Countershock, Cardioversion, and Defibrillation

• Study notes on circulatory assist devices, pacemakers, myocardial infarction (MI), and electrocardiogram (EKG) changes

Countershock

• Electric current to reset the heart rhythm
• Includes cardioversion and defibrillation

Elective Cardioversion vs. Defibrillation

• Elective Cardioversion: Delivers synchronized electric current to convert dysrhythmias, initially at 100-120 J, up to 200 J
• Defibrillation: Delivers unsynchronized electric shock, administering a larger number of joules up to 360 J
  • In defibrillation there is no discernible R wave to sync with

Indications for Countershocks

• Elective Cardioversion: Used when SVT is resistant to medication, in atrial fibrillation or flutter and for unstable patients with ventricular tachycardia,
• Symptoms for unstable ventricular tachycardia in patients can include hypotension, dyspnea, or chest pain
• May also be used if the person has symptoms suggestive of heart failure, myocardial infarction, or myocardial ischemia
• Defibrillation: Used for pulseless patients with ventricular tachycardia or ventricular fibrillation

Interventions for Countershocks

• Once patient is attached, push "sync" button before each cardioversion attempt
• Syncs with the R wave and the patient MUST HAVE an R or QS wave to sync with
• The machine discharges after the R wave and before the downstroke of the T wave
• During the absolute refractory period
• If performed during relative refractory period, ventricular fibrillation may result
• Apply conductive paste or gel pads on the chest wall at the apex and base of the heart
• Obtain continuous ECGs during the procedure
• Automatic External Defibrillator (AED) can be used by medical/lay persons
• AED uses IECG to detect patterns through large patches on the patient's chest
• If lethal dysrhythmia is detected, AED defibrillates
• AED will instruct the user on how to operate

Nursing Role in Preparation for Countershocks

• Start at low frequency, then increase
• Obtain informed consent and educate on the procedure
• Ensure an MD, RN, RT and RPh are present
• Obtain an ECG strip
• Check for atrium clots with a transesophageal echocardiogram
• Confirm IV access and meds are given
• If hemodynamically unstable then sedative meds are given
  • Light sedative
  • Conscious
  • Moderate sedation
  • Hypnotic sedative and narcotic analgesic: Etomidate, fentanyl
• Report any electrolyte imbalance (Ca, Mg, K)
• Remove oxygen and metallic objects
• Apply conductive pads to the right chest and midaxillary line
  • Can also be placed on anterior and posterior chest walls, which is better for people with atrial fibrillation
• Ensure that "All clear!" is called before the shock, and no one touches the bed or patient

Nursing Role after Countershocks

• Resume CPR and ACLS immediately after the shock
• Administer pharmacological interventions intravenously
• Every Q2 minutes (~5 cycles of CPR), check pulse and rhythm, continue if indicated
• Assess for Emboli(especially cerebral), respiratory depression, skin burns and dysrhythmias after the procedure

Pacemakers - Primary Function

• Pulse generator to provide electrical stimulus to the heart when the heart's conduction fails or doesn’t produce sufficient cardiac output
• Connected to wires that carry an electrical stimulus to the myocardial cells
• Used in addition to drug therapy when the conduction system fails or cannot initiate an impulse spontaneously, or is unable to maintain primary pacing control
• Can be programmed to pace when an intrinsic beat is not sensed as a backup trigger

Permanent Pacemakers

• Internally implants pulse generator in subcutaneous tissue of the chest wall
• Leads are passed transvenously to the heart and rest on the endocardium
• Pacer wires are inserted through subclavian or jugular veins into the atria and/or ventricles

Ventricle Pacing

• Most common type of pacemakers
• Appear as a spike before wide and bizarre QRS
• Impulses from the atria are blocked, such as in a complete heart block

Atrial Pacing

• Appears as a spike before the P wave
• Common with Sinus node disease leading to bradycardia
• The SA node fires improperly as a result of ischemic damage to SA node which leads to bradycardia

Single Chamber Pacemakers

• Can be in the atrium or the ventricle

Dual Chamber/Atrioventricular Pacemakers

• Requires 2 wires for atrioventricular pacing
• Used to synchronize heart depolarizations in order to maintain cardiac output

Temporary Pacemaker Types

• External Pacemakers
  • Pads are placed on anterior and posterior walls of the chest
  • Delivers direct electrical impulses through the skin and body tissues via transcutaneous pacemakers
  • Can be painful and requires medication with sedatives and analgesics
  • Analyze ECG strip before and after

Epicardial Pacemakers

• Temporary pacing used during open heart surgery
• 2 wires stitched in at the atrial level and 2 wires stitched in at the ventricular level
• Wires must be brought under the skin just under the sternum for access
• Lightly sutured to easily remove when unneeded
• Implanted to treat symptomatic bradycardia that arises post-surgery
• When not in use must be kept insulated by a sterile glove near the chest
• Nurse must keep sterile technique during the insertion site

Transvenous Pacemakers

• Electrical stimulation of the right ventricular or R atrial endocardium via electrode-tipped catheter
• Uses direct insertion of the pacing wire of a pulmonary artery catheter with embedded pacing port
• Temporary, sterile, bedside procedure, requires x-ray to confirm placement

