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392 P R I N C I P L E S A N D P R A C T I C E O F C R I T I C A L C A R E

Air leaks may cause conjunctival irritation and the high
flow of dry medical gas results in nasal congestion, TABLE 15.5 Set ventilator parameters
oral or nasal dryness and insufflation of air into the
stomach. Claustrophobia associated with the NIV inter- Parameter Description
face may also lead to agitation reducing the efficacy
of NIV treatment due to poor coordination of respira- Fraction of inspired The fraction of inspired oxygen
delivered on inspiration to the
oxygen (FiO 2 )
79
tory cycling between the patient and NIV unit. patient.
More serious, yet infrequent, complications include
aspiration pneumonia, haemodynamic compromise Tidal volume (V T ) Volume (mL) of each breath.
associated with increased intrathoracic pressures and Set breath rate (f) The clinician determined set rate of
pneumothorax. 80 breaths delivered by the ventilator
(bpm).
DETECTING NIV FAILURE Inspiratory trigger or Mechanism by which the ventilator
senses the patient’s inspiratory effort.
sensitivity
Failure to respond to NIV within 1–2 hours of com- May be measured in terms of a
mencement is demonstrated by unchanged or worsening change in pressure or flow.
gas exchange, as well as ongoing or new onset of rapid Inspiratory pressure Clinician determined pressure that is
shallow breathing and increased haemodynamic instabil- (P insp , P high ) targeted during inspiration.
111
ity. A decreased level of consciousness may be indicative Inspiratory time (T insp ) The duration of inspiration (sec).
of imminent respiratory arrest.
Inspiratory : expiratory The ratio of the inspiratory time to
ratio (I : E) expiratory time.
INVASIVE MECHANICAL Flow (V) The speed gas travels during inspiration.
VENTILATION (L/min).

Critically ill patients with persistent respiratory insuffi- Pressure support (PS) The flow of gas that augments a
ciency (hypoxaemia and/or hypercapnia), due to drugs, patient’s spontaneously initiated
disease or other conditions, may require intubation and breath to a clinician-determined
pressure (cmH 2 O).
mechanical ventilation to support oxygenation and ven-
tilatory demands. 115,116 Clinical criteria for intubation and Positive end- Application of airway pressure above
ventilation should be based on individual patient assess- expiratory pressure atmospheric pressure at the end of
expiration (cmH 2O).
(PEEP)
ment and patient response to measures aimed at revers-
ing hypoxaemia. Rise time Time to achieve maximal flow at the
onset of inspiration for pressure-
targeted breaths.
INDICATIONS Expiratory sensitivity During a spontaneous breath, the
Indications for intubation and mechanical ventilation ventilator cycles from inspiration to
include: expiration once flow has decelerated
to percentage of initial peak flow.
● apnoea Minute volume (VE) Generally not set directly but is
● inability to protect airway; e.g. loss of gag/cough reflex; determined by V T and f settings. Tidal
decreased Glasgow Coma Scale (GCS) score volume multiplied by the respiratory
● clinical signs indicating respiratory distress; e.g. rate over one minute (L/min).
117
tachypnoea, activation of accessory and expiratory Airway pressure (P aw ) The pressure measured in cmH 2 O by the
118
muscles, abnormal chest wall movements, tachycar- ventilator in the proximal airway.
dia and hypertension Plateau pressure (P plat ) The pressure, measured in cmH 2 O,
● inability to sustain adequate oxygenation for meta- applied to the small airways and
bolic demands; e.g. cyanosis, SpO 2 <88%, with sup- alveoli. P plat is not set but can be
plemental FiO 2 ≥0.5 measured by performing an
● respiratory acidosis (e.g. acute decrease in pH <7.25) inspiratory hold manoevre.
● postoperative respiratory failure
● shock.

The goals of mechanical ventilation are to achieve and
maintain adequate pulmonary gas exchange, minimise synchrony during both inspiratory and expiratory breath
the risk of lung injury, reduce patient work of breathing phases. Parameters commonly manipulated during
and optimise patient comfort. mechanical ventilation are detailed in Table 15.5. Para-
meters often observed and documented are discussed
MECHANICAL VENTILATORS below.
Contemporary ventilators use sophisticated microproces-
sor controls with sensitive detection, response and Fraction of Inspired Oxygen
control of pressure and gas flow characteristics. These The fraction of inspired oxygen (FiO 2 ) is expressed as a
mechanical ventilators are more sensitive to patient ven- decimal, between 0.21 and 1, when supplemental oxygen
tilatory demands, enabling improved patient–ventilator is applied. Room air has an oxygen content of 0.21 (21%).