Ventilator modes is best understood as a compilation of three settings: trigger, control, and cycling corresponding in function to: when to give a breath, what breath to give, and when to stop the breath.
Trigger is the parameter by which the ventilator initiates a breath. Common triggers include time, pressure, and flow. Time is appropriate when there is no respiratory drive and hence a breath is given at a given interval to achieve a determined respiratory rate. Pressure trigger follows the logic that a patient-initiated breath will generate negative pressure within the circuit. Mechanical ventilation delivered at this time will synchronize with the patient’s efforts. But in the presence of autoPEEP, generating a negative pressure can require considerable amount of work. The diaphragm generates only 2-3 cmH2O of pressure during quiet breathing. Hence, flow trigger was developed as a means to sense inspiratory flow within the circuit, which should theoretically decrease the work of breathing. Studies comparing flow and pressure triggering has been inconsistent except in COPD patients, which favors flow triggering.
Control refers to the type of breath given. The first ventilators provided a constant minute ventilation by controlling the tidal volume and respiratory rate. Today, this mode of control is termed volume targeted and is achieved by using variable airway pressures to maintain a constant flow rate. A deleterious consequence of this mode is pulmonary barotrauma, where excessive pressures leads to alveolar rupture. Plateau airway pressure < 35 cm H2O is associated with less events in ARDS. On volume targeted ventilation, the physician must manually titrate tidal volumes to assure plateau airway pressures < 35. However, pressure targeted ventilation performs this task automatically. This mode maintains a constant pressure at the price of variable tidal volumes. The downside to this mode is obvious when tidal volumes are too low from low compliance. However, high tidal volumes can also harm by introducing shear forces that produce ARDS. Comparing the two controls: there were no statistically differences in mortality, oxygenation, or work of breathing. But there was improved patient-ventilator synchrony and earlier liberation from MV with pressure targeted ventilation.
As an aside, I found it best to think of pressure vs volume as akin to shopping. Volume is the product that you are after and pressure is the price. You want the best quality product but you also want to pay as little as possible. This dilemma births two shopping strategies:
1) Set a budget and get the best quality item you can with this budget. This is pressure targeted. You set a pressure and see what volumes you get. If the volume is too low, you grudgingly increase the pressure until you find a good compromise.
2) Just pick up what you want and see how much it costs. This is volume targeted. You set a volume and see how much pressure that generates. If the pressure is too high, you grudgingly lower the volume until you find a good compromise.
You’ll get to the middle either way.
Cycling is the criteria by which the ventilator stops delivering a breath. This can occur when a set tidal volume is reached, when the flow rate drops to a specified level due to decreasing compliance at higher lung volumes, or after a specified amount of time.
Assist Control is most commonly volume-targeted; hence it’s commonly known as volume AC. One sets a tidal volume (8mL/kg x 70kg =560mL) and rate (10); the ventilator calculates the wavelength of each cycle (6 seconds per breath). When a patient initiates a breath sensed by flow or pressure, the ventilator assists by providing a set flow rate to reach 560mL(assist breath). If the patient does not take another breath in the next 6 seconds, the ventilator controls by giving 560mL (control breath). The minimum minute ventilation in this case is ~10L but the patient can exceed that by taking more breaths, which each totals 560mL.
There are in essence two different breaths: 1) controlled breaths (time triggered, volume targeted, time cycled), and 2) assisted breaths (pressure or flow triggered, volume targeted, time cycled).
The disadvantage all revolves around the tendency for this mode to over-breath for hyperventilating patient: respiratory alkalosis, auto-PEEP. This is especially pronounced in the patients who are not deeply sedated; they tend to stack breaths because this is probably the least comfortable of modes by virtue of being most invasive. The advantage is low work of breathing and the guarantee of a minimum minute ventilation no matter what the patient’s lung compliance is. Volume AC is the best studied mode and, for me, my initial mode.
Pressure Regulated Volume Control is identical to volume AC except that the flow rate is not a static number. Flow rate, which I had italicized above, ranges from 60-100Lpm. Increased flow rate decreases inspiratory time because air is forced into the lungs faster and is ideal in obstructive pathologies. However, this raises the peak airway pressure. It is debatable whether peak airway pressure causes any harm to the patient but it’s certainly very disruptive to nursing or RT because the vent alarm will never cease.
