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You are in Home >> Exams >> Mitchell Anaesthetic Notes


Cardiology

Created: 24/4/2006
Updated: 10/1/2007
 

Aortic incompetence

  • Aetiology
    • Acute
      • Endocarditis, aortic dissection, connective tissue disease, trauma
  • Pathophysiology
    • Acute
      • Sudden volume load on LV
      • Increased LVEDP, PAOP
      • Sympathetic response: increased HR, vasoconstriction
      • May cause acute decompensation and failure
    • Chronic
      • Progressive LV enlargement and hypertrophy
      • High compliance, high output
      • Late decompensation (10-20 years)
    • Classification
      • Regurgitant flow: <3 L/min mild, 3-6 L/min moderate, >6 L/min severe
      • Or by contrast findings on aortogram
  • Features
    • Tachycardia: reduced regurgitation, LVEDV, increased coronary perfusion
    • Relatively little increase in myocardial O2 demand until late
      • Volume work increases O2 demand less than pressure work
  • Management
    • Responsive to pressors, but vasodilators may increase forward flow
    • Diastolic hypotension and bradycardia to be avoided (decreased coronary perfusion)
    • IABP contraindicated (enhances retrograde flow)
  • Valve replacement
    • Retrograde flow of pump blood may distend LV until aortic clamping
    • Anterograde cardioplegia may be difficult

Mitral stenosis

  • Aetiology
    • Usually rheumatic heart disease
  • Clinical features
    • Symptomatic early, slow progression
    • Marked worsening with increased demand for CO (pregnancy) or development of AF
    • Severity
      • Pressure gradient not useful (dependent on HR, CO…)
      • Valve area (normal 4-6 cm2)
        • Mild 1.5-2.5 cm2
        • Moderate 1-1.5 cm2
        • Severe <1 cm2
  • Anaesthetic management
    • Heart rate maintained low-normal in sinus rhythm
      • Bradycardia decreased CO due to low SV
      • Tachycardia decreased CO due to slow diastolic filling
      • AF --> sudden decompensation, especially with rapid ventricular rate
    • Maximal tolerated LAP without pulmonary oedema
      • Risk of complications from PA catheter
        • Pulmonary hypertension: increased risk of PA rupture, unreliable PAOP
    • Risk of RV failure with pulmonary hypertension
      • Septal deviation impairs LV filling
      • Prevent with NO, lowering PVR

Pacemaker management

  • Identification code (5 letters)
    • Chamber paced: O, Atrium, Ventricle, Dual
    • Chamber sensed: O, Atrium, Ventricle, Dual
    • Mode of response: O, Triggered, Inhibited, Dual
    • Antitachycardic function: O, Pacing, Shock, Dual
    • Programmability: O, Program, Multi-program, Communicating, Rate-response
  • Modes
    • Asynchronous
      • AOO, VOO, DOO: pacing regardless of underlying activity
      • Fall-back mode only as wasteful of battery and may compete with intrinsic rhythm
    • Single chamber demand
      • AAI, VVI: single chamber pacing inhibited by intrinsic activity
      • Simple single-lead pacemakers
      • AAI requires intact A-V conduction
      • VVI does not maintain A-V synchrony
    • Dual chamber
      • A-V synchronous (VAT, VDD)
      • A-V sequential (DVI)
      • Universal (DDD)
        • Operate in AAI, VDD or DVI as required
      • A-V inhibited (DDI)
        • Used where atrial tachycardia causes rapid ventricular pacing in DDD
    • Rate responsive
      • Provide exercise response in patient who are pacemaker-dependent
      • Various sensors used
        • Temperature, SvO2, respiratory rate or minute volume, QT interval, vibration, acceleration
    • Antitachycardia functions
      • Simple shock devices
        • Recognize sustained tachycardia
        • Deliver 25-30 J shocks up to five times
      • Tiered (dual) therapy devices
        • Pacing for bradycardia
        • Overdrive pacing for atrial tachycardia
        • Low energy cardioversion for VT
        • High energy defibrillation for VF
      • Generate palpable but not dangerous voltage at the body surface
  • Issues
    • Maintain stable heart rate and rhythm throughout surgery
  • Preoperative
    • History
      • Reason for pacemaker insertion
      • Symptomatic arrhythmias or IHD
      • Medications
      • Other illnesses
      • Type of pacemaker, who manages it, last tested, history of problems
      • Previous anaesthetics
    • Examination
      • Routine, focussing on cardiorespiratory examination
      • Identify location of box
    • Investigation
      • ECG
        • Identify rhythm, presence of pacing spikes
    • Consultation
      • Cardiologist to determine pacing mode, rate-responsiveness
      • Rate-responsiveness and antitachycardia functions must be switched off prior to surgery
    • Premedication
      • Continue cardiac medications
    • Transport
      • Avoid excess movement if movement-responsive
    • Preparation
      • Chronotropic medications available, external pacemaker may be available
        • Atropine, isoprenaline
      • Routine access and monitoring
        • Additional heart rate monitor not susceptible to diathermy interference
          • Precordial stethoscope or palpable pulse
      • ECG leads short: can act as antennae
      • “Cardiac protected” theatre required
  • Intraoperative
    • Sensing pacemakers can be affected by myopotentials, movement, diathermy
    • Induction
      • Suxamethonium fasciculations produce myopotentials
      • Excess ventilation simulates exercise in movement-responsive pacemakers
      • No requirement for prophylactic antibiotics
    • Maintenance
      • Position so that pacemaker is not a pressure area
      • Diathermy
        • Problems
          • Direct damage to pacemaker from current
          • Microshock (VF) from current through lead
          • Inhibition of pacemaker by current if in demand mode
        • Precautions
          • No diathermy within 15 cm of pacemaker
          • Brief bursts of diathermy if interfering with pacing
          • Preferably bipolar diathermy with minimal current
          • Consider changing mode to asynchronous
          • Diathermy current distant from and at 90° to pacemaker
            • Grounding plate distant, but not if head & neck surgery
          • Avoidance of N2O with a recently implanted pacemaker
            • Expanding a gas pocket around the generator can cause loss of anode contact with a unipolar generator
      • Maintain temperature to avoid post-op shivering
    • Emergencies
      • Use of a ring magnet
        • In VVI will set a fixed rate
        • Effect is not predictable in programmable pacemakers
      • “Pacemaker syndrome”
        • Activation of VVI pacemaker causes hypotension
        • Loss of AV synchrony or retrograde conduction causes fall in CO
        • Atrial stretch causes reflex vasodilatation
      • Pacemaker failure
        • Interference from electrical activity or muscle potentials
        • Failure to capture due to electrolyte disturbance or drugs
          • Hypoxia, hypercarbia, increased intracellular K+, hypernatraemia
          • Verapamil, β-blockers, quinidine
      • Defibrillation
        • Paddles at least 12 cm from generator, orientated at 90° to AICD electrodes

