Chapter 11. Interpretation of ECGs PDF

Interpretation of the Electrocardiogram  What is an ECG? Recording of electrical activity in the heart. Obtained by placing electrodes on the surface of the patient’s body. 12-lead ECG allows 12 different views of the electrical activity in the heart. ECG monitoring...

Interpretation of the Electrocardiogram  What is an ECG? Recording of electrical activity in the heart. Obtained by placing electrodes on the surface of the patient’s body. 12-lead ECG allows 12 different views of the electrical activity in the heart. ECG monitoring to identify gross changes in heart rhythm by placing 3 or 5 leads.  EKG Lead Placement-Standard Leads Einthoven’s Triangle Leads I, II, III are limb leads Lead II best for heart rhythm Limb Leads or Wires  Value of an ECG Identification of primary cause of symptoms. Determination of severity of the problem. Evaluation of therapy. Does not measure pumping ability or cardiac output. Resting ECG tracing does not rule out acute problem such as MI.  Indications for ECG Chief Complaint/Physical exam – Chest pain – Orthopnea – Paroxysmal nocturnal dyspnea – Night sweats – Syncope – Palpitations – Pedal edema – Hypotension Past Medical Hx of heart disease or heart surgery Screening prior to surgery Cardiac Anatomy and Physiology  Types of Heart Cells Pacemaker cells High degree of automaticity and provide electrical power source for the heart Conducting cells Conduct electrical impulse throughout the heart Myocardial cells Contract in response to electrical stimuli and pump the blood Cardiac Anatomy and Physiology  Sinoatrial Node (SA) Electrical activity starts at the SA node. – SA node has the greatest degree of automaticity of all cardiac cells. – It is the pacemaker of the heart. – Controls the rate at which heart beats-60-100 beats/min. – Any impulse generated outside SA node: ectopic impulse; the focus is the site it originates (ie. Atria or ventricle) From SA node = impulse spreads across atria through internodal pathways causing depolarization = contraction.  Autonomic NS The SA node is richly innervated by – parasympathetic nervous system fibers Vagus Nerve – sympathetic nervous system fibers (T1-4, Spinal Nerves). Stimulation of the vagus nerve causes – a decrease in the SA node rate (thereby decreasing the heart rate and force of contraction). Stimulation via sympathetic fibers causes – an increase in the SA node rate (thereby increasing the heart rate and force of contraction).  Atrio Ventricular Junction (AV) The electrical impulse then reaches the AV junction. – Electrical bridge between atria and ventricles. – Contains the AV node and the bundle of His. Delay of 0.1 sec to allow for ventricular filling prior to contraction/systole. AV node is a backup pacemaker, if SA node fails, at a rate of 40-60 bpm. The more the AV node is stimulated the slower it conducts called decremental conduction – With Atrial fibrillation and flutter less AV stimulation and ventricles are less affected.  Bundle of His - Purkinje Fibers After leaving the AV node the impulse travels through bundle of His and then into the left and right bundle branches. At the end of the bundle branches there are projections called Purkinje fibers that take the impulse to the myocardium to create a coordinated contraction.  Dysrhythmias-Causes and Manifestations Hypoxia – Reduce oxygen to myocardium causing ischemia – Reduced PaO2, Hgb, blood flow Ischemia – Causes MI, injury of myocardium Sympathetic stimulation – Physical or emotional stress – Cardiac ischemia Dysrhythmias-Causes and Manifestations Drugs – Certain drugs irritate the heart due to improper levels or poor clearance of drug – Sympathomimetic agents irritate and may cause infarction (cocaine, Ritalin) Electrolyte balance – K+, magnesium and calcium most common to cause irritation Rate – Slow rate, irregular rhythm cause decrease CO Stretch – Atrial or ventricular hypertrophy because of increase workload Depolarization and Repolarization Polarized cells carry negative charge inside and positive charge outside. Sudden loss of negative charge is called depolarization. Return of electrical charge is called repolarization. The waves of this electrical activity are detected by the ECG electrodes. Depolarization and Repolarization Polarized cell is stimulated. K+ moves out Na+ moves into cell causes depolarization K+ moves into cell, Na+ moves out cell is repolarized Waves of electrical activity picked up by ECG. Size of wave determined by the voltage of a particular portion of the heart. Basic ECG Waves  Atrial Depolarization-P wave Depolarization P wave of the atria. Small atria = small wave. Size: < 2.5 mm height and < 3mm length. Repolarization obscured P wave by simultaneous depolarization of the ventricles.  