Understanding mechanical ventilation is a fundamental part of intensive care and perioperative medicine. Here’s an overview of the basics of mechanical ventilation. Keep in mind that clinically, there are many more things to consider, exceptions to rules, hybrid ventilator modes, and other strategies that I will not cover for the sake of maintaining simplicity. Please leave me questions in the comments section at the bottom!
First, it’s important to understand how a breath on the ventilator is initiated and terminated by phase variables (trigger, limit, cycling).
A trigger dictates what causes the ventilator to cycle to inspiration. To complicate things a bit further, modes can have more than one trigger.
- Pressure-triggered: the patient generate negative pressure (from diaphragmatic contraction)
- Volume-triggered: the patient inspires a small volume of air from the ventilator
- Flow-triggered: the patient generates a small amount of airflow into their lungs
- Time-triggered: a preset timer dictates how often the patient breathes
A limit restricts a variable before the end of inspiration.
- Pressure-limited: a pressure that will not be exceeded during inspiration
- Volume-limited: a volume that will not be exceeded during inspiration
- Flow-limited: a flow rate that will not be exceeded during inspiration
The cycling variable dictates how the ventilator “cycles” between inspiration and passive expiration.
- Pressure-cycled: after achieving the peak inspiratory pressure (PIP), exhalation occurs
- Volume-cycled: after the target volume is delivered, exhalation occurs
- Flow-cycled: exhalation occurs when the inspiratory flow rate falls below a preset threshold
- Time-cycled: purely based on time
The chest wall and the lungs are in a constant “tug-of-war” – the lungs want to collapse inward and the chest wall wants to expand outward. These two systems work together to influence respiratory mechanics. Compliance is mathematically defined as Δ volume / Δ pressure. In other words, a stiff system (low compliance) requires lots of pressure (large Δ pressure) to drive in a small volume of air (small Δ volume). This can be seen with interstitial pulmonary fibrosis, pneumonia, acute respiratory distress syndrome (ARDS), etc.
OXYGENATION AND VENTILATION
The lungs add oxygen and remove carbon dioxide (CO2) from the blood. These processes refer to oxygenation and ventilation, respectively. In terms of ventilator parameters, oxygenation can be improved by increasing the fraction of inspired oxygen (FiO₂) and positive end-expiratory pressure (PEEP). PEEP helps maintain alveolar recruitment between respiratory cycles and is often increased concurrently with FiO₂.
Minute ventilation is the product of respiratory rate (RR or ‘f‘) and tidal volume (VT). Increasing either of these parameters will therefore increase minute ventilation to remove more CO₂.
The inspiratory to expiratory ratio (I:E) refers to the amount of time spent in each phase of the respiratory cycle. Under physiologic conditions, we normally spent 2-3 times as much time in passive expiration (I:E of 1:2 or 1:3). If a patient is “riding the ventilator” (deeply sedated or paralyzed), he or she will breath at the RR set on the ventilator. Let’s assume this is at 15 breaths/minute. That means a full inspiratory-expiratory cycle lasts 4 seconds. An I:E ratio of 1:3 means that inspiration will last 1 second and expiration will last 3 seconds.
If this ratio changes to 1:1, then equal amounts of time are spent in inspiration and expiration. Inverse ratio ventilation occurs when more time is spent in inspiration than expiration (ie, 2:1). This mode of ventilation can be extremely uncomfortable to patients and can lead to “breath stacking” as there is inadequate time to exhale the previous VT. This can, in turn, lead to progressively higher mean airway pressures (the expiratory flow does not return to 0 L/m prior to inspiration) and alveolar trauma.
Now let’s talk about different modes of ventilation! Obviously these modes vary tremendously depending on the ventilator, but I’ll cover some of the basics.
ASSIST CONTROL (A/C)
Assist control (A/C) is routinely utilized in patients as a “rest mode” where the ventilator takes on the role of breathing (“control”). If the patient wants to breath over the dialed in RR, then he/she can “assist” with ventilation. Therefore, if the patient does not trigger any breaths, the ventilator will at least achieve the preset minimum. In the image above, the A/C mode (volume control) settings are: RR 16 bpm, VT 350 cc, FiO₂ 0.5, and PEEP 5 cm H2O.
- Volume Control (A/C – VC)
- VT is directly set. Driving pressure will vary depending on the compliance although a “maximum” pressure can be set to avoid barotrauma.
- PEEP, FiO₂, and RR can be adjusted.
- A patient-triggered breath will result in the full VT being received.
- Flow is traditionally constant and maintained until the VT has been reached (“square wave form”). A decelerating flow is seen in pressure-control, volume-guaranteed variants of A/C-VC (downsloping flow in the latter parts of inspiration). This decelerating flow pattern is also seen in pressure control ventilation (see below).
- Ventilator will cycle to expiration once VT is delivered.
- A fixed minute ventilation is guaranteed since both VT and RR are directly adjusted. To increase minute ventilation, I usually consider changing the RR first rather than increasing VT which can lead to alveolar overdistension (volutrauma).
- Pressure Control (A/C – PC)
- Inspiratory pressure is directly set. VT will vary depending on the lung compliance. For this reason, as the compliance changes (resolving pulmonary edema, worsening ARDS, etc.), the VT will change unless the inspiratory pressure is adjusted. Keep this in mind for any mode of ventilation where the inspiratory pressure is independently controlled.
- I/E ratio determines amount of time spent in each phase (see above explanation).
- PEEP, FiO₂, and RR can be adjusted.
- Usually cap the peak airway pressure around 30 cm H2O.
SYNCHRONIZED INTERMITTENT MANDATORY VENTILATION (SIMV)
Although SIMV delivers a preset minimum number of breaths per minute, it “synchronizes” itself with spontaneous breathing initiated by the patient to avoid delivering a breath as the patient is expiring. The mode of delivery can be anything (volume control, pressure control, hybrid modes).
The key difference between A/C and SIMV is how they handle spontaneous efforts made by the patient. If the ventilator is triggered by the patient while on A/C, he/she will receive a fully supported breath (ie, the full VT in volume-control). In SIMV, they get whatever VT they can pull without support from the ventilator. To a large extent, this is based on the patient’s respiratory strength and pulmonary compliance. If the patient is overbreathing their preset SIMV rate (ie, VT of 450 cc at a rate of 10 bpm), their VT can vary tremendously as some breaths are fully delivered by the ventilator and others are unsupported, spontaneous efforts.
PRESSURE SUPPORT VENTILATION (PSV)
What if the patient is triggering his/her own breaths but pulling pitiful VT? This is where the PSV comes in handy. In this mode, FiO₂, PEEP, and inspiratory pressure, and a backup rate can be directly set.
PSV provides just that – pressure support – with each breath spontaneously triggered by the patient who might be too weak to fly on their own (ie, respiratory weakness, under anesthesia, etc.) Remember that in SIMV, these same breaths would be unassisted. Just like A/C – PC, the actual delivered VT is dependent on the patient’s pulmonary compliance. With changing compliance, this can lead to hypo/hyperventilation. Volume support ventilation (VSV) aims to solve this by automatically scaling the amount of pressure support a patient receives to achieve the preset VT.
In either case, as respiratory strength recovers and/or pulmonary pathologies improve, compliance should improve (less pressure to deliver more VT). For example, at the end of a long anesthetic, patients may need 15 cm H2O or more of inspiratory pressure to maintain a decent VT. After volatile agents have been weaned down and neuromuscular relaxation has been reversed, they may only need 5-10 cm H2O to achieve the same VT.
Check out the following posts I’ve written dealing with pulmonology and mechanical ventilation: