Electrical Activity of the Heart: Electrocardiogram
electrical and mechanical order of events
1. Atrial Depolarization 2. Atrial Systole 3. Ventricular Depolarization 4. Ventricular Systole 5. Ventricular Repolarization 6. Ventricular Diastole
5 squares on the ECG = ________ sec time interval
5 squares on the ECG = 1 second time interval
A patient who is 5 weeks post-MI is participating in a cardiac rehab program. The PT is monitoring responses to increasing exercise intensity. Which finding is an indication that exercise should be immediately terminated? A. 1.5 mm of downsloping ST segment depression B. Peak exercise HR of 140 bpm C. Appearance of 1 PVC on the ECG D. BP of 140/80 mmHg
A. 1.5 mm of downsloping ST segment depression
SCENARIO: A patient with a recent history of a myocardial infarction (MI) was admitted to the hospital within the past 24 hours. His electrolyte levels are as following: Potassium (K): 2.2 mEq/L (3.7-5.1 mEq/L) Sodium (Na): 133 mEq/L (134-142 mEq/L) Calcium (Ca): 9.0 mg/dL (8.6-10.3 mg/dL) QUESTION: Therefore, which lab value is most concerning? A. Potassium B. Sodium C. Calcium D. None, all are within safe range
A. Potassium
Which of the following ECG changes is most specifically associated with hyperkalemia? A. Tall (spiked) T wave B. Shortened PR interval C. Shortened QRS interval D. Atrial fibrillation
A. Tall (spiked) T wave
ST Segment Elevation and Acute MI
Acute ST elevations present in leads over the infarcted area -Change in ST segment > 1.0 mm is considered significant
effects of myocardial ischemia and injury
Affects repolarization of the heart -Ventricles are not receiving enough oxygen -Takes longer for the ion pumps to restore membrane potential Manifests as abnormalities in the T wave and ST segment
atrial fibrillation
Atrial ectopic foci are firing in a random, chaotic manner -Atrial systole does not occur effectively because the atria are excited by many small random waves of depolarization AV node conducts APs whenever the wave of atrial excitation happens to reach it -Ventricles are activated in a "hit or miss" fashion, and therefore ventricular rate can be irregular Associated with numerous disease states but can also occur in otherwise healthy individuals -Ex: cardiomyopathy, pericarditis, hypertension, hyperthyroidism
Which of the following scenarios is most likely to be the least concerning in an individual at rest? A. Ventricular fibrillation B. Sinus bradycardia C. Atrial fibrillation accompanied by dizziness D. Seven consecutive PVCs
B. Sinus bradycardia
PVC on an ECG
Bizarre and wide QRS complex with no preceding P wave Followed by a long compensatory pause before the next normal P wave and QRS complex
An ST segment elevation visualized in leads II, III, and aVF would suggest myocardial infarction in which general section of the heart? A. Anterior B. Lateral C. Inferior D. Posterior
C. Inferior
what are some of the critical questions to ask about arrhythmias (due to variations in arrhythmias being asymptomatic or catastrophic)?
Clinical significance depends on the effect on cardiac output and blood pressure Critical questions: -What is the underlying cause? Is it correlated with exertion? -Is the arrhythmia negatively affecting tissue perfusion? -Is the arrhythmia associated with negative signs and symptoms? Correlated activities/situations?
third-degree (complete) AV block
Complete loss of communication between the atria and ventricles •Atria are electrically separate from the ventricles •AV node becomes the pacemaker for the ventricles, and the heart rate slows to that of the AV node •Ventricles may also generate some electrical impulses on their own
first degree AV block
Conduction of APs through the AV node experiences prolonged delay Marked by a lengthened PR interval (i.e., > 0.2 seconds) -All atrial excitations are eventually conducted to the ventricles Usually asymptomatic Generally no treatment needed -Correct electrolyte imbalances •Evaluate medications
SCENARIO: Long QT syndrome (LQTS) is a condition (either hereditary or acquired) in which delayed repolarization of the heart occurs. In these individuals, the QT interval on an ECG is abnormally long. The QT interval represents the time that elapses from ventricular depolarization to ventricular repolarization. One form of LQTS occurs because of a genetic error in the voltage-gated K+ channels in the heart. QUESTION: What would be a general physiological consequence of impaired cardiac electrical activity?
Development of arrhythmias, which could have significant implications for tissue perfusion
what is an arrhythmia?
