Lecture Notes- Heart
Cardiac Conduction
(1)The sinoatrial (SA) node and the remainder of the conduction system are at rest. (2)The SA node initiates the action potential, which sweeps across the atria. (3) After reaching the atrioventricular node, there is a delay of approximately 100 ms that allows the atria to complete pumping blood before the impulse is transmitted to the atrioventricular bundle. (4) Following the delay, the impulse travels through the atrioventricular bundle and bundle branches to the Purkinje fibers, and also reaches the right papillary muscle via the moderator band. (5) The impulse spreads to the contractile fibers of the ventricle. (6) Ventricular contraction begin
Defibrillators
(a) An external automatic defibrillator can be used by nonmedical personnel to reestablish a normal sinus rhythm in a person with fibrillation. (b) Defibrillator paddles are more commonly used in hospital settings.
Tachycardia
+ 100 beats/min
Heart contraction is intrinsic but there are two controlling systems
- Autonomic Nervous System - Intrinsic Conduction system: ensures sequential depolarization of the heart making sure that the heart beats as a coordinated unit.
Congestive heat failure is a progressive condition where the CO is so low that blood circulation is inadequate to meet tissue needs. It is caused by:
-Coronary atherosclerosis -Persistent high blood pressure -Multiple myocardial infarcts -Dilated cardiomyopathy (DCM)
isovolumetric relaxation phase
-ventricular diastole begins -AV valves are still closed -atrial diastole continues -SL valves close
The atrioventricular (AV) node delays the impulse approximately ____ second
0.1
Three main factors affect SV
1. Preload 2. Contractility 3. Afterload
3 Layers of the Heart Wal
1. The epicardium on the outside is the visceral layer of the serous pericardium. 2. The myocardium is the cardiac muscle layer which forms most of the heart. 3. The endocardium is the smooth inner layer which reduces friction
To keep oxygenated and deoxygenated blood flowing the heart has two pumping systems
1. The pulmonary circuit pumps blood through the lungs for oxygenation. 2. The systemic circuit sends oxygenated blood through the body
phases of cardiac cycle
1. ve
Junctional Rhythm
40-60 Regular! -impulse from AV node w/ retro/antegrade transmission - P wave often inverted/buried/follow QRS - slow rate - narrow QRS (not wide like ventricular)
Normal adult blood volume about ____ liters
5
The sinoatrial (SA) node generates impulses about ____ times/minute
75
electrocardiogram
A normal tracing shows the P wave, QRS complex, and T wave. Also indicated are the PR, QT, QRS, and ST intervals, plus the P-R and S-T segments
Dual System of the Human Blood Circulation
Blood flows from the right atrium to the right ventricle, where it is pumped into the pulmonary circuit. The blood in the pulmonary artery branches is low in oxygen but relatively high in carbon dioxide. Gas exchange occurs in the pulmonary capillaries (oxygen into the blood, carbon dioxide out), and blood high in oxygen and low in carbon dioxide is returned to the left atrium. From here, blood enters the left ventricle, which pumps it into the systemic circuit. Following exchange in the systemic capillaries (oxygen and nutrients out of the capillaries and carbon dioxide and wastes in), blood returns to the right atrium and the cycle is repeated.
Cardiac Reserve
Cardiac Reserve is resting minus maximal cardiac output 1. Nonathletes - 4 or 5 x normal 2. Athletes - Up to 7 x normal
Major Factors Influencing Cardiac Output
Cardiac output is influenced by heart rate and stroke volume, both of which are also variable.
