Page 412 - ACCCN's Critical Care Nursing

Ventilation and Oxygenation Management 389

rate and the density and viscosity of the inspired gas. 7. the resistance, compliance and dead space charac-
During mechanical ventilation, bronchospasm, airway teristics do not adversely affect spontaneous
oedema, endotracheal tube lumen size, increased secre- breathing modes
tions, and inappropriate setting of flow rates can influ- 8. the sterility of the inspired gas is not
ence airway resistance. Normal resistance for intubated compromised. 73
patients is 6 cmH 2 O/(L/sec). 68
Humidification is applied using either a heat–moisture
exchanger (HME) or a heated water bath reservoir device
VENTILATOR CIRCUITS in combination with a heated ventilator circuit.

Delivery of mechanical ventilation requires a ventilator
circuit to transport gas flow to the patient. To prevent Heat–moisture Exchanger
condensation from cooling of warm humidified gas, Heat–moisture exchangers conserve heat and moisture
inspired gas is heated via a wire inside the wall of the during expiration, and enable inspired gas to be heated
circuit in either the inspiratory limb alone or both the and humidified. Two types of HMEs exist: hygroscopic
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inspiratory and expiratory limbs. Historically ventilator and hydrophobic. Hygroscopic HMEs absorb moisture
circuits were changed frequently (48–72 hours) to onto a chemically impregnated foam or paper material
70
decrease the risk of VAP. Current guidelines for preven- and have been shown to be more effective than hydro-
tion of VAP found evidence that the frequency of ventila- phobic HMEs. HMEs are placed distally to the circuit
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tor circuit changes had no relationship to the incidence Y-piece in line with the endotracheal tube and increase
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of VAP and therefore recommended routine circuit dead space by an amount equal to their internal volume.
changes were not necessary and circuits should only be HMEs should be changed every 24 hours or when soiled
changed when soiled or damaged. 71 with secretions and are usually reserved for short term
humidification.
HUMIDIFICATION Heated Humidification
Humidification techniques warm and moisten gas to
facilitate cilia action and mucus removal as well as to Generally, heated humidification (HH) is used for patients
prevent drying and irritation of respiratory mucosa and requiring greater than 24 hours of mechanical ventila-
solidification of secretions. During endotracheal intuba- tion. Various models of heater bases and circuits are on
tion and mechanical ventilation, the normal humidifica- the market and we recommend their use in accordance
tion processes of the nasopharynx are bypassed. This, in with manufacturer instructions. A recent systematic
combination with the use of dry medical gas at high flow review and meta-analysis reported no overall effect on
rates, means alternative methods of humidification are artificial airway occlusion, mortality, pneumonia, or
required. The best conditions for mucosal health and respiratory complications when HMEs were compared to
function over prolonged periods are when inspired gas is HHs, although it noted that PaCO 2 and minute ventila-
warmed to core body temperature and is fully saturated tion were increased and body temperature was lower with
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with water. 72 the use of HMEs.
NON-INVASIVE VENTILATION
Absolute and Relative Humidity
Non-invasive ventilation (NIV) is an umbrella term
Absolute humidity refers to the amount of water vapour
in a given volume of gas at a given temperature. Absolute describing the delivery of mechanical ventilation without
humidity rises with increasing temperature; during the use of an invasive airway, via an interface such as an
mechanical ventilation gas is heated to increase the oronasal, nasal, or full face mask or helmet. NIV tech-
amount of water vapour it will hold. Relative humidity is niques include both negative and positive pressure
expressed as a percentage, and is the actual amount of ventilation, although in critical care positive pressure ven-
water vapour in a gas compared to the maximum amount tilation is primarily used.
this gas can hold (ratio of absolute to maximal humid-
ity). Ideal humidification is achieved when: TERMINOLOGY
Positive pressure NIV can be further categorised as non-
1. the inspired gas delivered into the trachea is at invasive positive pressure ventilation (NIPPV) or conti-
3
37°C with a water content of 30–43 g/m nuous positive airway pressure (CPAP). NIPPV is the
(relative humidity is 100% at 37°C in the provision of inspiratory pressure support (also referred to
bronchi) as inspiratory positive airway pressure [IPAP]) usually in
2. the set temperature remains constant without combination with positive end expiratory pressure (PEEP)
fluctuation (also referred to as expiratory positive airway pressure
3. humidification and temperature are unaffected by [EPAP]). CPAP does not actively assist inspiration but
a large or differing types of gas flow provides a constant positive airway pressure throughout
4. the device is simple to use inspiration and expiration. 77
5. the humidifier can be used with spontaneously
breathing and ventilated patients The terms Biphasic (or bilevel) positive airway pressure
6. safety alarms prevent overheating, overhydration (BiPAP®) and non-invasive pressure support ventilation
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and electrocution (NIPSV) are also used to refer to NIPPV. The acronym