Rafael Ortega, MD Boston University Medical Center External Pacemakers

From the Trenches  70-year-old woman  Returns to OR for expanding neck hematoma  Cardiac arrest after intubation  Management

OBJECTIVES  Review External Pacemakers (EP)  Summarize EP historical evolution  Clarify EP indications  Explain the Philips/Agilent HeartStart Unit

External Pacemaker Manual Defibrillator Automatic Defibrillator 3 in 1 Biphasic Unit

Question What is “Biphasic”?

A Few Words on “Biphasic”  Biphasic energy is delivered in two directions  Require less energy for defibrillation  Less myocardial injury and myocardial dysfunction  150 J should be used for defibrillating ventricular fibrillation (in adults)

Waveform Comparison  Monophasic  Biphasic

Indications for EP  Temporizing measure in patients with symptomatic bradycardia  Little benefit in pulseless situations  May be useful for overdrive-pacing in certain tachycardias.

Early History  1791: Galvani electrically stimulates dead frog’s heart  1872: Duchenne resuscitates child tapping precordium with an electrode  1932: Hyman designs external pulse generator: "artificial cardiac pacemaker“  1952: Zoll uses simultaneous precordial and transesophageal electrodes for pacing

46 Years later  Hesselvik JF. Ortega RA. Simultaneous transesophageal atrial pacing and transesophageal echocardiography in cardiac surgical patients. Journal of Cardiothoracic & Vascular Anesthesia. 12(3):281-3, 1998 Jun.

Who was Dr. Zoll?  A Boston native  Harvard Medical School  Internship BI Hospital  In 1939 joins a research group at BI  WW-ll on Dwight Harken's surgical team Paul Zoll Professor Emeritus HMS

Transvenous Pacing Effect  Temporary and permanent implantable transvenous pacemakers (late 1950’s) superseded the use of external models.  But, these took time to insert  There was still a need for external pacing

1981: A New Era  Zoll patents and introduces a transcutaneous external pacemaker  Longer pulse duration and larger electrode surface  Reduced current required for capture and increased comfort for the patient  New model could be applied much more rapidly

Pulse Duration and Current  Early models used short (1-2 msec) impulses resembling the action potential of skeletal muscles  Zoll increased duration to 4 msec with a 3-fold reduction in threshold  Stimulation 20% over threshold stimulates only the ventricles.  External pacing requires times greater current than transvenous pacing

Question What is the effect of EP onhemodynamics?

Hemodynamics  EP can simultaneously stimulate all 4 heart chambers (in dogs)  Madsen echo demonstrated that atrial stimulation was retrograde without opening the mitral valve.  Atrial-pacing threshold in humans is much higher than for ventricles  Net result: loss of the “atrial kick”  Cardiac output is reduced

Reducing Discomfort  Not a problem under GA  If awake, place electrodes in the midline chest and just below the left scapula  Considered sedation

Question Can you perform CPR and use EP at the same time?

EP During CPR  CPR can be performed with EP pads in place.  No electrical hazard to the person performing CPR  However, turning the unit off during CPR is advisable.

No Heart Damage  No enzymatic, EKG, or microscopic evidence of myocardial damage has been found after pacing (dogs and humans) for as long as 60 minutes  Low risk of triggering ventricular fibrillation

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