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Defibrillator: Principles, Types, and Application

1. Basic Principle

Key distinction: Defibrillation is NOT "jumpstarting" the heart. It is not indicated for asystole or pulseless electrical activity (PEA), which have no chaotic electrical activity to terminate.

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2. Electrical Physics

• Larger electrode surface area (optimal paddle diameter ~13 cm)
• Firm paddle pressure (~25 lb/11 kg)
• Conductive gel or saline pads
• Shocking at end-expiration (air is a poor conductor)
• Repeated shocks (impedance falls with each successive shock)
• Barash, Cullen & Stoelting's Clinical Anesthesia, 9e

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3. Waveform Types

Monophasic (legacy)

• Current flows in one direction only between electrodes
• Subtypes: Monophasic Damped Sinusoidal (MDS) and Monophasic Truncated Exponential
• Requires higher energy: 360 J recommended for all shocks (MDS)
• Historical use: stacked shocks at 200 J → 200-300 J → 300-360 J (pre-2005 AHA)
• Still found in some older equipment

Biphasic (current standard)

• Current flows forward then reverses between electrodes within the same shock
• Two subtypes in clinical use:
  • Biphasic Truncated Exponential (BTE): 150-200 J for first shock
  • Rectilinear Biphasic (RLB): 120 J for first shock
  • If waveform type unknown: default 200 J
• Lower energy requirements = less myocardial damage
• Higher first-shock success rate than monophasic
• All current AEDs and most hospital defibrillators use biphasic waveforms

"Current data do not support one waveform over another, but biphasic defibrillators appear to be more efficient in achieving defibrillation with the first shock." - Roberts and Hedges' Clinical Procedures in Emergency Medicine

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4. Types of Defibrillators

A. Manual External Defibrillator

• Requires trained operator to interpret the rhythm and manually deliver the shock
• Delivers shock via handheld paddles or self-adhesive pads
• Used in hospitals, ICUs, and by advanced paramedics
• Allows fine energy titration and cardioversion in synchronized mode

B. Automated External Defibrillator (AED)

• Monitors ECG, automatically recognizes VF, charges, and (in fully automatic models) delivers shock - or prompts the operator to press a button (semi-automatic)
• Designed for minimally trained lay rescuers
• VF-detection algorithms have near-perfect specificity (will not shock a non-shockable rhythm) but somewhat lower sensitivity for low-amplitude VF
• Rhythm analysis can take up to 90 seconds, during which CPR is interrupted - a recognized limitation
• Can misinterpret pacemaker spikes as QRS complexes
• Delivers biphasic waveform at a pre-set impedance-compensated energy level
• Roberts and Hedges' Clinical Procedures in Emergency Medicine

C. Implantable Cardioverter-Defibrillator (ICD)

• Surgically implanted device (subclavicular pocket, transvenous lead to RV) that continuously monitors rhythm
• Delivers tiered therapy:
  1. Anti-tachycardia pacing (ATP): overdrive pacing to terminate VT without a shock (patient often unaware)
  2. Low-energy cardioversion: synchronized shock for VT
  3. High-energy defibrillation: for VF
• Stores electrograms for later interrogation
• Appropriate shock occurs in ~50% of patients within 2 years of implantation
• Subcutaneous ICD (S-ICD) is available, requiring no transvenous leads

• Survived cardiac arrest due to VF/pVT (secondary prevention)
• EF ≤35% with symptomatic heart failure (NYHA II-III) on optimal therapy
• Hypertrophic cardiomyopathy with risk factors
• Channelopathies (Long QT, Brugada, catecholaminergic VT)

D. Wearable Cardioverter-Defibrillator (WCD)

• External vest worn by patients at temporary high risk (e.g., post-MI before reassessment of EF, new cardiomyopathy)
• Bridges to ICD implantation or recovery

E. Combined CRT-D (Cardiac Resynchronization Therapy with Defibrillator)

• Biventricular pacing device that also has ICD capability
• For patients with EF ≤35%, wide QRS/LBBB, and NYHA class II-IV symptoms
• Improves symptoms, exercise capacity, and reduces mortality

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5. Indications and Contraindications

Defibrillation (unsynchronized)

Cardioversion (synchronized)

• Timed to QRS complex to avoid R-on-T phenomenon (which would induce VF)
• Indications: Hemodynamically unstable VT with pulse, SVT, atrial flutter, atrial fibrillation
• Also used electively after failed pharmacologic cardioversion

IMPORTANT: Always confirm "SYNC" mode is OFF for defibrillation, and ON for cardioversion. If SYNC is accidentally left on during VF, the device searches for a QRS and will not fire.

