equation for cardiac output?
CO = SV x HR
CO = cardiac output SV = stroke volume HR = heart rate
What factors affect cardiac output?
Only factors which influence stroke volume or heart rate
What factors influence heart rate?
- Parasympathetic and sympathetic stimulation
- Alterations in rate of diastolic depolarization
- Alterations in the slope of phase 4 in the SA nodal cell action potential
What does sympathetic stimulation do to the heart rate?
Increase conduction velocity through the AV ode by increasing the slope of phase 4
Faster heart rate
What does parasympathetic stimulation do to the heart rate?
Decrease conduction velocity through the AV by decreasing the slope of phase 4
Slower heart rate
What factors regulate stroke volume?
- Inotropic state of myocytes
- Contraction and relaxation of myocytes
- Dimensions of the ventricles
- Structure of the ventricles
How do we determine stroke volume?
We look at preload and afterload
What is preload?
Diastolic load, that is, the load on the heart at the end of filling
The filling of the ventricle causes stretch, increasing the length of the sarcomere (contractile unit)
What is afterload?
Systolic load
The amount of work it takes to move the contents of the ventricles into the aorta and beyond
The afterload is related to the contractile EFFICIENCY… That is the force generated and the rate at which it is generated to deal with the preload
How would you theoretically determine the preload?
If we measure a single muscle fiber and add a weight, the muscle fiber will stretch corresponding to a particular tension… This is our preload
What does the Frank-Starling Law of the Heart describe?
The Frank-Starling Law of the Heart denotes that the work done by the heart is a function of the length of the muscle fiber.
The amount of work done by the heart is directly proportional to end diastolic volume
Force of ventricular contraction is a function of ventricular end-diastolic muscle length - Related to Ca++ sensitivity, within limits
Why would you measure the end diastolic volume?
End diastolic volume = the amount of blood back in the heart after filling
The more volume that returns to the heart, the more that can be expelled
If there is more end diastolic volume, the heart must generate a higher stroke volume to accommodate this larger volume
Describe the relationship between end diastolic volume and stroke volume
Fairly linear… If the end diastolic volume goes up, the stroke volume goes up
This is the case until the end diastolic volume gets very large and the heart cannot keep up
How does this linear relationship work? How does the heart know to increase stroke volume with increased end diastolic volume?
Changes in the end diastolic volume generate changes in the diastolic pressure because of the increase in tension
The increased tension is felt by the elastic components of the contractile apparatus, especially titin
What is meant by venous return?
The RATE of volume returning to the heart
What is the speed at which the blood returns to the heart dependent upon?
The pressure within the peripheral venous compartment and the pressure within the central venous compartement (superior and inferior vena cava and right atrium)
The difference in these pressures creates a “driving force” for the return
What is the effect of increased pressure within the central venous compartment?
If central venous pressure (or right atrial pressure) increases only a few mmHg, venous return will greatly diminish (the driving force will decrease)
When would the venous return cease?
When the central venous pressure equals the peripheral venous pressure
This point is called the MSFP - mean systolic filling pressure
What factors influence the MSFP (mean systolic filling pressure)?
Blood VOLUME
What influences venous return?
- Volume of blood
- Arteriolar tone
How does blood volume affect venous return?
more blood will require a larger rate of return than a smaller
Measurement of Cardiac Output Page 5
volume
How does arteriolar tone affect venous return?
increased arteriolar tone- or more vasoconstriction- will “hold onto” the volume and slow the rate of return; conversely, arteriolar dilation will allow more blood to flow into the venous circulation faster, thus increasing the rate it will come back to the heart
Does arteriolar tone alone change the MSFP or mean systolic filling pressure?
NO
Arteriolar tone usually does not change alone, meaning that it would usually change along with venous tone
Does venous tone affect the amount of blood returning to the heart?
Yes because the venous circulation serves a “storage” function.
Describe the generation of tension in the heart muscle
- The preload stretches the muscle fiber (left ventricle)
- Muscle contracts and generates tension in the muscle fiber
- The peak tension generated by the contraction is called the ACTIVE TENSION
Describe the end systolic volume of a health heart
really efficient heart can eject more volume and reduce end systolic volume
Describe the end systolic volume of a failing heart
A failing heart cannot efficiently remove as much of the starting (end diastolic) volume and therefore more is “leftover” in the heart after contraction- a larger end systolic volume
What is intropy?
