MODES OF MECHANICAL VENTILATION  

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Modes Of Ventilation

Mechanical Ventilation

 Devices designed to alter, transmit or directly apply energy in a predetermined way to assist or replace the work of respiration maintaining the gas exchange function of the lung.

The Various Modes Of Ventilation

Mandatory (Controlled Ventilation) / IPPV

Ø     Useful in patients with limited or absent respiratory drive

Ø     Patient cannot be weaned directly

Ø     Patient fights the ventilator once respiratory drive appears

Ventilator Parameters to be set are:

_ü        Tidal Volume: V_T

ü      Inspiratory Flow: Flow

ü      Frequency: f

ü      Inspiration Time: T_insp.

ü      O₂ concentration: FiO₂

ü      Positive End Expiratory Pressure: PEEP

  Synchronized Intermittent Mandatory Ventilation : SIMV

 Combination of machine ventilation and spontaneous breathing. The patient can breathe spontaneously between positive pressure breaths.

Ø     SIMV Enables the patient to breathe spontaneously in regular prescribed intervals between mandatory mechanical ventilation strokes that ensure a minimum ventilation

Ø     This minimum ventilation is defined by two set values, tidal volume (VT) and ventilation frequency (f). The minimum ventilation is the product of VT x f

Ø     The ventilation pattern is programmed by the following set values: tidal volume VT, Insp.Flow, frequency f and inspiration time Tinsp

Ø     To prevent the mandatory ventilation stroke being applied during spontaneous expiration, the Flow trigger of the machine ensures that the ventilation stroke is triggered within a "trigger window" and synchronized with the patient's spontaneous inspiration

Ø     During the spontaneous breathing phases, the patient can be assisted with pressure by ASB pressure support

Ø     SIMV is introduced in respiratory care for the smooth transition from IPPV support to independent breathing

Ø     In the course of progressively weaning the patient from artificial ventilation, the ventilation frequency f is further reduced while the spontaneous breathing time is increased, until finally the required total minute volume is supplied entirely by spontaneous breathing.

Ø     Spontaneous breathing can be assisted by ASB

Ventilator Parameters to be set are:

_ü        Tidal Volume: V_T

ü      Inspiratory Flow: Flow

ü      Frequency: f

ü      Inspiration Time: T_insp.

ü      O₂ concentration: FiO₂

ü      Positive End Expiratory Pressure: PEEP

ü      Sensitivity: Trigger

ü      Additionally for SIMV / ASB: Pressure support - PASB and Pressure Rise Time - Ramp

Mandatory Minute Volume Ventilation : MMV

Ø     In contrast to SIMV, the MMV ventilation mode gives mandatory breathing only if spontaneous breathing is not yet sufficient and has fallen below a pre-selected minimum ventilation.

Ø     This minimum ventilation is controlled by two set values tidal volume VT and frequency f, and results from the product VT x f

Ø     Unlike SIMV, the mandatory strokes are not given regularly but only in cases of insufficient ventilation

Ø     The frequency of mandatory strokes is determined by the level of spontaneous breathing

Ventilator Parameters to be set are:

_ü        Tidal Volume: V_T

ü      Inspiratory Flow: Flow

ü      Frequency: f

ü      Inspiration Time: T_insp.

ü      O₂ concentration: FiO₂

ü      Positive End Expiratory Pressure: PEEP

ü      Sensitivity: Trigger

ü      Additionally for MMV / ASB: Pressure support - PASB and Pressure Rise Time - Ramp

Biphasic Positive Airway Pressure : BIPAP

The BIPAP ventilation mode is a pressure / time-cycled ventilation mode in which the patient can always breathe spontaneously. BIPAP is therefore often described as a timed alteration between two CPAP levels.

Ø     The time-cycled change of pressure produces controlled ventilation corresponding to the pressure-controlled ventilation PCV

Ø     However, the constant option of spontaneous breathing allows the transition from controlled breathing to independent spontaneous breathing to take place smoothly over the course of the weaning phase, without requiring any change in the ventilation mode.

Ø     To adapt easily to the patient's spontaneous breathing pattern, the change-over from expiratory pressure level to inspiratory pressure level and also the change-over from inspiratory pressure level to expiratory pressure level, are synchronized with the patient's spontaneous breathing

Ø     The frequency of the change-over is kept constant, even with patient synchronization, by defining a trigger time window with a fixed time constant.