PRVC capitalizes on the fact that lung compliance is different at different lung volumes and uses the first few breaths to calibrate this nonlinear relationship. It then tweaks the flow rate by the millisecond to provide higher flow during periods of high lung compliance and lower flow during periods of low lung compliance. The net effect is a lower peak airway pressure. Please note that this does not change the plateau pressure. In many cases, in fact, the peak airway pressure becomes equal to the plateau pressure: a true testament to the effectiveness of PRVC.
I really like PRVC. It has all the advantages of Volume AC minus high plateau pressures. The only conceivable downside is that you lose control of I:E time, which may be important in obstructive pathologies. And like volume AC, you have to sedate the patient deeply because it is not the most comfortable.
Synchronized Intermitted Mechanical Ventilation (SIMV) is like assist control except that it incorporates periods of spontaneous or partially supported breathing to prevent progressive hyperinflation. One sets a tidal volume (560mL) and rate of 10; wavelength is 6 seconds per breath. First sensed breath is given a full 560mL (synchronized breath). A breath taken 2s later is at a tidal volume of however much the patient can draw in with a pre-programmed pressure support (assisted breath). If the patient does not draw breath 6s later, the ventilator delivers a 560mL (controlled breath).
Common SIMV settings actually incorporate three different breaths: 1) controlled breaths (time triggered, volume targeted, time cycled), 2) synchronized breath (pressure or flow triggered, volume targeted, time cycled), and 3) assisted breath (pressure or flow triggered, pressure targeted, flow cycled).
I have mixed feelings about SIMV. It is the hybrid mode of volume AC and pressure support. However, most patients do not need a transitional mode to switch between the two. Awake patients generally prefer pressure support and any controlled breaths from SIMV only aggravates from my observations. I suppose this is a good mode if you are worried about central apnea. If the patient has such waxing waning mental status or is over-narcotized that they intermittently become apneic, then the back-up controlled breaths become valuable.
Pressure Support is BiPAP. Each breath is initially solely by the patient and it is pressure supported with IPAP (inspiratory pressure) and PEEP (expiratory pressure). As such there is only assisted breath (pressure or flow triggered, pressure targeted, flow cycled).
As mentioned above, this mode maximizes patient-vent synchrony and is probably best in the awake, not deeply sedated patient. However, you can’t guarantee minute ventilation. You also can’t measure plateaus off this mode, which you shouldn’t have to unless you are trying an ARDS patient on PS.
Airway Pressure Release Ventilation (APRV) is a rescue mode for hypoxemia. Volume AC is the best studied mode for ARDS. However, at very high PEEP (20s) plateau pressures invariably become > 35. Sometimes PEEPs have to be even higher than 20 to maintain oxygenation. To give you some context, 20 is higher than most normal people’s peak airway pressure and it has now become an ARDS patient’s minimum airway pressure.
APRV permits usage of high PEEP by flipping the respiratory cycle on its head. Rather than moving air during inspiration, APRV predominately ventilates through expiration. The mode starts with a very high PEEP (hereby known as Phigh) to 28 and maintains this over a set time, usually 4-6s (hereby known as Thigh). Then it suddenly releases the pressure to Plow, usually 0, for a predefined amount of time (Tlow), usually 0.4-0.6 seconds.
This mode makes a lot of theoretical sense but only in extreme hypoxemia/ARDS. Consider using it if you cannot maintain oxygenation with PEEP at 20 on conventional settings or if you have difficulty with controlling plateau pressures. APRV sacrifices ventilation for oxygenation. But in extreme circumstances where you have down-titrated to super low tidal volumes to preserve plateau pressures, APRV may end up being better for ventilation.
All vent modes are subservient to vent settings. Titrate to the appropriate settings before flip-flopping on the modes.
Prioritize the problem list. It is entirely appropriate to sacrifice mental status with extra sedation if it promotes ventilator synchrony in a patient with diseased lungs.
I reserve pressure support for patients good enough to be extubated or were intubated for airway protection rather than respiratory insufficiency.
I will almost always pick volume AC first for patients with pulmonary disease. If there is refractory hypoxemia, I max out the settings then consider APRV.
If peak pressures are high, I switch to PRVC. If they are still high, I would consider PS or I would simply say: “screw it, peak pressures don’t matter anyways.”