Phys 20 Cardiac Output, Venous Return, and Regulations
what is the normal intrapleural pressure?
-4mmHg
even very neg pressures in the right atrium cannot increase venous return significantly above that which exists at a normal atrial pressure of
0mmHg
when the thorax is surgically opened the values of all pressures inside the chest become
0mmHg, which is the atmospheric pressure *the increased pressure in the chest tends to collapse the atria and decrease the transmural pressure in each atrium
If Oscar is transfused with a large amount of blood, what are 3 reasons why his increased cardiac output only last for 10-40 mins ?
1. Increased capillary pressure causing transudation of fluid 2. Increased pressure in veins causing a stress-relaxation reflex 3. Autoregulation causes increased resistance which decreases vascular return.
MOA of nervous system for preventing this arterial pressure drop?
1. Large vein constriction (increase venous return) 2. Increased HR 3. Increased contractility All together increase BF to muscles while maintaining MAP
the sympathetic nervous system can be blocked by inducing total spinal anesthesia or by using a drug such as hexamethonium that blocks transmission of nerve signals through the autonomic ganglia. What are the effects ?
1. Psf falls to about 4 mmHg 2. effectiveness of the heart as a pump decreases to about 80% of normal
what are two general reasons for low cardiac output?
1. abnormalities that decrease pumping effectiveness of the heart 2. those that decrease venous return
what factors directly affect the cardiac output
1. basic level of body metabolism 2. whether the person is exercising 3. person's age 4. the size of the body
what are some factors that interfere with venous return also can lead To decreased cardiac output ?
1. decreased blood volume 2. acute venous dilation 3. obstruction of large veins 4. decreased tissue mass, especially decreased skeletal muscle mass 5. decreased metabolic rate of tissues
how does the combination of sympathetic and parasympathetic inhibition increase the pumping effectiveness of the heart:
1. greatly increases the heart rate 2. increases the strength and contractility to 2x normal
sympathetic stimulation affects the heart and systemic circulation how??? (2)
1. it makes the heart a stronger pump 2. in the systemic circulation, it increases the Psf because of the contraction of the peripheral vessels , especially the veins, and it increases the resistance to venous return
what are some examples of when the heart becomes severely damaged, its limited level of pumping may fall below that needed for adequate blood flow to tissues?
1. severe coronary blood vessel blockage and consequent myocardial infarction 2. severe valvular heart disease 3. myocarditis 4. cardiac tamponade 5. cardiac metabolic derangement
about what age is the cardiac index for a person the highest?
10 years of age (graph figure 20.1)
31. If a patient has an oxygen consumption of 240 ml/min, a pulmonary vein oxygen concentration of 180ml/L of blood, and a pulmonary artery oxygen concentration of 160 mlIL of blood units, what is the cardiac output? (A) 8L/min (B) l0 L/min (C) 12 L/min (D) l6 L/min (E) 20 L/min
C. 12 fick principle : CO= oxygen absorbed per minute by the lungs/ AV oxygen diff
If peripheral resistance increases, what happens to cardiac output? If Arterial Pressure increases, what happens to cardiac output?
CO = Arterial Pressure / Total Peripheral Resistance Increase PR: Decrease CO Increase AP: Increase CO
In using the oxygen Fick Method to measure CO, what is the equation for CO?
CO = O2 absorbed per minute by the lungs (ml/min) / Arteriovenous O2 difference (ml/L of blood)
Little Miss Muffet, after being scared by a spider had her sympathetic nervous system shoot impulses down to her heart and blood vessels. This increased her CO to 10 L/min and her peripheral resistance increased to 15. What is her arterial pressure?
CO = arterial pressure / total peripheral resistance Arterial Pressure = CO x total peripheral resistance AP = 10 x 15 AP = 150 mmHg
What is Cardiac Index? What is the average cardiac index (assuming a 70 kg person with a body surface area of 1.7 m^2)?
Cardiac Index = CO / Body Surface Area (m^2) Average: 3 L/min/m^2
The sum of all of the venous return to the heart equals what?
Cardiac Output
In these scenarios, which direction does the cardiac output curve shift: Cyclical Respiration? Breathing Against Negative Pressure? Positive Pressure Breathing? Opening Thoracic Cage? Cardiac Temponade?
