Dr. E. Cardiac Lecture Flashcards

Question

Which artery crosses the crux (junction between the atria and ventricles) to feed the posterior descending coronary branch In 50% it is the RC In 20% it is the LC In 30% a balanced pattern exists

Answer 1

``` I. LCA Ant. descending branch R&L bundle branches Anterior and posterior papillary muscles of the mitral valve Anterolateral left ventricle II. Circ Lateral LV III. RCA SA & AV nodes RA RV Post. interventricular septum Interatrial septum ```

Answer 2

5% of CO or 250ml/min perfusion Flow is determined by: Duration of diastole CPP=Diastolic pressure-LVEDP LCA: flow occurs mostly during diastole RCA: flow occurs in both systole and diastole Myocardial O2 consumption is high with cardiac venous sat. lowest in the body (30%) {The O2 saturation of blood returning from the coronary sinus to the right atrium has the lowest saturation of any body organ (30%).}

Answer 3

Coronary blood flow is directly dependent upon perfusion pressure and inversely proportional to the resistance of the coronary vessel. Q ∞ Perfusion pressure / Vessel resistance Coronary perfusion occurs in diastole hence diastolic pressure is more important than systolic pressure in determining coronary perfusion. Coronary vessels are divided into epicardial or conductance vessels (R1), pre capillary (R2) and microvascular vessels (R3). The epicardial vessels, the site most commonly affected by atherosclerosis, offer negligible resistance to coronary flow. Resistance to flow occurs in the pre capillary (R2), and microvascular (R3) vessels which are termed resistance vessels. The increase coronary blood flow in response to increase myocardial oxygen demand (MVO2) is achieved by the dilatation of these resistance vessels. Three factors play a key role in modifying vascular tone; the accumulation of local metabolites, endothelial factors and neural tone. The accumulation of adenosine during ischemia is an example of local metabolic factors. The most important endothelial substance mediating vasodilatation is nitric oxide (NO). Other important mediators are bradykinin, endothelium derived 1 hyperpolarizing factor and prostacyclin. On the other hand, endothelin-1 (ET-1) is a well known vasoconstricting substance. Angiotensin II and thromboxane A2 are other well known endothelium derived constricting factors. Alpha receptor adrenergic stimulation results in coronary vasoconstriction whereas beta 1 receptor stimulation leads to vasodilatation.

Answer 4

CPP usually autoregulated at 50-120 mmHg Pressure dependent changes Myocardial oxygen demand alters autoregulation: O2 tenstion acting thru mediators, ie adenosine Greatest dilation occurs in smallest vessels LCA>RCA in autoregulation *Tufts( Coronary vascular resistance can be reduced to 1/5th of baseline resistance leading to a five fold increase in the volume of perfusion in response to an increase in need. Coronary reserve is the term used to reflect the amount of increase in coronary perfusion to accommodate increased demand. Autoregulation, mediated by changes in the vascular tone of the resistance vessels, allows distal coronary perfusion to remain unaltered in the face of changes in proximal perfusion pressures. Impaired endothelial function disrupts autoregulation and may lead to ischemia. Diseases known to impair endothelial function include atherosclerosis, dyslipidemias, diabetes mellitus, hypertension, smoking (both passive and active) and hyperhomocysteinemia.

Answer 5

``` Consists of: SA node Internodal tracts AV node AV bundle (bundle of His) Purkinje system ```