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6. Electrode Placement

• Sternal pad: below right clavicle, right of sternum
• Apical pad: midaxillary line, 5th-6th intercostal space (left)

• Anterior pad: left of sternum
• Posterior pad: directly behind, at the same level
• Preferred when pacemaker/ICD is present (avoids over the device) or in certain body habitus

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7. Step-by-Step Defibrillation Procedure

1. Confirm unresponsiveness, absent pulse (<10 sec check), call for help, start CPR
2. Apply pads/electrodes in sternum-apical position
3. Select energy: 200 J (biphasic default) or 360 J (monophasic)
4. Confirm SYNC mode is OFF
5. Press CHARGE, announce "I'm clear, you're clear, everybody's clear!"
6. Visually confirm all contacts broken, press SHOCK
7. Resume CPR immediately after shock - do not pause to check pulse first
8. Continue 5 cycles (2 minutes) of CPR, then reassess rhythm
9. If shockable rhythm persists: repeat shock (same or escalating energy), continue CPR, administer epinephrine, consider amiodarone

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8. Complications

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9. Special Considerations

• Pacemakers/ICDs: Place pads at least 8 cm from implanted device. Consider anterior-posterior placement. Interrogate device post-shock.
• Pregnancy: Standard defibrillation is safe; fetal monitors should be removed before shock.
• Children: Recommended dose 2-4 J/kg for biphasic; AEDs with pediatric attenuator pads preferred
• Low-amplitude VF: More likely to convert to asystole, associated with poor prognosis; epinephrine does NOT improve defibrillation threshold despite theoretical rationale
• ICD storm (≥3 appropriate shocks in 24 h): Indicates recurrent VT/VF; requires hospital admission, antiarrhythmics or ablation

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Application of Ultrasound in Anesthesia

Overview

"Echocardiography and point-of-care ultrasound (POCUS) have become essential parts of perioperative and peri-interventional anesthesia practice." - Barash, Cullen & Stoelting's Clinical Anesthesia, 9e

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1. Physics of Ultrasound Relevant to Anesthesia

Basic Principles

• Ultrasound is sound with frequency above 20 kHz; diagnostic ultrasound uses 2-10 MHz for cardiac/general imaging and >10 MHz for peripheral nerve imaging
• Transducers use piezoelectric elements (PZEs) - crystals that expand/contract with alternating current, generating sound pulses
• The machine measures time of return of reflected signals to calculate depth, assuming a fixed tissue sound velocity of 1,540 m/s
• The fundamental relationship: v = f × λ (velocity = frequency × wavelength)

Key Image Properties

Echogenicity

Artifacts in Anesthesia Practice

• Acoustic shadow: behind bone or calcified structures - limits imaging
• Acoustic enhancement: bright area deep to fluid (vessels) - can mimic nerve
• Reverberation / comet-tail: sound bouncing between parallel reflectors (common with needle shafts, pleural line, air)
• Mirror-image artifact: most relevant in TEE during aortic imaging - can mimic aortic dissection
• Bayonet artifact: apparent bending of needle due to local speed-of-sound heterogeneity

Imaging Modes

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2. Transducer Selection

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3. Regional Anesthesia

Why Ultrasound for Regional Blocks?

• Direct visualization of the peripheral nerve, needle tip, and local anesthetic spread in real time
• Identifies anatomic variations in nerve position - a major source of block failure with landmark technique
• Allows visualization of fascial planes for interfascial blocks
• Reduces procedure time and increases first-attempt success
• Detects intravascular injection (local anesthetic should not enter vessel lumen)

"Anatomic variation in nerve position and course, which is a potential source of block failure, can be directly visualized." - Miller's Anesthesia, 10e

Nerve Appearance on Ultrasound

• Frequencies ≥10 MHz can distinguish nerves from tendons based on echotexture alone
• Ultrahigh frequency (>20 MHz) can resolve 50-60% of individual fascicles
• Nerves are not easily compressed (unlike veins) and are mobile with extremity movement
• Doppler can detect the nerve's vascular supply (useful for proximal sciatic nerve)

Signs of Nerve Pathology (Caution Before Blocking)

• Fusiform enlargement with unclear nerve borders
• Fascicular crowding from edema
• Hyperemia on Doppler (seen in entrapment neuropathy, Charcot-Marie-Tooth, diabetic neuropathy)

