Modifying ventilator settings based on pressure readings
So far, after placing a patient on a mechanical ventilator using acceptable initial settings, we have identified which ventilator controls can be adjusted to correct blood gas abnormalities and ensure that the patient is receiving adequate ventilation and oxygenation.
But the amount of pressure the lungs are receiving is also important. So, we will now turn our attention to the ventilator to measure and monitor certain pressures inside the lung: peak inspiratory pressure (PIP) and plateau pressure.
What is peak inspiratory pressure?
Peak inspiratory pressure, or PIP, is how much pressure was created in order to move the volume of air from the ventilator into the patient’s lungs. The unit of measurement is expressed in cmH2O. And, if there is any abnormal interference between the ventilator and the lungs—such as excessive condensation in the inspiratory limb, kinking of the endotracheal tube, bronchospasm, alveolar damage, or even if the patient coughs—the PIP will increase. Remember too much pressure can cause harm to the lungs. In fact, in many cases, keeping the PIP below 35 cmH2O is an accepted practice to protect the lungs from injury. So, it’s important to monitor the PIP and assess for possible causes of, and solutions for, an increased PIP.
Figure 1. Peak inspiratory pressure (PIP) is the highest pressure applied to the lungs during inhalation and should be kept below 35 cmH2O.
Why is plateau pressure important?
But, just as important as, or perhaps even more important than, the monitoring of the PIP, is the monitoring of the plateau pressure. The plateau pressure is the pressure inside the lung once a breath has been delivered.
Now, unlike the peak pressure, the plateau pressure only monitors the pressure in the lung—not the pressure in the ventilator circuit or the ET tube, or even the volume flowing through the airways. There is no flow. It only measures lung or alveolar pressure when the lung is in a static state. So, for the ventilator to accurately measure the plateau pressure in the lung, the clinician must create an inspiratory pause by pressing the inspiratory hold button for about a second or so to keep the lung in a pause or inflated static state.
And remember, it's important to monitor this pressure regularly to ensure that the ventilator settings are not causing harm to the lung. When is the patient at risk for lung injury you ask? When the plateau pressure is greater than 30 cmH2O!
Figure 2. Plateau pressure is the pressure in the lungs during the peak inspiratory period and should be kept below 30 cmH2O.
Actually, this is another reason for choosing an initial tidal volume (VT) setting between 6 to 8 milliliters per kilogram. Typically, a VT in this range should create a plateau pressure of less than 30. But if it doesn't, and the VT created a plateau pressure of greater than 30, then you will find yourself having to decrease the VT. Or you may want to switch to a different mode altogether.
So it’s not only important to monitor your initial settings by obtaining an arterial blood gas to ensure adequate oxygenation and ventilation, but you should also be monitoring the PIP and the plateau pressure on the mechanical ventilator to make sure the lungs are not damaged due to pressure.
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