 

Management of heart block

  • Issues
    • Risk of development of AV block with bradycardia
      • Is a prophylactic pacemaker required?
    • Risk of associated cardiac disease
  • Preoperative
    • Assessment
      • Conduction defect
        • Symptoms: dizziness, drop attacks, palpitations
        • Cardiac examination: BP and HR, arrhythmia
        • Previous investigation: conduction studies, ECG
        • Stability of disease
      • Associated disease
        • History of Lev’s or Lenegre’s disease
          • Fibrosis of His bundle or terminal fibres
        • Cardiac surgery or His ablation
        • Cardiomyopathy
        • IHD
        • Previous investigations: stress test, echo, coronary angiogram etc.
        • Drug therapy either treating or causing block
          • e.g. digoxin, propranolol, quinidine, procainamide, verapamil
      • General assessment
        • Level of function
        • Routine anaesthetic assessment
      • Investigation
        • ECG, U&E, specific investigations as indicated
      • Consultation
        • Discussion with treating cardiologist
        • Optimize medication regimen
          • Defer surgery if time is needed
        • Decision whether preoperative pacing is required
          • Not usually necessary unless unstable and symptomatic
          • If unstable, insert transvenous pacing wire under LA
    • Premedication
      • Continue usual cardiac drugs
      • Anxiolytic, sedative premedication
    • Consent
      • Discuss possible requirement for temporary pacing
      • Anaesthetic plan
        • Regional with avoidance of hypotension preferable to GA
  • Intraoperative
    • Preparation
      • Routine anaesthetic equipment prepared
      • Drugs and equipment for rapid conversion to GA if necessary
      • Chronotropes available for CHB: atropine, isoprenaline
      • Transvenous or external pacing equipment available
    • Monitoring
      • Routine, plus
      • 5 lead ECG monitor prior to block insertion
      • Large bore IV access
    • Induction
      • Low spinal or epidural catheter
        • Depending on likely duration of surgery and stability of disease
      • Preloading with fluid
      • Pressors drawn up
    • Emergencies
      • Heart block with nodal rhythm
        • Usually responsive to atropine
      • Heart block with ventricular escape
        • Usually accompanied by bradycardia, hypotension
        • Immediate transvenous or external pacing if available
        • Otherwise
          • Airway control and ventilation with 100% O2 if unconscious
          • IV fluid
          • Rate support with isoprenaline and atropine until
          • Insertion of transvenous pacing wire

Kindly provided by Dr James Mitchell from his pharmacodynamics series


ArticleDate:20060424
SiteSection: Article
 
   
    
                                            
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