Cardiac Physiology and ECG Flashcards
What is an ECG?
-electrocardiography -provides a record of the net cardiac electrical activity measured between two points on the body surface as it changes with time -cell-cell propagation of cardiac AP occurs as a result of gap junctions
ECG is the primary clinical diagnostic tool for what 3 main things?
-cardiac arrhythmias -myocardial injury -disturbances in heart rate, rhythm, and wave-front propagation
what is diastole?
part of the cardiac cycle when the heart refills with blood
what is systole?
part of the cardiac cycle when ventricles contract
When does ventricular filling occur?
mid to late ventricular diastole (includes atrial systole)
what is ventricular systole?
isovolumetric contraction and ejection phase
what is the quiescent phase?
isovolumetric relaxation in early ventricular diastole until atrial contraction
What are characteristics of the first heart sound?
-S1 or “lub” -AV valves close at the beginning of ventricular contraction; systole
what are characteristics of the second heart sound?
-S2 or “dub” -produced by closure of semilunar valves; beginning of ventricular diastole
what are specialized (pacemaker) cells?
-cells responsible for the initiation and conduction of electrical signals (APs) through the heart -include SA and AV nodes, bundle of His, bundle branches, and purkinje fibers
what are working myocardial cells?
-responsible for contraction and relaxation -majority of the mass of the heart muscle
describe the normal activation sequence of the electrical conduction system of the heart
SA node –> atria –> AV node –> His bundle –> Bundle branches –> Purkinje fibers –> ventricles
What is the SA node?
normal pacemaker that initiates the AP that is conducted through the heart
what is the AV node?
slowly conducting cells that create delay between atrial contraction and ventricular contraction
what are purkinje fibers?
specialized for rapid conduction and ensure that all ventricular cells contract at nearly the same instant
which cells normally control heart rate?
the electrical activity of the SA nodal cells
Describe the 5 phases in the time course of the cardiac action potential
Phase 0 = upstroke, rapid depolarization
Phase 1 = rapid repolarization following the peak
Phase 2 = depolarized plateau
Phase 3 = rapid repolarization following the plateau
Phase 4 = period between max negativity
**most cardiac cells only exhibit phase 0-3, whereas pacemaker cells exhibit all phases
what is automaticity?
property of specialized cardiac cells to spontaneously fire APs
cardiac APs are the result of transient changes in what?
-the ionic permeability of the cell membrane -changes are triggered by initial depolarization in pacemaker
what is the absolute refractory period?
-working myocardial cells cannot be stimulated to fire another AP through most of the AP -precludes summated or tetanic contractions from occuring
what is the relative refractory period?
-myocardial cell can fire an AP, but it reqires a greater than normal stimulation to do so
in cardiac muscle, how much longer can the presence of a plateau in an AP last compare to in skeletal muscle?
up to 15x longer
in phase 0, once a fast-response AP is initiated, what happens to the high sodium permeability?
it is short-lived after initiation
what is the depolarization plateau state (phase 2) a result of?
the interactions of a sustained reduction in K+ permeability and a sustained increase in Ca2+ permeability
T or F: in pacemakers, there is no fast inward Na+ current, but rather a slower Ca2+ inward current (depolarization)
true
what causes repolarization (phase 3) in both fast and slow APs?
increase K+ permeability
what is the equation for cardiac output?
CO = (SV)(HR) stroke volume times heart rate stroke volume is calculated by end diastolic volume minus end systolic volume
what is TPR?
-total peripheral resistance -sum of the resistance of all peripheral vasculature in the circulatory system
how is blood pressure calculated?
BP = CO x TPR
what two factors can be altered to maintain blood pressure?
cardiac output and/or total peripheral resistance
what is the baroreceptor reflex a response to?
-a change in arterial pressure -reaction is to increase or decrease heart rate as needed
what is the bainbridge reflex a response to?
-changes in blood volume -protective reflex increases heart rate when there is an increase in atrial pressure
what is normal sinus rhythm?
when the SA node is acting as the pacemaker
what is tachycardia and bradychardia?
normal heart rate is 60-100 beats/min
- tachycardia - HR > 100 beats/min
- bradycardia - HR < 60 beats/min
if heart rate is > 100 beats/min or < 60 beats/min
sinus tachycardia and sinus brachycardia
describe what is happening in this figure
ECG waves and intervals
- P wave: atrial depolarization
- PR interval: atrioventricular conduction
- QRS complex: duration ventricular depolarization
- ST segment: approximate index of ventricular AP plateau; isoelectric
T-wave: ventricular repolarization
QT interval: ventricular AP
describe steps 1-8
1) SA node initiates impulse
2) atrial muscle depolarization
3) atrial contraction
4) depolarization of AV node and common bundle
5) atrial muscle repolarization
6) ventricular muscle depolarization
7) ventricular muscle contraction
8) ventricular muscle repolarization
who created the ECG?
Dr. Willem Einthoven
T or F:
conduction through the atria and AV node is fast compared to the His bundle, bundle branches, and purkinje fibers
false
it is slow
when does the ECG return to baseline?
when all cells are depolarized (or hyperpolarized)
what would happen if all ventricular cells depolarized at once?
there would be no QRS wave
name 4 major ECG abnormalities that occur as a result of acute myocardial infarction
1) ST elevation
2) T wave inversion; indicates ischemia
3) exaggerated Q waves
4) ST depression; may also indicate ischemia
what is the R-R interval?
- the interval in the ECG that is between ventricular beats
- it is used to calculate HR
- the faster the HR, the shorter the R-R interval
how does electrode position effect direction and size of ECG waveform?
- there is an electric dipole caused by the surface charge separation during depolarization and repolarization
- think of leads as dipoles
- when electrode paiirs are oriented parallel to the dipole, the recorded potential difference will be maximal
- dipole will normally travel toward negative end
what will happen if the positive recording electrode faces a wave of depolarization?
it will record an upward signal
if the positive recording electrode approaches the negative electrode, what will happen?
the deflection will be negative (downard)
describe voltage and time scale from an ECG
- each large box is equivalent to 0.5 mV and 0.2 sec
- each large box is subdivided into 5x5 smaller squares such that each small box is equivalent to 0.1 mV and 0.04 sec
describe Einthoven’s triangle
-it is a 3 bipolar lead ECG
- Lead I: (-) electrode on RA and (+) electrode on LA
- Lead II: (-) electrode on RA and (+) electrode on LL
- Lead III: (-) electrode on LA and (+) electrode on LL
what is Einthoven’s law?
dipole calculation:
lead I + lead III = lead II
what are the 3 types of augmented leads?
- aVR (Augmented Vector Right), is perpendicular to Lead III and looks towards the right upper portion of the heart.
- aVL (Augmented Vector Left) is perpendicular to Lead II and looks towards the left upper portion of the heart.
- aVF (Augmented Vector Feet) is perpendicular to Lead I and looks towards the inferior portion of the heart.
what are precordial leads?
- views the horizontal plane of the heart
- includes 6 leads (V1-V6)
what are common ECG signs of supraventricular tachycardia?
- atria are abnormally excited and drive the ventricles at a very rapid rate
- QRS complexes may or may not appear normal; P and T waves may be superimposed when the HR is very high
- low BP and dizziness due to insufficient diastolic time for ventricular filling
what are common ECG signs for atrial fibrillation?
- irregular heart rhythm
- no clear P waves; cannot determine PR interval
- absence of an isoelectric baseline
- irregular ventricular (R wave) rate
- QRS complexes may be prolonged but do not have to be prolonged
what are common ECG signs for first degree AV block?
- P and R waves both present and regular pattern
- PR interval > 0.2 sec
- ‘marked’ first degree block if PR interval >0.3 sec
- this is typically benign
what are common ECG signs in type I second degree AV block?
- HR is irregular
- P waves present but not regular
- R waves present but not regular
- PR interval >0.2 sec
- cyclic lengthening and then QRS drops
- can be treated with atropine
what are common ECG signs in type II second degree AV block?
- HR is irregular
- P wave present and regular
- R waves present but not regular
- PR interval can be normal, but doesn’t have to be (doesn’t prolong before the QRS drop)
what are common ECG signs in third degree AV block?
- HR is irregular
- P waves present and regular
- R waves present but not regular
- dropped QRS’s; ex 7 Ps but only 4 Rs
- PR interval irregular
- P not linked to QRS (ventricles and atria contracting at own rates aka ventricular escape)
- ventricular escape may lead to syncope or sudden cardiac death
- needs pacemaker immediately
what does the prolongation of the QT interval, or ventricular repolarization, lead to?
- spontaneous ventricular tachyarrhythmias, such as ventricular tachycardia
- can be inherited or aquired
what is torsade de pointes?
- arrhythmia associated with patients with long QT syndrome
- form of ventricular tachycardia
- ECG shows undulation or twisting about baseline
- high risk of sudden cardiac death if sustained; rapidly degenerates into ventricular fibrillation and then hemodynamic collapse
