8.2 ECG Rhythm Interpretation, Arrhythmias & Nursing Interventions

7.2 ECG Rhythm Interpretation, Arrhythmias & Nursing Interventions

Electrocardiography is an indispensable clinical tool in acute nursing practice. The ability to systematically analyze a 12-lead ECG and telemetry rhythm strip, promptly identify lethal dysrhythmias, and execute evidence-based nursing interventions is a core competency assessed in the RCSI FNM Overseas Aptitude Test. This section delivers a rigorous guide to systematic ECG analysis, explores common supraventricular and ventricular dysrhythmias, details atrioventricular blocks, and outlines life-saving clinical interventions as set out in Irish Resuscitation Council (IRC) and European Resuscitation Council (ERC) guidelines.

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1. The Systematic 6-Step ECG Analysis Framework

To prevent diagnostic errors and cognitive bias, every ECG must be evaluated sequentially using a standardized 6-step framework. Standard ECG paper runs at a speed of 25 mm/second with a voltage calibration of 10 mm = 1 mV:

• 1 Small Square: 1 mm = 0.04 seconds (40 ms) horizontally; 0.1 mV vertically.
• 1 Large Square: 5 small squares = 5 mm = 0.20 seconds (200 ms) horizontally; 0.5 mV vertically.
• 5 Large Squares: 25 mm = 1.0 second.

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|                       SYSTEMATIC 6-STEP ECG ANALYSIS MATRIX                     |
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| STEP 1: RATE         - Regular: 300 / (large squares between R-R).              |
|                        Irregular: Count QRS on 10-sec rhythm strip x 6.         |
| STEP 2: RHYTHM       - Regular, regularly irregular, or irregularly irregular.  |
| STEP 3: P WAVES      - Present, upright in Lead II, uniform, 1:1 with QRS.      |
| STEP 4: PR INTERVAL  - 0.12 to 0.20 seconds (3 to 5 small squares).             |
| STEP 5: QRS DURATION - Narrow (<0.12 s / <3 small squares) vs Broad (≥0.12 s).  |
| STEP 6: ST & T WAVES - Isoelectric ST segment, T-wave concordance, QTc interval. |
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Step 1: Cardiac Rate Determination

• Regular Rhythms: Use the Sequence Method or mathematical division: Count the number of large squares between two consecutive R waves and divide 300 by this number: $\text{Heart Rate} = \frac{300}{\text{Number of Large Squares between R-R}}$ (Alternatively, divide 1500 by the number of small squares for precise calculation). Memory sequence for large squares: 1 square = 300 bpm, 2 squares = 150 bpm, 3 squares = 100 bpm, 4 squares = 75 bpm, 5 squares = 60 bpm, 6 squares = 50 bpm.
• Irregular Rhythms (Essential for Atrial Fibrillation): The large-square rule fails in irregular rhythms. Instead, count the total number of complete QRS complexes across the standard 10-second rhythm strip (typically Lead II at the base of the tracing, spanning 50 large squares) and multiply by 6: $\text{Heart Rate} = \text{Total QRS complexes in 10 seconds} \times 6$

Step 2: Rhythm Regularity

• Assess the spacing between consecutive R waves (R-R interval) across the strip using ECG calipers or the paper-and-pencil method.
• Classify the rhythm as:
  • Regular: R-R intervals are identical throughout.
  • Regularly Irregular: An irregular pattern that repeats predictably (e.g., ventricular bigeminy, second-degree AV block Type I).
  • Irregularly Irregular: Completely chaotic R-R intervals with no predictable pattern (hallmark of Atrial Fibrillation).

Step 3: P-Wave Morphology & Atrial Activity

• Inspect Lead II and Lead V1 where atrial depolarization is most distinct.
• Normal P waves are upright, smooth, rounded, and uniform in morphology, preceding every QRS complex with a 1:1 ratio. Inverted P waves in Lead II indicate retrograde junctional or ectopic atrial depolarization. Saw-tooth flutter waves indicate atrial flutter; fibrillatory baseline waves indicate atrial fibrillation.

