VENTILATION
If the patient’s ABGs deteriorate to P a
O
2
< 8kPa or P a
CO
2
> 10.5kPa (type 2), or they become fatigued or fail to improve with just oxygen therapy then consider ventilation respiratory support. The purpose of ventilation is to improve gas exchange and thereby help to deliver more oxygen. There are essentially two types of support:
1) Non-invasive ventilation
2) Invasive mechanical ventilation
Non-invasive ventilation o A positive pressure ventilator is connected to a mask that delivers positive pressure to the nose or mouth o Indicated for acute type 2 RF caused by chest wall or neuromuscular disease or COPD patients with pH < 7.35 o Patients must be alert and able to breathe naturally o Avoids the need of endotracheal intubation such as in the immunocompromised o Advantages include: o Lower P a
CO
2
and respiratory rate o Improves pH, oxygenation and dyspnoea o Preserves speech, swallowing and cough o Relaxes respiratory muscles o There are 3 main types:
- Continuous positive airways pressure (CPAP)
- Bilevel positive airway pressure (BIPAP)
- Nasal intermittent positive pressure ventilation (NIPPV)
CPAP
- A mask fits tightly over the patient’s nose and or mouth to prevent air entering
- An inlet to deliver O
2 may be available and there are exhalation ports to remove expired air
- Positive pressure is delivered throughout spontaneous inspiration and expiration
- Pressure re-inflates collapsed alveoli (recruitment)
- Lung compliance is also enhanced
Typical pressures are 5-12 cmH2O and higher pressures may risk gastric dilatation then aspiration
BIPAP
- Similar to CPAP but delivers two different levels of positive pressure
- Changes in pressure cycle between normal breaths or is fixed
- At high pressures CO
2
is cleared as alveolar ventilation enhances, and at low pressures lung inflation is preserved

NIPPV
- Positive pressure is delivered at certain times during inspiration
- Reduces work of breathing and removes CO
2
Typical pressures are 10-30 cmH
2
O
Invasive mechanical ventilation o Mechanical machine delivers ventilatory support via an endotracheal tube or tracheostomy o General indications include severe hypoxaemia despite FiO
2
> 40%, lack of spontaneous ventilatory drive, insufficient lung expansion, increased work of breathing (tachypnoea, fast shallow breaths) o Indications in acute RF include:
- Unresponsive, unconscious, confused patient
- absent airway protection from aspiration
- excess respiratory secretions requiring suction o Positive end expiratory pressure (PEEP) raises the functional residual capacity and thus prevents end-expiratory alveolar collapse in many lung diseases (e.g. COPD). This improves oxygenation, which means
PEEP can be added to the support ventilators if required o Mechanical supports can be neatly divided into:
1. Full support
2. Partial support o However, modern ventilatory machines can switch between the two
Full support
This is where respiration is completely dependent upon the machine and spontaneous respiration from the patient is not possible (sedated):
 Tidal volume controlled mechanical ventilation - all breaths delivered by the ventilator are at a set volume and rate
 Pressure controlled mechanical ventilation - inflation pressures and rates are fixed but lung compliance determines the tidal volume
Partial support
This is where any spontaneous breathing from the patient is detected by the machine that further provides support:
 Synchronised intermittent mandatory ventilation (SIMV)
– patient can breathe between set breaths that are delivered after patient exhales
 Assisted mechanical ventilation – breaths delivered at a preset volume but any inspiratory effort detected from the patient can activate more breaths
 Pressure support ventilation (PSV) - patient’s spontaneous breaths determines the delivery rate of a preset pressure. Usually given with
CPAP.