Basics of Mechanical Ventilation in the ICU

Modes of Mechanical Ventilation

• Two primary breath strategies exist: volume-controlled ventilation (flow-targeted) and pressure-controlled ventilation (pressure-targeted). Most modes on ICU ventilators are variations on either of these strategies for delivering each breath.
• Three phases are present in a breath cycle that define ventilatory modes. The trigger describes the signal that initiates inhalation and can be the ventilator or the patient. The target defines the parameter that the ventilator achieves during inspiration (most commonly either flow or pressure). The cycle is the event that initiates exhalation.^1,2
• Volume-controlled ventilation: inflation volume (tidal volume [V_T]) is set, and the ventilator will adjust the delivered flow to reach that volume
  • Parameters that must be set: V_T (the amount of air delivered in each breath), respiratory rate, flow rate, fraction of inspired oxygen (FiO₂), and positive end-expiratory pressure (PEEP)
  • Trigger: ventilator or patient
  • Target: flow
  • Cycle: volume (inspiration ends once the target volume has been reached)
• Pressure-controlled ventilation: the inflation pressure and duration are set
  • Parameters that must be set: Respiratory rate, inspiratory pressure, inspiratory time, FiO₂, and PEEP
  • Trigger: ventilator or patient
  • Target: pressure
  • Cycle: time (inspiration ends after the set amount of inspiratory time)

• The goals of mechanical ventilation in the ICU differ from those in the operating room due to the level of patient participation in their respiratory mechanics and the typical overall duration of mechanical ventilation. In the operating room, intubated patients are unconscious and often pharmacologically paralyzed, so control over every aspect of their breath cycle is desirable. However, ventilated ICU patients are often under light to moderate sedation and can initiate their own breaths. In this setting, enhancing patient synchrony with the ventilator is critical. Several standard modes are available in ICU ventilators, with either a volume- or pressure-controlled strategy, and each can be selected to tailor care to the patient’s specific needs. Examples of ICU ventilator modes include:
  • Continuous mandatory ventilation (CMV): Each breath is initiated and controlled by the ventilator, with no patient effort required.²
    • Uses: heavy sedation, coma, patients who need full pharmacologic paralysis
  • Assist control ventilation (AC): Breaths can be triggered by the patient or the ventilator. Each breath, regardless of trigger, is still fully supported by the ventilator.²
    • Uses: patients with ventilator desynchrony on CMV mode, patients under light to moderate sedation who can initiate their own breaths
  • Adaptive pressure control: A variant of either CMV or AC ventilator modes that allows breath-to-breath variation in inspiratory pressures to achieve a set V_T, allowing for automatic adaptation to the mechanics of the patient.²
    • Uses: patients with light sedation whose respiratory mechanics can change throughout the day (most ICU patients), patients with reduced lung compliance or obstructive disease
  • Synchronized intermittent mandatory ventilation (SIMV): Ventilator has a minimum respiratory rate with fully supported breath mechanics (either volume control or pressure control), but the patient may increase minute ventilation with their own spontaneous breaths. These patient-triggered breaths can be set to be pressure-supported.²
    • Uses: weaning from mechanical ventilation (allows partial respiratory support while the patient gradually rebuilds respiratory endurance)

Setting Ventilator Parameters

• Ventilator settings must be highly individualized for each patient, taking into consideration the patient’s ventilation-perfusion matching and pressure-volume relationship of their lungs.¹ Some common factors to consider are the indication for mechanical ventilation, along with patient comorbidities. Several common strategies include:
• Lung protective strategy: Now the standard of care for most ICU patients, this mode is particularly beneficial for patients at risk of progressing to acute respiratory distress syndrome (ARDS) (including those with sepsis, trauma, pneumonia, pancreatitis, large volume transfusions, and severe aspiration).¹ The goal of this mode is to limit lung injury induced by high V_Ts (volutrauma), high airway pressures (barotrauma), and the frequent collapse of small airways (atelectrauma). In acute lung injury, functional lung volume is significantly reduced due to edema and atelectasis, causing shunt physiology.
  • V_T – 6 mL/kg predicted body weight
  • Respiratory rate (RR) – based on the patient’s acid-base status, but typically attempts to achieve a goal pH of 7.3-7.45
  • Inspiratory flow rate – 60 mL/min
  • FiO₂: adjust to achieve a goal oxygenation of SpO₂ 88-95% or PaO₂ of 55-80 mmHg
  • PEEP: minimum of 5 cm H₂0
  • ARDSnet PEEP/FiO₂ protocol: You must frequently assess the impact of mechanical ventilation on the patient’s alveoli by monitoring plateau pressure (goal less than 30) and driving pressure (plateau pressure – PEEP less than or equal to 14). If driving pressure gets too high, reduce V_T goals to 4 mL/kg and increase respiratory rate to maintain minute ventilation.^1,5
• Obstructive strategy: Intended for patients with COPD and asthma, whose obstructive disease narrows and collapses small airways. The increased resistance causes residual air trapped in the chest at the start of inhalation, resulting in increased intrathoracic pressure (auto-PEEP) and progressive hyperinflation. This decreases lung compliance and increases the work of breathing.
  • V_T – 8 mL/kg
  • RR – 10 breaths/minute
  • Inspiratory flow rate – 60 mL/min
  • FiO₂ – 40%
  • Confirm that the ventilator waveform returns to baseline by the start of the next breath cycle.
• Intermediate strategy: As found in the PReVENT trial⁶, patients without the risk of acute lung injury or current obstructive disease tolerate the intermediate volume strategy of 8-10 mL/kg V_Ts
  • V_T – 8 mL/kg
  • RR – 16
  • Inspiratory flow rate – 60 mL/min
  • FiO₂ – 40%
  • PEEP – 5 cm H₂0

Weaning Ventilatory Support

• Prolonged ventilatory support is not without its risks, as intubated patients have increased deconditioning, mechanical lung trauma, and higher rates of ventilator-associated pneumonia. Therefore, weaning from mechanical ventilatory support is critical to achieving extubation and expediting patient discharge from the ICU.

Ventilatory Modes to Increase Patient Independence

• Pressure support ventilation (PSV): patient triggers breaths and controls minute ventilation with the ventilator only assisting with a fixed driving pressure (primarily to overcome the resistance of the endotracheal tube).²
• Please see the OA summary on pressure support ventilation for more details. Link
• SIMV: discussed above; provides partial ventilatory support while allowing the patient to rebuild respiratory endurance

Examining Readiness for Extubation

• Spontaneous breathing trial (SBT): This should be performed daily in most ICU patients; patients should be able to breathe for 30 minutes without significant changes in hemodynamics or oxygen saturation. It should be performed with minimal sedation and on a low level of PSV support (less than 5-8 mmHg). Other considerations should include the quality and quantity of respiratory secretions, along with the ability to cough and maintain adequate wakefulness when determining safety for extubation.⁷
• Rapid shallow breathing index: calculated ratio of respiratory rate to V_T achieved during SBT, a ratio of less than 105 is suggestive of readiness for extubation⁷
• Cuff leak test: This test is performed by deflating the endotracheal tube cuff and assessing for adequate air leak. This demonstrates airway patency and no significant laryngeal edema around the tube.⁷