Step 4: The PR Interval (PRI)

• Measured from the beginning of the P wave to the beginning of the QRS complex (representing total physiological conduction time from the sinoatrial node, through atrial myocardium, across the AV node, bundle of His, and bundle branches).
• Normal Range: 0.12 to 0.20 seconds (3 to 5 small squares).
• Pathology: A prolonged PRI ($>0.20\text{ s}$) indicates delayed AV nodal conduction (First-Degree AV Block). A shortened PRI ($<0.12\text{ s}$) indicates an accessory bypass tract pre-exciting the ventricles (Wolff-Parkinson-White syndrome) or an ectopic pacemaker originating in the lower atria or AV junction.

Step 5: QRS Complex Duration & Morphology

• Measured from the beginning of the Q (or R) wave to the J-point (end of the S wave), reflecting total ventricular depolarization time.
• Normal Range: $<0.12\text{ seconds}$ ($<3\text{ small squares}$, typically 0.08–0.10 s).
• Narrow Complex ($<0.12\text{ s}$): Confirms normal, rapid conduction through the specialized His-Purkinje network, indicating a supraventricular origin (SA node, atria, or AV junction).
• Broad / Wide Complex ($\ge 0.12\text{ s}$): Reflects delayed, slow cell-to-cell myocardial conduction. Causes include Ventricular Tachycardia (VT), ventricular escape rhythms, ventricular pacing, or supraventricular rhythms with Bundle Branch Block (BBB) or aberrant intraventricular conduction.

Step 6: ST Segment, T Wave & QTc Interval

• ST Segment: Evaluate from the J-point to the onset of the T wave relative to the preceding isoelectric PR or TP baseline. Elevation indicates acute transmural myocardial infarction or pericarditis; depression indicates subendocardial ischemia, reciprocal change, or digoxin toxicity.
• T Wave: Represents ventricular repolarization. Normally upright in leads with positive QRS complexes (concordant). Symmetrical, deep T-wave inversions indicate ischemia; tall, peaked, "tented" T waves indicate acute hyperkalemia ($K^+ > 6.0\text{ mmol/L}$) or early hyperacute phase of STEMI.
• Corrected QT Interval (QTc): Bazett's formula: $\text{QTc} = \text{QT} / \sqrt{\text{RR}}$. Normal is $<450\text{ ms}$ in men and $<460\text{ ms}$ in women. A prolonged QTc ($>480\text{ to }500\text{ ms}$) predisposes to early after-depolarizations triggering Torsades de Pointes.

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2. Sinus Node Dysrhythmias & Symptomatic Bradycardia

Sinus Bradycardia (Heart Rate <60 bpm)

• Etiology: High vagal tone in athletic conditioning, acute inferior STEMI (RCA ischemia affecting the sinoatrial nodal artery), hypothermia, hypothyroidism, obstructive jaundice, raised intracranial pressure (Cushing's triad: bradycardia, hypertension, irregular respirations), and medications (beta-blockers, non-dihydropyridine CCBs, digoxin, amiodarone).
• Physiological Assessment: Asymptomatic sinus bradycardia requires no treatment. Bradycardia becomes pathological when cardiac output is insufficient to meet tissue metabolic demands.
• Signs of Hemodynamic Compromise ("Adverse Features"): Shock (SBP $<90\text{ mmHg}$, pallor, diaphoresis), syncope or presyncope, acute heart failure (pulmonary crackles), or myocardial ischemia (angina).
• Emergency Management Algorithm (ERC / IRC Guidelines):
  1. First-Line Pharmacotherapy: Atropine Sulfate 500 micrograms (0.5 mg) IV bolus. Atropine competitively antagonizes muscarinic acetylcholine receptors, blocking parasympathetic vagal inhibition of the SA and AV nodes. Repeat every 3 to 5 minutes as needed to a maximum cumulative dose of 3 mg (which achieves full vagal blockade).
  2. Caution on Dosing: Doses $<500\text{ mcg}$ must be avoided because subtherapeutic doses paradoxically cause central vagal stimulation, worsening bradycardia.
  3. Second-Line Interventions (if Atropine ineffective or high-degree AV block):
     • Transcutaneous Pacing (TCP): Immediate non-invasive electrical capture.
     • Intravenous Chronotropic Infusions: Adrenaline infusion (2–10 micrograms/minute) or Isoprenaline infusion (5 micrograms/minute titrated).