Intrinsic contractile state of the heart
What is intropy influenced by?
Extrinsic factors
- i.e. sympathetics
Intrinsic factors
- i.e. stiffness of the ventricles
What is the effect of changing the contractility of the heart?
A INCREASED stroke volume
This means that muscle efficiency increases the amount of blood propelled into the peripheral circulation
When would you want to change the contractility of the heart?
When there is a need to increase blood (oxygen, nutrient) delivery when the body senses it is needed most (e.g. when the sympathetic nervous system (“fight-or-flight” response) is activated.)
What are Frank-Starling curves? What are Sarnoff curves?
the relationship between stroke volume and contractility is very complex- there are an infinite number of Frank-Starling curves for the heart, demonstrated experimentally by Dr. Sarnoff (Sarnoff curves.)
How does a change in heart rate affect cardiac output?
Changing heart rate will change cardiac output
- Higher heart rate, higher cardiac output
- Lower heart rate, lower cardiac output
How does this relationship play out in real life?
Heart rate is never exclusively changed… The stroke volume is also affected by an increase or decrease in heart rate
Give an example of how cardiac output stays constant during increases or decreases in heart rate
Dramatic changes in heart rate have little overall effect on cardiac output; if the heart rate increases, the reduction in stroke volume (the heart doesn’t have enough time to pump out the blood) will offset to buffer the change in CO.
Conversely, if HR decreases, the heart has longer time to fill and thus a greater EDV will increase SV.
How do vagal efferents affect heart rate and therefore cardiac output?
- Vagal efferents activate muscarinic receptors on nodal cells
- This reduces the slope of diastolic depolarization
- This increases the time between action potentials
- This decreases the heart rate and therefore the cardiac output
How does the activation of beta adrenergic receptors affect cardiac output?
- Activate beta adrenergic receptors on the nodal tissue
- This increases the slope of diastolic depolarization
- This decreases the time between action potentials
- This increases the heart rate and therefore increases cardiac output
Summarize the effect of muscarinic activation and adrenergic activation on cardiac output
Muscarinic activation = decreased HR and CO
Adrenergic activation = increased HR and CO
What is mass balance? What is it used to describe?
Mass balance (or conservation of mass) would describe that from adding a known quantity of something (dye) into an unknown volume of liquid, one can calculate the volume from the volume from the final concentration of the dye in the liquid
This principle is used to measure cardiac output
Describe the reasoning behind the Fick method of determining cardiac output
- The “material” used was oxygen
- The rate of oxygen consumption by an organ (or person) is equal to the rate at which the oxygen comes to the organ (person) multiplied by the amount of oxygen in the tissue (estimated by the difference of what came in arterially and went out venously)
How did Fick use this rationale to test for cardiac output?
- We can measure the oxygen consumption by having our patient inhale and exhale into a spirometer
- We can sample the amount of oxygen exiting the lungs via the pulmonary vein (or MUCH easier in an arterial blood sample) and the amount of oxygen entering the lungs via a sampling of the right ventricle/ pulmonary artery
Then we use this equation…
Cardiac output (L/min) = (oxygen consumption [mL/min])/ (arterial-venous oxygen difference [mL/L])
Describe the indicator dilution technique of determining cardiac output
- Injection of a known amount of indicator into the circulation (usually into the R ventricle, pulmonary artery or L ventricle)
- A downstream blood sample is then taken
- The timing and amount of indicator is used to calculate cardiac output
Describe the thermal dilution technique
A common variation of the indicator dilution technique
- Inject a known volume of ice-cold saline into the right ventricle
- A temperature sensor is placed in the pulmonary artery
- The solution reduces the blood temperature
- The amount of temperature change is related to the volume of blood the cold saline mixes with
- The cardiac output can then be calculated based on timing and temperature
What is the last method that can be used to approximate cardiac output?
Electrocardiogram
How does an ECG determine cardiac output?
- The ultrasound image can be quantified to assess chamber dimensions (during systole and diastole) and Doppler shift can assess the blood flow velocity through the semilunar valves
- output is related to the conversion of blood velocity to stroke volume and heart rate
Any other methods?
- Impedence cardiography
- Pulse contour analysis
- Flow probe placed in aorta (labs)