QRS complex Depolarization of the ventricles. Large ventricular mass = larger waves. – Q= first negative deflection – R= first positive deflection. – S= second negative deflection.  T wave T wave indicates ventricular repolarization. U wave : final phase of repolarization.  U wave U wave : final phase of repolarization. Heart is getting ready for depolarization of atria’s.  ECG Paper and Measurements Paper has grid like boxes with light and dark lines. Each small box with light line (1mm x 1mm). Each large box with dark line (5mm x 5mm). Time is measured on horizontal axis. Paper moves at 25 mm/sec. – Each small square = 0.04 sec – Each large square = 0.2 sec – Five large boxes = 1 sec ECG Paper and Measurements Voltage or amplitude is measured on the vertical axis. – 1 small box = 0.1 mv – 10 small boxes = 1 mv Isoelectric baseline must be identified before measuring amplitude. – Flat line before the P wave Isoelectric line  Intervals and Segments Segment-a straight line between 2 waves Interval-contains at least 1 wave plus the connecting straight line P-R interval QRS interval S-T segment RR interval QT interval  PR Interval P-R interval – Time it takes for impulse to go from SA node to the AV junction. – Normal: 0.12-0.2 sec  QRS Interval QRS interval – Time it takes for impulse to travel throughout the ventricles. – Normal: ≤ 0.10 sec  S-T Segment S-T segment – Should be isoelectric or not more than 1 mm above or below. – Abnormally depressed or elevated in MI (evaluate for ischemia or injury). – Figure 10-9 or 11-9 shows elevation and depression.  RR Interval R-R interval – Useful in determining rate and regularity of ventricular contraction. – If RR falls in precise number of seconds = HR easy to calculate. – If RR is variable = arrhythmia.  QT Interval Q-T interval – As HR increases = QT shortens. – As HR decreases=QT lengthens – Common causes of abnormally prolonged Q-T interval Hypokalemia and hypocalcemia  Measuring Heart Rate Number of large boxes between QRS (if no 6 second strip) # boxes divided into 300. – 1 large box between QRS = 300/1 = 300 1=300 – 2 large boxes between QRS = 300/2=150 2=150 – 3 large boxes between QRS = 300/3 =100 3=100 – 4 large boxes = 75 4=75 – 5 large boxes = 60 5=60 – 6 large boxes = 50 6=50 Count # of QRS complexes in 6 seconds x 10 = heart rate Just look at the monitor-my favorite  Heart Rate What is HR of this 6 second strip?  ECG Leads LIMB LEADS – I, II, III, AVR, AVL, AVF. – I, II, III are bipolar (voltage difference between two electrodes). I: L arm (+) R arm (-) II: L leg (+) R arm (-) III: L leg (+) L arm (-) – AVR, AVL, AVF are augmented and unipolar leads. Machine needs to amplify tracings to get adequate recordings. One limb positive, the other limbs negative. AVR: R arm (+) AVL: L arm (+) AVF: L leg (+)  ECG Leads – The 6 limb leads allow a view of the the heart in a vertical plane called a frontal plane. Giant circle that surrounds the patient and lies in the same plane as the patient.  Chest Leads CHEST LEADS – V1 through V6. – Unipolar leads placed in a horizontal plane. – Under normal conditions: V1 and V2 view interventricular septum V3 and V4 view anterior wall of left ventricle V5 and V6 view lateral wall of the left ventricle Normal Tracings  Rate I. Identify atrial and ventricular rates. – Tachycardia > 100/min – Bradycardia < 60/min – Take heart rate and compare to monitor – Is the rate irregular? If so, a strip should be printed.  Rhythm II. Evaluate the rhythm. – Is spacing between QRS equal? – Should be 1 block (.04 sec). More than this irregular rhythm. – Small variations are normal with breathing.  P wave Normal: < 2.5 mm height / < 0.11 sec. and positive. Oddly shaped P wave may indicate atrial enlargement. Only 1 P wave should precede QRS complex.  