Cardiac AP + ECG Interpretation Flashcards
What four ions determine the electro-chemical gradient in cardiac cells?
K+
Na+
Ca2+
Cl-
What is the resting membrane potential in cardiac cells?
-90mV
What ions are primarily responsible for the resting membrane potential of cardiac muscle?
K+ equilibrium potential
At rest only these channels are open and the RMP is -90mV
K+ channels
What causes voltage gated Na+ ion channels to open?
Wave of depolarization from one end of the membrane causes the adjacent sodium channels to open.
What does cardiac AP depend on?
Similar to skeletal muscle it depends on time varying membrane conductance.
What are the 5 phases of the cardiac action potential?
Phase 0 - Upstroke
Phase 1 -Early Repolarization
Phase 2 - Plataeu
Phase 3 - Actual Repolarization
Phase 4 - RMP
What happens in Phase 0 of Cardiac AP
Heart muscle reaches threshold to generate AP.
Fast upstroke = fast AP = fast conduction
Membrane permeability = Na+
What happens in phase 1 of cardiac AP
Early repolarization. VG K+ channels open just a little and K+ eflux causes small repolarization to happen. Little drop in MP
What happens in phase 2 of cardiac AP?
Plateau. There is abalance between Ca2+ influx and K+ eflux so slight balance between depolarization and repolarization. This is the main difference from skeletal muscle APs
What happens in phase 3 of cardiac AP?
Actual Repolarization = VG K+ channels open for real and no more Ca2+ influx. Moving back to K+ equilibrium.
What happens in phase 4 of cardiac AP?
Background K+ channels are open again so cells reach RMP again. Back at around -90mv
What phases of cardiac AP correspond with EKG reading?
Phase 0 = QRS complex
Phase 1 = end of QRS complex
Phase 2 = ST segment
Phase 3 = T wave
The QRS is primarily caused by movement of what ion?
Myocardial Na+ ion.
The T wave is a result of movement of what ion?
K+ ion movement
What part of the Cardiac AP is the refractory period?
During the plateau phase and during first part of the actual repolarization phase — this part the myocardium cannot be stimulated again.
Phase 2 - Phase 3 of AP graph
During abnormal conditions the myocardium CAN be stimulated here?
Later part of actual repolarization phase (phase 3) and hyperpolarization
Abnormal conditions like: ischemia, re-entrant currents, and altered electrolytes
A patient with Hypokalemia might affect what phases of cardiac AP?
Slower time course of phase 2 and 3 due to not enough K+ so repolarization takes longer.
What direction is the deflection when depolarization current moves toward the electrode?
Positive deflection above isoelectric line
What direction is the deflection when depolarization current moves away from the electrode?
Negative deflection below isoelectric line
AT REST, what channels are open and what is equilibrium potential?
K+ channels open
Na+ channels close
Eq Potential = -90mV
During DEPOLARIZATION what channels are open and what is membrane potential?
K+ stay open
Na+ open
Membrane potential = +30mV
When an ECG is calibrated how much does the little block measure? And the big block?
Little block measures 0.1mV vertical and measures 0.04 seconds horizontally
Big block measures 0.5mV vertically and measures 0.2 seconds horizontally
What are the 12 leads for a 12 lead EKG?
- 6 Limb leads = bipolar leads 1, 2, 3 and unipolar leads AVr, AVl, AVf
- 6 chest leads = V1-V6
What is the sequence for the conduction of the heart?
SA node -> AV node -> slight pause in AV node -> Bundle of His -> L/R bundle branches -> purkinje fibers -> spread depolarization to myocytes.
How does depolarization spread to R and L atrias?
Via inter- and intra- atrial tracts called Bachman’s bundle
Damage to the inter and intra atrial bundles can lead to?
Disruption of the P wave
Atrial Fibrillation
What are the components to a normal wave form on an EKG?
P wave - atrial depolarization
QRS - ventricular depolarization
T wave - Ventricular repolarization
What are the intervals of an EKG?
PR interval and QT interval
What does the PR interval show?
