So now that we know how to set the initial settings and make adjustments, let's walk through a case study together.
Here we go. A 55-year-old male patient with an ideal body weight of 70 kg has just been rushed to the emergency room. He was found to be unresponsive and apneic. And so the decision is made to intubate and mechanically ventilate the patient.
The patient was placed on the following settings:
- Assist-control (AC) mode with volume control
- Tidal volume (VT) of 500 mL
- Respiratory rate (RR) of 15 breaths / min
- Positive end-expiratory pressure (PEEP) set to 5 cmH2O
- Fraction of inspired oxygen (FIO2) of 100%
- Flow rate of 60 L / min
An arterial blood gas (ABG) has just been ordered. With sedation on board, the patient appears comfortable with our current ventilator settings.
After several minutes, the ABG results come back and they reveal the following:
- pH of 7.30
- Arterial carbon dioxide level (PaCO2) of 55 mmHg
- Arterial oxygen level (PaO2) of 310 mmHg
You also note on the ventilator that the positive inspiratory pressure, or PIP, was 19 cmH2O and, after performing an inspiratory pause maneuver, the plateau pressure was 12 cmH2O. According to the blood gas, the patient is clearly not removing an acceptable amount of CO2 as noted by the high PaCO2.
So, you decide to increase the tidal volume to blow off more CO2, which should also bring the pH closer to an acceptable range. Additionally, this will bring the tidal volume to within the acceptable 6–8 mL / kg range. And since the PaO2 is really high, you also decide to incrementally decrease the FIO2 safely to 50% while at the same time monitoring the oxygen saturation level (SpO2).
Another ABG is obtained from the patient on these settings and the following results are noted:
- pH has normalized
- PaCO2 is 47 mmHg
- PaO2 is 98 mmHg
- Positive inspiratory pressure, or PIP, is 24 cmH2O
- Plateau pressure is 17 cmH2O
Since these values now look pretty good, you decide to allow the patient to rest on the current settings and reevaluate in a couple of hours. In the meantime, it's important to obtain diagnostic tests, such as a chest x-ray, and other pertinent information to help determine the diagnosis and prognosis of the patient.
- Esteban, A, Ferguson, ND, Meade, MO, et al. 2008. Evolution of mechanical ventilation in response to clinical research. Am J Respir Crit Care Med. 177: 170–177. PMID: 17962636
- Hess, D. 2001. Ventilator modes used in weaning. Chest. 120: 474S-476S. PMID: 11742968
- Tobin, MJ, and Lodato, RF. 1989. PEEP, auto-PEEP, and waterfalls. Chest. 96: 449–451. PMID: 2670461
- Mechanical ventilation protocol summary. NIH-NHLBI ARDS Clinical Network. http://www.ardsnet.org