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 provide a number of such methods called Modes (derived from Modus operandi) to enable the most suitable respiratory support for the individual patient.[2] Each Mode has its particular 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[3].

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 functions and 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)

While technological differences exist among devices to implement these two functions , their use is clear and the terminology is widely accepted. In contrast, the terminology of breath delivery methods can be quite confusing[3].

Basic breath delivery modes

In the basic breath delivery modes, the ventilator acts as a high-fidelity delivery device. The clinician's settgins are direct commands to the inspiratory and expiratory valve controls. The ventilator uses fast sensors for intra-breath control to match the clinicians settings exactly. Breath delivery follows the phases of breathing,[4] i.e., inhalation and exhalation. Each Mode defines how breath delivery and timing are controlled by the clinician[1].

Inhalation trigger

The start of inhalation is initiated by the machine or the patient and that point in time is called Inspiratory Trigger. 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.

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[5].

Breath delivery

Once the ventilators is triggered to deliver respiratory gas, two methods to deliver the gas mixture are technically possible: flow controlled or pressure controlled[6] 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.

Cycling to exhalation

Inhalation must eventually stop and cycle to exhalation to 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: The pressure maintained throughout exhalation is termed Positive End-Expiratory Pressure PEEP. CPAP differs from PEEP because the patient can inhale and exhale in CPAP.

Table of basic modes and their acronyms

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

  • Breath delivery mechanism: Flow means that the ventilator controls the valve to maintain a set flow independent of pressure or volume. Pressure means that the ventilator maintains a set pressure, independent of flow and volume.
  • Trigger: start of inhalation
  • Cycle: start of exhalation
  • Vent means controlled by ventilator based on settings by clinician.
  • Pat means controlled by patient, based on measurements of flow , pressure or muscle activity[7].
Mode examples, not exhaustive! Trigger Breath delivery

Mechanism

Cycle Remark
Volume Controlled Ventilation, VC CMV, VCV, A/C Vent or Pat Flow Vent Clinicians sets tidal volume, flow waveform and timing. Ventilator adjusts the valves to deliver that exact volume irrespective of changing resitance and compliance of the patient[8].
Pressure Controlled Ventilation PC Vent or Pat Pressure Vent Clinician sets a pressure trajectory (inspiratory pressure level and inspiratory time). Ventilator uses a pressure transducer to servo-control the valve, creating the desired pressure wave (usually a square wave) at the airway opening[9].
Synchronized Intermittent Mandatory Ventilation SIMV (volume cycled) Vent or Pat Flow (mandatory breath) or Pressure (spontaneous breath) Vent or Pat Combination of VC and PS. Basis is Volume Controlled Ventilation. Patient can take breaths in between.
Synchronized Intermittent Mandatory Ventilation SIMV (pressure limited) Vent or Pat Pressure Vent or Pat Combination of PC and PS. Basis is Volume Controlled Ventilation. Patient can take breaths in between.
Continuous Positive Airway Pressure CPAP Pat Pressure Pat Ventilator maintains a set pressure (PEEP) independent of patient effort.
Pressure Support PS Pat Pressure Pat Clinician sets a pressure level. The control loop is identical to PCV for maintaining pressure, but breath cycling is determined by patient flow decay[10]. The level of support is fixed until changed by the clinician.
Airway Pressure Release Ventilation, APRV, Bilevel Positive Airway Pressure, BiPAP, DuoPAP Vent or Pat Pressure Vent or Pat Clinician sets two levels of pressure and the time to switch between the two. Ventilator maintains each level independet of patient effort. Patient can breathe on both levels.
Proportional Assist Ventilation PAV[7] Pat Pressure Pat Advanced basic mode. Clinician sets a fixed degree of support (assist %) rather than a pressure level. Pressure level is calculated by ventilator as a proportion of delivered volume and flow[11].
Neurally Adjusted Ventilation Assist NAVA[7] Pat Pressure Pat The diaphragm's electrical activity (EAdi) is the commanding waveform; the ventilator delivers pressure in proportion to the EAdi signal with a fixed gain set by the clinician[12].

PAV and NAVA provide superior synchrony with the patient's breathing by using a physiological signal as the command source.

Dual control modes

Dual control modes introduce an outer control loop that wraps around a Basic Mode. The clinician sets a performance target (for example Vt and respiratory rate) and the ventilator then uses a Basic Mode (usually a pressure-controlled or pressure-support breath) as its actuator and automatically adjusts the pressure level from breath to breath to meet the performance target.

This represents a significant user-interface advantage. The clinician can manage the fundamental goals of ventilation—for example tidal volume or respiratory rate —while the machine handles the technical translation into the required pressure, adapting automatically to changes in the patient's respiratory system compliance and resistance and spontaneous activity[13].

Table of dual control modes and their acronyms

The table below lists the working principles of some of the dual control modes. The clinician set a desired target, for example the tidal volume Vt and the ventilator adjusts one of the variables of provided by the basic mode it uses[14].

  • The Target Variable is set by the clinician and fixed.
  • The Controlled Variable is adjusted by the ventilator to achieve the Target Value.
Mode examples, not exhaustive! Underlying basic mode Target Variable Controlled Variable Remark
Pressure-Regulated Volume Control PRVC, VC+, AutoFlow Pressure Controlled Tidal volume Inspiratory pressure Ventilator operates in a Pressure Control (Basic Mode) scheme. After each mandatory breath, it compares the delivered Vt to the clinician-set target Vt. It then adjusts the inspiratory pressure level for the next mandatory breath upward or downward[13].
Volume Support VS Pressure Supported Tidal volume Pressure support level This is the Targeted Spontaneous Mode counterpart to PRVC. It uses Pressure Support (Basic Mode) as its actuator. After each spontaneous breath, ventilator adjusts the PS level for the subsequent breath to achieve the set Vt target[15].
Mandatory Rate Ventilation MRV Pressure Supported Respiratory Rate Pressure support level This is the Respiratory Rate counterpart to VS. Uses Pressure Support (Basic Mode) as its actuator. After each spontaneous breath, ventilator adjusts the PS level for the subsequent breath to achieve the set Respiratory Rate target[16].
Mandatory Minute Ventilation MMV Pressure Supported Minute Ventilation Number of Mandatory Breaths Ventilator provides a spontaneous mode with or without inspiratory support set by the clinician. Clinician also sets desired Minute Ventilation MVtarget. The ventilator monitors Minute Ventilation taken by the patient (MVpat). If MVpat drops below MVtarget, ventilator provides a VC breath to the patient with volume and timing as preset by the clinican[17].
Volume Assured Pressure Support Ventilation VAPSV Pressure Control and Volume Control Tidal Volume Pressure support level plus volume add This mode represents intra-breath target enforcement. Ventilator primarily delivers Pressure Support (Basic Mode). Within the ongoing inspiratory phase, if the algorithm predicts that the decaying flow of the PS breath will not meet the set Vt target, ventilator automatically switches the control logic to a Volume Control (Basic Mode) waveform for the remainder of that breath to guarantee the volume. This is sophisticated targeting, but the goal remains achieving the clinician's predefined Vt[18].
Automode Selectable Mode 1 and Mode 2 Switching criterium Mode 1 or Mode 2 This is a mode-manager that operates on top of dual control modes. It pairs two Dual Control Modes (e.g., PRVC and VS) and ventilator automatically switches between them based on the presence or absence of patient triggering, all while maintaining the clinician-set volume targets[19].

See also

References

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  10. ^ Brochard, Laurent (2013). Pressure support ventilation. In: Tobin MJ, ed. Principles and Practice of Mechanical Ventilation. McGraw-Hill. ISBN 9780071736268.
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