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Aim

Effect = Improved oxygenation + carbon dioxide elimination

Use must be appropriate given the prognosis of the disease

Modes of Ventilation

1. Pressure control:
   • Pre-set inspiratory pressure is delivered or cycle changes from inspiration to expiration when a certain pressure is reached
2. Volume control
   • Fixed tidal volume delivered and is generally used by older and simpler circuits
3. Assisted modes
   • Ventilator augments each inspiratory effort initiated by patient – either pressure or volume support
   • Eg. PSV: pressure support ventilation / SIMV synchronized intermittent mandatory ventilation

Indications

1. Inadequate oxygenation
   • PaO2 < 8kPa with FiO2 > 0.6
2. Inadequate ventilation:
   • PaCO2 > 8kPa
   • RR > 35 with imminent exhaustion
   • Tidal volume < 5mls/kg
   • Vital capacity < 10-15 ml/k
3. Special circumstances:
   • Raised ICP: Keeping PaCO2 at 4.0-4.5kPa > cerebral vasoconstriction > reduces ICP (this may occur at expense of reducing oxygenation)

Parameters

1. Fraction inspired oxygen (FiO2): 0.21-1.0
2. Respiratory rate
3. Tidal Volume 5-7ml/kg
4. Flow waveform: sinusoidal flow during respiratory cycle reduces mean airway pressures
5. Inspiratory:Expiratory ratio; Usually 1:2
6. Pressure limit
7. Additional PEEP / CPAP – delivers additional pressure at end of cycle to splint airways open

Adjustments

1. Improve oxygenation: Increasing FiO2 / increase PEEP / Increase I:E ratio (more "inspiration")
2. Improving ventilation (CO2 elimination): Increase RR / increase TV / Increase peak pressure

PEEP: Positive End Expiratory Pressure

1. Used in conjunction with IPPV; delivery of additional pressure (5-20cmH2O) at end of respirator cycle à prevent alveolar collapse à improve oxygenation when additional alveoli recruited
2. Physiological effects: increased compliance
3. Increased FRC
4. Reduced shunting with increased V/Q ratio

Complications of IPPV

1. Cardiovascular: Makes intrathoracic pressure “less negative” (ie more positive) à reduces venous return to heart à reduced Cardiac output + arterial pressure // + Lung expansion à distorts alveolar capillaries à increased pulmonary vascular resistance
2. Respiratory: Barotrauma from excessive distension à alveolar rupture à pneumothorax/pneumomediastinum + increased risk of nosocomial pneumonia
3. Renal: reduces renal perfusion pressure + urine output
4. Paralytic ileus: uncertain mechanism

Weaning from ventilation

1. Original disease process treated
2. Adequate lung function
   • RR < 35
   • PaO2 > 11kPa FiO2 < 0.5
   • Adequate ventilation
3. Haemodynamic stability
4. Adequate cerebral function
5. Nutrition adequate
   • Effects respiratory muscle strenght and fatigability
   • Excess carbohydrate metabolism produces loads of CO₂ requiring it to be blown off (and raised ventilatory demands)

Ventilation Strategy

1. T-piece
2. T-piece and CPAP
3. SIMV - Intermittent mandatory ventilation - supports patients own breaths
4. Pressure support ventilation - patient breaths on own, but each breath augmented with positive inspiratory pressure
5. Extubate + CPAP