Mechanical
Ventilation
Mechanical ventilation is usually used
to prevent or treat type 2 respiratory (ventilatory) failure. The main
indications in adults are listed in Fig. 42a.
Types of mechanical ventilation (Fig. 42c)
Inspiratory muscle paralysis by
poliomyelitis was a common reason for mechanical ventilation in the firs half
of the twentieth century. It was usually performed by intermittent negative
pressure ventilation (INPV), which is still occasionally used today. Patients
are placed inside a tank ventilator sealed at the neck, and tank
pressure is in-termittently lowered, expanding the chest and lowering
intrapleural pressure as in spontaneous breathing. Disadvantages of this iron
lung include claustrophobia, discomfort, diff cult nursing care and the
bulk and expense of the equipment. Jacket and cuirass ventilators produce
a negative pressure just around the chest, but diff culty in achieving a
satisfactory seal limits their use to patients only needing ventilatory augmentation.
From the 1950s, intermittent
positive pressure ventilation (IPPV; controlled mechanical
ventilation, CMV) quickly replaced INPV for most purposes. Air is driven
into the lungs by raising airway pressure, usually via an endotracheal or
tracheostomy tube. Expiration is achieved by allowing pressure to fall to zero.
This simple form of IPPV is used during routine surgery. Typical initial adult
settings for IPPV are:
Tidal volume, VT =
8-12 mL/kg Respiratory frequency, f = 8-14 breaths/min
Minute ventilation, V(= VT
× f) ≈ 6000 mL/min Inspiratory time/expiratory time = 1 : 2-1 : 3
Minute ventilation is adjusted to
maintain Paco2 at about 5 kPa (37 mmHg). A
slightly lower Paco2 may be used initially in the
presence of raised intracranial pressure. Accepting a higher Paco2
(permissive hypercapnia) may prevent the need for excessively high
airway pressures. Pao2 is maintained above 10 kPa
(75 mmHg) by adjusting inspired Fo2. The lowest concentration
needed is used, usually in the range 30-60%. It may be preferable to accept a
slightly lower Po2 than to use more than 60% for long
periods.
Microprocessor control of ventilators
has permitted development of numerous variations of IPPV. For example, in
non-paralysed patients, the positive pressure may be synchronized with
spontaneous breaths and a mandatory breath given if no spontaneous breaths
occur in a preset time (synchronized intermittent mandatory ventilation,
SIMV). In another form, the ventilator operates only where spontaneous
ventilation falls below a preset minimum (mandatory minute ventilation, MMV).
If, instead of allowing airway
pressure to fall to zero, a small positive pressure is maintained throughout
expiration (positive end-expiratory pressure, PEEP), there is a
reduction in VA/Q mismatching and an improvement in Pao2
in some conditions, such as acute respiratory distress syndrome (ARDS). This
occurs because PEEP increases functional residual capacity (FRC) and reduces
the closure of airways and alveoli towards the end of expiration. Unfortunately,
intrathoracic pressure is raised, impairing venous return, and occasionally the
fall in cardiac output can reduce tissue oxygen delivery despite the increased Pao2.
The increased mean airway pressure
caused by PEEP also increases the risk of barotrauma. A good compromise is to
use the minimum PEEP required to keep Po2 at an acceptable
level (>8 kPa, 60 mmHg) when breathing 50-60% oxygen.
Non-invasive respiratory support
Non-invasive ventilation avoids the
use of tracheal intubation or tracheostomy. An example is INPV (above), but
this is no longer widely used. In contrast, non-invasive positive pressure
techniques using either a nasal mask (Fig. 42b) or sometimes a full face mask
are increasingly being used.
In continuous positive airway
pressure (CPAP), a standing pressure of 5-10 cmH2O is applied to
a nasal or face mask in a spontaneously breathing patient (Fig. 42c). This has
several potential benefi cial effects. First, it helps prevent upper airway
collapse in obstructive sleep apnoea. In interstitial diseases such as ARDS,
it recruits alveoli, reducing VA/Q mismatching. FRC is increased,
and this may increase lung compliance by moving the patient onto the steep part
of the pressure-volume curve (Chapter 6). CO2 retention may be a
problem during CPAP, which may be improved by using biphasic or bilevel
positive pressure ventilation (BiPAP). BiPAP alternates between high and low
pressure either for f xed time periods (Fig. 42c) or between inspiration and
expiration, making expiration easier and improving the emptying of the lungs.
CPAP may improve oxygenation and may
aid the patient's own respiratory efforts, but it cannot produce ventilation by
itself. In contrast, non-invasive intermittent positive pressure ventilation
(NIPPV) is IPPV delivered by face or, more usually, nasal mask. For it to
be used successfully, the patient must be cooperative and introduced to the
technique gradually, to allow synchronization of his or her breathing with the
ventilator. Its use includes nocturnal ventilation of patients with chronic
respiratory failure due to neuromuscular disease or thoracic deformity. It is
well established for the treatment of respiratory failure caused by acute
exacerbations of chronic obstructive pulmonary disease (COPD), avoiding the need
for intubation and improving survival. It is increasingly being used for a
range of other conditions as an alternative to standard IPPV.
In summary, the main beneficia effect
of CPAP is recruitment of alveoli. The reduction in collapse sometimes
also gives rise to a reduction in the work of breathing. In contrast, NIPPV is
used to reduce or take over the work of breathing rather than to recruit
alveoli. This may be of benefi in the tired (e.g. COPD) patient.
Weaning the patient off the ventilator following surgery
is usually achieved by reversing neuromuscular blockade and lightening the
anaesthetic level. In ICU patients, weaning may be more difficult Several
techniques are used, including removing mechanical ventilation for
progressively longer periods, or by using a spontaneously breathing mode (e.g.
SIMV), and pressure support in which support is progressively reduced. CPAP
applied via the endotracheal tube may also help the weaning process.
Problems are hard to predict
accurately, but are most likely following prolonged ventilation, in debilitated
patients, or in those with neuromuscular or chronic respiratory disease. A
pattern of rapid shallow breathing 5 minutes after disconnection from the
ventilator is one of the more useful predictors of failure.
Complications of mechanical
ventilation are numerous, and are
listed in Fig. 42d.
