Cardiovascular Response to Stress Flashcards
What is stroke volume?
The volume of blood that leads the heart with each beat
What factor determines the value for stroke volume?
The force of contraction of the heart
What is heart rate?
The number of heart beats per minute
What is the equation for cardiac output?
Cardiac output = heart rate x stroke volume
What is normal cardiac output and how is this reflected in the CO equation?
Heart rate = 72bpm
Stroke volume = 0.07L
72 x 0.07 = 5 L/min
Cardiac output is 5 L/min
What is total peripheral resistance?
The resistance to blood flow offered by the systemic circulation
What will increase total peripheral resistance?
narrowing the lumen of blood vessels through constriction of arteries
What is the equation for blood pressure?
Mean systemic arterial (blood) pressure =
cardiac output x TPR
What is on the axes of the Frank-Starling curve?
x axis - preload
y-axis - stroke volume
How does stroke volume change as preload increases initially?
As preload increases, stroke volume increases up until a certain point
What is meant by stroke volume increasing as preload increases?
The more blood that enters the heart, the more it stretches
More blood will be ejected when the heart recoils
When does stroke volume begin to decline as preload increases?
If the heart is stretched too much, the stroke volume begins to decline
What is preload best measured as?
How is it measured?
Preload is best measured as end-diastolic volume
It is measured using an echocardiogram
What is end diastolic volume?
The volume of blood in the ventricle before systole
How can central venous pressure be increased?
How does this affect preload?
Increasing the circulating volume of blood increases central venous pressure
This increases preload
Why will drinking large volumes of water increase preload?
Extra fluid enters the venous system
This increases central venous pressure
What other mechanism can be used to increase central venous pressure?
Breathing more quickly
This is because more blood is moving into the heart, increasing the preload
What other factor, involving the veins, will increase preload?
Decreased venous compliance
How will a decreased heart rate affect preload and why?
It will increase preload
Diastole is prolonged, giving longer for the ventricle to fill up before systole
How does aortic and pulmonary pressure influence preload?
Preload is increased with increased aortic or pulmonary pressure
Why will increased ventricular compliance increase preload?
If the ventricles are stiff, more energy is needed to expand them
Less blood will enter the ventricles
On the Bowditch effect graph, what are the axes labels?
y - axis - cardiac performance
x-axis - heart rate
How are the Bowditch effect and Frank-Starling graphs related?
They have the same shape
What is the basis behind the Bowditch effect?
An increase in heart rate increases the force of contraction up until a point
When the heart rate is too high, this eventually leads to a decreased force of contraction
Why is the Bowditch effect independent of the Frank Starling curve?
The length of the muscle is not affected
How does increasing the heart rate affect stroke volume?
Increasing the heart rate directly increases stroke volume
This increases cardiac output, and eventually blood pressure
How is the Bainbridge reflex initiated?
- Increased venous return
- Baroreceptors in the atria detect the increased stretch
- Heart rate is INCREASED via sympathetic stimulation to the SA node
How does the Bainbridge reflex compare to the carotid baroreceptor response?
They are antagonistic
The carotid baroreceptors detect an increase in blood pressure and cause the heart rate to DECREASE
How is the Bainbridge reflex involved in sinus arrhythmia?
During inspiration, the heart rate increases
The pressure in the thorax decreases, so more blood can enter the heart
How does the sympathetic nervous system influence the SA node?
Stimulation leads to positive chronotropy
What does chronotropy describe?
It describes how fast the heart is going
How do positive chronotropic drugs work?
They increase the heart rate through action on the SA node
This increases heart rate, cardiac output and blood pressure
How does the sympathetic nervous system influence the AV node?
Stimulation leads to positive dromotropy
It opens the gate in the AV node to allow more signals to flow through
What does dromotropy describe?
The ability of electrical signals to be conducted through the heart properly
How does the sympathetic nervous system influence the atria and ventricles?
Stimulation leads to positive inotropy and positive lusitropy
What is meant by positive inotropy?
An increase in the force of contraction of the ventricles
What is meant by positive lusitropy?
It allows for relaxation of the atria to let the ventricles fill with blood
What shape does the inotropy (x) vs. cardiac performance (y) graph follow?
The same shape as the Frank-Starling and the Bainbridge effect graphs
What is shown by the inotropy graph?
The greater the force of contraction (inotropy), the greater the cardiac output, until a certain point
Which patients are given dobutamine?
What does it do?
Patients with reduced cardiac output
It activates the sympathetic NS to increase inotropy, and consequently cardiac output
Why is dobutamine described as a ‘positive inotrope’?
It increases the force of contraction, allowing more blood to leave the heart
What could replace ‘inotropy’ on the x-axis of the graph, and still produce the same shape?
either preload or chronotropy (heart rate)
What are the systemic actions of the sympathetic nervous system on the heart?