Disturbance in heart rate or rhythm caused by an abnormal rate of electrical impulse generation by the SA node or the abnormal conduction of impulses throughout the cardiac electrical conduction system
ECG versus Echo
ECG = electrical activity of the heart Echo = ultrasound to view structures of the heart
conditions associated with hypocalcemia
ETOH abuse poor dietary intake limited GI absorption pancreatitis laxative use
atrioventricular (AV) blocks
Electrical signals are partially or completely blocked as they travel through the AV node, Bundle of His, and/or bundle branches •Bradycardia may also be present
TRUE OR FALSE: Atrial systole occurs before the P wave.
FALSE ATRIAL SYSTOLE OCCURS AFTER THE P WAVE
SCENARIO: Long QT syndrome (LQTS) is a condition (either hereditary or acquired) in which delayed repolarization of the heart occurs. In these individuals, the QT interval on an ECG is abnormally long. The QT interval represents the time that elapses from ventricular depolarization to ventricular repolarization. One form of LQTS occurs because of a genetic error in the voltage-gated K+ channels in the heart. QUESTION: What are some possible symptoms of LQTS? What might predispose a person with LQTS to experiencing these symptoms?
Fainting, seizures (if the brain is deprived of oxygen for a period of time), sudden cardiac death (if cardiac electrical activity can't be restored) Symptoms can be exacerbated by stress (e.g. emotional stress or physical exertion) Arrhythmias and their associated symptoms might be more likely to develop when the body is placed under extra demand.
ECG abnormalities associated with hypokalemia
Flattened T wave followed by a secondary repolarization (U wave) ST segment depression Apparently long QT interval (due to fusion of T and U waves)
what are chest (precordial) leads trying to see?
Gives a 2D picture of the electrical activity of the heart as it would be viewed from above or below a HORIZONTAL PLANE bisecting the heart Leads V1-V6 Lie very close to the surface of the heart -Give detailed information about the electrical activity in the small, specific portion of the heart that lie beneath each electrode
what are limb leads trying to see?
Gives a 2D picture of the electrical activity of the heart as it would be viewed in the FRONTAL PLANE of the body RA, LA, LL electrodes form the 3-pairs RL electrode is an electrical ground
sinus bradycardia
HR < 60 bpm Example: endurance-trained athlete at rest
sinus tachycardia
HR > 100 bpm Example: a person experiencing hyperthyroidism Example: a person taking certain medications (e.g. certain asthma medications, decongestants, SNRI antidepressants)
electrocardiogram (ECG or EKG)
Impulse conduction through the heart generates electrical currents that can be detected at the surface of the body A recording of the electrical changes that accompany each cardiac cycle (heartbeat) is called an electrocardiogram (ECG or EKG) -Non-invasive diagnostic tool The ECG helps to determine if the conduction pathway is abnormal, if the heart is enlarged, if certain regions are damaged, etc.
hyperkalemia
Increased potassium Serum K+ > 5.5 mEq/L Also consider serum K+ trending upward over time
SCENARIO: Consider two individuals with a resting HR of 45 bpm. One is a marathon runner and the other has a 3rd degree heart block. QUESTION: What are the physiological mechanisms contributing to this low resting HR for each individual? Which individual would be symptomatic and why?
Marathon runner --> LV chamber hypertrophy due to endurance exercise training. SA node is the primary pacemaker --> Increase in SV --> Q and MAP can be maintained with a lower HR --> Heart is healthy; Hemodynamic control and tissue perfusion are normal --> No symptoms Patient with 3rd degree AV block --> AV node becomes primary pacemaker; Intrinsic cardiac pacing slows considerably --> No increase in SV --> Q and MAP cannot be maintained with a lower HR --> Heart is not healthy; Hemodynamic control and tissue perfusion are compromised --> Presence of symptoms
3rd degree heart block on ECG
Marked by a P wave, QRS complex, and a T wave that are present but separated
atrial fibrillation on an ECG
NO TRUE OR DISCERNABLE P WAVES Unless other abnormalities are present, conduction through the AV node and ventricles is normal -Cardiac output is usually maintained if rate is controlled -May precipitate ventricular failure in an abnormal heart
ST Segment Depression
Occurs due to impaired coronary perfusion Digitalis medication may also depress the ST segment Can be upsloping, horizontal, or downsloping -Change in ST segment > 1.0 mm is considered significant
1st degree heart block on ECG
PR interval that lasts longer than 0.2 seconds
second-degree AV blocks
Partially blocked conduction through AV node, Bundle of His, and/or bundle branches -Only some of the atrial excitations are conducted by the AV node -Portion of the heart with the blocked branch receives delayed activation from the unblocked side of the ventricles
Connecting QT interval with events of the ventricular AP
Phase 0 of ventricular AP = QRS complex = ventricular depolarization Phase 2 of ventricular AP = ST segment = plateau Phase 3 of ventricular AP = T wave = ventricular repolarization
SCENARIO: Long QT syndrome (LQTS) is a condition (either hereditary or acquired) in which delayed repolarization of the heart occurs. In these individuals, the QT interval on an ECG is abnormally long. The QT interval represents the time that elapses from ventricular depolarization to ventricular repolarization. One form of LQTS occurs because of a genetic error in the voltage-gated K+ channels in the heart. QUESTION: Can you think of any special considerations regarding exercise that may be relevant for a person with LQTS?
