What are the components of the cardiac conduction system?
Sinoatrial (SA) node
Atrioventricular (AV) junctional area
Bundle branch system
What is the sinoatrial (SA) node?
Located close to the surface of the right atrium near its junction with superior vena cava
Rate 60-100 bpm
Has greatest degree of automaticity
Impulses move directly through atrial muscle and lead to atrial depolarization, which is reflected in a P wave on a ECG
What is the atrioventricular (AV) junctional area?
Consists of a transitional cell zone, the AV node, and the bundle of His
Lies just beneath the R atrial endocardium, btwn the tricuspid valve and the ostium of the coronary sinus
T-cells cause impulses to slow down or be delayed in the AV node before proceeding to the ventricles
Delay is reflected in the PR segment on the ECG
The slow conduction allows the atria to contract and the ventricles to fill
Contraction is known as "atrial kick" and contributes to additional blood volume for a greater CO
What is the bundle of His?
Connects with the distal portion of the AV node and continues through intraventricular septum
Extends as a right bundle branch down the right side of intraventricular septum to the apex of the right ventricle
On the left side, it extends as a left bundle branch, which further divides
What are the Purkinjie fibers?
At ends of both R and L bundle branch systems are the Purkinjie fibers
Interweaving network located on the endocardial surface of both ventricles, from apex to base
Fibers then partially penetrate into the myocardium
Makes up the bundle of Hiis, bundle branches and terminal Purkinjie fibers
Responsible for rapid conduction of electrical impulses throughut the ventricles, leading to ventricular depolarization and the subsequent ventricular muscle contraction
When SA and AV nodes fail, can initiate impulses at a rate of 20-40 bpm
What are the valves of the heart?
What is the atrioventricular valve?
Separates atria from the ventricles
Control blood flow btwn atria and ventricles
What is the tricuspid valve?
Located btwn the right atrium and the right ventricle
Has three leaflets
What is the mitral valve?
Located bten the left atrium and left ventricle
Has two leaflets
What are the leaflets of the AV valves?
Connected to the papillary muscles by the chordae tendinae to prevent backflow
S1, the first heart sound, is heard when AV valves close
What are the semilunar valves?
Separate the cardiac chambers from the great vessels
Control blood flow out of the cardiac chambers
Each valve has three cusps that prevent backflow
S2, the second heart sound, is heard when the semilunar valves close
What is the pulmonic valve?
Located btwn the right ventricle and the pulmonic artery
Unoxygenated blood flows through this valve to the lungs
What is the aortic valve?
Located btwn the left ventricle and the aorta
Oxygenated blood is pumped from the heart through this valve into systemic circulation
Blood flows from the body into the R atrium and from the lungs into the L atrium
With atrial contraction, the blood is pumped from the atria into the ventricles
With ventricular contraction, the blood is pumped from the R ventricle into the pulmonary artery and lungs, and from the L ventricle into the aorta and the arterial circulation
Right heart circulation- deoxygenated blood
Venous syatem -> right atrium -> right ventricle -> lungs for oxygenation via pulmonary artery
Left heart circulation- oxygenated blood
Lungs via pulmonary veins -> left atrium -> left ventricle -> aorta -> systemic circulation for tissue perfusion
Right main coronary artery
Left main coronary artery
2 main branches-
Circumflex coronary artery
Left anterior descending coronary artery
Right coronary artery
R atrium, R ventricle, inferior L ventricle, posterior septum, SA node
Left coronary artery
2 main branches
LAD: descends toward anterior wall and apex of L ventricle, supplies portions of L ventricle, ventricular septum, chordae tendinae, papillary muscle, to lesser extent R ventricle
Circumflex: descends toward lateral wall of L ventricle and apex, supplies L atria, lateral and posterior surfaces of L ventricle, sometimes portions of intraventricular septum
Coronary vessel blood flow
75% of coronary blood flow occurs during diastole
Need diastolic pressure of 60 to maintain adequate flow to the heart muscle
Flow increases with activity and sympathetic stimulation
What is the P wave?
A deflection representing atrial depolarization
What is the PR segment?
The isoelectric line from the end of the P wave to the beginning of the QRS complex, when the impulse is traveling through the AV node, where it is delayed
It then travels through the ventricular conduction system to the Purkinjie fibers
What is the PR interval?
Measured from the beginning of the P wave to the end of the PR segment
Represents the time required for atrial depolarization as well as the impulse delay in the AV node and the travel time to the Purkinjie fibers
Normally measures from 0.12 to 0.20 sec (5 small blocks)
what is the QRS complex?
Represents ventricular depolarization
Q wave is the first negative deflection
It's small and it represents septal depolarization
When Q wave is abnormally present it represents myocardial necrosis (cell death)
The R wave is the first positive deflection-may be small, large, or absent
The S wave is a negative deflection following the R wave
What is the QRS duration?
Represents the time required for depolarization of both ventricles
Measured from the beginning of QRS complex to the J point (the junction where the QRS complex ends and the ST segment begins)
Normally measures from 0.04-0.10 sec (up to three small blocks)
What is the ST segment?
Normally an isoelectric line and represents early ventricular repolarization
Occurs from the J point to the beginning of the T wave
Length varies with changes in HR, administration of meds, and electrolyte disturbances
Normally not elevated more than 1mm or depressed more than 0.5mm from baseline
Its amplitude is measured at a point 1.5 - 2mm after the J point
ST elevation or depression can be caused by myocardial injury, ischemia or infarction, conduction abnormalities, or the administration of meds
What is the T wave?
Follows the ST segment and represents ventricular repolarization
Usually positive, rounded, and slightly asymmetric
If an ectopic stimulus excites the ventricles it may cause ventricular irritability, lethal dysrhythmias, possible cardiac arrest in the vulnerable heart-known as the R-on-T phenomenon
Waves may become tall and peaked, inverted (negative), or flat as a result of myocardial ischemia, potassium or calcium imbalances, meds, or ANS effects
What is the U wave?
When present, follows the T wave and may result from slow repolarization of ventricular Purkinjie fibers
Of the same polarity as the T wave, although it is generally smaller
Abnormal wave may suggest an electrolyte abnormality (particularly hypokalemia) or other disturbance
Correct ID is important so that it is not mistaken for a P wave
What is the QT interval?
Represents the total time required for ventricular depolarization and repolarization
Is meaured from the beginning of the QRS complex to the end of the T wave
Interval varies with age, gender, and changes in HR, lengthening with slower HR and shortening with faster rates
May be prolonged by certain meds, electrolyte disturbences, Prinzmetal's angina, or subarachnoid hemorrhage
Prolonged QT may lead to unique type of ventricular tachycardia called torsades de pointes
What is artifact?
Interference seen on the monitor or rhythm strip, which may look like a wandering or fuzzy baseline
May be caused by pt mvmnt, loose or defective electrodes, improper grounding, faulty ECG equip, such as broken wires or cables
Some artifact can mimic lethal dysrhythmias like v tach or v fib
Assess pt to differentiate artifact from lethal rhythms
ECG rhythm analysis
Determine heart rate
Determine heart rhythm
Analyze P waves
Measure PR interval
Measure QRS duration
Interpret the rhythm
Normal sinus rhythm (NSR)
What is NSR?
Rate: a/v rates of 60-100bpm
Rhythm: a/v rhythms regular
P waves: present, consistent configuration, one P wave before each QRS complex
PR interval: 0.12-0.20 sec and constant
QRS duration: 0.04-0.10 sec and constant
What is sinus arrhythmia?
Variant of NSR
All the characteristics of NSR except for its irregularity
Rate: a/v rates btwn 60-100bpm
Rhythm: a/v rhythms are irregular, with shortest PP or RR interval varying at least 0.12sec from the longest PP or RR interval
P waves: one P wave before ea QRS complex, consistent configuration
PR interval: normal, constant
QRS duration: normal, constant
What are dysrhythmias?
Any disorder of the heartbeat
A disturbance in the relationship btwn electrical conductivity and the mechanical response of the myocardium
A disturbance in impulse formation (either from an abnormal rate or from an ectopic focus)
A disturbance in impulse conduction (delays and blocks)
The combination of several mechanisms
What are tachydysrhythmias?
HR > 100bpm
Major concern in adult pt with coronary artery disease (CAD)
Coronary artery blood flow occurs mostly during diastole when the aortic valve is closed and is determined by diastolic time and BP in the root of the aorta
They shorten the time and therefore the coronary perfusion time (the amount of time available for blood to flow through the coronary arteries to the myocardium)
Initially increases CO and BP; however, a continued rise in HR decreases the ventricular filling time because of a shortened diiastole , decreasing the stroke volume. Consequently, CO and BP will begin to decrease, reducing aortic pressure and therefore coronary perfusion pressure
Increase the work of the heart, increasing myocardial oxygen demand
Pt may have: palpitations, chest discomfort, restlessness/anxiety, pale/cool skin, syncope from hypotension