Lippincott Nursing Pocket Card

Mechanical Ventilation: Settings and Basic Modes

icon-howtouse.jpg

About Mechanical Ventilation

Mechanical ventilation is utilized in intensive care and long-term care settings to assist patients who require additional respiratory support. It is indicated for acute or chronic hypoxemic respiratory failure, which is defined as insufficient oxygenation, insufficient alveolar ventilation, or both.
 
Benefits of mechanical ventilation are improved gas exchange and decreased work of breathing. Use this handy reference guide to help you safely manage oxygenation and ventilation goals for your patients on ventilator therapy.

icon-info.jpg

Definitions

MECHANICAL VENTILATION: IMPORTANT TERMS TO KNOW
TERM CLINICAL CONSIDERATIONS
Fraction of inspired oxygen (FiO2)
  • Concentration of oxygen in the inspired air
  • Use the lowest FiO2 that achieves the targeted oxygenation
  • Avoid prolonged FiO2 > 0.60, as this may cause oxygen toxicity
Frequency (f) or respiratory rate (RR)
(10-20 breaths/min)
  • Set number of ventilator breaths per minute
  • Actual RR includes the spontaneous breaths taken by the patient
  • Hypoventilation may cause respiratory acidosis; hyperventilation may cause respiratory alkalosis
Inspiratory:expiratory (I:E) ratio
  • Normal: longer expiratory phase than inspiratory phase (1:2, 1:3)
  • Inverse ratios provide a longer inspiratory phase (1:1, 2:1, 3:1, 4:1)
  • Reduced I:E allows more time for exhalation and reduces breath stacking; used for patients who have obstructive airway disease with acute respiratory acidosis
Minute ventilation (VE)
(5-10 L/minute)
  • Volume of gas exchanged per minute
  • VE=RR x VT
Peak flow rate
  • Maximum flow delivered by the ventilator during inspiration
Peak inspiratory pressure (PIP)
  • Highest proximal airway pressure reached during inspiration
  • Target PIP is < 35 cm H2O.
  • Low PIP may result in hypoventilation; high PIP may cause lung damage
Plateau pressure (Pplat)
  • Reflects pulmonary compliance and is measured by applying a brief inspiratory pause after ventilation
  • Assess Pplat with peak inspiratory pressure (PIP):
    • A high PIP with normal Pplat = increased resistance to flow (i.e., endotracheal tube obstruction or bronchospasm) 
    • High PIP and high Pplat = decreased lung compliance (i.e., interstitial pulmonary fibrosis, pneumonia, ARDS, pulmonary edema)
Positive end-expiratory pressure (PEEP)
(3-10 cm H2O)
  • Pressure remaining in the lungs at end expiration
  • Used to keep alveoli open and “recruit” more alveoli to improve oxygenation for patients
  • High levels may cause barotrauma, increased intracranial pressure, and decreased cardiac output
Pressure support (PS)
(8-20 cm H2O)
  • Provides additional pressure during inspiration to ensure a larger VT with minimal patient effort
  • Used to help overcome the work of breathing through ventilator tubing
Target
  • Flow of air into the lung can target a predetermined flow rate (i.e. peak inspiratory flow rate) or pressure limit
Termination 
  • Signal for a ventilator to end inspiration
  • May be related to volume (i.e. tidal volume), time (i.e. predetermined duration of inspiration), or flow (decrease in inspiratory flow to a percentage of peak value)
Tidal volume (VT)
[6-8 mL/kg of ideal body weight (IBW) to prevent barotrauma]
  • Volume of gas exchanged with each breath
  • A lower VT is indicated in patients with stiff, non-compliant lungs
  • Higher VT may cause tachycardia, decreased blood pressure, and lung injury
Trigger
  • Breaths can be triggered by:
    • Timer (ventilator-initiated breaths); occur at the set respiratory rate or frequency
    • Patient effort (patient-initiated breaths); occur when the patient causes sufficient change in either the pressure or flow in the circuit