Regulation of Stroke Volume: CONTRACTILITY
Contractility refers to the contractile strength at a givenmuscle length, independent of muscle stretch and EDV. • Positive inotropic agents increase contractility - Increased Ca2+ influx due to sympathetic stimulation -Hormones (thyroxine, glucagon, and epinephrine) •Negative inotropic agents decrease contractility -Acidosis -Increased extracellular K+ -Calcium channel blockers
systole
Contraction of the heart
Frank-Starling Law
Critical factor controlling stroke volume is stretch of cardiac muscle
myocardial infarction
Heart attack; Death of cardiac muscle
CPR Technique
If the heart should stop, CPR can maintain the flow of blood until the heart resumes beating. By applying pressure to the sternum, the blood within the heart will be squeezed out of the heart and into the circulation. Proper positioning of the hands on the sternum to perform CPR would be between the lines at T4 and T9
Standard Placement of ECG Lead
In a 12-lead ECG, six electrodes are placed on the chest, and four electrodes are placed on the limb
Relationship between the Cardiac Cycle and ECG
Initially, both the atria and ventricles are relaxed (diastole). The P wave represents depolarization of the atria and is followed by atrial contraction (systole). Atrial systole extends until the QRS complex, at which point, the atria relax. The QRS complex represents depolarization of the ventricles and is followed by ventricular contraction. The T wave represents the repolarization of the ventricles and marks the beginning of ventricular relaxation.
Hypoxia
Low oxygen saturation of the body, not enough oxygen in the blood
Heart Sounds
Lub-dub. 1st- a-v valves close. 2nd- aortic and pulmonary valves close
Intervals
PQ - 0.16 sec. - time between atrial excitation and ventricular excitation QT - 0.365 sec. - Time between beginning of ventricular depolarization and repolarization; includes ventricular contraction
Stethoscope Placement for Auscultation
Proper placement of the bell of the stethoscope facilitates auscultation. At each of the four locations on the chest, a different valve can be heard
path of impulse
SA node AV node AV Bundle Bundle branches Purkinje fibers
Regulation of Stroke Volume (SV)
SV = EDV* -ESV* 120 ml - 50 ml = 70 ml/beat EDV = amount of blood collected in a ventricle during diastole ESV = amount of blood remaining in a ventricle after contraction
Conduction System of the Heart
Specialized conducting components of the heart include the sinoatrial node, the internodal pathways, the atrioventricular node, the atrioventricular bundle, the right and left bundle branches, and the Purkinje fibers
Sympathetic Nervous System (SNS)
The component of the autonomic nervous system that responds to stressful situations by initiating the fight-or-flight response.
Chambers of the Heart
The heart has four chambers: two atria and two ventricles
SA node
The pacemaker of the heart, located in the right atrium. Serves as the stimulus for contraction since it has highest discharge rate. It sets rate of depolarization
Development of the Human Heart
This diagram outlines the embryological development of the human heart during the first eight weeks and the subsequent formation of the four heart chambers
Chemical Regulation
a. The hormones epinephrine and thyroxine increase heart rate. b. Ions must be present in the proper amounts. Imbalances lead to serious problems. c. Hypocalcemia (low calcium levels decreases heart rate. d. Hyperkalemia may lead to heart block and cardiac arrest . It disrupts depolarization by lowering the resting potential making depolarization difficult.
arrhythmias
abnormal heart rhythms
ectopic focus
abnormal pacemaker
Other factors that influence heart rate
age, gender, exercise, body temperature
isovolumetric contraction phase
all valves are closed
dicrotic notch
brief rise in aortic pressure caused by backflow of blood rebounding off semilunar valves
Purkinje fibers
carry the impulse to the heart apex and ventricular walls
bundle branches
carry the impulse toward the apex of the heart
Coronary Circulation
circulation of blood through the coronary blood vessels to deliver oxygen and nutrients to the heart muscle tissue
electrocardiography
electrical currents which spread through body as heart beats are measured as an electrocardiogram
Heart Valves
ensure unidirectional blood flow through the heart
Action Potential at the SA Nod
he prepotential is due to a slow influx of sodium ions until the threshold is reached followed by a rapid depolarization and repolarization. The prepotential accounts for the membrane reaching threshold and initiates the spontaneous depolarization and contraction of the cell. Note the lack of a resting potential
Damage to AV node leads to
heart block which interferes with the ability of the ventricles to receive signals from the SA node
The heart is located
in the mediastinum and occupies an area extending from the second rib to fifth intercostal space. The organ is tipped to left with the base pointed toward the right shoulder and the apex pointing toward hip and resting on diaphragm. The heart is located within the thoracic cavity, medially between the lungs in the mediastinum. It is about the size of a fist, is broad at the top, and tapers toward the base
ischemia
lack of blood flow
bradycardia
less than 60 beats/min
Atrioventricular (AV) valves
lie between the atria and the ventricles and prevent backflow into the atria when ventricles contract.