Possible avoidance of strenuous physical exertion, or contact sports Exercising with a partner Staying hydrated Special considerations associated with various medications (e.g. beta-blockers), pacemakers, ICDs
premature ventricular contraction (PVC)
Premature beat arising from the ventricles -Occurs occasionally in the majority of the normal population Spontaneous generation of an AP in the ventricular tissue -Ectopic focus in the Purkinje fibers or ventricular myocardium that reaches threshold before being depolarized by the normal wave of excitation Probability of increased by factors such as cigarette smoking, nicotine, physical or emotional stress, caffeine, and fatigue
ECG abnormalities associated with hypocalcemia
Prolonged QT interval *Calcium is one of the triggers for ventricular repolarization during phase 3 of the ventricular AP *Low calcium levels can delay ventricular repolarization
ventricular fibrillation (V-fib)
Random, chaotic, uncoordinated activation of ventricular cells originating from multiple foci -Ventricles cannot fill or pump -No pulse -No effective cardiac output Medical emergency -Requires CPR, defibrillation, medications -If a regular rhythm cannot be established, clinical death will result in 4-6 minutes
how do you calculate HR from an ECG?
Recognize that 5 squares on the ECG = 1 second time interval Count the number of squares on the ECG strip and convert to a time interval for the strip (Ex: 30 squares/5 squares per sec = 6 second time interval) Count the number of QRS peaks in that time interval (Ex: 7 QRS peaks = 7 heartbeats in the strip of 30 squares) Convert the number of beats per time interval in the strip to beats per minute (bpm) *Remember that 60 seconds = 1 minute (Ex: 7 beats/6 seconds * 60 seconds/1 minute = 70 bpm)
clinical implications of the QT interval
Represents the combined influence of all ventricular APs Measures the total duration of ventricular activation Delayed repolarization is reflected in a lengthened QT interval, which is associated with the genesis of ventricular arrhythmias
normal sinus rhythm
Resting heart rate of 60-100 bpm P wave, followed by QRS complex, followed by T wave •Uniform shape and size for each type of wave •Uniform distances between each type of wave •Intervals are in the normal range WHEN YOU HEAR SINUS THINK SA NODE -- SA node is driving electrical activity
electrode placements for a 12-lead ECG
Right arm (RA): upper R side of torso, beneath the clavicle Left arm (LA): upper L side of torso, beneath the clavicle Right leg (RL): vertical with RA, on the right side of the navel Left leg (LL): vertical with LA, on the left side of the navel V1: 4th intercostal space, R sternal border V2: 4th intercostal space, L sternal border V3: Between V2 and V4 V4: 5th intercostal space on the left side in the mid-clavicular line V5: Horizontally parallel to V4 in the anterior axillary line V6: Horizontally parallel to V4 and V5 in the mid axillary line
ventricular tachycardia (V-tach)
Run of ≥ 4 PVCs occurring sequentially Abnormally high rate of ventricular activation generated from ventricular ectopic foci -Usually the result of an ischemic ventricle, and may occur as an abrupt onset -150-200 bpm IMPORTANT***Serious compromise to cardiac output -Extremely high HR limits ventricular filling because heart is contracting too rapidly
When does ventricular systole occur?
ST segment after the S wave to the beginning of the T wave
hypocalcemia
Serum calcium < 8.6 mg/dL Also consider serum calcium that is trending downward over time
2nd degree heart block on ECG
Several different possibilities for a 2nd degree heart block ECG abnormality *Progressive elongation of the PR interval *QRS complex is dropped *Widened QRS interval; abnormalities in the shape of the QRS complex
ECG abnormalities associated with hyperkalemia
Spiking of T waves (Tall T waves) Flattened P waves -> prolonged PR intervals -> loss of P waves Wide QRS complexes Sine wave pattern Ventricular fibrillation
TRUE OR FALSE: Electrical activity of the heart controls mechanical activity of the heart.
TRUE
TRUE OR FALSE: With ventricular tachycardia, the SA node is no longer running things --> ectopic foci are generating ventricular activation
TRUE
TRUE OR FALSE: Ventricular systole occurs after the S wave.
TRUE VENTRICULAR SYSTOLE OCCURS AFTER THE S WAVE.
echocardiogram (Echo)
Use of ultrasound to observe the structure and function of the heart, heart valves, and vessels attached to the heart
sinus arrhythmias
Variation in HR from cardiac cycle to cardiac cycle •Resting HR < 60 bpm or > 100 bpm Everything else is normal SA node is still driving electrical activity Generally no contraindications to exercise 2 major types: sinus bradycardia, sinus tachycardia
ECG abnormalities associated with hypercalcemia
Widened QRS complex Shortened QT interval *Calcium is one of the triggers for ventricular repolarization during phase 3 of the ventricular AP *High calcium levels can speed up ventricular repolarization
non-sustained ventricular tachycardia (NSVT)
a run of ≥ 4 PVCs occurring sequentially, terminating spontaneously in less than 30 seconds
how are arrhythmias classified?
according to origin, pattern, and rate at which they occur •Supraventricular (atrial) vs. ventricular •Fibrillation vs. flutter •Tachycardia vs. bradycardia
When does ventricular diastole occur?
after the T wave
major signs and symptoms of hypocalcemia
anxiety confusion agitation seizure fatigue numbness/tingling increased reflexes muscle cramps *Symptoms-based approach when determining appropriateness of activity
What is occurring during the P wave?
atrial depolarization
P wave
atrial depolarization depolarization of the SA node and atrial fibers typically lasts 0.06-0.1 seconds
What is occurring during the PR segment?
atrial systole
PR segment
atrial systole occurs here after the P wave
Lead aVL
augmented left arm midpoint of Lead II (from RA to LL) to LA
Lead aVF
augmented left foot midpoint of Lead I (RA to LA) to LL
Lead aVR
augmented right arm midpoint of Lead III (from the LA to LL) to RA
2nd degree heart block: if ventricular rate is slowed, then ______________________ is decreased.
cardiac output *may need to be treated with a pacemaker
3rd degree heart block: cardiac output is ____________________.
compromised **treatment requires medications (atropine) and pacemaker
list possible causes of AV blocks
congenital presence aging previous MI heart failure heart valve abnormalities heart valve surgery certain medications (e.g. digitalis, beta blockers, calcium channel blockers) exposures to toxic substances Lyme disease electrolyte disturbances AV node disease
arrhythmias: medical treatment goals
control ventricular rate, prevent thromboembolism, and restore normal sinus rhythm •Medications •Pacemakers •Implantable cardioverter defibrillators (ICDs) •Radio-wave ablation
hypokalemia
decreased potassium Serum K+ < 3.5 mEq/L Also consider serum K+ trending downward over time
conditions associated with hypokalemia
diarrhea/vomiting GI impairment diuretics Cushing's Syndrome malnutrition restrictive diet ETOH abuse
multifocal PVC
electrical activity arises from multiple ectopic sites in the ventricles
unifocal PVC
electrical activity arises from the same ectopic site in the ventricles
conditions associated with hypercalcemia
excessive calcium supplements/antacids bone destruction/tumor cancer immobilization fracture excessive vitamin D renal failure
major signs and symptoms associated with hypokalemia
extremity weakness decreased reflexes paresthesia leg cramps constipation hypotension cardiac arrest *Symptoms-based approach when determining appropriateness of activity *K+ < 2.5 mEq/L: consult with interprofessional team
hypercalcemia
increased calcium Serum calcium > 10.3 mg/dL, s Also consider serum calcium that is trending upward over time
Lead III
line of sight along the frontal plane: LA to LL
Lead I
line of sight along the frontal plane: RA to LA
Lead II
line of sight along the frontal plane: RA to LL
major signs and symptoms of hypercalcemia
muscle weakness decreased reflexes lethargy nausea/vomiting constipation coma heart block ventricular dysrhythmias *Symptoms-based approach when determining appropriateness of activity
major signs and symptoms of hyperkalemia
muscle weakness/paralysis paresthesia bradycardia heart block ventricular fibrillation cardiac arrest Risk for cardiac issues when K+ > 5 mEq/L *Systems based approach when determining appropriateness of activity
Asystole
no cardiac electrical activity
ST segment
portion between the end of the S wave and the beginning of the T wave Usually flat Ventricular systole and atrial diastole are occurring here
ECG provides information about __________________________________________.
rate rhythm of excitation pattern of conduction can tell whether the heart has hypertrophy or not
conditions associated with hyperkalemia
renal failure metabolic acidosis diabetic ketoacidosis Addison's disease excess K+ supplements blood transfusion
sustained ventricular tachycardia (VT)
run of sequential PVCs that lasts > 30 seconds and/or requires termination due to hemodynamic compromise in < 30 seconds
If the QRS interval is lengthened, then _____________________________________________.