Pacemaker Box/Generator

• Essential to ensure the pacemaker is set correctly
• Pacing rate: Top dial in beats per minute (BPM)
• Output: Middle dial, Energy amount in milliamperes
• Sensitivity: Bottom dial, amount of electrical activity pacer senses to fire
• Blue caps underneath connect wires

Pacemaker Terms

• Know how to identify on an ECG
• Triggered: Fires an impulse in response to sensing
• Inhibited: Only initiates pacing, will not activate if the pacemaker senses an intrinsic beat
• Double: Pacemaker reacts to both inhibition and triggering
• Firing: Sends out an electrical impulse
• Capture: A stimulus strong enough to produce depolarization
• Sensing: Pacemaker "senses" the patient's own impulse generation

Three Letter Codes for Pacemakers

• Chamber Paced: represents the chamber that will be paced - Atrium (A), Ventricle (V), or Dual (D)
• Chamber Sensed: represents the chamber that the pacemaker will react to - Atrium (A), Ventricle (V), or Dual (D)
• Pacemaker Response: represents what the pacemaker will do upon reaction - Triggered, Inhibited, or Double
• Example for VVI: Ventricle, Ventricle, Inhibited - the ventricles will be paced only if necessary, if no intrinsic beat

Pacemaker Malfunctions

• Failure to sense: Pacemaker competes with the patient's own impulses, it may discharge an impulse during the refractory period
• Failure to capture: Initiates an impulse, but not strong enough for depolarization
• Failure to fire: Unable to send an impulse, indicated by absence of a pacemaker spike

ICDs

• Function in "Countershock" area
• Used to prevent sudden cardiac arrest as well as sustained ventricular tachycardia
• Prophylactically used in high risk groups; cardiomyopathy.
• Pacemaker capabilities: To defibrillate or cardiovert and anti-tachycardia pacing to avoid conversion, treats bradycardia

Hypothermia Therapy and Cardiac Arrest

• Cooling technique for those who have already experienced cardiac arrest to 90-93 degrees F
• Slows body metabolism to protect the brain and reduce mortality
• 80% of patients who received hypothermia resulted in a more favorable neurological outcome than normothermia,
• Externally cools via cooling blanket device, cool pads, or blankets
• Internally cools via cold intravenous saline infusion which circulates fluid percutaneously through jugular catheters
• Measures skin for cold burns
• Hypothermia leads to Atrial and ventricular dysrhythmias with PR, QRS, QT elongations
  • Also leads to common hypokalemia

Nursing Care when Rewarming from Hypothermia

• Occurs after 12-24 hours of therapy
• Can lead to hyperkalemia and vasodilation, electrolyte shifts
• Closely follow CVP, ScVO2, and urine output
• Fluid shifts are an indicator, if CVP and UO declines, may need fluid therapy

MI Patterns on a 12 Lead ECG

• Anterior MI: Front wall, supplied by the left anterior descending artery
• Inferior MI: Bottom and right side wall, supplied by the right coronary artery
• Lateral MI: Left side wall, supplied by the circumflex artery
• Posterior MI: Back wall, supplied by the right coronary artery
• Anterior leads have ST elevation in V2-V4 and reciprocal changes in II, III, aVF
• Inferior leads have ST elevations in II, III, aVF and reciprocal changes in 1, aVL, and V1-V6

ECG Changes with Ischemia, Injury and Infarction

• Ischemia: Heart starves for blood and oxygen, appears as an inverted T wave and pale or whitish tissue
• Injury: Tissue injured by lack of perfusion, appears as ST segment elevation and blueish tissue
• Infarction: Tissue necrosis, appears as significant Q wave and black tissue

ST Segment Abnormalities

• Normal ST: At the same baseline as PR
• ST Depression: ST below baseline indicates ischemia or reciprocal changes of an infarct
• Concave ST elevation: ST above the baseline with rounded edges, may indicate pericarditis or myocardial infarction (STEMIs)
• Convex ST elevation: ST above the baseline with coved edges, typically associated with myocardial injury

MI Locations + CA distribution

• Anterior MI
  • ST elevation in V2-V4
  • Reciprocal changes II, III, aVF
  • CA: left anterior descending (LAD)
• Inferior ΜΙ
  • ST elevations in II, III, avF
  • Reciprocal changes in I, aVL, and V1-V6
  • CA: right coronary artery (RCA)
• Lateral MI
  • ST changes in I, aVL, V5, V6
  • Reciprocal changes II, III, aVF
  • CA: circumflex
• Posterior MI
  • ST depression with tall R wave and upright T waves
  • Reciprocal changes in V1 & V2
  • CA: RCA or circumflex
• Extensive Anterior MI
  • Indicative changes in I, aVL, V1 to V6
  • Reciprocal changes in II, III, aVF
  • CA: LAD or left main
• Anteroseptal MI
  • Indicative changes in V1 plus any anterior leads
  • Usually no reciprocal changes
  • CA: LAD
• MIs can extend across into other walls, besides the left ventricle