Cardiac Muscle
made up of branched, multinucleate fibers. Intercalated discs mark the boundaries between cells
total heart block
no signal passes through and the ventricle beats at its intrinsic rate (too slow). In partial heart block, some of the impulses get through. In either case, a pacemaker is used to regulate the beating of the heart.
a second-degree or partial block
one- half of the P waves are not followed by the QRS complex and T waves while the other half are
ventricular filling phase
period during which blood drains from atria into ventricles
pectinate muscles
prominent muscular ridges along the inner surface of the auricle and across the adjacent anterior atrial wall
fibrillation
rapid, random, and ineffective contractions of the heart
Regulation of Stroke Volume - PRELOAD
refers to the degree of stretch of cardiac muscle cells before they contract (Frank-Starling law of the heart). Cardiac muscle exhibits a length-tension relationship. At rest, cardiac muscle cells are shorter than optimal length. Slow heartbeat and exercise increase venous return. Increased venous return distends (stretches) the ventricles and increases contraction force
Regulation of Stroke Volume: AFTERLOAD
refers to the pressure that must be overcome for ventricles to eject blood. Hypertension increases afterload, resulting in increased ESV and reduced SV
Diastole
relaxation of heart muscle
A-V (atrioventricular) node
specialized cardiac tissue that provides normal conduction pathway between atrial and ventrizular synctia
AV bundle
splits into two pathways in the interventricular septum (bundle branches)
Parasympathetic nervous system (PNS)
stimulation is mediated by acetylcholine and opposes the SNS
congenital heart defects
structural abnormalities caused by the failure of the heart to develop normally before birth
Cardiac Output (CO)
the amount of blood pumped/ventricle / minute CO = Heart Rate* x Stroke Volume (SV) CO = 75 beats/min x 70 ml/beat= 5.2L/min *number of beats per minute
atrial fibrillation
the electrical pattern is abnormal prior to the QRS complex, and the frequency between the QRS complexes has increase
Atria
the receiving chambers of the heart. Each atrium has a protruding auricle which increases its volume Blood enters right atria from superior and inferior venae cavae and coronary sinus Blood enters left atria from pulmonary veins
ventricular tachycardia
the shape of the QRS complex is abnormal
Ventricles
the two lower chambers of the heart, and they pump blood out to the lungs and body.
third-degree block
there is no correlation between atrial activity (the P wave) and ventricular activity (the QRS complex)
ventricular fibrillation
there is no normal electrical activity
The pericardium serves to:
• Protect and anchor the heart • Prevent overfilling of the heart with blood • Allow for the heart to work in a relatively friction-free environment
Right and left pulmonary veins Vessels carrying blood away from the heart include:
• Pulmonary trunk, which splits into right and left pulmonary arteries • Ascending aorta (three branches) - brachiocephalic, left common carotid, and subclavian arteries
Vessels returning blood to the heart include:
• Superior and inferior venae cavae • Right and left pulmonary veins
The pericardium is a double-walled sac surrounding the heart. It is composed of a superficial fibrous pericardium and a deep two-layer serous pericardium:
• The parietal layer lines the internal surface of the fibrous pericardium. • The visceral layer or epicardium lines the surface of the heart. • They are separated by the fluid-filled pericardial cavity