Cyclical Respirations: Shift both ways +-2 mmHg (can be as much as -+50mmHg during strenuous breathing) Negative Pressure: Shifts to the left Positive Pressure: Shifts to the right Opening Thoracic Cage: Shifts to the right 4 mmHg Cardiac Temponade: Shift to the right
From the list below, indicate if the condition would increase or decrease CO? Why? MI? Anxiety? AV Shunt? (fistula) Shock? BeriBeri? Anemia? Pulmonary Disease? Decreased Tissue Mass (skeletal muscle)?
MI: Decrease, decrease contraction force Anxiety: Increase, increase sympathetics/glucocorticoids AV Shunt: Increase, decrease PR and increased BP in vein. Shock: Decrease, very low BP Beriberi: Increase, low thiamine increases vasodilation decreasing PR Anemia: Increase, low O2 to vasodilation to decreased PR, and decreased viscosity Pulmonary Disease: Increase, same as above. Muscle Mass Decrease: Decrease, decrease metabolic need for more blood (O2)
What is the average CO for a young, healthy man? young, healthy woman? What is the average CO for all adults (old, young, etc.)?
Man: 5.6 L/min Woman: 4.9 L/min Adults: 5 L/min
During exercise, Peripheral Vascular Resistance is decreased to supply adequate O2 and nutrients needed for muscle contraction (arterioles relax). This would normally decrease arterial pressure, but it DOESN'T. How is this prevented?
Nervous system compensation, this drop in pressure is prevented bc the same brain activity that sends motor signals to the muscles sends simultaneous signals into the autonomic nervous centers of the brain to excite circulatory activity , causing large vein constriction, increased HR and increased contractility of the heart
Flip to the other side before reading these answers! A: Equilibrium point B: AV fistula (shunt): Decrease in resistance leading to larger venous return leading to greater cardiac output C: 1 minute later: sympathetics reflexes returning BP to normal thus causing increased CO and increased Psf D: Several Weeks Later: Decreased BP caused decrease kidney output causing increased blood volume. Also the heart has hypertrophied giving a larger CO.
What does A indicate? What does B indicate? When does C occur? When does D occur?
what is beriberi?
disease caused by insufficient quantity of the vitamin thiamine (vitamin B1) ; lack of this vitamin causes diminished ability of the tissues to use some cellular nutrients and the local tissues blood flow control mechanisms in turn cause marked compensatory peripheral vasodilation
the greater the volume of blood in the circulation , the _____ (greater/ less) the mean circulatory filling pressure because extra blood volume stretches the walls of the vasculature
greater *at still higher volumes, the mean circulatory filling pressure increases almost linearly (see pic above)
what does a high Psf mean vs low Psf
higher the Psf (which also means the greater the "tightness" with which the circulatory system is filled with blood)= the more the venous return shifts upward and to the right lower Psf= curve shifts down and to the left Put in more simple terms - the greater the degree to which the system is filled, the easier it is for blood to flow into the heart. The lesser the degree to which the system is filled, the more difficult it is for the blood to flow into the heart
any factor that decreases the hearts ability to pump blood causes ____
hypoeffectivity (increased arterial pressure against which the heart must pump, such as severe HTN, inhibition of nervous excitation of the heart, pathological factors that cause abnormal heart rhythm or rate of heartbeat, coronary artery blockage causing a heart attack, valvular heart disease, congenital heart disease, myocarditis, cardiac hypoxia )
What are two ways one can increase the mean systemic filling pressure (Psf)?
increased volume increased sympathetic stimulation (constricts all systemic blood vessels and larger pulmonary blood vessels and even the chambers of the heart--> so capacity of the system decreases so that at each level of blood volume, the mean circulatory filling pressure is increased )
sympathetic stimulation affect on psf
it increases the Psf because of the contraction of the peripheral vessels , especially the veins, and it increases the resistance to venous return
how is the mean systemic filling pressure diff from mean circulatory filling pressure?
it is the pressure measured everywhere in the systemic circulation after blood flow has been stopped by clamping the large blood vessels at the heart so that the pressures in the systemic circulation can be measured independently of the pulmonary circulation **almost impossible to measure in a LIVE animal!! but almost always equal to mean circulatory filling pressure
without blood flow, the pressures everywhere in the circulation become equal (ex heart stopped with electricity)
mean circulatory filling pressure
how do you measure the degree of filling of systemic circulation?
