Peri-operative Accidents & Emergencies

Physiological
1. Respiratory Arrest/ Dyspnoea/Apnoea
  1. Apneoa (Total arrest of breathing)
    1. Results in severe Hypoxia & Hypercapnia => Cardiopulmonary Arresst
    2. 1) De novo: Acute event that stops breathing)
    3. 2) End-result of progressive unintended hypoventilation
      1. A) Neural unresponsiveness to blood-gas dereangements
        Signs of severe CNS depression
        B) Mechanical obstruction/restriction to gas flow (or ineffective ventilation)
        Lung inflation:
        Low Pressures
        Upper airway obstruction or CNS depression
        High Pressures (Stiff lungs)
        Pneumothorax or Bronchospasm
    4. 3) Sign of cardiac arrest
  2. TX: (Cause dependent)
    1. CNS Depression
      1. Ventilation should be supported mechanically or manually until cause is identified and amerliorated
      2. End Anaesthesia if cranial nerves indicate overdosage
      3. IV Anesthetic Overdose:
        Ventilate (DON'T Hyperventilate) the lungs until the drug redistributes to skeletal muscle
        If no breathing after 5 min -> Hyperventialtion may have cause Hypocapnia => Reduce Resp. rate to 2 breaths/min
        Last resort: Opiod antagonists (Naloxone) & Analeptics (Doxapram)
    2. Obstructed Gas Flow
      1. Cleared or bypassed (tracheostomy)
      2. Vomit aspiration:
        Positioned head down, (Conscious) mouth gagged, oropharynx cleared of material using dry swabs (held with towel forceps or haemostats), moistened swabs
        Unconscious: Endotracheal intubation & PPV w/ O2, Endotracheal suction & lavage
        Will be life saving in most cases, however the application of O2 alone is useless if the victim is not breathing
2. Cardiopulmonary Arrest
  1. Causes:
    1. Myocardial Hypoxia
      1. Systemic Hypoxia
        Insufficient O2 in the arterial blood perfusing the myocardium
      2. Local Cardiac Effect
        CO may be too low to ensure it's own oxygenation under normal working conditions
        Hypotension
        Inferred Clinically: Imperceptible pulses in the periphery, diminished blood flow at the surgical site
        Severe or prolonged hypotension diminishes perfusion in splanchic & renal vasculature => Post-op failure in these organs
        Equine: Post-anaesthetic myositis => May result from prolonged muscle hypoperfusion
        Myocardium depends on adequate Diastolic BP to maintain its own perfusion
        (Severe) => Diminishes coronary blood flow => Cardiac arrest
        (arterial)BP = CO x SVR
        CO = SV x HR
        Tx:
        Infuse fluids rapidly
        Decreased contractility: Inotropes (Dobutamine)
        Decreased SVR: Alpha 1 agonists (Phenylephrine)
        Bradycardia
        Very low HR => insufficient to maintain an adequate Diastolic BP required for coronary perfusion
        Arrythmias
        Result of myocardial hypoxia, or (by reducing cardiac output) be the cause of hypotension
        Often indicate poor management
        Causes:
        SNS activation: Light anaesthesia, Hypoxia, Hypercapnia, Hypotension, Hypoglycaemia
        Drugs: Alpha2 agonists, halogenated hydrocarbons + adrenaline
        Electrolyte abnormalities (esp. K+)
        Certain surgical procedures
        Pre-existing heart disease
        Medical conditions e.g. gastric dilation volvulus complex
        Tx:
        ECG for Dx
        Drugs required depend on arrhythmia present
        Adequate anaesthesia & Ventilation restores normal rhythm
        In Electro-mechanical dissociation (EMD) the ECG appears normal, but there is no mechanical activity of the heart, No palpable pulses
        Workload of the heart is overwhelmingly increased
        Hypertension
        Tachycardia
        Myocardium O2 requirement > delivery
        Most common cause of negative myocardial O2 balance
        Difficult to define/ Tx if:
        The trend is dangerous & unchanging
        The rate is of Haemodynamic significance
        The ECG shows signs of myocardial hypoxia
        If steps not taken to restore a +ve myocardial O2 balnce, ventricular premature complexes (VPCs) will arise => Ventricular fibrilation (VF)
3. Toxaemia
  1. Depresses myocardial contractility & excitability
  2. Feature of many diseases (require extensive pre-op preparation)
  3. Azotemia in renal diseases
    1. Lactacidaemia in Hypovolemic shock
      1. Diabetic keto-acidosis
        Endotoxaemia in canine pyometria
  4. Tx:
    1. Correcting underlying condition & restoring perfusion (mainly by restoring effective circulating blood volume)
  5. Anaesthetic overdose can be regarded as a toxaemia
4. Electrolyte Disturbances
  1. Hyperkalaemia
    1. Most Important!
    2. Addison's disease
      1. Iatrogenic
        Renal failure
        Major injury/burns
        Acidaemia
    3. Tx:
      1. Resolving initiating cause & rapidly lowering [K+] levels by:
        Hyperventilation
        Fluids
        HCO3-
        Ca gluconate
        Insulin glucose
        Peritoneal dialysis
5. Hypothermia
  1. Under anaesthesia:
    1. Hypothalamic thermoregulation impaired
      1. Blood vessels (skin) vasodilate
        Skeletal muscle activity ceases
        Shivering is inhibited
        Visceral surfaces exposed
        Inspired gases are cold & dry
  2. Animals most at risk:
    1. High surface to volume ratios (e.g. neonates, birds, small lab animals
    2. Underdeveloped/impaired thermoregulatory reflexes (Age extremes)
  3. Effects:
    1. Reduced alveolar ventilation (VA)
      1. Reduced HR & CO
        LT-shifted oxyhaemoglobin dissociation curve
        Increased blood viscosity
        Shivering (recovery) elevates O2 consumption & plasma catecholamines
  4. Consequences:
    1. Prolonged recoveries
      1. Decreased elimination of volatile agents
        Decreased drug redistribution, elimination
        Decreased BMR
        Self-reinforcing cycle
    2. Cardiac arrest
      1. Ventricular fibrilation is likely when T falls below 28 degrees C
  5. Prevention:
    1. Physical Factors
      1. Increase operating room environment T
        Insulate surfaces animals lay on
        BAIR Hugger, aluminum foil, bubble wrap, heater blankets, hot water bottles, heat lamps
        Heat incoming fluids
    2. Anaesthetic Factors
      1. Use short acting anaesthetics
        Ensure anaesthetic depth is not excessively deep
        Adequate, not excessive, ventilation
        Rebreathing systems where appropriate
    3. Surgical Factors
      1. Avoid unnecessary wetting, clipping, volatile preparations (alcohol)
        Minimize surgical time
        Technique (exposed visceral surfaces moistened w/ warm irrigation fluids)
        Avoid wetting non-surgical areas
        Minimize incision size, Viscera replaced in body cavity ASAP
  6. Tx:
    1. Post-op: thoroughly dry animal with towels, heaters (plastic bags for small lab animals)

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Peri-operative Accidents & Emergencies

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