Ø     This smooth adaptation to the patient's spontaneous breathing requires less sedation, so that the patient returns to spontaneous breathing more rapidly

Ø     As in all pressure-controlled ventilation modes, the patient is not prescribed a fixed tidal volume (VT)

Ø     The tidal volume results principally from the pressure difference between the settings for PEEP and Pinsp

Ø     The display of the tidal volume measured on expiration, VTe, is used to set the required difference between the two pressure levels. Any increase in this difference will cause an increased BIPAP ventilation stroke.

Ø     Changes in lung compliance and airways, as well as active 'fighting' by the patient can lead to changes in tidal volume. This is a desired effect in this ventilation mode. with the knowledge that the tidal volume, and therefore the minute volume, are not constant, the alarm limits for minute volume must be adjusted with care.

Ventilator Parameters to be set are:

ü      Inspiratory Pressure: Pinsp

ü      Frequency: f

ü      Inspiration Time: T_insp.

ü      O₂ concentration: FiO₂

ü      Positive End Expiratory Pressure: PEEP

ü      Additionally for BIPAP / ASB: Pressure support - PASB and Pressure Rise Time - Ramp

Continuous Positive Airway Pressure : CPAP

Ø     The CPAP mode of ventilation allows spontaneous breathing

Ø     The spontaneous breathing is at a raised pressure level, to increase the functional residual capacity FRC.

Ø     Spontaneous breathing can be assisted with additional pressure by ASB

Ventilator Parameters to be set are:

ü      O₂ concentration: FiO₂

ü      Positive End Expiratory Pressure: PEEP

ü      Additionally for CPAP / ASB: Pressure support - PASB and Pressure Rise Time - Ramp

Assisted Spontaneous Breathing : ASB

Ø     Pressure support for insufficient spontaneous breathing

Ø     The function of the machine in assisting insufficient spontaneous breathing

Ø     (similar to that of the anaesthetis; who manually assists and monitors the patient's spontaneous breathing by feeling the breathing bag)

Ø     The machine takes over part of the inhalation function, with the patient maintaining control of spontaneous breathing

Ø     The pressure support of the ASB system is started:

-  when the spontaneous inspiration flow reaches the

   set value of the Flow trigger, or at the latest

-   when the spontaneous inspired volume exceeds,

    25 mL (12 mL in pediatric mode).

Ø     The machine then produces an increase in pressure up to the pre selected ASB pressure PASB, which is adjustable to the breathing requirement of the patient

Airway Pressure Release Ventilation : APRV

Ø     Spontaneous breathing at continuous positive airway pressure with short term pressure releases.

Ø     Meant to open up and stabilize collapsed alveoli associated with acute lung injury with out excessive peak pressures

Ø     Airway pressure is intermittently reduced from preset CPAP levels to a lower ambient pressure

Ø     Patient continues to breathe spontaneously

 Future Ventilatory Developments

Lung Acoustics

Ø     While the study of lung acoustics is not new, several new advancements use this field to develop cheaper, safer, and more practical methods of helping patients

Ø     One such method for diagnosing lung disease and setting appropriate ventilatory modes uses TSS (transmitted sound signals)

This method is simple because it involves the patient repeating several vowel sounds such as iii, while just one transducer measures the resultant sound signals from within the chest. Under normal conditions, the low frequency sounds of iii will be returned as iii to the transducer. High frequencies are usually filtered out by healthy lung tissue. In cases where diseased tissue is present, the high frequencies are enhanced and the iii sounds more like an aaa sound through the transducer. Lung acoustics also help in mapping the regions of the lung. Combining the different frequencies and microphone locations determines a map of the lungs

Medical Imaging

Ø     Electrical Impedance Imaging (EIT) measures conductivity distribution on the surface of the chest when imaging the lungs

Ø     Although this method is not very good for anatomical images, its noninvasive and inexpensive characteristics make it ideal for obtaining functional images of the respiratory system

Ø     On comparing and analyzing the results physicians / machines can determine lung ventilation

Thank You

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Last modified: February 28, 2002