mean systemic filling pressure *forces the systemic blood toward the heart (this is the pressure measured everywhere in the systemic circulation when all flow of blood is stopped)
cardiac output is regulated throughout life almost directly in proportion to overall _____
metabolic activity *therefore the declining cardiac index is indicative of declining activity and/ or declining muscle mass with age
what is the effect of hyperthyroidism on metabolism?
metabolism of most tissues of the body becomes greatly increased, oxygen usage increases and vasodilator products are released from tissues --> TPR decreases and VR and CO increase to 40-80%
how does acute venous dilation occur?
most often when the sympathetic nervous suddenly becomes inactive ex. fainting (blood pools in vessels and does not return to the heart as rapidly as normal)
under most normal unstressed conditions, the cardiac output is controlled mainly by what?
peripheral factors that determine venous return limitation: if the returning blood does become the limiting factor that determines the cardiac output
why is the mean systemic filling pressure almost nearly equal to mean circulatory filling pressure?
pulmonary circulation has less than 1/8th as much capacitance as the systemic circulation and only about 1/10 as much blood volume
venous return is defined as
quantity of blood flowing from the veins into the right atrium each minute
how does decreased skeletal tissue muscle mass affect CO?
reduction in muscle mass decreases the total oxygen consumption and blood flow needs of the muscles, resulting in decreases in skeletal muscles blood flow and CO
a rise in intrapleural pressure to -2mmHg shifts the cardiac output curve to the right or left?
right shift bc filling the cardiac chambers with blood requires an extra 2mmHg of right atrial pressure to overcome the increased pressure on the outside of the heart
why does stretching the heart cause an increased heart rate?
stretch of the sinus node in the wall of the right atrium has a direct effect on the rhythmicity of the node to increase the HR as much as 10-15%
what does a normal venous return curve show?
that when the heart pumping capability becomes diminished and causes the right atrial pressure on the veins of the systemic circulation decreases venous return of blood to the heart
what is the main reason why peripheral factors are usually so important in controlling cardiac output?
the heart has a built in mechanism that normally allows it to pump automatically the amount of blood that flows from the veins into the right atrium
Why does the venous return curve plateau when the right atrial pressure reaching -2 mmHg or lower?
the plateau is caused by a collapse of veins entering the chest, inhibiting any more flow of venous return. neg pressure in the right atrium sucks the walls of the veins together where they enter the chest, which prevents any additional flow of blood from the peripheral veins
cardiac out put Is defined as
the quantity of blood pumped into the aorta each minute by the heart
what happens when the right atrial pressure rises to equal the Psf?
there is no longer any pressure difference between the peripheral vessels and right atrium *there can no longer be any blood flow from peripheral to the right atrium
mixed venous blood is usually obtained how
through a catheter inserted up the brachial vein of the forearm through subclavian vein down to right atrium finally into RV or Pulmonary artery
how does decreased metabolic rate affect CO?
tissue metabolic rate decreases, (like bed rest) the oxygen consumption and nutrition needs of the tissues will also be lower which decreases blood flow to the tissues resulting in reduced cardiac output
the sympathetic nervous system can be blocked by 2 main ways
total spinal anesthesia using drugs such as HEXAMTHONIUM (blocks transmission of nerve signals through the autonomic ganglia)
what is the diff in resistance for arteries and veins?
venous pressures rise very little because the veins are highly distensible when arteriolar and smalll artery resistances increase, the blood accumulates in the arteries bc they have a capacitance 1/13th as great as that of the veins
immediately after opening a large fistula why does venous return and CO increase
venous return=large decrease in resistance to venous return bc blood is allowed to flow with almost no impediment directly from the large arteries into the venous system bypassing the resistant elements CO= opening of fistula decreases the peripheral resistance and allows an acute fall in arterial pressure against which the heart can pump more easily Net result = increases CO to 13L/min and Right atrial pressure P to +3mmHG
at each increasing level of work output exercise, do oxygen consumption and cardiac output increase in parallel?
yes!!
should the cardiac out put and venous return equal each other?
yes, they should equal each other except for a few heart beats when blood is temporarily stored in or removed from the heart and lungs
what is the normal Psf?
about 7mmHg
whaat two locations can be used to measure CO with a flowmeter?