Needle Approaches

1. Surround and clarify the nerve border ("doughnut sign")
2. Track along the nerve path and branches
3. Separate the nerve from adjacent structures (e.g., adjacent artery)

Common Ultrasound-Guided Blocks in Anesthesia

Upper Extremity

Lower Extremity

Trunk / Neuraxial

Neuraxial (Ultrasound-Assisted)

• Pre-procedure scanning of the lumbar spine to identify midline, intervertebral level, and depth to ligamentum flavum - especially in obese patients and those with scoliosis
• Does NOT eliminate fluoroscopy for epidurals but reduces attempts and improves success

Local Anesthetic Considerations

• Bupivacaine / Ropivacaine: long-acting (up to 24 hours); standard for most peripheral blocks
• Epinephrine (1:200,000-1:400,000): prolongs block, reduces systemic absorption; contraindicated for ring blocks, ankle blocks, penile blocks (vasoconstriction risk)
• Additives (dexamethasone, clonidine, dexmedetomidine): can prolong block by 4-8 hours

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4. Vascular Access

Central Venous Access

• Internal jugular vein (IJV): most studied; short-axis approach allows visualization of IJV collapsibility and differentiation from carotid artery
• Subclavian/infraclavicular axillary: reduces pneumothorax risk vs. landmark technique
• Femoral: useful in cardiac arrest; less affected by body habitus

• Reduced number of attempts
• Lower rate of arterial puncture
• Lower pneumothorax rate (for subclavian)
• Fewer overall mechanical complications

Arterial Line Placement

• Radial, femoral, and brachial arteries visualized directly
• Reduces hematoma formation and multiple attempts, especially in hypotensive/vasoconstricted patients

Peripheral IV Access

• Valuable in patients with difficult venous access
• Basilic and cephalic veins in the upper arm can be cannulated under direct vision

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5. Perioperative Echocardiography

Transesophageal Echocardiography (TEE)

• Cardiac surgery (valve repair/replacement, CABG, aortic surgery, LVAD, ECMO)
• Hemodynamically unstable patients of unknown cause
• Diagnosis of suspected intraoperative air embolism
• Guide to intracardiac procedures (ASD closure, TAVR, MitraClip)
• Assess adequacy of valve repair immediately after surgery

• LV systolic function (qualitative EF assessment, wall motion abnormalities)
• RV function and size
• Valve morphology and regurgitation/stenosis severity
• Fluid responsiveness (IVC collapsibility index, LVOT VTI changes with passive leg raise)
• Detection of aortic pathology (dissection, atheroma)

Transthoracic Echocardiography (TTE) / POCUS

• Qualitative LV systolic function (visual EF)
• Detection of severe LV/RV dysfunction, tamponade, massive PE
• IVC assessment for volume status
• Gross valvular pathology
• Rule out intracardiac thrombus or masses

1. Parasternal long axis (PLAX)
2. Parasternal short axis (PSAX)
3. Apical four-chamber (A4C)
4. Subcostal four-chamber
5. Subcostal IVC view

Doppler in Perioperative Echo

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6. Pulmonary / Lung Ultrasound

• Confirm bilateral lung sliding after intubation (rules out right mainstem intubation and pneumothorax)
• Diagnose cause of intraoperative desaturation (atelectasis vs. pneumothorax vs. pulmonary edema)
• Guide thoracentesis and chest drain placement

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7. Airway Assessment

• Measure soft tissue thickness from skin to anterior trachea at vocal cord level (predicts difficult laryngoscopy in obese patients)
• Identify cricothyroid membrane for emergency front-of-neck access (FONA)
• Confirm tracheal intubation vs. esophageal intubation by visualizing tracheal ring movement and absence of "double-tract" sign (two air columns = esophageal intubation)
• Assess for subglottic stenosis and neck masses displacing the trachea
• Confirm ETT tip position (bilateral lung sliding confirms bilateral ventilation)

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8. Gastric Ultrasound

• Performed with a curvilinear probe in the right lateral decubitus position, viewing the antrum
• Empty stomach: flat collapsed antrum (Grade 0)
• Clear fluid only: single layer (Grade 1)
• Solid content or thick fluid: complex content (Grade 2) = high aspiration risk; requires RSI or delay

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9. Limitations and Safety

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10. Training and Competency

• Didactic physics and anatomy education
• Supervised clinical procedures (minimum case numbers vary by block)
• Image acquisition and interpretation assessment
• The PERSEUS Guidelines (ESAIC, 2024) provide current evidence-based recommendations for ultrasound in vascular access and regional anesthesia

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