MODES OF MECHANICAL VENTILATION
MODE DESCRIPTION
VOLUME-LIMITED VENTILATION
Clinician sets the peak flow rate, flow pattern, VT, RR, PEEP, and FiO2; inspiration ends once the set inspiratory time has elapsed.
Controlled mechanical ventilation (CMV)
  • VE is determined entirely by the set RR and VT.
  • No patient effort required.
Assist control (AC)
  • Minimal VE is determined by setting the RR and VT.
  • Patient can increase the VE by triggering additional breaths.
  • Each patient-initiated breath receives the set VT from the ventilator.
Pressure regulated volume control (PRVC) (also called volume control plus [VC+])
  • VT is set and the applied airway pressure changes to attain the target VT.
  • The initial applied inspiratory pressure is determined by the change in pressure required by the previous breath to attain the VT.
Intermittent mandatory ventilation (IMV)
  • Minimal VE is determined by setting the RR and VT; patient can increase the VE by spontaneous breathing, rather than patient-initiated ventilator breaths.
Synchronized intermittent mandatory ventilation (SIMV)
  • A variation of IMV; ventilator breaths are synchronized with patient inspiratory effort; can be used to titrate the level of ventilatory support.
PRESSURE-LIMITED VENTILATION
Clinician sets the inspiratory pressure level, I:E ratio, RR, PEEP, and FiO2; inspiration ends after delivery of the set inspiratory pressure.

Pressure-limited CMV (also called pressure control ventilation)

  • VE is determined entirely by the set RR and inspiratory pressure.
  • Patient does not initiate additional VE above that set on the ventilator.

Pressure-limited AC

  • Set RR and inspiratory pressure determine the minimum VE.
  • The patient can increase the VE by triggering additional ventilator-assisted, pressure-limited breaths.

Pressure-limited IMV or pressure-limited SIMV

  • The set RR and inspiratory pressure determine the minimum VE.
  • The patient can increase the VE by initiating spontaneous breaths.

Pressure support

  • Flow-limited mode that delivers inspiratory pressure until the inspiratory flow decreases to a predetermined percentage of its peak value.
  • Clinician sets the pressure support (inspiratory pressure), applied PEEP, and FiO2. The patient must trigger each breath because there is no set RR. The VT, RR, and VE are dependent on multiple factors, including ventilator settings and patient-related variables.
  • The work of breathing is inversely proportional to the pressure support level and the inspiratory flow rate.

Continuous positive airway pressure (CPAP)

  • Delivery of a continuous level of positive airway pressure.
  • Functionally similar to PEEP.
  • The ventilator does not cycle during CPAP and no additional pressure above the level of CPAP is provided; patients must initiate all breaths.

Bilevel positive airway pressure (BPAP)

  • Noninvasive positive pressure ventilation (NPPV) that delivers a preset inspiratory positive airway pressure (IPAP) and expiratory positive airway pressure (EPAP).
  • The VT correlates with the difference between the IPAP and the EPAP.

Airway pressure release ventilation (APRV)

  • High continuous positive airway pressure (P high) is delivered for a long duration (T high) and then falls to a lower pressure (P low) for a shorter duration (T low).
  • Alveolar recruitment is maximized by the high continuous positive airway pressure.
  • VT is related to both the driving pressure and the compliance.

Inverse ratio ventilation (IRV)

  • The inspiratory time exceeds the expiratory time.
  • Strategy employed during volume-limited or pressure-limited mechanical ventilation to increase the mean airway pressure and potentially improve oxygenation.

References:

Grossbach, I., Chlan, L., & Tracy, M. F. (2011). Overview of mechanical ventilatory support and management of patient- and ventilator-related responses. Critical Care Nurse, 31(3), 30–44. https://doi.org/10.4037/ccn2011595

Hyzy, R. & MsSparron, J. (2020, April 5). Overview of mechanical ventilation. UpToDate. https://www.uptodate.com/contents/overview-of-initiating-invasive-mechanical-ventilation-in-adults-in-the-intensive-care-unit
 
Hyzy, R. (2019, May 30). Modes of mechanical ventilation. UpToDate. https://www.uptodate.com/contents/modes-of-mechanical- ventilation