aorta or pulmonary artery
Under many conditions, the long term cardiac output level varies reciprocally with changes in total peripheral vascular resistance as long as _____ is unchanged
arterial pressure ** when the total peripheral resistance increases above normal, the CO falls; conversely when the total peripheral resistance decreases, the CO increases
via the indicator dilution method - dye injected into large systemic vein or into r atrium- passes through the heart and lungs in a normal adult in ___
average concentration of 0.25mg/dL of blood and duration average was 12 seconds
what is an AV shunt?
b/w major artery and major vein ; large amounts of blood flow directly from the artery into the vein *decreases TPR and increases VR and CO
what happens when the right atrial pressure falls progressively lower than the Psf?
blood flow to the heart increases proportionately *the greater the diff between the Psf and right atrial pressure the greater the venous return
if the cardiac output ever falls below the level required for adequate nutrition of the tissues, the person is said to experience ____ ____
circulatory shock
What is the major determining factor of the declining cardiac index with age?
declining metabolism (activity or muscle mass)
what is the most common non cardiac peripheral factor that leads to decreased cardiac output?
decreased blood volume, often from hemorrhage loss of blood = decrease the filling of the vascular system = low level of not enough blood in periphery to create peripheral vascular pressures high enough to push the blood back to the heart
any factor that _____(decreases/ increases) TPR chronically increases CO if arterial pressure does not fall too much
decreases **autonomics control arterial pressure to try to maintain around 100 mmHg
if all nervous circulatory reflexes are prevented from acting, venous return decreases to zero when the right atrial pressure rises to about _____
+7mmHg
Here's another number Q: What is the average resistance to venous return in a healthy adult? A. 1.2 mmHg/L/min B. 1.4 mmHg/L/min C. 12 mmHg/L/min D. 14 mmHg/L/min
B. 1.4mmHg/L/min
Conditions that decrease peripheral resistance and increase CO with MAP not decreasing too much?
1. Beriberi 2. Arteriovenous fistula (shunt) 3. Hyperthyroidism 4. Anemia
What are two factors that cause a heart to be a hypereffective heart? What are a few factors that cause the heart to be a hypoeffective heart?
1. Hypereffective: Nervous Stimulation, 2. Hypertrophy (~2.5x average CO) Hypoeffective: Increased arterial pressure, inhibition of nervous excitation, pathological factors, coronary artery blockage, valvular heart disease, congenital heart disease, myocarditis, cardiac hypoxia
blood oxygen concentration entering the right heart has an oxygen concentration of ____ and blood oxygen concentration leaving the left heart has an oxygen concentration of ____
160mL 200mL *every liter of blood passing through the lungs absorbs 40mL of oxygen , so 5 mL of blood passes through the pulmonary circulation each min
immediately upon infusion of blood the increased filling of the system causes the Psf to increase to about
16mmHg which shifts the venous return curve to the right
what is the effects of anemia on CO?
2 peripheral effects greatly decrease total peripheral resistance 1. reduced viscosity of the blood , resulting from the decreased viscosity of the blood, resulting from the decreased concentration of RBCs 2. diminished delivery of oxygen to the tissues=> local vasodilation overal CO greatly increases
a sudden increase in blood volume of about 20% increases the cardiac output to about __ to ___ normal
2.5 to 3 x normal
How much more effective can the heart become under normal conditions before it becomes the limiting factor in blood flow?
