What are mechanical ventilators? Part 2: vent management

In this post, I discuss vent management. The overarching principle is that you must constantly adjust different parts of the ventilator depending on the patient’s respiratory/mental/overall status. Think: dynamism.

To read and understand a vent, look at the multiple parts on the screen. Anything on the x-axis is “things we adjust.” Anything on the y-axis is “things we use to measure response.”

Screen Shot 2015-12-07 at 8.38.18 PM

First look at the mode—is it pressure control? Pressure support? AC? Volume controlled? These mean different things.

  1. Pressure control—patient’s pressure settings are titrated and number of breaths are controlled by the machine. Breaths are triggered by the machine. Therefore, you can adjust the respiratory rate. If the patient ends up triggering their own breath, the vent acts like PS.
  2. Pressure support/CPAP (“what’s the difference between BiPAP and PS/CPAP?”)—patient’s pressure settings are controlled by the machine, but the patient can trigger their own breaths
  3. Volume controlled—the patient is set to a certain tidal volume, and the machine adjusts pressure accordingly to make sure the TV is maintained. Oftentimes, this is found in low-volume ventilation patients, like ARDS. “Low-volume” is thought to be 4-6 ml/kg of tidal volume, based on IBW (ideal body weight). For reference In a person my size, my low-volume TV is anywhere between 320-420 cc/min, with a minute ventilation up to 12 L. “Normal” volumes would be about 10 ml/kg.

Next, look at the pressure settings: usually read “delta/PEEP”

  1. peak=the pressure of the airway at max inspiration. If peak is high, that means there’s airway resistance. Goal peak airway pressure <30. Around 60 mm H2O is when you start worrying that intubation isn’t working.
  2. delta=plateau=peak-PEEP. Tells you how much extra inspiratory pressure the patient requires, or in other words, “the pressure greater than pressure support.” 8 is normal for non-ventilated patients. If plateau is high, this means the patient has stiff lungs (at risk for ARDS), so you should use low volume. Goal O2 sat 88-95%.
  3. PEEP=end expiratory pressure, left at the end of the breath, or how much support the patient requires during expiration. 3-5 cm H2O is normal for non-ventilated patients.
    1. AUTO-PEEPING: is the phenomenon that applies to someone with air hunger, especially in COPD or asthma. The PEEP may be higher than indicated on vent settings, because of mucus plugging, bronchoconstriction, or shortened expiration time. Regardless of cause, auto-PEEP causes more work of breathing—they have to generate a higher inspiratory negative pressure to open up the alveoli during inspiration. Ways to get around auto-PEEP: lengthen the expiration time, shorten the inspiration time, decrease the RR, or use CPAP to create a “waterfall effect”: as long as the positive pressure of the CPAP is higher than the auto-PEEP, you’ve created a “negative” gradient that air can flow down to the alveoli.

Another thing you can adjust is the FiO2. 40% is considered “normal” for a vent; 21% is room air.

To adjust vent pressures, there are several tricks respiratory therapy uses to decide whether to change the settings:

  1. Is it O2 or CO2 status that needs to be improved? Patients with hypoxic respiratory failure benefit more from O2-increasing mechanisms like FiO2. Patients with hypercapnic failure benefit more from CO2-reducing mechanisms like upping the RR or PEEP.
  2. ABG: You should always look at the O2 sat on the monitor, obviously! But ABG is more accurate and tells you pH and CO2 (those are directly measured; the other values are calculated from those). Can get daily or q12h to measure response to the vent. 3.5 bicarcb to 10 PCO2. If there is compensated respiratory acidosis, you don’t need to adjust the vent because we allow “permissive hypercapnia” in ventilated patients in order to maintain them at low lung volumes. It is not infrequent to get hourly ABGs in the intensive care unit.
  3. “recruitment maneuver”=a brief blast of PEEP, like 35-40 cm H2O. This opens alveoli, which theoretically improves oxygenation, but may cause hypotension.
  4. Inspiratory hold=used to determine plateau pressure. Plateau pressure is directly correlated with alveolar pressure, so it’s used to titrate tidal volume.
  5. Expiratory hold=used to see if PEEP should be increased because of impaired elastic recoil or airway resistance (emphysema, bronchospasm, airway collapse). An exp hold sees how much auto-PEEP there is.
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