Sinus Tachycardia (Heart Rate >100 bpm)

• Etiology: A physiological homeostatic response driven by elevated sympathetic tone or vagal withdrawal: pain, fever, sepsis, hypovolemia/dehydration, acute blood loss, hypoxia, pulmonary embolism, hyperthyroidism, alcohol withdrawal, or anxiety.
• Clinical Principle: Sinus tachycardia is a secondary sign, not a primary arrhythmia. Never administer beta-blockers blindly to suppress sinus tachycardia without identifying the cause. Suppressing compensatory tachycardia in hypovolemic or septic shock precipitates sudden cardiovascular collapse.

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3. Atrial Fibrillation (AF) & Atrial Flutter

Atrial Fibrillation (AF)

• Pathophysiology: The most common sustained cardiac arrhythmia. Characterized by chaotic, disorganized electrical activity within the atria at rates of 350 to 600 bpm, originating predominantly from ectopic foci located within the muscular sleeves of the pulmonary veins in the left atrium. The atria undergo fibrillatory quivering rather than coordinated mechanical systole, causing complete loss of the "atrial kick" (which normally accounts for 20–30% of ventricular filling volume) and stasis of blood, particularly within the left atrial appendage (LAA).
• ECG Hallmarks:
  1. Absent P waves: Replaced by fine, irregular fibrillatory (f) baseline undulations.
  2. Irregularly irregular QRS complexes: The AV node acts as an electrical filter, bombarding the ventricles with random, irregular impulses.
  3. Variable ventricular rate: Rapid (AF with Rapid Ventricular Response [RVR], HR $>100\text{ bpm}$), controlled (HR 60–100 bpm), or slow (HR $<60\text{ bpm}$).

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|                      ATRIAL FIBRILLATION MANAGEMENT MATRIX                      |
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| 1. STROKE RISK STRATIFICATION : Calculate CHA2DS2-VASc Score.                   |
|                                Score ≥1 in men or ≥2 in women -> DOAC indicated.|
| 2. BLEEDING RISK STRATIFICATION: Calculate HAS-BLED Score.                      |
|                                Identifies and addresses modifiable risks.       |
| 3. RATE CONTROL STRATEGY      : Beta-blocker (Bisoprolol) or CCB (Diltiazem).   |
|                                Digoxin added for sedentary / heart failure.     |
| 4. RHYTHM CONTROL STRATEGY    : Synchronized Cardioversion or Antiarrhythmics.  |
|                                *MANDATORY: Anticoagulate ≥3 weeks before if AF  |
|                                duration >48 hours (or perform TOE).             |
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• Stroke Risk Stratification: CHA2DS2-VASc Score:
  • C: Congestive Heart Failure (+1)
  • H: Hypertension (+1)
  • A2: Age $\ge 75\text{ years}$ (+2)
  • D: Diabetes Mellitus (+1)
  • S2: Stroke / TIA / Thromboembolism history (+2)
  • V: Vascular disease (prior MI, PAD, or aortic plaque) (+1)
  • A: Age 65 to 74 years (+1)
  • Sc: Sex category (Female) (+1) Clinical Application: Oral anticoagulation with a Direct Oral Anticoagulant (DOAC) is indicated for patients with a score of $\ge 2$ in females or $\ge 1$ in males.
• Bleeding Risk Stratification: HAS-BLED Score: Hypertension (uncontrolled SBP $>160$), Abnormal renal/liver function (1 pt each), Stroke history, Bleeding history or predisposition, Labile INRs, Elderly ($>65$), Drugs (antiplatelets, NSAIDs) or Alcohol excess (1 pt each). A score $\ge 3$ indicates high bleeding risk, warranting close surveillance and correction of modifiable bleeding factors, but is not a reason to withhold anticoagulation.
• Rate Control vs. Rhythm Control:
  • Rate Control: First-line for most stable elderly patients. Target resting heart rate $<110\text{ bpm}$ (lenient control). Agents include cardioselective beta-blockers (bisoprolol 2.5–10 mg daily) or non-dihydropyridine calcium channel blockers (diltiazem 120–360 mg daily, verapamil). Digoxin is added as second-line therapy in sedentary patients or those with concomitant HFrEF.
  • Rhythm Control (Restoration of Sinus Rhythm): Indicated for younger patients, highly symptomatic individuals, or those with heart failure exacerbated by AF. Modalities include electrical synchronized cardioversion or pharmacological cardioversion (IV Amiodarone, oral Flecainide in the absence of structural heart disease).
  • CRITICAL CARDIOVERSION SAFETY RULE (The 48-Hour Rule): If Atrial Fibrillation has been present for greater than 48 hours (or if the onset time is unknown), cardioversion must not be attempted until the patient has received therapeutic anticoagulation for at least 3 consecutive weeks, or until a Transesophageal Echocardiogram (TOE) has definitively excluded thrombus in the left atrial appendage. Restoring sinus rhythm dislodges pre-existing atrial thrombi, precipitating massive ischemic stroke. Anticoagulation must continue for at least 4 weeks post-cardioversion.

