Guidelines for Mechanical Ventilation and Non‑Invasive Ventilation

Fundamental Principles

• Mechanical ventilators maintain adequate gas exchange while minimizing ventilator‑induced lung injury by controlling pressure, volume, or flow. 1

Ventilator Classification

• Three primary control mechanisms are used:
  
  • Pressure‑based – a set inspiratory pressure is delivered; tidal volume varies with lung compliance, airway resistance, and chest wall mechanics. 2
  • Volume‑based – a set tidal volume is delivered; the ventilator generates whatever pressure is needed to achieve that volume. 2
  • Flow‑based – inspiratory flow patterns are controlled, providing detailed monitoring of leaks, airway obstruction, and circuit problems. 1

Alarm Systems

• Comprehensive alarm systems are mandatory for patient safety and include:
  
  • Low‑pressure alarms – detect circuit disconnection or excessive leak that prevents reaching the set pressure. 1
  • High‑pressure alarms – warn of excessive pressure on volume‑controlled modes, indicating worsening compliance or obstruction. 1
  • Flow alarms – identify changing leaks, worsening airflow obstruction, or partially occluded tubing. 1

Controlled Mechanical Ventilation (CMV)

• CMV provides full ventilatory support with no patient effort; preset inflation pressure or tidal volume, respiratory rate, and timing are delivered regardless of patient drive. 2
• Indicated for patients lacking respiratory drive (e.g., severe neurological injury, deep sedation, neuromuscular blockade). 2
• Guarantees minute ventilation independent of patient effort. 2

Assist/Control Ventilation (ACV)

• Delivers a preset number of mandatory breaths; patients may trigger additional breaths that receive the same preset parameters. 2
• A lock‑out period prevents breath stacking; patient‑triggered breaths delay the next machine‑determined breath, improving synchrony. 2
• Recommended for patients with variable respiratory drive who still require guaranteed minute ventilation. 2
• Potential for patient‑ventilator dyssynchrony if the lock‑out period is set incorrectly. 2
• Excessively long expiratory times can create prolonged lock‑out periods, reducing tolerance. 2

Synchronized Intermittent Mandatory Ventilation (SIMV)

• Provides a preset number of mandatory breaths while allowing spontaneous breaths between them; spontaneous breaths delay the next mandatory breath to maintain synchrony. 2
• Can be configured as volume‑controlled or pressure‑controlled and is also known as “spontaneous/timed” (S/T) or IE mode on some devices. 2
• More complex than other modes and requires understanding of synchronization windows. 2
• Terminology varies among manufacturers, which may cause confusion. 2

Pressure Support Ventilation (PSV)

• Patient effort initiates and terminates each breath; respiratory frequency and timing are entirely patient‑determined. [2][3]
• Most devices include a backup rate of 6–8 breaths /min if the patient becomes apneic. [2][3]
• Indicated for patients with adequate drive who need partial ventilatory support. 3
• No ventilation occurs if the patient fails to generate effort, risking apnea. [2][3]

Bi‑Level Positive Airway Pressure (BiPAP/NIV)

• Guideline society: British Thoracic Society recommends bi‑level pressure‑support ventilators as a simple, cost‑effective, and flexible option, used in most NIV randomized trials. 4
• Combines pressure support with CPAP using two pressure levels:
  
  • IPAP – provides ventilation during inspiration.
  • EPAP – recruits underventilated lung and offsets intrinsic PEEP. [5][3]
• Indications include:
  
  • Acute COPD exacerbation with respiratory acidosis (pH < 7.35) despite maximal medical therapy. 4
  • Acute or acute‑on‑chronic hypercapnic failure from chest‑wall deformity or neuromuscular disease. 4
  • Decompensated obstructive sleep apnea with respiratory acidosis. 4
  • Cardiogenic pulmonary edema when CPAP alone is insufficient. 4
• Approximately 20–30 % of patients with acute respiratory failure cannot be managed with NIV due to mask‑fit problems or asynchrony. 1
• Overtightening headgear to reduce leaks can cause skin ulceration, especially over the nasal bridge. 1

Continuous Positive Airway Pressure (CPAP)

• Maintains a constant positive airway pressure throughout the respiratory cycle, recruiting underventilated lung similarly to PEEP. [5][3]
• Benefits include improved oxygenation, reduced inspiratory muscle work, and, in COPD, offsetting intrinsic PEEP to lower PaCO₂. [5][3]
• Indications:
  
