How to read and set a temporary pacing box after cardiac surgery: modes (AAI, VVI, DDD, asynchronous), output and sensitivity thresholds, daily checks, troubleshooting failure to capture or sense, and safe wire removal.
At the bedside
Timed steps- 1
Identify the wires
ArrivalConfirm which wires are atrial and which ventricular (by convention atrial often exit right of the sternum and ventricular left — but always check the operation note/labels). Connect to the pacing box with labelled cables; keep unused wires insulated and dry.
- 2
Record the settings
ArrivalDocument mode, rate, atrial and ventricular output (mA), sensitivity (mV), AV delay and underlying rhythm on the chart. Check the box battery and have a spare battery and transcutaneous pacing pads available.
- 3
Select the mode
ChooseSinus bradycardia with intact AV conduction → AAI. AF/flutter with slow ventricular response → VVI. Heart block with atrial activity → DDD. Asynchronous (AOO/VOO/DOO) only temporarily (e.g. diathermy interference) — risk of R-on-T VF.
- 4
Threshold testing
DailyCapture threshold: at a rate above intrinsic, reduce output until capture is lost; set output at least 2–3 × threshold. Sensing threshold: set rate below intrinsic, increase the mV number until sensing fails (pacing spikes appear); set sensitivity at about half that value. Do NOT test capture in pacing-dependent patients without senior help.
- 5
Failure to capture
ProblemSpikes without P/QRS: increase output, check connections and battery, reverse polarity, correct K⁺/acidosis/ischaemia; if wires failing, use a skin needle as an indifferent electrode (unipolar), start transcutaneous pacing or isoprenaline, arrange transvenous pacing.
- 6
Failure to sense / oversensing
ProblemUndersensing (inappropriate spikes on intrinsic beats; risk of R-on-T): make more sensitive (lower mV). Oversensing (pauses because noise/T-waves inhibit pacing; crosstalk in DDD): make less sensitive, check leads, shorten ventricular blanking per expert advice.
- 7
Overdrive pacing (expert)
Atrial flutterType 1 atrial flutter post-op may be terminated by rapid atrial pacing via atrial wires (e.g. 10–20% above flutter rate) — only with ventricular wire backup and senior/cardiology supervision; never deliver rapid pacing through ventricular wires.
- 8
Removal
Day 3–5Remove when no longer needed (often day 3–5, before discharge). Check INR, platelets and that heparin/LMWH is timed appropriately; pull gently with steady traction (cut flush at skin if resistant, per surgeon). Bed rest and monitor BP/HR for ~1 h; watch for tamponade.
Bradycardia while pacing is being set up
Atropine
- Dose
- 0.5–1 mg; repeat every 3–5 min to max 3 mg
- Worked example
- 0.6 mg ampoule IV, repeat to 3 mg.
Bradycardia with failing wires — bridge to transvenous pacing
Isoprenaline (isoproterenol)
- Dose
- 0.5–10 mcg/min
- Range
- 0.5–10 mcg/min
- Preparation
- 2 mg in 50 mL = 40 mcg/mL (check ampoule strength)
- Worked example
- 2 mcg/min = 120 mcg/h = 3 mL/h of 40 mcg/mL.
Pacing code (NBG) — first three letters
| Position | Meaning | Letters |
|---|---|---|
| 1st | Chamber paced | A = atrium, V = ventricle, D = dual, O = none |
| 2nd | Chamber sensed | A, V, D, O |
| 3rd | Response to sensing | I = inhibited, T = triggered, D = dual (inhibit + trigger), O = none |
Examples: AAI = paces and senses atrium, inhibited by native P waves. VVI = paces/senses ventricle. DDD = paces and senses both, tracks atrial activity. DOO = asynchronous dual-chamber.
Typical starting settings after cardiac surgery (guide)
| Setting | Atrial | Ventricular |
|---|---|---|
| Rate | 80–90/min (AAI/DDD) | 80–90/min (VVI backup often 50–60 if intrinsic rhythm adequate) |
| Output | ≥2–3 × capture threshold (often 5–10 mA initially) | ≥2–3 × capture threshold (often 5–10 mA initially) |
| Sensitivity | About half the sensing threshold (often 0.5–1 mV) | About half the sensing threshold (often 2–3 mV) |
| AV delay (DDD) | Commonly 150–180 ms (optimise to cardiac output) | — |
Settings are institution-specific; record the measured thresholds daily.
Troubleshooting
| Problem | ECG appearance | Action |
|---|---|---|
| Failure to capture | Spikes without P or QRS | ↑ output, check connections/battery, reverse polarity, correct K⁺, unipolar via skin needle, transcutaneous pacing |
| Failure to pace (no output) | No spikes, bradycardia | Check battery/box/cables; oversensing? reduce sensitivity; asynchronous mode if needed |
| Undersensing | Spikes on top of intrinsic beats/T waves | Lower the mV value (more sensitive); consider turning pacing off if intrinsic rhythm adequate |
| Oversensing / crosstalk | Pauses, irregular pacing | Raise the mV value (less sensitive), check for electrical interference |
| Pacemaker-mediated tachycardia (DDD) | Fast paced rhythm at upper rate | Change to VVI/DVI or adjust PVARP per expert |
Calculators for this topic
References
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Bradycardia or pacing failure after cardiac surgery
Step 1 of ~4
Bedside pathway.
Decision: Epicardial wires present and connected?
Based on Reade MC; Reade MC. Educational use; follow local protocol.
Pathophysiology
Epicardial electrodes sense and stimulate local myocardium; the capture threshold rises with local oedema, inflammation and fibrosis, plus metabolic factors (acidosis, hyperkalaemia, ischaemia, antiarrhythmic drugs). The post-operative ventricle is stiff and benefits from AV synchrony and a higher rate (80–90/min) to maintain cardiac output.
Evidence
- Reade 2007 (two-part practical review, Anaesthesia): indications, wire placement, pacing codes, mode selection, threshold testing, troubleshooting, overdrive pacing for atrial flutter and removal complications.
- Bradyarrhythmias requiring pacing are commonest after valve surgery (especially aortic and tricuspid) because of proximity of the conduction system; persistent complete heart block beyond ~5–7 days usually needs a permanent pacemaker.
- Complications of wire removal (bleeding, tamponade, arrhythmia, retained wire) are uncommon but potentially fatal.
Controversies
- Routine placement of wires in all patients vs selective (e.g. omitted in uncomplicated CABG in some centres).
- Optimal timing of removal relative to anticoagulation for mechanical valves.
🇮🇳 India notes
- Temporary pacing boxes may be shared between ICU beds in Indian units; label each box with the patient, mode and thresholds and charge/replace batteries per schedule.
- Transvenous pacing expertise is typically in cardiology on call; agree escalation pathways in advance.
- Rheumatic and congenital valve surgery in young Indian patients leads to frequent conduction problems post-op.
References
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Educational reference for trained clinicians. Check doses against your hospital protocol and the patient in front of you.