MANAGING A VENTILATOR

Indications for ventilation

The most important determinant is the clinical assessment of the patient, looking for apnea, tachypnea, or respiratory failure (increased work of breathing with respiratory muscle fatigue) that cannot be corrected otherwise. ABGs that show severe hypoxemia (Pa_O2 < 50) despite F_IO2 > 60%, or CO₂ retention (Pa_CO2 > 50 with pH < 7.35), and poor PFTs (VC < 15 ml/kg, inspiratory force < -25 cm H₂O, or FEV₁ < 10 ml/kg) may also be of assistance.

Common ventilator modes

1. ACV (assist control): produces a ventilator-delivered breath for every patient-initiated breath. If the patient’s respiratory rate decreases past a preset rate, the ventilator delivers tidal breaths at a preset rate. Should be the initial mode of ventilation in most patients with respiratory failure.

2. IMV (intermittent mandatory ventilation): allows the patient to breathe at a spontaneous rate and tidal volume without triggering a ventilator; the ventilator also adds additional mechanical breaths at a preset rate and tidal volume. Indicated in the majority of spontaneously breathing patients becaue it maintains respiratory muscle tone and results in less depression of cardiac output than assist control.

3. SIMV (synchronized IMV): a hybrid of assist control and IMV – the ventilator becomes coordinated with the patient’s respiratory cycle, waiting for patient effort to deliver a positive pressure breath at the appropriate interval (every 6 seconds if the machine rate is 10/min). This prevents inadvertent stacking of a mechanical breath on top of a spontaneous breath. Advantages include less respiratory alkalosis, fewer adverse CV effects due to lower intrathoracic pressures, less requirement for sedation/paralysis, maintenance of respiratory muscle function, and facilitation of long-term weaning.

4. CMV (controlled mechanical ventilation): a mode in which the patient does not breathe spontaneously – the respiratory rate and tidal volumes are determined by the physician. May be used with sedation or paralysis, but paralyzed patients need to be monitored closely, and the ventilator cannot respond to ventilatory needs.

5. PSV (pressure support ventilation): the patient breathes at his own frequency, and the ventilator augments each patient-initiated breath with a set pressure; tidal volumes are determined by patient effort and the mechanical properties of the lung. Advantages include increased patient comfort and decreased work of breathing.

Selection of settings

1. Tidal volume (V_T): usually 10-15 ml/kg body weight, but should be lower in patients with decreased lung compliance/ARDS (6-8 ml/kg)

2. Rate: begin with 10-15 breaths per minute and adjust to achieve desired Pa_CO2 or pH.

3. Oxygen concentration (F_IO2): initially set to 100%, then decrease to the lowest level that will maintain a Pa_O2 > 60 mmHg or S_aO₂ > 90%.

4. PEEP: used to prevent airway collapse at the end of expiration (especially in those with COPD, diffuse lung edema, or ARDS); use is indicated when S_aO₂ < 90% despite an F_IO2 > 50%. Start with a PEEP of 5 cm H₂O and increase in increments of 2-5 cm to maintain Pa_O2 > 60 mmHg. May result in pulmonary barotrauma and hemodynamic compromise (decreased RV filling).

Get an ABG 15-30 minutes after initiating ventilation and a CXR to check for placement of the endotracheal tube. Other considerations include PTE prophylaxis (heparin, anti-embolic stockings, etc.), GI bleeding prophylaxis (IV ranitidine), pulmonary toilet to avoid accumulation of secretions, and placement in a semi-recumbent position (45^o angle) to decrease risk for nosocomial pneumonia.

Other notes about oxygen management

² Room air is 21% oxygen.

² If a patient is on nasal cannula, for each L/min of O₂ being given, add 4% to the F_IO2.

² Nasal cannula is only effective up to about 6 L/min O₂ (F_IO2 ~ 45%).

² Open face masks can deliver 40-60% O₂ effectively.

² Non-rebreather masks with O₂ reservoir are as close as you can get to deliver F_IO2 ~ 100% without intubation.