Physiology Terms
Airway resistance is the resistive forces encountered during the mechanical respiratory cycle and is ≤5 cm H2O.
Lung compliance describes the ease with which lungs stretch and expand to accommodate a change in volume or pressure. People with high lung compliance have more difficulty with the exhalation process while those with low compliance have difficulty with inhalation.
Derecruitment is the loss of gas exchange surface area due to atelectasis. It can be minimized by increasing PEEP.
Recruitment is the restoration of gas exchange surface area by applying pressure.
Phases of Mechanical Breathing
Initiation phase is the start of the mechanical breath, whether triggered by the patient or the machine.
Inspiratory phase is the portion of mechanical breathing during which there is a flow of air into the patient’s lungs to achieve a maximal pressure, the peak airway pressure (PIP or Ppeak), and a tidal volume (TV or VT).
Plateau phase does not routinely occur in mechanically ventilated breaths but may be checked as an important diagnostic maneuver to assess the plateau pressure (Pplat). With cessation of air flow, the plateau pressure and the tidal volume (TV or VT) are briefly held constant.
Exhalation is a passive process in mechanical breathing. The start of the exhalation process can be either volume cycled (when a maximum tidal volume is achieved), time cycled (after a set number of seconds), or flow cycled (after achieving a certain flow rate).
Ventilator Settings
Peak inspiratory pressure is the maximum pressure in the airway at the end of the inspiratory phase. Since this value is generated during a time of airflow, the PIP is determined by both airway resistance and compliance. By convention, all pressures in mechanical ventilation are reported in “cm H2O.” It is best to target a PIP <35 cm H2O.
Plateau pressure is the pressure that remains in the alveoli during the plateau phase, during which there is a cessation of air flow, or with a breath-hold. To calculate this value, the clinician can push the “inspiratory hold” button on the ventilator. The plateau pressure is effectively the pressure at the alveoli with each mechanical breath and reflects the compliance in the airways. To prevent lung injury, the Pplat should be maintained at <30 cm H2O.
Positive end-expiratory pressure (PEEP) is the positive pressure that remains at the end of exhalation. This additional applied positive pressure helps prevent atelectasis by preventing the end-expiratory alveolar collapse. PEEP is usually set at 5 cm H2O or greater, as part of the initial ventilator settings. PEEP set by the clinician is also known as extrinsic PEEP, or ePEEP , to distinguish it from the pressure than can arise with air trapping.
Intrinsic PEEP (iPEEP or auto-PEEP) is the pressure that remains in the lungs due to incomplete exhalation, as can occur in patients with obstructive lung diseases. This value can be measured by holding the “expiratory pause” or “expiratory hold” button on the mechanical ventilator.
Driving pressure (ΔP) is the term that describes the pressure changes that occur during inspiration, and is equal to the difference between the plateau pressure and PEEP (Pplat – PEEP). It is also known as the pressure required to expand the lungs.
Inspiratory time (iTime) is the time allotted to deliver the set tidal volume (in volume control settings) or set pressure (in pressure control settings).
Expiratory time (eTime) is the time allotted to fully exhale the delivered mechanical breath.
I:E ratio, or the inspiratory to expiratory ratio, is usually expressed as 1:2, 1:3, etc. The I:E ratio can be set directly or indirectly on the ventilator by changing the inspiratory time, the inspiratory flow rate, or the respiratory rate. By convention, decreasing the ratio means increasing the expiratory time.
Peak inspiratory flow is the rate at which the breath is delivered, expressed in L/min. A common rate is 60 L/min. Increasing and decreasing the inspiratory flow is a means of indirectly affecting the I:E ratio. A patient with a respiratory rate set at 20, who is not overbreathing, has 3 s for each complete cycle of breath. If you increase the inspiratory flow, the breath is given faster, and that leaves more time for exhalation. Thus, inspiratory flow indirectly changes the I:E ratio.
Tidal volume (TV) is the volume of gas delivered to the patient with each breath. The tidal volume is best expressed in both milliliters (ex: 450 mL) and milliliters/kilogram of predicted body weight, much as one might describe a drug dosage in pediatrics. Clinicians can choose to set the ventilator in a volume control mode, where the tidal volume will be constant for each breath. In pressure control modes, the pressure is constant, but the tidal volume will vary slightly with each breath.