T wave – Normally (+) in leads where QRS is (+). – T wave inversion is common in the evolving phase of the myocardial infarction. – Abnormally tall and peaked T waves are common with electrolyte imbalances. – Changes in T waves seen when levels > 5.5 mEq/L of potassium  Spiked T wave Normal ECG Spiked T wave of a patient with 6.8 mEq/L potassium level Common Dysrhythmias  Sinus Bradycardia < 60 bpm. – Absolute bradycardia-HR 100-150/min – Rhythm-regular – P waves-p wave present and precedes QRS – PR interval-0.2 or less – QRS complex-< 0.12  Sinus Dysrhythmia Benign dysrhythmia. Meets criteria for sinus rhythm but rhythm is irregular. Usually asymptomatic and does not require treatment. Irregularities associated with respiratory pattern. Common in children and some adults with certain respiratory patterns. No problems with CO and does not lead to more serious arrhythmias.  Sinus Dysrhythmia Rate: 60-100. May also be bradycardia Rhythm: irregular P waves: Normal and QRS follows each P wave PR interval: 0.2 or less QRS complex: < 0.12  Atrial Flutter “Sawtooth”-pattern between normal appearing QRS complexes. Pattern indicates minimal atrial function in ventricular diastole. Atrial contraction causes reduce atrial filling that reduces ventricular filling. Thrombi formation in atria from reduced blood flow.  Atrial Flutter Atrial flutter is usually a short- lived dysrhythmia. – Deteriorates to atrial fibrillation – Returns to previous rhythm. Commonly associated with pulmonary disease Causes – MI – Valve disease  Atrial Flutter Atrial rate: 180-400/min Ventricular rate: varies but always less than atrial rate. Rhythm: regular P waves: saw-tooth appearance. 3-4 flutter waves to one QRS pattern  Atrial Fibrillation Atrial electrical activity completely chaotic. Quivering of atrial myocardium and loss of atrial pumping ability. Higher risk of atrial thrombi formation and embolization. Reduced filling time of atria and less ventricular filling  Atrial Fibrillation Does not severely reduce CO Pulmonary disease QRS complexes have normal appearance, but are irregular.  Atrial Fibrillation Atrial rate: > 400/min Ventricular rate: always less than atrial rate Rhythm: irregular P waves: waves having different appearance PR interval: can’t measure QRS: 0.12sec). – No P wave preceding QRS complex. – Large looping ST segment opposite in direction of QRS complex – Followed by full compensatory pause  PVC’s Unifocal-PVC’s with origination from same site and look the same < 6 PVC’s /min-give oxygen, monitor > 6/min-treat. Lidocaine to slow rate and oxygen.  PVC’s to WATCH Multifocal PVC’s – QRS complexes with more than one configuration. Couplets – Paired or 2 PVC’s in a row, then normal rhythm that repeats. Salvos – Three PVC’s in a row, then normal rhythm that repeats. R-on-T phenomenon – PVC occurs during the T wave of preceding beat. It can precipitate V tach. PVC’s to WATCH Bigimeny PVC’s  Ventricular Tachycardia Series of broad QRS complexes, at a rapid rate (140-300), without identifiable P waves. Reduced CO. – Regular rhythm – Uniform appearance  Ventricular Tachycardia Usually caused by severe ischemia. Patients deteriorate rapidly; treated as an emergency. – Hypotension – Without treatment = ventricular fibrillation = death. – Treatment-lidocaine given quickly, oxygen – Pulseless and patient is unconscious V tach-start CPR  Ventricular Fibrillation Chaotic, completely unorganized ventricular electrical activity. Wavy, irregular pattern. Cardiac output drops to ZERO= unconsciousness. Life-threatening, needs immediate defibrillation.  V fib Ventricular fibrillation less chaotic  V Fib PVC going into V Fib Successful defibrillation  Asystole Cardiac standstill= straight or almost straight line. Fatal unless acceptable rhythm restored. One of the criteria to confirm death. Always assess patient and check leads before initiating therapy. Asystole  Pulseless Electrical Activity (PEA) Clinically diagnosed. ECG pattern does not reflect the mechanical activity of the heart. Palpable pulse does not match what is on the monitor – No pulse but electrical activity Rare, precipitated by a problem: – Tension pneumothorax – Cardiac trauma – Severe electrolyte imbalance or acid-base status