Invasive ventilation: Intermittent positive pressure ventilation (IPPV)
IPPV is invasive and non-physiological, and hence reserved for cases where non-invasive ventilation is not suitable.
Terminology
PEEP: positive end expiratory pressure
Cycling: change from inspiration to expiration or the reverse. It can be volume cycled, pessure cycled, time cycled or flow cycled.
Modes of ventilation: controlled mode, assist controlled mode, assist mode
Controlled mode – every breath by the ventilator; even if the subject wants breath spontaneously, it is not permitted. Volume and pressure controlled modes are available.
Assist control mode – IMV and SIMV (synchronized intermittent mandatory ventilation). SIMV removes the chance of fighting with the ventilator. It is a popular weaning method. Both these modes are volume cycled.
Assist mode – ventilator only supports patient’s effort. eg. Pressure support ventilation (PSV). This mode is for weaning in a conscious patient. It will be disastrous in those prone for apnea.
While using volume controlled ventilation, peak airway pressure has to be monitored. In pressure controlled ventilation, the tidal volume has to be monitored for adequacy.
Positive end expiratory pressure (PEEP): Begin with 5 cm and step up if necessary. PEEP improves oxygenation by preventing alveolar collapse and recruits lung units. It increases FRC and prevents cyclical collapse of alveoli. PEEP can reduce cardiac output and overdistension of normal units.
Sedation / muscle relaxation for IPPV
Sedation is necessary for the person to tolerate ventilation. Midozolam or propofol are preferred. Instead of bolus doses, infusions may be better. Muscle relaxants are seldom used now-a-days in the ICU setting as relaxatants have their on problems. Relaxants may be used initially. But good sedation has to be given before giving relaxants.
Ventilator induced lung injury (VILI)
Causes: Barotrauma, Volutrauma, Biotrauma, Shear injury, PEEP, Peak pressure
Ventilator associated pneumonia (VAP) has a mortality of about 30%
Prevention of VILI:
Permissive hypercapnia – accept higher PaCO2 upto 60 mm Hg, limit airway pressue (<35 cms), low tidal volume (6 ml/kg)
Permissive hypoxia – accept lower level of PO2.
Weaning: Graduallly reduce ventilatory support using SIMV or PSV or T piece ventilation.
