NICU:  Respiratory: Ventilator Adjustments

Ventilation: To affect pH or pCO2

The goal is to change the pCO2 to treat a respiratory acidosis or alkalosis.  Ventilation (CO2 elimination) is a function of minute ventilation, which is the produce of the tidal volume (TV) and respiratory rate (RR).  When using pressure limited ventilation, you can change the tidal volume by raising or lowering the PIP.  Alternatively, you can change the RR (alter the IMV).  In summary, to decrease the pCO2, increase the PIP or IMV.

Oxygenation:  To affect the pO2

The pO2 is mainly affected by FiO2 and MAP.  In most cases, the most profound effect on MAP can be exerted by changing the PEEP.  So, to increase pO2, increase the FiO2 or the PEEP.  There are specfic exceptions to this guideline.

Weaning

When weaning ventilatory support, your goal should be to maintain approximately normal (not perfect) ABGs.  Usually, with pH greater than 7.25 to 7.28, and pCO2 stable or gradually rising are considered weanable gases (this depends on the clinical setting, of course).

Weaning CMV:
When the pCO2 is acceptable, first wean the PIP in increments of 1-2 cm H2O to the minimum PIP that moves the chest wall.  Then step down the IMV in increments of 2-5 and follow ABGs and clinical exam (watch for tachypnea and retractions).  When the IMV is around 10, you can usually extubate to oxygen or nasal continuous positive airway pressure (NCPAP).  The usual starting NCPAP setting is 5-8 cm H2O.  If blood gases are acceptable and no respiratory distress is present, wean NCPAP 5-6 down to 5-6 cm H2O and then begin to cycle it beginning with one hour on:2 hours off.  If tolerated, increase the time off  NCPAP in one hour increments until it is discontinued.

Rapid Deterioration

Rapid unexpected rises in pCO2 (greater than 10 in less than 12 hours) suggest that the ET tube is plugged, displaced or that there is a pneumothorax.  Infants can deteriorate rapidly from these addressable causes.  Less rapid deterioration can result from insufficient distending pressures and worsening atelectasis and ventilation/perfusion mismatch.

Persistent Pulmonary Hypertension

A common exception to the above recommendations is PPHN.  The goal in PPHN is to induce respiratory alkalosis with PaO2's greater than 80-100 mmHg.  In this case, ventilator settings are weaned very conservatively (i.e., PIP by 1 cm at a time, and FiO2 in 2-3% increments).  An overly aggressive wean can allow pulmonary blood pressure to rebound, and the infant flips back into PPHN.