Cardiac Output & Venous Return Flashcards
True or False:
The cardiovascular system is a closed hydraulic circuit that includes the heart, arteries, arterioles, capillaries, and veins.
True
What is cardiac output?
The total amount of blood ejected into the aorta from the left ventricle per minute.
Can also be described as the quantity of blood that flows through the circulation.
What is the equation for cardiac output?
CO = Heart Beat Rate x Stroke Volume
What is stroke volume?
Stroke volume is the volume of blood pumped by the ventricle each heartbeat.
What is the average stroke volume in a normal adult male?
5 L/minute
What is venous return?
The amount of blood flowing back to the atria each minute.
Also referred to as the “preload”
According to the Frank Starling mechanism, cardiac output and _______ must be equal.
Cardiac output & venous return must be equal.
True or False:
More preload = More cardiac output?
True
What is systemic vascular resistance (SVR)?
SVR is the total amount of resistance in the blood vessels throughout the body.
It is the same as the total peripheral resistance (TPR).
What is central venous pressure (CVP)?
Central venous pressure is an estimate of right trial pressure (RTP; the right atrium is the one that receives the blood upon venous return).
What is mean arterial pressure (MAP)?
Mean Arterial Pressure is the average pressure of the blood in your arteries during a cardiac cycle.
It is a better indicator of how well vital organs are being perfused than than referring to the systolic blood pressure (SBP)
What is the equation for Mean Arterial Pressure (MAP)?
MAP = (CO x SVR) + CVP OR MAP = CO x SVR
Where,
CO = Cardiac output
SVR = Systemic vascular resistance (aka total peripheral resistance)
CVP = Central venous pressure (aka right atrial pressure)
What is afterload?
The resistance that is met when blood is being pumped out of the heart through the aorta.
It is the load that the heart has to work against to pump blood out to the periphery.
The best marker for afterload is the Total Peripheral Resistance (TPR) or the Systemic Vascular Resistance (SVR)
Cardiac output (CO) & Contractility have a __________ relationship.
Direct relationship.
An increase in contractility = an increase in ventricular output/cardiac output (CO)
Cardiac output (CO) & Afterload have a __________ relationship.
Inverse relationship.
An increase in the afterload (total peripheral or systemic vascular resistance) = a decrease in ventricular output/cardiac output (CO).
Cardiac output (CO) & Preload have a __________ relationship.
Direct relationship.
An increase in preload (venous return, which should be equal to cardiac output) = an increase in ventricular output/cardiac output (CO).
What are the invasive methods of measuring cardiac output?
Fick Method - indirectly calculates the cardiac output by measuring oxygen consumption; based on law of conservation of mass (amount of oxygen delivered to body must equal oxygen consumed
Thermodilution Method via the Swan-Ganz Catheter - a cold solution is inserted into the heart & is observed for warming up over time. High functioning heart w/ high cardiac output = solution warms more rapidly.
What are the non-invasive methods of measuring cardiac output?
Echocardiography -
Lithium Dilution -
Esophageal Doppler -
True or False:
The degree of tension developed by cardiac muscle depends upon both ‘preload’ and ‘afterload’.
The heart is able to generate more pressure when more blood is ‘presented’ to it.
True
Preload; which is a measure of left ventricular end-diastolic pressure (LVEDP) & correlates to left ventricular end-diastolic volume(LVEDV); directly correlates with what measure?
Preload directly correlates with Stroke volume.
Stroke volume = EDV - ESV
Preload directly correlates with EDV, and thusstroke volume as well.
So if preload increases, EDV increases, and Stroke volume increases.
If preload decreases, EDV decreases, and stroke volume decreases.
In a cardiac function curve, if contractility is unchanged, then end-systolic volume (ESV) will remain unchanged as well, allowing for the direct relationship between preload & stroke volume to be true.
What factors can alter the cardiac function curve?
What factors determine venous return? What is the equation for venous return?
Venous Return is determined by the pressure gradient between the venous pressure (Pv) and right atrial pressure (PRA) & resistance to venous return (RV).
The equations for venous return (VR) is:
VR= (Pv - PRA) / Rv
What is the driving force for venous return?