  • Cardiogenic pulmonary edema with persistent hypoxia despite optimal medical therapy. 4
  • Chest‑wall trauma with ongoing hypoxia despite adequate regional anesthesia and high‑flow oxygen. 4
  • Diffuse pneumonia with hypoxia resistant to high‑flow oxygen. 4
  • Obstructive sleep apnea without respiratory acidosis. 4
• Caution: In hyperinflated patients with airflow obstruction, further lung‑volume increase may impair inspiratory muscle function. [5][3]
• CPAP does not provide ventilatory support; it only maintains positive pressure. [5][3]
• Requires high‑flow capability (> 60 L/min) for distressed COPD patients with high minute ventilation. [5][3]
• Patients with chest‑wall trauma on CPAP should be monitored in ICU due to pneumothorax risk. 4
• CPAP trials for acute pneumonia should be limited to HDU/ICU settings because of high intubation risk. 4
• Low‑flow CPAP generators suitable for sleep apnea are inadequate for acute respiratory failure. [5][3]

Proportional Assist Ventilation (PAV)

• Adjusts flow assistance to counter airway resistance and volume assistance to counter lung/chest‑wall compliance, allowing the patient to set breathing pattern and depth. [5][3]
• May improve patient comfort and compliance with NIV, but clear clinical benefit has not yet been demonstrated. [5][3]
• Requires sophisticated operator understanding. 1

Minimum Requirements for Non‑Invasive Ventilators

• Pressure capability ≥ 30 cmH₂O.
• Support inspiratory flow ≥ 60 L/min. 1
• Disconnection alarm. 1
• Adjustable short pressure rise time. 1

Clinical Decision‑Making Algorithm for NIV

• Before initiating NIV, determine whether the patient would require immediate intubation if NIV fails. 4
• Immediate intubation is indicated for absent airway reflexes, severe upper‑airway obstruction, high oxygenation requirements, or hemodynamic instability. 4
• NIV is appropriate for COPD exacerbation with pH < 7.35, cardiogenic pulmonary edema, or neuromuscular disease with hypercapnia. 4

Interface Selection for NIV

• Use a full‑face mask initially in acute settings; switch to a nasal mask after 24 h as the patient improves. 4
• Proper mask fit is essential for comfort and effective ventilation. 1
• Apply barrier dressings from the start to reduce nasal‑bridge ulceration risk. 1
• Avoid overtightening headgear, which can cause skin damage and reduce compliance. 1

Mechanical Ventilation Modes and Strategies

Core Ventilatory Modes

• The American Thoracic Society recommends starting with volume-cycled Assist/Control Ventilation when initiating mechanical ventilation, as it provides complete ventilatory support immediately after intubation and prevents central apneas 6
• Controlled Mechanical Ventilation (CMV) provides full ventilatory support with no patient effort required, delivering preset breaths at fixed intervals regardless of patient respiratory drive 7
• In pressure control CMV, the resulting tidal volume varies based on airway resistance, airflow limitation, and lung/chest wall compliance 7
• In volume control CMV, tidal volume is fixed and the ventilator generates whatever pressure is necessary to deliver this volume, determined by circuit compliance and thoracic mechanics 7
• Assist/Control Ventilation (AC) mode guarantees a preset number of mandatory breaths per minute while allowing patient-triggered breaths, with all breaths delivering identical preset parameters 6, 8
• Synchronized Intermittent Mandatory Ventilation (SIMV) synchronizes patient-triggered breaths with machine-delivered breaths, delaying the next mandatory breath when a patient triggers 7
• In Pressure Support Ventilation (PSV), the patient's respiratory effort triggers the ventilator both on and off, with the patient determining respiratory frequency and timing of each breath 7, 9
• Continuous Positive Airway Pressure (CPAP) is employed to correct hypoxaemia by maintaining constant positive pressure throughout the respiratory cycle, recruiting underventilated lung similar to PEEP 10

Critical Initial Settings

• The American College of Chest Physicians recommends targeting 6 mL/kg predicted body weight to reduce mortality in ARDS and sepsis-induced respiratory failure 6
• Maintain plateau pressure ≤30 cmH₂O to prevent alveolar overdistension and ventilator-induced lung injury 6
• For ARDS, use AC with low tidal volumes (6 mL/kg PBW) and plateau pressure ≤30 cmH₂O, adding prone positioning for >12 hours/day 6
• For post-cardiac arrest, avoid hyperventilation and target normocapnia with PaCO₂ 40-45 mmHg 6
• For sepsis-induced respiratory failure, follow the same lung-protective strategy as ARDS 6

Common Pitfalls to Avoid

• Do not use actual body weight for tidal volume calculations—always use predicted body weight 6
• Do not hyperventilate patients, as this causes cerebral vasoconstriction, hemodynamic instability, and increased mortality 6
• Be aware that terminology for ventilation modes varies between ventilator manufacturers, potentially causing confusion 8, 9

