Cardiovascular physiology Flashcards
Circulatory systems function
The circulatory system functions as the transport system
Delivers oxygen, nutrients, hormones and regulatory chemicals to all cells
Transports carbon dioxide and other products of metabolism from the cells to the lungs, liver and kidneys
Distributes heat from inside the body to the extremities and vice versa
Circulatory system consists of
Heart
Vessels
Blood
Arteries
Carry blood away from the heart
Veins
Carry blood towards the heart
Lymphatic vessels carry
Carry tissue fluid
Formula for cardiac output
stroke volume x heart rate
Cardiac output is effected by
Preload
Contractibility
Afterload
Heart rate
Preload is determined and effected by
Determined by ventricular filling
End diastolic volume
Affected by
Leaky semilunar valves
Incomplete emptying due to poor contractility
Contractibility is determined by
Major determinants are
Preload
Sympathetic stimulation – increases contractility
Blood and tissue calcium concentrations
The ruminant heart is particularly sensitive to low calcium
Afterload or peripheral resistance is and is determined by
The resistance against which the ventricles pump
Basically, the tone in the arterial system
Measured as blood pressure
Affected by input from the
Sympathetic NS (vasoconstriction)
Parasympathetic NS (vasodilation)
By local mediators
Reducing afterload with vasodilators is beneficial in heart failure
Systemic circulations
High -pressure
Requires hydrostatic pressure to force blood through capillaries in tissues.
Also needs to pump against gravity to reach organs such as the brain
Pulmonary circulation is
Low -pressure
Very little resistance in vessels of lungs
Can easily get fluid leakage in the delicate capillaries if pressure too high
Total blood volume distribution
Lungs: 15%
Body: 80%
Heart 5%
The circulatory system is divided into
systemic and pulmonary, each with arterial (away from the heart) and venous (toward the heart) components
Systole def
Contraction of ventricles causing the heart to eject blood into ciculation
Contractibility def
Contractility: ability of the heart to fully contract
Afterload def
Afterload: the force the ventricles need to overcome to push blood forward
Preload def
Preload : the amount of blood in the heart before contraction (end diastolic volume)
Stroke volume def
Stroke volume (aka systolic discharge): the amount of blood ejected out of the heart with ventricular contraction
Cardiac output def
Cardiac output: the volume of blood the heart is able to pump forward in one minute (stroke volume X heart rate)
Starlings principle is
Starling’s principle: increase contractility of cardiac muscle if fibers are stretched
More blood in ventricles results in stronger contraction
Diastole def
Diastole: relaxation of ventricles causing the heart to fill
Systolic blood pressure def
Systolic blood pressure: maximum pressure in arteries, occurs during ventricular contraction
Diastolic blood pressure def
Diastolic blood pressure: minimum pressure in arteries, occurs during ventricular relaxation
Pulse pressure or systematic pressure def
Pulse pressure or systematic resistance: the difference between systolic and diastolic pressures (when the ventricle contracts, the arteries stretch and recoil to normal size – this is felt as the pulse)
Peripheral or systemic resistance def
Peripheral or systemic resistance: friction in the arteries that limits the flow of blood
Heart rate is controlled by
Body temperature
Decreased (subnormal) temperature means decreased heart rate
Autonomic NS
Cardioregulatory center in the medulla oblongata in the brainstem
Receives input from baroreceptors in the vessels (primarily the arch of the aorta and the carotid sinuses) and heart chambers
Atropine block
Acetylcholine and can be used to correct low heart rate
Hormones effect heart rate by
Sympathetic stimulation to adrenal gland causes release of epinephrine –increases contractility and therefore stroke volume.
Increased thyroid hormone will increase heart rate (very important for hyperthyroid cats!)
Decreased T4 will decrease HR – helpful to Dx hypoT4
Low heart rates are a sign of hypothyroidism
Ion levels effect heart rate
Hyperkalemia (excess potassium) decreases heart rate.
Cardiovascular center in the medulla effects on heart rate
If pressure drops, feedback increases sympathetic stimulation with the release of the neurotransmitter norepinephrine – increases heart rate and contractility
Norepinephrine and epinephrine are also hormones released by the adrenal medulla
Sympathetic stimulation also increases BP through vasoconstriction
If pressure rises, feedback increases parasympathetic tone through the vagus nerves to reverse the above
Vegas nerves affect the heart rate by
Vagus nerves innervate the SA and AV nodes
Stimulation causes the release of acetylcholine
slows the rate of depolarization to decrease heart rate
Shock is defined by
Defined as a failure of tissue perfusion
In some types of shock vessels dilate to a volume that exceeds that of the blood
In some types of shock vessels dilate to a volume that exceeds that of the blood which causes
Results in a fall in BP.