The pressure gradient between central venous pressure and the right atrial pressure is the driving force for venous return.
What four major factors can influence venous return?
Transient changes in venous return can occur in response toseveral factors:
Muscle contraction. Rhythmical contraction of limb muscles occurringduring normal locomotory activity (walking, running, swimming) promotes venous return by the muscle pump mechanism.
Decreased venous compliance.Sympathetic activation of veins decreases venous compliance, increases central venous pressureand promotes venous return indirectly by augmenting cardiac output through the Frank-Starling mechanism, which increases the total blood flow through the circulatory system.
Respiratory activity. Duringrespiratory inspiration, the venous return transiently increases because of a decrease in right atrial pressure. The opposite occurs duringexpiration.
Vena cava compression. An increase in the resistance of the vena cava, as occurs when the thoracic vena cava becomes compressed during a Valsalva maneuveror during late pregnancy, decreases venous return.
True or False:
Venous return is a major determinant of cardiac output: filling pressure of the heart.
True.
What point on a venous return graph represents the mean systemic filling pressure
The mean systemic pressure is the point on the vascular function curve where the venous return is zero.
True or False:
Right atrial pressure (RAP) ~ central venous pressure (CVP)
True
What affect would increasing or decreasing blood volume or venous compliance have on the vascular function curve?
If, for example, blood volume is increased due to renal retention of sodium and water, or venous compliance is decreased due to sympathetic activation of the veins, there is a parallel shift to the right in the vascular function curve, which leads to an increase in the MSFPwhen the heart is stopped.
The opposite shift occurs with decreased blood volume or increased venous compliance.
What affect would increasing the systemic vascular resistance (SVR; aka “afterload”) have on the vascular function curve?
If SVR is increased by administering an arterial vasoconstrictor drug, the slope of the systemic vascular function curve decreases, but there is little or no change in the MSFP.
What affect would increasing the systemic vascular resistance (SVR; aka “afterload”) have on the vascular function curve?
If SVR is increased by administering an arterial vasoconstrictor drug, the slope of the systemic vascular function curve decreases, but there is little or no change in the MSFP.
What affect would decreasing the systemic vascular resistance (SVR; aka “afterload”) have on the vascular function curve?
The opposite occurs with a decrease in SVR. The MSFPdoes not change appreciably with arterial constriction or dilation because arterial diameter changes required to change resistance causes only a small change in total vascular compliance.
How would the vascular function curve be altered if there was resistance/ constriction in both the arteries & the veins?
Ifboth arteries and veins are constricted during sympathetic activation, then the curve will shift to the right as shown in Panel C (increased MSFPdue to decreased CV) and the slope will decrease due to the increase in SVR.
How can steady state cardiac output/venous return be determined using vascular function curve & cardiac output graphs?
The point at which the curves intersect is the equilibrium point of the system in the steady state. In the steady state, cardiac output and venous return are equal (blue circle) at the point of intersection. Combining the vascular function curve with the cardiac function curve allows us to predict changes in various cardiovascular parameters on cardiac output, venous return and RAP. The intersection will determine a new steady state.
How will steady state cardiac output/venous return be altered in response to an increase or decrease in blood volume?
An increase in blood volume increases the stressed volume and thus an increase in mean systemic pressure. Without a change in total peripheral resistance, the curve shifts in a parallel manner. Increased blood volume shifts the curve to the right – cardiac output is increased and RAP is increased. A decrease in blood volume shifts the curve to the left – there is a corresponding decrease in CO and RAP.
How will steady state cardiac output/venous return be altered in response to positive or negative inotropic agents?
Positive inotropic agent = increases contractility = increase in cardiac output = increase in stroke volume
Negative inotropic agents have the opposite affect.
Contractility changes will only affect the cardiac output curve/graph; it will have no affect on vascular function curve
How will steady state cardiac output/venous return be altered in response to an increase or decrease in total peripheral resistance (TPR)?
Increased TPR = shifts both graphs & steady state down
Decreased TPR = shifts both graphs & steady state up
What is Myocardial Ischemia?