Mechanical Ventilation Modes and Settings

Fundamental Classification: Volume vs. Pressure Targeting

• The clinician sets a specific tidal volume and inspiratory time, and the ventilator generates whatever pressure is necessary to deliver this volume within the set time, in volume-targeted ventilation, as described by the Thorax journal 11
• The clinician sets the inspiratory pressure level, and the delivered tidal volume becomes a function of lung impedance, airway resistance, and inspiratory time, in pressure-targeted ventilation, according to the Thorax journal 11
• This mode offers several advantages, including constant pressure delivery, compensation for air leaks, and positive expiratory pressure, as noted in the Thorax journal 11
• In the UK and many centers, volume ventilators are rarely employed for non-invasive ventilation outside specialist centers, as mentioned in the Thorax journal 11

Evidence‑Based Comparison of SIMV + Pressure Support and PRVC

Mechanical Characteristics

• In mechanically ventilated patients, the mandatory breaths of SIMV + Pressure Support are delivered at preset intervals and can be synchronized with the patient’s effort. 12
• Between mandatory breaths, SIMV + Pressure Support provides spontaneous breaths that are patient‑triggered and patient‑cycled, receiving only pressure support. 13

Clinical Implications

• The use of SIMV + Pressure Support leads to more variable oxygen saturation (SpO₂) because spontaneous breaths often deliver lower tidal volumes than mandatory breaths. 12
• An incorrectly set “lock‑out” period intended to prevent breath stacking in SIMV + Pressure Support can cause poor patient tolerance. 12, 13

Indications for PRVC

• PRVC is especially beneficial for patients with injured lungs (e.g., acute respiratory distress syndrome or pneumonia), offering guaranteed tidal volume delivery with minimal patient‑ventilator asynchrony. 12

Ventilator Mode Classification and Clinical Application Guidelines

1. Primary Classification of Mechanical Ventilators

Mechanical ventilators are classified by their control variable: pressure‑targeted modes deliver a preset inspiratory pressure with a variable tidal volume, whereas volume‑targeted modes deliver a preset tidal volume with a variable inspiratory pressure. 14

2. Pressure‑Targeted Ventilation (PTV)

• In PTV the clinician sets the inspiratory pressure; the resulting tidal volume varies according to lung compliance, airway resistance, and inspiratory time. 14
• PTV provides constant pressure throughout the breath, automatically compensates for circuit leaks, and maintains positive expiratory pressure during the entire respiratory cycle. 14
• By using pressure control, PTV avoids the abrupt pressure spikes that can occur with volume‑controlled ventilation, improving patient comfort. 14

3. Volume‑Targeted Ventilation (VTV)

• In VTV the clinician sets the desired tidal volume and inspiratory time; the ventilator generates whatever pressure is required to achieve that volume. 14
• VTV is infrequently used for non‑invasive ventilation in the United Kingdom outside specialist centres. 14

4. Synchronized Intermittent Mandatory Ventilation (SIMV)

• SIMV can be delivered in either volume‑controlled or pressure‑controlled mode, offering flexibility to match patient needs. 14
• Terminology for SIMV varies among manufacturers (e.g., “spontaneous/timed” or “IE” mode), which may cause confusion in clinical practice. 14

5. Assist/Control Ventilation (ACV) vs. Pressure Support Ventilation (PSV)

5.1. Assist/Control Ventilation

• ACV (volume‑cycled) is recommended to prevent central apneas during sleep, providing more reliable protection than PSV. 15

5.2. Pressure Support Ventilation

• High levels of pressure support can disrupt sleep by inducing periodic breathing and central apneas. 15
• Excessive pressure support may cause hyperventilation, hypocapnia, and subsequent apnea. 15
• In one study, 54 % of patients experienced central apneas (≈53 events / h) during PSV, leading to frequent arousals and marked sleep fragmentation. 15
• Overall sleep quantity and quality are poorest during PSV compared with assist‑control modes. 15
• To mitigate these effects, clinicians should titrate pressure support to avoid hyperventilation, especially in patients with chronic heart failure, and consider adding dead space or switching to ACV if central apneas develop. 15

6. Non‑Invasive Ventilation (NIV) – Mask Fit and Failure Rates

• Proper mask fit is essential for patient comfort and effective ventilation. 16
• Approximately 20 %–30 % of patients with acute respiratory failure cannot be managed with NIV because of mask‑fit problems or patient‑ventilator asynchrony. 16
• Overtightening the headgear to reduce leaks frequently causes skin ulceration, particularly over the nasal bridge. 16

7. Minimum and Desirable Ventilator Specifications for NIV

8. Clinical Decision Algorithm for Initiating NIV

Before starting NIV, clinicians must assess whether the patient would require immediate intubation if NIV fails; this assessment should be documented early and confirmed by senior staff. 16

• Immediate intubation is indicated when the patient has absent airway reflexes, severe upper‑airway obstruction, high oxygenation requirements, or hemodynamic instability.
• NIV is appropriate for conditions such as COPD exacerbation with acidemia, cardiogenic pulmonary edema, or neuromuscular disease with hypercapnia.

Two strategic approaches to NIV: