Table of modes of mechanical ventilation

Modes of mechanical ventilation refers to the methods a Ventilator offers to assist or replace spontaneous breathing ^[1] . Modern Ventilators offer a number of such methods (Modus operandi or Mode) to enable the most suitable respiratory support for the individual patient. ^[2] Each Mode has its set of Controls to adapt the breath delivery to the patient. CAVEAT: Although manufacturers may offer identical breath delivery methods, the names of the Modes may be different.

Composition of breathing gas mixture

Most ventilators can deliver higher oxygen concentration than available in room air and can provide increased levels of pressure at the end of exhalation. Thus, the following controls are available to the clinician:

• Composition of gas mixture delivered (Control: fraction of inspired oxygen, FiO2)
• Pressure at the end of exhalation (Control: Positive End-Expiratory Pressure PEEP, also called CPAP or EPAP)

While technological differences exist among devices, terminology and effect are clear.

Breath delivery

Breath delivery follows the phases of breathing, ^[3] i.e., inhalation and exhalation. Each Mode defines how breath delivery and timing are controlled by the clinician ^[1] .

Inhalation

The start of inhalation is initiated by the machine or the patient and that point in time is called InspiratoryTrigger. The ventilator needs to know when to start delivering gas to the patient. If the patient does not breathe at all, a timer starts inhalation. If the patient has some breathing activity, the ventilator can sense this effort by measuring pressure or flow and start inhalation if pressure or flow drop below a certain threshold. That threshold is called Trigger Sensitivity. Thus, the controls available to the clinician are respiratory rate and trigger sensitivity.

Once the ventilators is triggered to deliver respiratory gas, two methods to deliver the gas mixture are technically possible: flow controlled or pressure controlled ^[4] gas gelivery. Both methods have their advantages and disadvantages. If flow controlled gas delivery is chosen, it is often combined with a Volume limit which stops gas delivery when a set volume is reached. Thus, the term Volume Controlled Ventilation is often used. The controls available to the clinician are inspiratory pressure, inspiratory flow and/or inspiratory volume.

End of inhalation and cycling to exhalation

Inhalation must eventually stop and enable the lungs to exhale. If the patient does not breathe, the ventilator must switch to exhalation after a pre-set time (time cycled) has elapsed, a certain pressure is exceeded (pressure cycled) or a pre-set volume (volume cycled) has been delivered ^[1] . If the patient has some breathing activity, the ventilator can sense this by measuring flow and start exhalation, for example, if flow drops below a certain threshold. That threshold may be termed "Expiratory Trigger Sensitivity". The controls available to the clinician are inspiratory time, inspiratory volume, inspiratory flow, maximum pressure and/or expiratory trigger sensitivity.

NOTE: Inspiratory flow can be expressed as V'I = Vt/Ti and respiratory rate f = 60/(Ti+Te). Both formulas have three variables and two degrees of freedom. This means that only two variables can be controlled independently, the third variable follows.

Exhalation

Emtpying the lungs requires time which starts with the onset of exhalation and ends with the start of the subsequent inhalation. If the patient is passive, the exhalation is terminated by a timer. If the patient has some breathing activity, exhalation may be terminated by the subsequent inhalation effort of the patient. Controls include a selection of expiratory time, respiratory rate and/or trigger sensitivity.

NOTE: Trigger sensitivity plays a double role. Evidently, it determines the start of inhalation and, by the same toke, it ends expiration. For example, if trigger sensitiviy is set too sensive, it may influence respiratory rate and create tachypnea.

Table of modes and their acronyms

The table below lists the working principles of some of the common modes of ventilation.

• Vent means controlled by ventilator
• Pat means controlled by patient, based on measurements of flow , pressure or muscle activity ^[5] .
• AUTO means that the ventilator automatically adjusts controls that are conventionally adjusted by the clinician. For example: When Pressure Vontrolled Ventilation (PCV) is used, the volume delivered by the Ventilator may vary depending on the patient's lungs and the patient's efforts. Hypothetically, a clinician may sit next to the Ventilator and adjust the pressure control knob breath-by-breath to maintain a certain target volume. In other words, the clinician inputs the target volume and the Ventilator adjusts the pressure control variable to meet the volume target.

Mode examples, not exhaustive!	Trigger	Inhalation Mechanism	Cycle	Exhalation Mechanism	AUTO
Volume Controlled Ventilation, CMV, VCV, A/C)	Vent or Pat	Flow	Vent	Vent	no
Pressure Controlled Ventilation PCV	Vent or Pat	Pressure	Vent	Vent	no
Synchronized Intermittent Mandatory Ventilation SIMV (volume cycled)	Vent or Pat	Flow	Vent or Pat	Vent	no
Synchronized Intermittent Mandatory Ventilation SIMV (pressure limited)	Vent or Pat	Pressure	Vent or Pat	Vent	no
Synchronized Intermittent Mandatory Ventilation plus Pressure Support, SIMV+PS (volume cycled)	Vent or Pat	Flow	Vent or Pat	Vent	no
Synchronized Intermittent Mandatory Ventilation plus Pressure Support,SIMV+PS (pressure limited)	Vent or Pat	Pressure	Vent or Pat	Vent	no
Continuous Positive Airway Pressure CPAP	Pat	Pressure	Pat	Vent	no
Continuous Positive Airway Pressure plus Pressure Support CPAP+PS	Pat	Pressure	Pat	Pat	no
Airway Pressure Release Ventilation, APRV, two level CPAP	Vent or Pat	Pressure	Vent or Pat	Vent	no
Inversed Ratio Ventilation (usually PCV based)	Vent or Pat	Pressure	Vent	Vent	no
Volume Support [5]	Pat	Pressure	Pat	Pat	Yes (Adjusts Pinsp breath-by-breath to meet a Volume target set by clinician)
Proportional Assist Ventilation PAV [5]	Pat	Pressure	Pat	Pat	Yes (Instantaneous control of pressure proportional to elastic load set by clinician)
Neurally Adjusted Ventilation Assist NAVA [5]	Pat	Pressure	Pat	Pat	Yes (Instantaneous pressure proportional to measured patient effort, clinician sets %)

See also

• Respiratory therapy

References

1. Cairo, J.M. (2020). Pilbeam's Mechanical Ventilation. Physiological and clinical applications. St. Louis, Missouri: Elsevier. pp. 62ff. ISBN   978-0-323-87164-8.
2. Esteban A, Alía I, Ibañez J, Benito S, Tobin MJ (1994). "Modes of mechanical ventilation and weaning. A national survey of Spanish hospitals. The Spanish Lung Failure Collaborative Group". Chest. 106 (4): 1188–93. doi:10.1378/chest.106.4.1188. PMID   7924494.
3. Brunner, J.; Wolff, G.; Langenstein, H.; Cumming, G. (December 1985). "Reliable detection of inspiration and expiration by computer". International Journal of Clinical Monitoring and Computing. 1 (4): 221–226. doi:10.1007/BF01720186. ISSN   0167-9945.
4. Garnero, A.J.; Abbona, H.; Gordo-Vidal, F.; Hermosa-Gelbard, C. (2013). "Modos controlados por presión versus volumen en la ventilación mecánica invasiva". Medicina Intensiva (in Spanish). 37 (4): 292–298. doi:10.1016/j.medin.2012.10.007.
5. Navalesi P, Costa R (2003). "New modes of mechanical ventilation: proportional assist ventilation, neurally adjusted ventilatory assist, and fractal ventilation". Curr Opin Crit Care. 9 (1): 51–8. doi:10.1097/00075198-200302000-00010. PMID   12548030.