Atrial Flutter

• Mechanism: A macro-reentrant electrical circuit typically confined to the right atrium, circulating counter-clockwise around the tricuspid valve annulus (cavotricuspid isthmus).
• ECG Hallmarks: Absence of normal P waves; replaced by regular, rapid, saw-tooth flutter waves (F waves), classically seen inverted in inferior leads (II, III, aVF) at an atrial rate of $\approx 300\text{ bpm}$.
• AV Conduction Ratios: The AV node cannot conduct at 300 bpm and establishes a fixed physiological block: a 2:1 AV block produces a ventricular rate of exactly 150 bpm (a clinical red flag: any regular narrow-complex tachycardia at 150 bpm is atrial flutter until proven otherwise); a 3:1 block yields 100 bpm; a 4:1 block yields 75 bpm. Thromboembolic risk and anticoagulation indications are identical to atrial fibrillation.

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4. Supraventricular Tachycardia (SVT / AVNRT) & Stepwise Protocol

Paroxysmal Supraventricular Tachycardia (SVT) refers to rapid, regular tachyarrhythmias originating above the bifurcation of the bundle of His. The most common mechanism is Atrioventricular Nodal Reentrant Tachycardia (AVNRT), caused by dual functional pathways (a slow pathway and a fast pathway) within the AV node.

• ECG Characteristics: Rapid regular ventricular rate (150 to 250 bpm), narrow QRS complexes ($<0.12\text{ s}$), and P waves that are either absent, buried within the QRS, or visible immediately following the QRS as pseudo-$R'$ waves in V1 or pseudo-$S$ waves in inferior leads.

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|                        STEPWISE SVT ESCALATION PROTOCOL                         |
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| STEP 1: ASSESS HEMODYNAMIC STABILITY (Adverse Features: Shock, Syncope, Chest   |
|         Pain, Heart Failure).                                                   |
|         • IF UNSTABLE -> Immediate Synchronized Electrical Cardioversion        |
|         • IF STABLE   -> Proceed to Step 2                                      |
| STEP 2: VAGAL MANOEUVRES                                                        |
|         • Execute Modified Valsalva Manoeuvre (Blow into 10 mL syringe 15 sec   |
|           then lie flat with passive leg raise to 45° for 15 sec).              |
| STEP 3: INTRAVENOUS ADENOSINE (First-Line Pharmacological)                      |
|         • 6 mg rapid IV push via large antecubital cannula + 20 mL saline flush |
|         • If unsuccessful after 1-2 min: 12 mg rapid IV push + 20 mL flush      |
|         • If unsuccessful after 1-2 min: second 12 mg rapid IV push + 20 mL     |
| STEP 4: SECOND-LINE PHARMACOLOGICAL (IV Verapamil or Diltiazem)                 |
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Stepwise Nursing & Medical Interventions

1. Hemodynamic Assessment: If adverse features are present (hypotension SBP $<90\text{ mmHg}$, acute heart failure, ischemic chest pain, altered consciousness), prepare for immediate Synchronized Electrical Cardioversion (50–100 J biphasic).
2. Vagal Manoeuvres (The Modified Valsalva Manoeuvre):
   • Standard Valsalva: The patient strains by blowing into a 10 mL or 20 mL syringe (generating 40 mmHg pressure) for 15 seconds in a semi-recumbent position. This increases intrathoracic pressure, stimulates aortic and carotid baroreceptors, and triggers high vagal parasympathetic outflow to slow AV nodal conduction.
   • The Postural Modification (REVERT Trial): Immediately upon completion of the 15-second strain, the patient is laid completely flat (supine) and their legs are passively elevated to 45 degrees by a nurse for 15 seconds. This drastically enhances venous return, augmenting cardiac filling and maximizing vagal stimulation. The modified technique achieves cardioversion success rates of 43%, compared to only 17% for standard Valsalva.
   • Carotid Sinus Massage: Alternative for trained medical practitioners; massage the carotid bifurcation at the angle of the jaw for 5 seconds. Contraindicated if carotid bruits are present or if the patient has a history of stroke/TIA within the past 3 months.
3. Intravenous Adenosine Protocol:
   • Mechanism: An endogenous purine nucleoside that binds to vascular and cardiac A1 receptors, opening potassium channels and inhibiting calcium influx. This produces profound, transient depression of the SA node and complete blockade of AV nodal conduction for 1 to 2 seconds, terminating reentrant circuits that incorporate the AV node.
   • Pharmacokinetics: Extremely short plasma half-life ($<10\text{ seconds}$) due to rapid cellular uptake and enzymatic deamination by erythrocytes and vascular endothelium.
   • Administration Technique (Strict Nursing Protocol):
     • Cannulation: Establish a large-bore cannula (18-gauge or 16-gauge) in the antecubital fossa (closest accessible peripheral vein to the central circulation). Do not use small-gauge wrist or hand veins.
     • Setup: Connect a three-way stopcock directly to the cannula hub, or use two connected ports.
     • Bolus Dosing: Administer 6 mg rapid IV push in $<2\text{ seconds}$, followed immediately by a vigorous 20 mL bolus of 0.9% sodium chloride flush, and immediately elevate the patient's arm to accelerate delivery to the heart.
     • Dose Titration: If sinus rhythm is not restored within 1 to 2 minutes, administer a 12 mg rapid IV bolus followed by a 20 mL flush. If unsuccessful after another 1 to 2 minutes, administer a final second 12 mg bolus.
   • Crucial Patient Communication: Before injection, warn the alert patient of distressing but transient adverse effects: "You will feel a sudden, intense sensation of heat, flushing, chest tightness, shortness of breath, and a feeling of impending doom. This lasts only a few seconds as your heart resets." Run a continuous 12-lead ECG recording during injection. The monitor typically displays a brief period of transient asystole or sinus arrest before sinus rhythm resumes.
   • Contraindications: Severe asthma or active reactive bronchospasm (adenosine triggers severe bronchoconstriction; verapamil is preferred); second- or third-degree AV block; known long-QT syndrome.

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4. Ventricular Tachycardia (VT) & Ventricular Fibrillation (VF)

Broad-complex tachyarrhythmias (QRS duration $\ge 0.12\text{ s}$) originate within the ventricular myocardium or His-Purkinje conduction system. They carry high mortality and demand immediate pulse verification.

Monomorphic vs. Polymorphic Ventricular Tachycardia

• Monomorphic VT: Successive QRS complexes exhibit identical, uniform morphology at rates of 140 to 220 bpm, resulting from a stable reentrant circuit around scarred, infracted tissue. ECG shows broad complexes, AV dissociation (independent P waves dissociated from QRS), and fusion/capture beats.
• Polymorphic VT (Torsades de Pointes): QRS complexes continuously twist in amplitude and morphology around the isoelectric baseline ("twisting of the points"), associated with prolonged QTc intervals, hypokalemia, or hypomagnesemia.
  • Treatment for Torsades de Pointes: Intravenous Magnesium Sulfate 2 grams infused over 10 to 15 minutes, even if serum magnesium levels are normal, alongside correcting serum potassium to $>4.5\text{ mmol/L}$.

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|                     VENTRICULAR TACHYCARDIA CLINICAL ALGORITHM                  |
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|                             BROAD-COMPLEX TACHYCARDIA                           |
|                                         │                                       |
|                                         ▼                                       |
|                               CHECK CAROTID PULSE                               |
|                                         │                                       |
|                     ┌───────────────────┴───────────────────┐                   |
|                     ▼                                       ▼                   |
|              PULSE ABSENT                            PULSE PRESENT              |
|         (CARDIAC ARREST - PEA/VF)                           │                   |
|                     │                     ┌─────────────────┴─────────────────┐ |
|                     ▼                     ▼                                   ▼ |
|         IMMEDIATE DEFIBRILLATION     UNSTABLE (Hypotension,              STABLE |
|         • Crash Call 2222             Chest Pain, Shock)            • IV Amiodarone|
|         • CPR 30:2                   • Synchronized Cardioversion     300 mg over |
|         • 150-200 J Shock              (100-150 J Biphasic)           20-60 mins  |
|         • Adrenaline & Amiodarone                                               |
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Clinical Management: The Pulse Dictates the Protocol

