Operating Modes of Mechanical Ventilation

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Operating Modes of Mechanical Ventilation
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Introduction

• Ventilator mode can be defined as a set of
  operating characteristics that control how the
  ventilator functions
• Operating mode can be described by the way a
  ventilator is triggered into inspir. and cycled
  into exhalation what variables are limited
  during inspiration and whether or not the mode
  allows mand. spont. breaths or both.

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Introduction (cont.)

• 13 essential modes - spontaneous - PEEP -
  CPAP - BiPAP - CMV - AC - IMV -
  SIMV - MMV (mandatory minute ventilation) -
  PSV - PCV - APRV (airway press. release
  ventilat.) - IRV (inverse ratio ventilation)

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SPONTANEOUS

• Not an actual mode since rate and tidal volume
  during spont. breathing are determined by patient
• Role of ventilator during spont. vent. is to
  provide the (1) flow to the pt. in a timely
  manner (2) flow adequate to fulfill a patients
  insp. demand and (3) provide adjunctive modes
  such as PEEP to complement the spont. effort

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SPONTANEOUS (contd.)

• Apnea ventilation is a safety feature used for
  spontaneous mode

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PEEP

• PEEP increases the end-expiratory or baseline
  airway press. to a value greater than atmos. and
  is often used to improve the pt.s O2 status
  esp. if refractory
• PEEP is not a stand-alone mode but is used in
  conjunction with other modes
• When PEEP is applied to spont. breathing pt.
  then called CPAP

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PEEP (contd.)

• Two major indication for PEEP are -
  intrapulmonary shunt leading to refractory
  hypoxemia - decreased functional residual
  capacity and lung compliance
• Complications assoc. with PEEP include -
  decrease venous return and C. O. - barotrauma -
  incr. ICP - alterations of renal hepatic
  et.al.

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CPAP

• Is PEEP applied to a spont. breathing pt.
• Indications are same as PEEP but in addition pt.
  must have adequate lung function to sustain
  eucapnic ventilation
• Can use in adult with ET tube or facemask and in
  neonates with ET or nasal

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BiPAP

• Allows one to apply IPAP and EPAP
• IPAP provides positive pressure breaths and it
  improves hypoxemia and/or hypercapnia
• EPAP (essentially PEEP) improves oxygenation by
  increasing the FRC and enhancing alveolar
  recruitment

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BiPAP (contd.)

• Indications for BiPAP - preventing
  intubation of end-stage COPD patient -
  supporting patients with chronic ventilatory
  failure - patients with restrictive chest
  wall disease - neuromuscular disease -
  nocturnal hypoventilation

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BiPAP (contd)

• Three modes include - spontaneous -
  timed - spontaneous/timed
• Initial settings - if pt. breathing spont.
  set at 8 and 3 - use spont./timed as backup and
  set 2-5 breaths below pt.s spont.
• IPAP levels are determined by monitoring pt.s
  clinical physiologic response notTV

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BiPAP (contd)

• EPAP should be increased by 2 cmH2O increments to
  increase FRC and oxygenation
• not possible to increase EPAP higher than IPAP

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CMV

• Ventilator delivers the preset tidal volume at a
  set time interval
• should only be used when the pt. is properly
  medicated with a combination of sedatives
  respiratory depressants and neuromuscular
  blockers
• Indicated if pt. fights the vent. tetanus or
  other seizure complete rest for pt. for 24 hr.
  crushed chest where spont. paradoxical mvmt.

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Assist Control

• Pt. always receives a mechanical breath either
  by timed or by assist
• Indicated when full ventilatory support is
  needed used when pt. has a stable respiratory
  drive (10-12 spont. rate)
• The generally accepted set minimum rate is 2-4
  breaths less than the pt.s assist rate or a
  minimum of 8 - 10 breaths

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AC (contd)

• Advantages include a very small WOB when sens.
  and flow is set properly and this mode allows
  the pt. to control the RR
• Disadvantage include alveolar hypervent.

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IMV

• Pt. breaths spont. at any tidal volume between
  the mechanical breaths
• Primary disadvantage is chance for breath
  stacking therefore care should be taken to set
  high press. limit properly to reduce risk of
  barotrauma

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SIMV

• A mode in which the vent. delivers mandatory
  breaths to the pt. at or near the beginning of a
  spont. breath mandatory breaths are synchronized
  with the pt.s spont. efforts to avoid breath
  stacking
• synchronized window refers to the time just
  prior to time triggering in which the vent. is
  responsive to the pt.s effort (0.5 sec is
  typical)

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SIMV (contd)

• gas source for spontaneous breathing is usually a
  demand valve that allows pt. to determine their
  tidal volume
• Indications include providing partial vent.
  support usually after 24 hr. of full vent.
  support
• Advantages include maintaining resp. muscle
  strength reduces V/Q mismatch decreases mean
  airway press. helps wean pt

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SIMV (contd)

• disadvantage usually has to do with trying to
  wean pt. too rapidly leading to incr. WOB and
  muscle fatique

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MMV (Mandatory Minute Ventilation)

• also called minimum minute ventilation
• Provides a predetermined minute ventilation when
  the patients spontaneous breathing effort
  becomes inadequate
• Useful for preventing hypoventilation and
  respiratory acidosis in the final stages of
  weaning with SIMV
• Need to keep watch spontaneous minute volume
  (distressed pt. may increase RR with lower tidal
  volume)

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PSV

• Used to lower the WOB and augment a patients
  spont. tidal volume
• When PSV is used with SIMV it lowers the O2
  consumption because of the decr. WOB
• PSV applies a preset pressure plateau to the pt.
  during a spont. breath
• PSV breaths are patient triggered pressure
  limited and flow cycled

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PSV (contd)

• Typically used in the SIMV mode to help weaning
  by (1) increasing spont. tidal volume (2)
  decreasing spont. RR (3)decreasing WOB
• May see M.D. adjust press. to get a desired
  spont. tidal volume or may see M.D. adjust
  press. to get a target RR i.e.

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PCV

• The pressure controlled breaths are time
  triggered by a preset resp. rate
• Once inspir. begins a pressure plateau is
  created and maintained for a preset inspir. time
• Typically used in ARDS where it takes excessive
  press. in volume cycled modes to ventilate a pt.
  leading to barotrauma

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APRV (airway pressure release ventilation)

• Is similar to CPAP in that the pt. is allowed to
  breathe spont. without restriction
• Combines two separate levels of CPAP and the pt.
  may breathe spont. from both levels
• Periodically pressure is dropped to the lower
  level reducing mean airway press.
• During spont. expir. the CPAP is dropped
  (released) to a lower level which simulates an
  effective exhalation

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APRV (contd)

• See this mode used like PCV in ARDS

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IRV (inverse ratio ventilation)

• The inverse IE ratio used is 21 - 41 and often
  used in conjunction with PCV
• IRV improves oxygenation by (1) reducing
  intrapulmonary shunting (2) improvement of V/Q
  matching and (3) decreasing deadspace ventilation
• See increase of mean airway pressure and presence
  of auto PEEP and may worsen pulmonary edema

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Flow-by

• Similar to the add-on IMV systems used on other
  ventilators
• set sensitivity flow and base flow