Ventilation Modes

Goals of Mechanical Ventilation

• oxygenation (paO2 & SpO2)

• ventilation (pH & pCO2)

• comfort (vent synchrony, minimize sedation)

• weaning (minimize muscle loss, promote readiness to wean)

Modes of Ventilation

• ventilator modes are some variation (or combination of) pressure and/or volume, all have their pros & cons, and each has three features:

  1. trigger: what initiates a breath

  2. cycle: what ends a breath

  3. limit: what stops a breath early

• VC (volume control): every breath delivered is the same set tidal volume (TV)

  1. pros: ensures a minimum minute ventilation (MV), good for lung-protective ventilation (LPV)

  2. cons: requires monitoring pressure to avoid barotrauma

  3. settings: RR / TV / PEEP / FiO2

  4. monitor: peak inspiratory pressure (PIP) / plateau pressure (PPlat)

• PC (pressure control): every breath delivered at a set inspiratory pressure (IP) for a set inspiratory time (Ti)

  1. pros: limits pressure, may be more comfortable for select pts, can also be used for LPV

  2. cons: requires monitoring volumes to avoid volutrauma or hypoventilation

  3. settings: RR / IP / Ti / Risetime / PEEP / FiO2

  4. monitor: TV / MV

• AC (assist control): an adjunct to VC & PC to allow breathing over the set RR; still guarantees a minimal TV or IP

  1. pros: takes over WOB, provider can set a TV or IP

  2. cons: tachypneic pts will still receive full TV or IP every breath, which (without adequate sedation) can lead to respiratory alkalosis or air trapping esp. in COPD or asthma

• PRVC (pressure-regulated volume control): dynamically changes IP to deliver a desired TV

  1. pros: guarantees TV with pressure-controlled breaths (i.e., low-risk of causing VILI)

  2. cons: provides less support for pts w/a high WOB

  3. settings: RR / TV / Ti / Risetime / Pmax / PEEP / FiO2

  4. monitor: Ppeak / Pplat / TV / MV

• SIMV (synchronous intermittent mechanical ventilation): delivers mandatory? breaths w/a fixed volume (pt cannot? trigger); can be combined w/PSV for additional support

  1. pros: can take over WOB, allows more spontaneous breathing than AC, useful for weaning support gradually

  2. cons: seldom used, not effective for weaning, often uncomfortable

  3. settings: RR / TV / PEEP / FiO2

  4. monitor: Ppeak / Pplat

• PSV (pressure support ventilation): allows pt to breathe on own & only provides pressure (all breaths are patient-initiated, no back-up RR)

  1. pros: ideal for weaning, is most comfortable

  2. cons: does not guarantee a rate, unstable pts will rapidly fatigue, need to closely monitor

  3. settings: PS / PEEP / FiO2

     • when expressing the PS, it is only the pressure above PEEP, ex. 10 / 5 / 40% implies the PIP is 15

  4. monitor: TV / MV

• APRV (airway pressure release ventilation, aka Bi-Vent or Inverse Ratio Ventilation): allows pt to breath spontaneously

  1. pros: great for ARDS pts who are spontaneously breathing, may improve comfort & oxygenation (but no mortality benefit)

  2. cons: complex mode, risk of VILI if settings are done improperly, does not make sense if paralyzed

  3. settings: Thigh / Tlow / Phigh / Plow / FiO2

  4. monitor: TV / MV / paCO2 / EtCO2

• HFOV (high frequency oscillatory ventilation): 

Selecting a Mode

• Restrictive lung disease (ex. ARDS, pneumonitis, PNA, pulmonary fibrosis, pulmonary edema, alveolar hemorrhage, chest trauma) is associated w/reduced lung compliance i.e., hard to get air in but easy to get air out; the prudent ventilation strategy:

  • recruits vulnerable alveoli

  • prevents alveoli closure

  • provides adequate oxygenation

  • minimizes volutrauma

  • typically, VC-AC or PC-AC

• Obstructive lung disease (ex. COPD, asthma) is associated w/increased lung compliance i.e., easy to get air in but hard to get air out; the prudent ventilation strategy

  • rests the respiratory muscles

  • provides adequate oxygenation

  • reduces hyperinflation

  • typically, VC-AC

• Severe metabolic acidosis (ex. septic shock, acute renal failure, DKA, toxic exposure) is associated w/compensatory hyperventilation and high MV; it is difficult to accomplish an MV by setting a high RR and/or TV; the prudent ventilation strategy let's the pt's respiratory drive work in their favor:

  • minimum sedation for intubation

  • avoid NMB entirely

  • typically, PSV

• LV cardiac failure: the LV likes PEEP; increasing intra-thoracic pressure (i.e., increasing PEEP) reduces preload & reduces afterload, which is beneficial in acute cardiac failure due to left ventricular dysfunction (either systolic or diastolic)

• RV cardiac failure: the RV doesn't like PEEP; reducing intra-thoracic pressure (i.e., minimizing PEEP) reduces pulmonary vascular pressures & RV stress; use more FiO2 than PEEP in the setting of RV failure (i.e., massive PE, acute pulmonary HTN

  • PEEP < 10

• Brain injury (ex. TBI, stroke, hemorrhage): hypoxemia increases ICP, so the priority is adequate oxygenation; keep in mind that PEEP can increase ICP

  • SpO2 > 94%

  • paO2 > 80

  • PEEP < 15

• Hyperventilation (paCO2 < 32) is an adjunct to lower ICP by causing cerebral vasoconstriction, which means it also makes the brain ischemic; this can be helpful in the setting of acute increases in ICP where you need to temporize impending herniation for 5-10 minutes before giving mannitol, hypertonic saline, or go the OR; prolonged hyperventilation, however, worsens brain ischemia & has no lasting effect on intracranial HTN

Initial Settings