Systolic BP < 40 is generally considered incompatible with life!
The body tries to maintain flow through vital organs like the brain and heart and may shut down peripheral circulation (skin, kidneys).
Requires treatment with high IV fluid rates
Stroke volume definition
(SV): The amount of blood pumped out of the ventricles during each contraction
Isovolumetric contraction def
Isovolumetric contraction: Just before the actual pumping forward of the blood, the ventricle begins to contract but all the valves are closed and the pressure in the ventricle increases
Ventricular ejection def
Ventricular ejection: semilunar valves open and ventricles empty
Cronatrope
Time
Ionotrope
Force of contraction
First heart sound
S1 or lub, is the closing of the AV valve during the initial ventricular contraction.
Prevents backflow of blood into the atria
Second heart sound
S2 or dup, is the closing of the semilunar valves at the end of systole
Prevents blood flowing back into the ventricles
Third heart sound
Is generated by the passive filling of he ventricle
In ventricular diastole pressure is lower in the ventricle than atria, therefore AV valves open and blood flows into the ventricles
Passive filling results in about 80% of the normal filling of the ventricles
Normal in horses (not always heard)
Abnormal in dogs and cats
Fourth heart sound
Generated by atrial contraction
Final amount of blood pushed into the ventricles – about 20% of total ventricular volume
Normal in horses (not consistently heard)
Abnormal in dogs and cats
Cardiac cycle steps
Atrial contraction (S4) to complete filling of ventricles
Ventricular contraction (Systole)
AV valves close (S1) at beginning of ventricular systole to stop backflow.
The semilunar valves open as the contracting ventricles develop pressure and blood is ejected into the major arteries. These are elastic and initially distend to accommodate the blood
Ventricular relaxation (diastole)
Semilunar valves close (S2)
Ventricular pressure falls
Blood passively enters the ventricles (S3) as the AV valves open
Go back to S4 and repeat for the rest of your life without resting
The shorter pause is between S1 and S2
Properties of cardiac muscle fibers
Striated, involuntary
Impulses travel from cell to cell through the intercalated disc , not through nervous stimulation of each individual cell
Intercalated disks are composed of desmosomes (strongly bond the cells together) and gap junctions (allow fast movement of ions and transmission of action potentials)
Automaciatity : is innate ability of cardiac muscle fibers to contract at a certain rhythm without external stimulation
Longer refractory period than skeletal muscle.
Network of specialized cardiac muscle cells
Generate action potentials which initiate contraction
Propagate the contraction in a coordinated spread of cardiac muscle cell excitation
What is the action potential of the heart
Action potentials are depolarization of the resting membrane potential due to influx or efflux of ions - mostly sodium, calcium, and potassiumAction potentials are depolarization of the resting membrane potential due to influx or efflux of ions - mostly sodium, calcium, and potassium
What can affect the contraction of the heart
Minor changes of serum potassium or calcium concentrations of only a few mmoles have major effects on cardiac contraction.
Repolarization requires
Repolarization requires pumping of ions back to their original concentrations to re-establish the resting membrane potential
Components of the conduction system
Sinoatrial (SA) Node (pacemaker) in the right atrium
Atrioventricular(AV) Node
Atrioventricular (AV) Bundle (or Bundle of His)
Right and Left Bundle Branches
Conduction Myofibers (Purkinje Fibers)
SA node is the
Small mass of cells embedded in the right atrial wall near the opening of the cranial vena cava
Depolarize spontaneously at a rate of 100 bpm in humans
Modified by input from the autonomic NS (decreases resting HR to about 70 bpm in humans)
It is the pacemaker because it spontaneously depolarizes faster than any other area of heart
Sequence of activation
SA node spontaneously depolarizes
AV node: slow conduction to allow atria to finish filling the ventricles before ventricles begin to contract
From AV Node, the signal is transmitted along the left and right AV Bundle Branches (or bundle of His)
The AV bundle branches bring the signal very quickly to the tip or apex of the heart – at the bottom of the ventricles
The bundle branches divide into smaller fibers called Purkinje fibers that radiate upwards and spread through the ventricular muscle
Causes a contraction wave to spread through the ventricles starting at the septum and papillary muscles and moving towards the base - squeezing blood upward and out through the pulmonary and aorta valves into the arteries
Signal transmission of heart contraction
Both atria contract from upper → atria downward to squeeze blood into ventricles → impulse hits a band of nonconducting fibrous tissue at the junction between atria and ventricles and can only get through via the AV node
During relaxation of the heart
During relaxation the cell cannot contract
i.e. the cell is in its refractory period and cannot respond to another stimuli
This is important in allowing refilling of the heart chambers
Prevents tetanic contractions (would be deadly) and arrhythmia
Electrocardiogram is the
The electrical impulse is predictable in both magnitude and direction in a healthy heart
Body fluids can conduct electrical impulses, by setting up electrodes on the skin at strategic points, we can measure the relative magnitude and direction of the depolarization of the heart muscle through the use of the electrocardiogram.