Time for the SA node to fire, get to AV node, AV node to pause for a sec, and start signal to ventricles .
Duration of the PR interval depends on conduction velocity at the AV node
What does the QT interval show?
Full time it takes for the ventricles to fully contract and fully relax.
What does the QRS reflect?
Conduction through the myocardium. Due to extensive branching of the purkinje fibers.
What does the ST segment represent?
Delay between depolarization and repolarization of the ventricles. Normally at the isoelectric line
How can you determine HR from an EKG?
- If you get a 6 second strip - count number of QRS and multiply by 10 — just an estimate
- Can count down from the a QRS on a darker line and count each big box down 300-150-100-75-60.
- Can count number of small boxes from one QRS to the next and divide into 1500
What is a good sequence to interpret an ECG?
- Is it regular?
- Is the HR normal - between 60-100
- Look at P and QRS - are there p waves, and is it a 1:1 ratio
- Look at QRS complex - is it too long?
- Look at PR interval - is it too long?
What can sinus bradycardia be caused by?
Normal in athletes due to increased stroke volume
- can be caused by beta blockers, calcium channel blockers, antiarrhythmic drugs, or vagal stimulation
What is sinus arrhythmia?
Irregularity caused by SA node often due to altered vagal stimulation.
What does a left axis deviation suggest?
LV Hypertrophy
What does a R axis shift suggest?
RV Hypertrophy or MI
What is the first thing checked when MI is suspected?
If T wave is inverted or ST segment depression is present.
What are the 3 important determinants of CO?
- Preload
- Afterload
- Contractility
What is preload
Volume with which ventricle is loaded
- determined by venous return and atrial kick
- starling law = greater stretch= greater force
What is afterload
Resistance to blood flow
- Diastolic BP is primary determinant
What is contractility?
State of the cardiac muscle with regard to ability to generate force
- In heart failure contractility goes down
- in Acute and chronic exercise contractility goes up
How is contractility typically measured?
Ejection Fraction
What does the contractile state of the heart depend on?
- Intrinsic factors = training, disease, structure
- Autonomic Nervous System - ACh for vagus N, NE for cardiac nns
- Hormonal factors = epi, angiotensin, other hormones
How is contractility regulated?
Through Ca2+
- amount of calcium influx determines the level of activation of cardiac muscle
- drugs like calcium channel blockers, beta blockers, digitalis affect contractility by altering Ca2+ delivery
What is the baroreceptor cardiac reflex?
- Carotid body senses increases in BP and alters vasodilation, HR, and contractility to normalize BP
What is the bainbridge Reflex?
R Atrium senses increases in blood volume and modulates HR
* responsible for respiratory ECG rhythm
What is the chemoreceptor cardiac reflex?
Brain stem sense CO2, H+, and O2 levels and alters HR, BP, contractility, and respiration
Where is the primary site of resistance in the body’s circulatory system?
Arterioles
Smaller radius
What is most of the body’s resistance set up?
In parallel. Loss of a part of the body like amputation increases the total peripheral resistance
What is the radius of a blood vessel determined by?
- Local metabolic needs
- Vasoactive substances - adenosine, K+, H+
- CNS - sympathetic NS
- Hormones - Epi, angiotensin, antidiuretic H
Where does nutrient and waste exchange occur in circulatory system?
Capillaries and venules by 2 different mechanisms - diffusion and filtration
What is diffusion vs. Filtration really important for?
Diffusion = most important for nutrient/waste exchange
Filtration = most important for fluid balance
What are the effects of exercise on contractility and HR?
Contractility and heart rate increase at the same time that resistance decreases causing a net effect of SBP increase
What happens to DBP when large mm groups are exercised vs when small mm groups are exercised?
- large mm groups = DBP either remains constant or may decrease
- small mm groups = DBP may increase slightly
Resistance exercise produces an increase in both SBP and DBP
True
What are some abnormal responses to exercise on HR?
- HR rises very rapidly — deconditioning or CV problem limiting the Stroke Volume
- HR rises just a little — often due to cardiac meds or heart disease
- Decrease in HR — indicative of severe disease and/or arrhythmias