there are problems conducting the APs through the His-Purkinje system
QT interval
time between the beginning of the QRS complex and the end of the T wave -Represents ventricular depolarization through repolarization -Affected by drugs or conditions that alter the rate of ventricular repolarization
PR Interval
time from the beginning of the P-wave to the start of the QRS complex -Represents the amount of time it takes for the APs to travel from the SA node through the AV node -Typically lasts 0.12-0.2 seconds -Represents the delay of the APs at the AV node, giving the atria time to contract and time for the ventricles to fill -A lengthened PR interval may suggest inhibited conduction through the AV node
What is occurring during the PR interval?
time it takes for the APs to travel from the SA node through the AV node represents time of all atrial events
QRS Interval
time it takes for the APs to travel through the His-Purkinje system and the ventricles -Rapid, synchronous excitation of the ventricles -Typically lasts 0.06-0.1 seconds (same duration as the P wave) -Lengthened QRS interval may suggest inhibited conduction through the His-Purkinje system
What is occurring during the QRS complex?
ventricular depolarization
QRS Complex
ventricular depolarization repolarization of the atria also occur here, but this is not specifically visualized on the ECG *beep on the monitor is timed with the QRS Complex
What is occurring during the QT interval?
ventricular depolarization through repolarization time it takes to achieve all ventricular events
T wave
ventricular repolarization
What is occurring during the T wave?
ventricular repolarization
What is occurring during the ST segment?
ventricular systole and atrial diastole
list leads and associated area where the ischemia or infarction occurs
•Anterior wall: changes in V1-V6 •Anteroseptal: changes in V1-V2 •Anteroapical: changes in V3-V4 •Anterolateral: changes in V5-V6, I, aVL •Lateral wall: changes in I, aVL •Inferior wall: II, III, aVF •Posterior wall: not seen on a typical ECG
major symptoms of 3rd degree heart block
•Bradycardia •Hypotension •SOB •Angina •Lightheadedness, weakness, dizziness, syncope •Fatigue, exercise intolerance
clinical significance of reading ECGs
•Differentiating normal from abnormal •Underlying mechanisms •Effects on cardiac output and hemodynamic control •Signs/symptoms •Exercise safety
exercise and arrhythmias
•Exercise can increase certain arrhythmias because of increased SNS activity and catecholamine levels •Certain arrhythmias may disappear with exercise and increased perfusion •Importance of proper pre-exercise screening •Careful monitoring during and after exercise •Take a symptoms-based approach when determining appropriateness of activity
PVC possible symptoms
•Heart feels like it is pounding or jumping, fluttering or flip-flopping •Skipped heart beats •Increased awareness of heart beat •Dizziness, weakness
2nd degree heart block variation of symptoms
•May be asymptomatic •Lightheadedness, dizziness, syncope •Chest pain •Irregular HR •Bradycardia •Hypotension
List the ECG Rhythms we Need to Know
•Normal sinus rhythm •Sinus arrhythmias •Atrial fibrillation •Premature ventricular contraction •Ventricular tachycardia and fibrillation •Atrioventricular blocks •Effects of altered serum potassium and calcium •Effects of myocardial ischemia and injury
Although exercise is beneficial for individuals with A-fib, what are some of the signs and symptoms indicating that exercise intensity is too strenuous?
•Racing heart •Feeling faint •Shooting arm pain •Confusion, disorientation, difficulty thinking clearly, slurred speech (or other changes in speech patterns) •Sudden weakness on one side of the body, unsteadiness
List some S&S that would indicate that a patient's exertion level is too high.
•Racing heart rate, emergence of arrhythmias, spike in BP •Heart rate and SBP that does not increase (or decreases) with increased exertion •Desaturation •Feeling faint •SOB •Shooting arm pain, angina •Sudden weakness on one side of the body •Altered mental status, confusion, disorientation, slurred speech or other changes in speech patterns •Pallor, flushing, profuse sweating
signs and symptoms of ventricular tachycardia (V-tach)
•Rapid heart beat, palpitations •Angina •Neck tightness •SOB •Dizziness, lightheadedness, fainting •Cardiac arrest
PVCs may not be a problem, but they are meaningful if:
•They occur in paired or sequential runs, or > 6 occur per minute •They increase in frequency over a period of time, or if they become a regular occurrence •They are multifocal •They occur early after the T wave (R on T phenomenon) **Meaningful because a PVC does not pump blood effectively