Unhealthy hearts can show characteristic changes in electrical activity that indicate the nature of the problem
ECG sections and what they represent
P-wave: depolarization (i.e.: contraction) of atria
QRS complex: ventricular depolarization (i.e.: contraction).
Atrial repolarization (relaxation) is “hidden” by the large QRS complex
T-wave: ventricular repolarization (relaxation).
Sinus rhythm
When the heart is depolarizing normally and controlled by the sinoatrial (SA) node
Regularly spaced beats, normal HR
Sinus arrhythmia
A consistently irregular rhythm that changes with stage of respiration: increases rate with inspiration and decreases with expiration
NORMAL, especially in dogs.
Bradyarrhythmeia may be
Sinus bradycardia (normal conduction, but slow rate)
Sinus arrest (no evidence of normal SA node initiated depolarizing)
AV block (the atria keep contracting at a regular rhythm, but the signal is ‘blocked’ at the AV node)
Tachyarrhythemia may be
Supraventricular – signal for depolarization originates above the AV junction
Ventricular – signal for depolarization originates below the AV junction, or in the ventricles
Atrial fibrillation
Extremely rapid contraction of the atria in an uncoordinated manner
Results in a very rapid but ineffective atrial rate
Ventricular contractions are irregular and much slower because the AV node is only sporadically stimulated and is refractory to impulses that arrive close together.
Can be a problem in large dog breeds and racing horses
Ventricular fibrillation
Extremely rapid uncoordinated contraction of the ventricles
Results in a very rapid but ineffective ventricular rate that produces NO PULSE. Will die very quickly if CPR or electrical defibrillation is not attempted immediately
A murmur during systole can be caused by
A leak (insufficiency) in an AV valve (which is supposed to be closed)
A narrowing (stenosis) in a semilunar valve (which is supposed to be open)
Normal turbulence (a flow murmur) in some large animals with wide vessels.
Diagnostic tests for heart issues
Radiography detects cardiac enlargement
Will not detect a heart murmur
Ultrasound (echocardiography) can show both lesions (thickened valves) and abnormal flow (murmurs).
Electrocardiography detects abnormal rhythms
Causes of diastolic murmur
AV valve stenosis or insufficiency of semilunar valve causes the murmur
Physical exam to feel the pulse
When you feel a pulse, the pulse rate is equal to the number of ventricular contractions per minute.
The pulse pressure (amplitude) is the difference between the systolic and diastolic pressures.
The pressure required to completely block the vessel is equal to systolic pressure
In late shock the pulse is low amplitude and easy to occlude.
Sites for auscultating heart sounds
Three valves can be heard on the left. From cranial to caudal these are:
Pulmonic, Aortic and Mitral (PAM).
There is one intercostal space between each valve and the aortic is higher than the other two.
In dogs, cattle and horses the pulmonic is found at approximately the third IC space, cats at the fourth IC space.
In large animals the heart stands more vertically and the heart sounds are heard more dorsally than in small animals.
The tricuspid (T) is heard on the right
Sites for feeling the pulse
Cats: Femoral artery
Dogs: Femoral artery or Lingual artery
Horses and Cows: Facial or median artery
Difference in arteries between species
In carnivores and the pig the left subclavian branches off aorta
In ungulates the left subclavian branches off the brachiocephalic trunk
Atrial septal defects
(patent foramen ovale)
The foramen ovale fails to close at birth so blood still passes through from one atrium to the other
This is known as “shunting”
Unless the opening is large, there may be no detectable clinical signs.
Ventricular septal defect
Range from small openings to complete absence of the septum.
