These are the most commonly used ventilator modes or settings that you may encounter in the intensive care unit.
“Assist control” basically means the ventilator is breathing for the patient. The ventilator ensures that the patient receives a certain number of breaths per minute (e.g. 18 breaths per minute), and will initiate breaths to achieve the prescribed breathing rate. For when the patient attempts a breath on his or her own, the ventilator is usually programmed to assist the patient take the additional breath.
Assist control has different modes. The most commonly used ones are volume control and pressure control. Each type has many sub-variations which are not covered here.
There is no ideal mode. What is good for one patient and one clinical scenario may not be good for another. It is as much art as science. The different modes allow respiratory therapists (RTs) the freedom to try different solutions if the patient is not doing well on a particular mode of ventilation (i.e. poor oxygenation or asynchronous with the ventilator)
Volume control is a type of assist control ventilation. In volume control, the RT specifies the tidal volume—the target volume of inspired air for each breath. This is usually between 6 to 8 ml/kg based on the patient’s ideal body weight given the patient’s actual height. For adults, this works out to usually around 400 to 600 ml per breath. The ventilator will apply adequate inspiratory pressure to deliver the prescribed tidal volume.
Pressure-regulated volume control
Pressure-regulated volume control is a subtype of volume control whereby the RT specifies both the tidal volume as well as the desired maximum inspiratory pressure. This is theoretically a very good mode for ventilation. In RT school, some instructors say this should be the “go-to mode”. However, in practice, it has its challenges. This mode can cause ventilators to behave erratically and aggravate patient-ventilator dysynchrony if—for example—the ventilator is not able to accurately measure the inspiratory plateau pressures or if the patient over breathes.
In pressure control, the RT specifies the desired maximum inspiratory pressure. The ventilator applies an inspiratory breath until that pressure is reached. That pressure ultimately generates a tidal volume—we won’t know how much before each cycle—and these tidal volumes can potentially vary from breath to breath, and will vary from patient to patient. Pressure control is apparently more comfortable for patients who are not medically paralyzed and can spontaneously breathe. They are also useful when the RT wants tighter control over the pressures that are being applied on the patient’s lungs.
“Pressure support” is a mode of ventilation whereby the ventilator does not breathe for the patient. Rather, the ventilator is programmed to apply a baseline pressure (called the PEEP, or positive end expiratory pressure) with or without additional pressure when the patient tries to take a breath.
In BIPAP pressure support, the ventilator applies a PEEP and additional pressure when the patient tries to take a breath. In a patient who is fully weaned off the ventilator, this is usually a PEEP of 5 mmH2O, with an inspiratory pressure boost of an additional 5 mmH2O. RTs refer to this as “on pressure support, 5 on 5.”
In CPAP pressure support, the ventilator applies a PEEP without any additional pressure boost during inspiration.
The peak pressure is the highest pressure applied by the ventilator upon inspiration. It occurs near the end of the inspiratory cycle, just before the plateau pressure is obtained. Plateau pressures are more physiologically relevant than the peak pressures when it comes to assessing the highest pressure applied on the patient’s lungs.
Plateau pressures are normally ≤ 30 mmH2O. Very high plateau pressures can result in acute respiratory distress syndrome (ARDS) and other forms of lung injury.
This is the fraction of inspired oxygen. For ambient air, the comparable FiO2 would be around 25% or 0.25. The maximum FiO2 is 100% or 1.0. High FiO2 for prolonged periods of time can cause oxygen-related injury to the airway and lungs.
The positive end expiratory pressure is the pressure felt within the ventilatory circuit at the end of exhalation and just before inhalation. Normally the PEEP is set to 5 mmH2O. Higher PEEPs (e.g. PEEP of 8 mmH2O) can improve alveolar recruitment and be good for patients with COPD. However, high PEEPs can also reduce alveolar perfusion and hence worsen oxygenation, as well as cause lung injury. Conversely lower PEEPs can be good for patients with asthma.
Heliox is a mixture of oxygen and helium, a noble gas. Heliox is less dense than ambient air. This results in less turbulence during ventilation and potentially better oxygenation in tenuous patients. It comes with 70% oxygen and 80% oxygen mixtures.