Respiratory rate (RR or f) is the mandatory number of breaths delivered by the ventilator per minute. However, it is important to be mindful that the patient can breathe over this set rate, and therefore one must report both your set RR and the patient’s actual RR; both of these values can be found on the ventilator screen. In addition, it is important to remember that the RR is a key factor in determining time for exhalation. For example, if a patient has a RR of 10 breaths per minute (bpm), he will have 6 s per breath: ((60 s/min) / 10 bpm = 6 s/breath). A RR of 20 bpm only allows 3 s for the entire respiratory cycle.
Minute ventilation (MV) is the ventilation the patient receives in 1 min, calculated as the tidal volume multiplied by the respiratory rate (TV x RR), and expressed in liters per minute (L/min). Most healthy adults have a baseline minute ventilation of 4–6 L/min, but critically ill patients, such as those attempting to compensate for a metabolic acidosis, may require a minute ventilation of 12–15 L/min, or even higher, to meet their demands.
Fraction of inspired oxygen (FiO2) is a measure of the oxygen delivered by the ventilator during inspiration, expressed at a percentage.
Ventilator Modes
Assist control (AC) is a commonly used mode of ventilation and one of the safest modes of ventilation in the emergency department. Patients receive the same breath, with the same parameters as set by the clinician, with every breath. They may take additional breaths, or over-breathe, but every breath will deliver the same set parameters. Assist-control can be volume-targeted (where volume is set) or pressure-targeted (where pressure is set).
Synchronized intermittent mandatory ventilation (SIMV) is a type of intermittent mandatory ventilation, or IMV. The set parameters are similar to those in AC, and the settings can be volume controlled (SIMV-VC) or pressure controlled (SIMV-PC). Similar to AC, each mandatory breath in SIMV will deliver the identical set parameters. However, with additional spontaneous breaths, the patient will only receive pressure support or CPAP.
Pressure regulated volume control (PRVC) is a type of assist control that combines the best attributes of volume control and pressure control. The clinician selects a desired tidal volume, and the ventilator gives that tidal volume with each breath, at the lowest possible pressure. If the pressure gets too high and reaches a predefined maximum level, the ventilator will stop the air flow and cycle into the exhalation phase to prevent excessive airway pressure and resulting lung injury.
Pressure support is a partial support mode of ventilation in which the patient receives a constant pressure (the PEEP) as well as a supplemental, “supporting” pressure when the ventilator breath is triggered. In this mode, the clinicians can set the PEEP and the additional desired pressure over the PEEP. However, the peak inspiratory airflow, the respiratory rate, and the tidal volume are all dependent variables and determined by the patient’s effort. The patient triggers every breath, and when the patient stops exerting effort, the ventilator stops administering the driving pressure, or the desired pressure over PEEP. Therefore, patients placed on this mode of ventilation must be able to take spontaneous breaths.
Noninvasive positive pressure ventilation (NIPPV) refers to two noninvasive modes of ventilation, in which the patient’s airway is not secured with an endotracheal tube. Rather, these modes of ventilation are delivered through a tight-fitting facemask or nasal prongs.
Continuous positive airway pressure (CPAP) is a partial support mode of ventilation, in which the patient receives a constant airway pressure throughout the respiratory cycle. The peak inspiratory airflow, respiratory rate, and tidal volume are all dependent variables and determined by the patient’s effort. Therefore, the patient must be awake, minimally sedated, and able to take spontaneous breaths during this mode of ventilation.
Bilevel positive airway pressure (BPAP) is a partial support mode of ventilation, in which the patient receives two levels of airway pressure throughout the respiratory cycle. A high inspiratory pressure (iPAP) is similar to the peak airway pressure setting. The lower expiratory pressure (ePAP) , similar to PEEP, is clinically apparent at the end of expiration and helps to maintain alveolar distention. The patient must be awake, minimally sedated, and able to take spontaneous breaths during this mode of ventilation.