Electrocardiography Flashcards
What does an ECG do?
traces electrical impulses from the heart over time (summed surface potential changes that take place)
- determines HR and conduction disturbances
- gives info regarding size of chambers, myocardial hypoxia, and electrolyte imbalances
What are some indications for performing an ECG?
- exact diagnosis of arrhythmias heard on auscultation
- understanding onset of dyspnea or respiratory distress
- monitoring during shock
- understanding if there is a cardiac component to collapse, fainting, or seizures
- monitoring during/after surgery
- determining chamber size and severity of murmurs
- cyanosis
- pre-op in older patients to make sure there is not an issue prior to anesthesia
- evaluates the effect of certain cardiac drugs - digoxin, quinidine, propranolol, atenolol, sotalol
- electrolyte disturbances
- serial ECG for prognosis and monitoring heart disease
What are some causes of arrhythmias?
- electrolyte abnormalities - calcium (eclampsia), postassium (blockage, Addisons)
- hypoxia
- GDV
- pancreatitis, splenic disease
- intracranial disease
- myocardial trauma/blunt chest trauma
- heat stroke (DIC)
- hypothermia
- intoxication/drug-related
- infectious disease
- endocrine disease
- vagotonia/dysautonomia
- autoimmune disease
- electrocution
What 3 criteria are used to determine immediate danger of arrhythmias?
- rate
- duration
- effect on cardiac function
What is the purpose of pacemaker cells?
create electrical impulses and transmit them in organized manner to the rest of the myocardium
(captured by ECG electrodes)
What is the difference between depolarization and repolarization?
DEPOLARIZATION - heart muscle contraction in response to electrical stimulus, occurring when electrolytes move across the cell membrane via the sodium potassium pump
REPOLARIZATION - heart muscle relaxation occurs when the electrolytes move back across the cell membrane, rendering the cell ready for the nect electrical impulse
What are the 5 physiologic properties of cardiac muscle?
- AUTOMATICITY - SA node is the primary pacemaker, but any cells can initiate their own impulses (further down the system = slower rate of automaticity)
- EXCITABILITY - electrical stimulus reduces resting potential to threshold; all or none
- REFRACTORINESS - will not respond to external stimuli while contracting
- CONDUCTIVITY
- CONTRACTILITY
What is conductivity? How does velocity differ?
activation of an individual muscle cell produces activity in the neighboring cells
varies based on different portions of the specialized conduction system and muscle fiber - Purkinje fibers > mid portion of AV node
What is contractility?
tension developed and velocity of shortening of myocardial fibers at a given preload and afterload as a result of electrical currents
- ECG measures stimulus, not actual contraction so ECHO is the tool of co=hoice for assessing it
What are the 5 stages of cardiac myocyte propagation?
- pacemaker cells spontaneously depolarize
- generates current moves cell to cell via gap junctions
- current stimulates opening of Na channels in cardiac myocytes
- cells reach threshold and fire (all or none depolarization)
- cells re-establish resting membrane potential (repolarization)
What augments pacemaker spontaneous diastolic depolarization (SDD)? What are the 2 responses?
sympathetic and parasympathetic tone
- SYMPATHETIC = increases rate of spontaneous depolarization and HR
- PARASYMPATHETIC (vagal) = decreases rate of spontaneous depolarization and HR
What are the 5 anatomic sites of cardiac conduction?
- SA node (right-sided heart failure more likely to develop arrhythmias)
- AV node
- bundle of His
- bundle branches
- Purkinje fibers
How do the waves on ECGs correlate to cardiac conduction?
- P wave = through atria —> SA node, intraatrial/intranodal conduction tracts, AV node
- Q wave = bundle of His, L and R bundle branches
- R wave = ventricular wall, Purkinje fibers
- S wave = ventricular base depolarization
- T wave = ventricular repolarization
ECG and cardiac conduction:
What happens if the SA node fails?
other cells take over —> fasted pacemaker cell will control HR
- SA node = 80-100 bpm
- atrial cells = 55-60 bpm
- AV node = 40-60 bpm
- His bundle = 40-60 bpm
- bundle branches = 20-40 bpm
- Purkinje fibers = 20-40 bpm
6 lead ECG:
What are the 2 standard rates of ECG paper speed? How are boxes used for measurement at each speed?
- 25 mm/s - 25 small boxes = 1 second (1 mm box is 0.4 s)
- 50 mm/s - 50 small boxes = 1 second (1 mm box is 0.2 s)
If the paper is going 25 mm/s, what is the heart rate of this patient?
5 large boxes = 1 second
10 large boxes = 2 seconds
(3 beats/2 seconds) x (60s/min) = 90 bpm
If the paper is going 50 mm/s, what is the heart rate of this patient?
5 large boxes = 0.5 s
10 large boxes = 1 s
(3 beats/s) x (60s/min) = 180 bpm
How can a BIC pen be used to calculate HR based off of ECGs?
a BIC pen will measure 3 seconds on 50 mm/s paper and 6 seconds on 25 mm/s paper
count the amount of beats within the pen margins and multiply them by 20 and 10 to get how many beats in 60 seconds
Comparing paper speeds:
can increase paper speed to observe bunched up ECG waves due to high HRs
If the paper is going 25 mm/s, what is the heart rate?
(13 beats/6 s) x (60s/min) = 130 bpm
(can just multiple 13 by 10)
What is the difference between P pulmonale and P mitrale?
P PULMONALE = tall P wave suggesting a large RA (peak)
P MITRALE = wide P wave suggesting a large LA (mound)
(bi-atrial enlargement = BOTH)
P mitrale vs. P pulmonale vs. both
- LA enlargement = wide P wave
- RA enlargement = tall P wave
- bi-atrial enlargement = wide AND tall P waves
What does the PR interval measure?
AV nodal depolarization time (longest phase!)
(beginning of P wave to beginning of QRS)
What does the QRS complex measure?
ventricular depolarization
Tall R waves:
LV enlargement = more muscle to pass through
Deep S waves:
RV enlargement/hypertrophy
ST segment depression:
myocardial ischemia