- activation of the RAAS
2. suprarenal stimulation leads to release of catecholamines from the adrenal gland
Where is angiotensinogen produced?
In the liver
Where is renin produced?
What does it do?
It is produced in the kidney
It converts angiotensinogen into angiotensin I
What will convert angiotensin I into angiotensin II?
Angiotensin converting enzyme (ACE)
This is found in the lungs
What effects does angiotensin II have on the adrenal glands?
It causes them to produce aldosterone
Aldosterone leads to increased Na+ and H2O retention
What hormone will angiotensin II induce the production of?
What is the function of this hormone?
Vasopressin fro the pituitary gland
It causes the contraction of blood vessel walls
What is the overall effect of the renin-angiotensin-aldosterone system?
It causes an increase in blood pressure through vasoconstriction of blood vessels and increased Na+ and H2O retention
What is the role of catecholamines on the heart?
Adrenaline has direct effects on the heart to increase heart rate and stroke volume
How will adrenaline affect the kidneys?
It causes renin release from the kidneys
How does adrenaline affect the blood vessels?
It is a potent vasoconstrictor
How does innervation differ in the parasympathetic NS, compared to the sympathetic?
How does its effects differ?
There is no innervation to the kidney or adrenal gland
It has no effect on ventricular contractility or the RAAS
What are the effects of the parasympathetic nervous system on the heart?
- negative chronotropy - decreased heart rate
- negative inotropy - reduced atrial contractility
- negative dromotropy - decreased AV conduction
How is the autonomic nervous system regulated locally or otherwise?
It is regulated through baroreceptors
It is regulated locally or through ‘higher’ centres in the brain
Where are baroreceptors located?
In the carotid sinuses and the aortic arch
How does arterial pressure affect the baroreceptors?
Decreased arterial pressure leads to decreased firing
Increased arterial pressure leads to increased firing
What is the result of increased baroreceptor firing?
Increased baroreceptor firing will decrease sympathetic tone and increase parasympathetic tone
This will lower the blood pressure
What is the initial effect of losing large amounts of blood?
Loss of blood decreases central venous pressure
This decreases preload
What does a reduction in preload through blood loss lead to?
Decreased stroke volume
What does a decreased stroke volume, due to blood loss, lead to?
Decreased cardiac output
This leads to a lower blood pressure
What is the response of the body to blood loss and how is it brought about?
- Fall in blood pressure leads to less firing of baroreceptors
- increase in sympathetic tone to try and increase blood pressure
- compensatory effects increase heart rate and contractility
What else will increased sympathetic tone lead to to try and increase blood pressure?
Increase in venous tone
Increase in systemic vascular resistance
How is the RAAS involved in increasing blood pressure after blood loss?
- increased renin production by kidney
- volume of blood is increased through salt and water retention
- increase in blood pressure
How are other organs in the RAAS affected by blood loss?
- increased angiotensin II production
leads to increased vasopressin and aldosterone production
- increased catecholamine (adrenalin) production
What is normal blood pressure?
What is low blood pressure?
Normal - 120/80
Low - 90/50
What are the symptoms elicited by a patient with haemorrhagic shock?
- low blood pressure - 90/50
- raised pulse - 130bpm
- confused due to lack of oxygen in the brain
- poor urine output due to reduced blood supply to kidneys
- tachypnoea
What is the first intervention used to treat haemorrhagic shock and why?
Fluids are given to increase blood pressure by increasing preload
Blood transfusions take some time to arrive
What is the response after a patient with haemorrhagic shock is given fluids?
- increased central venous pressure increases preload
2. this increases stroke volume, and improves cardiac output
What happens to a patient in haemorrhagic shock if too much fluid is given?
Central venous pressure is raised very high
The preload is increased by such a large amount, that stroke volume begins to decline
What will the decrease in stroke volume, after too much fluid is given, lead to?
A decrease in cardiac output and a decrease in blood pressure
What happens to blood vessels in the lungs if too much fluid is given?
Why?
High left ventricular pressure leads to back pressure in the lungs
This increases alveolar capillary pressure
What is the result of high LV pressure on the lungs, if too much fluid is given?
Fluid in the lungs cannot get out and remains there
It leaks into surrounding tissues, rather than entering blood vessels
This causes pulmonary oedema
What is the main treatment for pulmonary oedema?
Diuretics (Furosemide)
These get rid of the fluid that is causing respiratory failure
What is measured in cardiopulmonary exercise testing?
Oxygen consumption at rest and during exercise
Why is cardiopulmonary exercise testing used?
To evaluate the functional aspects of the heart and lungs and physical fitness
It also works out whether someone will be fit for major surgery
What is the anaerobic threshold?
The level of exercise when the muscles have begun anaerobic respiration
They begin to release lactic acid
What is the major measurement taken in cardiopulmonary exercise testing?
VO2 max
This is maximum oxygen usage by the tissues