2.5x greater (13 L/min) A normal heart can pump a venous return up to 2.5 x normal venous return without any stimulation
according to the Fick principle, ___ml of Oxygen are being absorbed from the lungs into he pulmonary blood each minute
200ml
at normal blood volume, max sympathetic stimulation increases the mean circulatory filling pressure from 7mmHg to about _____
2x that value or 14 mmHg
slight accumulation of blood in the arteries raises the pressure greatly ___X as much as in the veins
30x
*every liter of blood passing through the lungs absorbs __mL of oxygen , so 5 mL of blood passes through the pulmonary circulation each min
40mL
completely inhibition of the sympathetic nervous system relaxes both the blood vessels and heart , decreasing the mean circulatory filling pressure from the normal value of 7mmHg down to
4mmHg
a long term increased workload, but not so much excess load that it damages the heart, causes the heart muscle to increase in mass and contractile strength in the same way that heavy exercise causes skeletal muscles to hypertrophy. For example, the hearts of marathon runners have an increase heart mass of ___ to ___ %
50-75%
*every liter of blood passing through the lungs absorbs 40mL of oxygen , so ___mL of blood passes through the pulmonary circulation each min
5mL (which is also a measure of CO) 200/40
an increase in intrapleural pressure of +2mmHg requires a ____mmHg increase in right atrial pressure form the normal -4mmHg
6mmHg shifts the entire cardiac output curve 6mmHg to the right
what's impedance cardiomyography
AKA thoracic electrical bioimpedance non invasive technology used to measure changes in total electrical conductivity of the thorax as an indirect assessment of hemodynamic parameters such as cardiac output *detects the impedance changes caused by a high frequency low mag current flowing through the thorax between additional two pairs of electrodes located outside the measured segment
If the thorax of a normal, healthy patient is urgically opened, what happens to the cardiac output curve? (A) It shifts 2 mmHg to the left (B) It shif ts 4 mmHg to the left (C) It shifts 2 mmHg to the light (D) It shifts 4 mmHg to the right (E) It does not shift
D. the right atrial transmural pressure gradient decreases about 4mmHg so CO shifts 4 mmHg to the right
In most tissues, blood flow increases mainly in proportion to what?
Each tissue's metabolism
What is the most common way to estimate CO with a machine? What are two other ways to measure CO?
Estimate: Echocardiography Measure: Oxygen Fick method Indicator Dilution method
Maintenance of normal arterial pressures by nervous reflexes is essential to achieve high or low CO when peripheral tissues dilate their vessels to increase venous return with no change in arterial pressure?
High **if autonomic control of the nerve system was blocked, vasodilation of the blood vessels caused a profound decrease in arterial pressure
Flip to the other side before reading these answers Blue: Hypereffective heart + Increased Intrapleural Pressure Green: Normal + Normal Yellow: Hypoeffective heart + Reduced Intrapleural Pressure
In this cardiac output curve vs. right atrial pressure chart indicate what each color means. Blue Green Yellow
What is the highest the right atrial pressure can rise?
Psf (mean systemic filling pressure)
Oliver was injected with total spinal anesthesia how does this affect his Psf? effectiveness of the heart as a pump? CO?
Psf= falls to a bout 4 mmHG effectiveness of heart as a pump to 80% below normal CO decreases to about 60% below normal
what is the value of the Right atrial pressure and cardiac output at equilibrium point
RAP=0mmHg CO=5L/min
So, during exercise, nervous system actually provides additional signals to ______ arterial pressure above normal as means for compensating the increase in CO?
Raise
What are the three factors determining venous return?
Right Atrial Pressure (backward force on the veins to impede flow of blood form the veins into the R atrium) Mean Systemic Filling Pressure Resistance to Blood Flow
The pressure in the right atrium is equal to what factor? What is this pressure normally?
Right Atrium Pressure = intrapleural pressure Pressure = -4 mmHg
CO =
SV x HR
What is the mechanism of the Bainbridge Reflex?
The stretched right atrium initiates a nervous reflex to the vasomotor centers of the heart to increase the HR sympathetically.
What is the equation for venous return?
VR = (Psf - PRA) / RVR VR = venous return Psf = mean systemic filling pressure PRA = Pressure in right atrium RVR = Resistance to Venous Return
Nicole, after arguing about how many chapters should be assigned, finds that her mean systemic filling pressure is 14 mmHg, her right atrial pressure is 0 mmHg and and resistance to venous return equals 2 mmHg. What is her venous return?
VR = (Psf - PRA) / RVR VR = venous return Psf = mean systemic filling pressure PRA = Pressure in right atrium RVR = Resistance to Venous Return VR = (14-0)/2 = 7 L/min
What is circulatory shock?
When cardiac output falls below the level required for adequate nutrition of the tissues.
What is mean circulatory filling pressure? What is the normal mean systemic filling pressure (Psf)?
When heart pumping is stopped by shocking the heart with electricity to cause ventricular fibrillation or is stopped in any other way, flow of blood everywhere in the circulation ceases a few seconds later. the pressure at which the arterial and venous pressures reach equilibrium if all flow in the systemic circulation ceased. Average: 7 mmHg
What is the mechanism of the Frank-Starling Law of the Heart?
When increased blood flows into the heart, it stretches the the chamber walls, causing an increased contraction of the cardiac muscles.