Blood is transmitted between ventricles with considerable force
Shunt is usually left to right
Depending on the size of the opening between the ventricles, there can be no outward signs of heart disease or cyanosis, weakness, dyspnea and anorexia.
This is the most common congenital defect in cattle and horses. Also, common in terriers and large breed dogs.
Persistent ductus arteriosus
During fetal life the ductus arteriosus shunts blood from the pulmonary artery into the aorta.
When this opening fails to close at birth it is referred to as a persistent or patent ductus arteriosus or PDA
Blood is continuously shunted, usually from the aorta into the pulmonary artery
The heart murmur sound is a typical “machinery murmur” (a continuous murmur.)
One of the most common defects of the dog and is inherited in miniature and toy poodles
Persistent right aortic arch
The embryonic arch persists and displaces the esophagus and sometimes the trachea
Often traps them in a “ring” formed by the arch of the aorta on the right, the pulmonary artery below, the base of the heart ventrally and the ductus arteriosus dorsally and to the left
This ring may compress the trachea and esophagus with resulting dyspnea and regurgitation
Pulmonic stenosis
A narrowing at or just below the pulmonary semilunar valve.
The murmur is produced during the systole part of cardiac cycle
Aortic stenosis
Narrowing of the region of outflow from the left ventricle, so there is difficulty in emptying
Fairly common in dogs
Tetralogy of Fallot
A complex malformation consisting of Pulmonic stenosis (usually below the valve), an interventricular septal defect, malpositioning of the aorta and right ventricular hypertrophy.
Reported in many species
Heart disease
Heart conditions, unless they are severe, are generally first noticed when listening to the heart (auscultation)
Heart defects and disease conditions often produce abnormal heart sounds which are referred to as murmurs, or abnormal rhythms or rates which are referred to as arrhythmias.
Heartworm
(Dirofilaria immitus)
Found in dogs, and rarely cats and ferrets,
Can cause congestion in the right side of the heart
Worms grow to maturity in pulmonary arteries, right ventricle and right atrium
Leads to right ventricular dilation, hypertrophy of right ventricle and right-sided heart failure.
Signs are exercise intolerance, coughing, weakness, fainting, heart murmur.
Heart insufficiency and failure
Disease and/or defects of the heart result in a decrease of pumping efficiency.
Clinically cardiac failure is recognized as left-sided, right-sided or generalized.
Back pressure increases pressure at end of pulmonary capillaries and forces fluid out causing edema
Exercise tolerance decreases, dyspnea is seen with exercise or excitement.
Lung congestion and coughing are seen.
Increased venous pressure in systemic circulation
Fluid can’t get out of organs so organs swell and lose function.
Jugular veins are engorged and even superficial veins may be distended.
The liver and spleen are enlarged.
Fluid builds up - in the abdomen (ascites) in dogs, in the chest (hydrothorax) in cats, and under the skin in large animals (subcutaneous edema)
Generalized failure
Both left and right-sided failure occur and most or all of their associated symptoms will be seen
Left sided failure of the heart
congestive heart failure
Right sided failure signs
lung congestion and ocughing are seen
Heart insuffiecency and failure are caused by
Disease and/or defects of the heart result in a decrease of pumping efficiency.
Back pressure increases pressure at end of pulmonary capillaries and forces fluid out causing edemaIncreased venous pressure in systemic circulation
Fluid can’t get out of organs so organs swell and lose function.
Jugular veins are engorged and even superficial veins may be distended.
The liver and spleen are enlarged.
Signs of heart failure
Exercise tolerance decreases, dyspnea is seen with exercise or excitement.
Lung congestion and coughing are seen.
Increased venous pressure in systemic circulation
Fluid can’t get out of organs so organs swell and lose function.
Jugular veins are engorged and even superficial veins may be distended.
The liver and spleen are enlarged.
Fluid builds up - in the abdomen (ascites) in dogs, in the chest (hydrothorax) in cats, and under the skin in large animals (subcutaneous edema)
Acquired cardiovascular disease
Dilated Cardiomyopathy
* Mitral insufficiency
* Parasites
* Strongylus vulgaris in horses
* Dirofilaria immitus (heartworm) in dogs
Dilated cardiomyopathy common breeds and signs
Dogs and cats
Dogs: Inherited in Dobermans, Great Danes,
Boxer, Cocker Spaniels
Recent dietary link to grain-free diets
Cats: Taurine deficiency
Signs include coughing, weakness, collapse, weight loss, murmur, arrhythmias
