Physiology and pathophysiology of the cardiovascular system Flashcards
What is the cardiac cycle
The cardiac cycle is a term that is used to encompass the electrical and mechanical stages that comprise each heartbeat
- it is broken down into seven steps
What are the steps that compose the systole
Systole somprises 4 steps:
- step 1 : atrial contraction
- step 2: isovolumetric contraction
- step 3: rapide ejection phase
- step 4: reduced ejection phase
What are the steps that compose the diastole
Diastole comprises 3 steps:
- step 5: isovolumetric relaxation
- step 6: rapid filling
- step 7: reduced filling
What are the electrical and mechanical implications of step 1 (= atrial contraction)
Atrial contraction is marked by the P wave and the onset of QRS on the ECG
- it represents electrical depolarization of the atria and thence the ventricles
The atria contract, which causes blood to flow into the ventricles via the atrioventricular valves
Pressures are low in all chambers in the heart
- the elastic recoil of the walls of the aorta, plus the peripheral vascular resistance maintain pressure in this area at about 80 mm HG
What are the electrical and mechanical implications of step 2(= isovolumetric contraction)
Isovolumetric contraction is marked by the end of the QRS complex on the ECG
The ventricles begin to contract against the closed semi-lunar valves
The abrupt rise in pressure causes the atrioventricular valves to snap shut
- as the semilunar valves have not yet opened, blood pressures start to rise rapidly in the ventricles
What are the electrical and mechanical implications of step 3 (= rapid ejection phase)
The rapid ejection phase occurs once the pressures within the heart exceed the pressures within the aorta and pulmonic arteries
- the semi-lunar valves open and blood is ejected from the ventricles
Blood pressures within the LV and aorta reach their peak as blood rushes out of the heart
What are the electrical and mechanical implicatiions of step 4 (= reduced ejection phase)
The reduced ejection phase is indicated by the T wave on the ECG and is characterized by ventricular repolarization
Blood pressures start to drop in the ventricles and arteries
- but blood still flows forward due to kinetic energy
What are the electrical and mechanical implications of step 5 (= isovolumetric relaxation)
Isovolumetric relaxation is characetrized by a a substantial decrease in intraventricular pressure as the ventricles relax
- the semilunar valves snap shut, creating the second heart sound
Blood pressure drops rapidly in the ventricles but remains relatively constant in the aorta
What are the electrical and mechanical implications of step 6 (= rapid filling)
As the intraventricular pressures continue to fall the AV valves open and blood rushes into the ventricles from the atria
Blood pressures lowers in all chambers as the heart relaxes to allow diastolic filling
What are the elctrical and mechanical implications of step 7 (= reduced filling)
Step 7 marks the final phase of the cardiac cycle and is caused by the fact that the ventricles have started to reach their expansile limit
The reduced pressure gradient across the AV valves causes the rate of ventricular filling to drop
- blood pressures in the heart chambers increase marginally during this phase
What are the major causes of heart failure
The major causes of heart failure are:
- systolic myocardial dysfunction
- diastolic myocardial dysfunction
- chronic systolic mechanical overload:
- pressure overload
- volume overload
- hyperkinetic circulation
What is systolic myocardial dysfunction
The ability of the ventricle to maintain normal cardiac output is decreased because of a reduction in myocardial contractility
This is characterized by increased ventricular blood volumes at the end of systole and diastole (eccentric hypertrophy)
The disorder can be:
- primary (e.g., idiopathic dilated cardiomyopathy)
- secondary (e.g. taurine deficiency)
What is diastolic myocardial dysfunction
It is due to increased ventricular stiffness and the development of relaxation abnormalities
- it is the pathomechanism behind most of the feline myocardial diseases (e.g., hypertrophic, restrictive, unclassified cardiomyopathy)
As the pressures required to fill the ventricle become higher, left atrial pressures also begin to rise
- this causes left atrial distension and increased pulmonary venous pressures
What is chronic systolic pressure overload
Pressure overload occurs when pumping against increased resistance (afterload) leads to chronic increase in ventricular wall stress which results in concentric hypertrophy (i.e., thickened walls, normal chamber dimensions)
Causes of pressure overload are:
- obstructive lesions in the left or right ventricular outflow tracts (e.g. aortic or pulmonic stenosis)
- increased vascular resistance (e.g., pulmonary or systemic arterial hypertension)
What is chronic systolic volume overload
Volume overload occurs when handling excessive amount of venous return (preload) results in increased blood volume and pressure within the ventricle at the end of diastole which in turn leads to eccentric hypertrophy (i.e., increased chamber dimensions, usually with normal wall thickness)
Causes of increased blood volume are:
- leaks (e.g., valvular insufficiency)
- shunting lesions (e.g., atrial or ventricular septal defects)
What is chronic systolic mechanical overload due to hyperkinetic circulation (i.e., high output state)
A reduction in peripheral vascular resistance forces the heart to increase cardiac output, which imposes ventricular volume overloading
Causes of this are:
- thyrotoxicosis
- chronic anemia