Answer 6

``` Mass of specialized cells Junction of SVC and RA 2 Cell types I. P cells (pacemaker cells) II. Transitional or intermediate cells- conduct impulses within and away from the node ``` *The SA node is richly innervated by parasympathetic nervous system fibers (CN X: vagus nerve) and by sympathetic nervous system fibers (T1-4, spinal nerves)

Answer 7

Within the atria Conduction pathways b/w the SA & AV Also contain P cells and transitional cells 3 Major tracts: 1. Anterior (Buchmann’s bundle)-septum 2. Middle (Wenckebach’s tract)-SVC 3. Posterior (Thorel’s tract)-septum

Answer 8

Supplied by nerve endings including vagal ganglionic cells. Causes a delay in the transmission of the action potential: Size of cells: smaller Resting memb. potential: more neg (-60 vs -50 for SA node). Gap junctions: very few Resistance to action potential: incr. Rate of about 50bpm *The atrioventricular node (abbreviated AV node) is a part of the electrical control system of the heart that coordinates the top of the heart. It electrically connects atrial and ventricular chambers.[ The blood supply of the AV node is via the AV nodal artery. The origin of this artery is most commonly (about 90% of hearts) a branch of the right coronary artery, with the remainder originating from the left circumflex artery.[5][6][7] This is associated with the dominance of the coronary artery circulation. In right-dominant individuals the blood supply is from the right coronary artery while in left dominant individuals it originates from the left circumflex artery. Contraction of myocytes (heart muscle cells) requires depolarization and repolarization of their cell membranes. Movement of ions across cell membranes causes these events. The cardiac conduction system (and AV node part of it) coordinates myocyte mechanical activity. A wave of excitation spreads out from the sinoatrial node through the atria along specialized conduction channels. This activates the AV node.[1] The atrioventricular node delays impulses by approximately 0.12s. This delay in the cardiac pulse is extremely important: It ensures that the atria have ejected their blood into the ventricles first before the ventricles contract.[9] This also protects the ventricles from excessively fast rate response to atrial arrhythmias (see below).[10] AV conduction during normal cardiac rhythm occurs through two different pathways: the first “pathway” has a slow conduction velocity but shorter refractory period the second “pathway” has a faster conduction velocity but longer refractory period.[11] An important property that is unique to the AV node is decremental conduction,[12] in which the more frequently the node is stimulated the slower it conducts. This is the property of the AV node that prevents rapid conduction to the ventricle in cases of rapid atrial rhythms, such as atrial fibrillation or atrial flutter. The AV node's normal intrinsic firing rate without stimulation (such as that from the SA node) is 40-60 times/minute.[13]

Answer 9

Extends from the AV Node Enters the posterior part of the ventricle and Purkinje system. Preferential channel for conduction from the atria to the ventricles *The bundle of His is an important part of the electrical conduction system of the heart, as it transmits impulses from the atrioventricular node, located at the inferior end of the interatrial septum, to the ventricles of the heart. The intrinsic rate of the bundle of His is 20 or less beats per minute.[1] The bundle of His branches into the left and the right bundle branches, which run along the interventricular septum. The left bundle branch further divides into the left anterior and the left posterior fascicles. These bundles and fascicles give rise to thin filaments known as Purkinje fibers. These fibers distribute the impulse to the ventricular muscle. The ventricular conduction system comprises the bundle branches and the Purkinje network. It takes about 0.03–0.04 seconds for the impulse to travel from the bundle of His to the ventricular muscle.

Answer 10

``` 2 systems: Left and Right 1. Left: Spreads under the endocardium Forms several fascicles-branch over the left ventricle 2. Right: Travels under the endocardium Base of the anterior papillary muscle ``` *During the ventricular contraction portion of the cardiac cycle, the Purkinje fibers carry the contraction impulse from both the left and right bundle branch to the myocardium of the ventricles. This causes the muscle tissue of the ventricles to contract and generate force to eject blood out of the heart, either to the pulmonary circulation from the right ventricle or to the systemic circulation from the left ventricle. Purkinje fibers also have the ability of firing at a rate of 15-40 beats per minute if upstream conduction or pacemaking ability is compromised. In contrast, the SA node in normal state can fire at 60-100 beats per minute. In short, they generate action potentials, but at a slower rate than sinoatrial node. This capability is normally suppressed. Thus, they serve as the last resort when other pacemakers fail. When a Purkinje fiber does fire, it is called a premature ventricular contraction or PVC, or in other situations can be a ventricular escape.

Answer 11

The cell at rest: The resting cell is relatively permeable to potassium and much less to either sodium or calcium Thus the resting membrane potential of the heart is most dependent on potassium

Answer 12

Five phases: Phase 0-depolarization Fast Na+ channels Phase I- repolarization Na+ influx ends Phase II-plateau Slow Ca+ channels open allowing an influx of Ca+ Phase III-terminal repolarization Slow Ca+ channels are inactivated and efflux of K+ occurs Phase IV-diastolic phase Na+ - K+ pump Refractory Pds Long lasting action potentials prevent premature excitation Absolute: No response occurs during phase 0- middle of phase III Relative: Middle of phase III to phase IV when a second stimulus will cause a weaker action potential than the first

Answer 13

Sympathetic: Arise from: Stellate ganglion and caudal cervical fibers Turn into: The right dorsal medial and lateral cardiac nerves They unite to form one large nerve that follows the course of the L main CA They then branch along the ant. descending and circumflex arteries. Cholinergic fibers-ventricle

Dr. E. Cardiac Lecture Flashcards

(38 cards)