1. Pulseless VT & Ventricular Fibrillation (VF):
   • Represents cardiac arrest. VF is characterized by totally chaotic, disorganized ventricular quivering with zero mechanical stroke volume or cardiac output.
   • Action: Initiate high-quality chest compressions (30:2), call the Cardiac Arrest Team (dial 2222 in Irish acute hospitals), and attach the manual defibrillator.
   • Both rhythms are Shockable: Deliver an immediate unsynchronized shock at 150 to 200 Joules biphasic (or 360 J monophasic). Resume CPR immediately for 2 minutes without pausing to check the rhythm or pulse. Administer Adrenaline 1 mg IV after the 2nd shock (then every 3–5 minutes) and Amiodarone 300 mg IV bolus after the 3rd shock (with an additional 150 mg after the 5th shock).
2. Unstable VT with a Pulse:
   • Signs of hemodynamic collapse: SBP $<90\text{ mmHg}$, acute pulmonary edema, angina, syncope.
   • Action: Emergency Synchronized Electrical Cardioversion (initial shock 100 J biphasic, escalating to 150 J and 200 J). Ensure sedation/analgesia if consciousness allows. The synchronizer mode must be engaged to prevent the shock from firing on the vulnerable T wave (which induces VF via the R-on-T phenomenon).
3. Stable VT with a Pulse:
   • Patient is alert, oriented, with maintained blood pressure ($>90\text{ mmHg}$) and no chest pain or pulmonary edema.
   • Action: Pharmacological cardioversion. First-line agent: Intravenous Amiodarone 300 mg infused over 20 to 60 minutes, followed by a continuous maintenance infusion of 900 mg over 24 hours via a central venous line or dedicated large peripheral cannula (using a volumetric pump and in-line filter). Avoid rapid bolus injection in stable patients, which precipitates severe hypotension and bradycardia.

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5. Atrioventricular (AV) Conduction Blocks

AV blocks represent impairment of electrical impulse transmission from the atria to the ventricles through the AV node, bundle of His, or Purkinje system.

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|                          ATRIOVENTRICULAR (AV) BLOCKS                           |
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| 1. FIRST-DEGREE AV BLOCK    : Prolonged PR interval (>0.20 s), fixed & constant.|
|                               Every P wave conducts (1:1). Benign.              |
| 2. SECOND-DEGREE TYPE I     : Progressive PR lengthening until a QRS is dropped.|
|    (Wenckebach)               AV nodal fatigue. Usually stable.                 |
| 3. SECOND-DEGREE TYPE II    : Fixed PR intervals with sudden, non-conducted P   |
|    (Mobitz II)                waves. Infranodal (His-Purkinje). High-risk block!|
| 4. THIRD-DEGREE (COMPLETE)  : Complete AV dissociation. P waves & QRS complexes |
|                               beat independently. Ventricular rate 30-40 bpm.   |
|                               EMERGENCY PACING MANDATORY.                       |
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Comprehensive Comparison of AV Blocks

Transcutaneous Pacing (TCP) Procedure (OSCE Skill Standard)

1. Attach Electrodes: Apply multifunction pacing pads in the anterior-posterior (AP) position (anterior pad over left precordium; posterior pad on left infrascapular back), which minimizes transthoracic electrical impedance.
2. Sedation & Analgesia: Electrical pacing causes painful skeletal muscle contractions. Administer IV analgesia (Morphine or Fentanyl) and IV sedation (Midazolam) if the patient is conscious and time allows.
3. Set Pacemaker Mode & Rate: Turn defibrillator to PACER mode. Set the demand rate between 60 and 70 bpm.
4. Current Output Titration (Milliamperes [mA]): Start at 0 mA and increase current output gradually (in increments of 5–10 mA) until electrical capture is identified on the monitor (manifested by a sharp pacing spike followed immediately by a broad QRS complex and a prominent, broad T wave).
5. Confirm Mechanical Capture: Electrical capture does not equal mechanical perfusion! Immediately palpate the right femoral pulse or right radial pulse to verify mechanical capture (do not palpate the left carotid pulse, as skeletal muscle twitching from pacing currents can mimic a pulse). Measure blood pressure and check peripheral capillary refill.
6. Set Safety Margin: Once mechanical capture threshold is established, set the pacing current 5 to 10 mA above threshold to maintain consistent ventricular capture.

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6. Clinical Traps & OSCE Pitfalls