Non-Invasive Ventilation (NIV): A Comprehensive Review for MD Anesthesia

1. Definition and Historical Background

• Roberts and Hedges' Clinical Procedures in Emergency Medicine, p. 230
• Fishman's Pulmonary Diseases and Disorders, p. 2620

2. Methods of Non-Invasive Ventilation

A. Negative-Pressure Ventilation

• Historically used (tank ventilator/"iron lung," cuirass shell, poncho wrap)
• Generates subatmospheric pressure around the chest, causing passive lung expansion
• Rarely used today except in select neuromuscular conditions
• Major limitations: immobility, poor access to patient, risk of upper-airway obstruction

B. Positive-Pressure Noninvasive Ventilation (NPPV)

i. Continuous Positive Airway Pressure (CPAP)

• Delivers a single, fixed positive pressure throughout the entire respiratory cycle (both inspiration and expiration)
• A fixed pressure, variable flow mode
• Relies entirely on the patient's intrinsic respiratory drive; does not augment tidal volume or minute ventilation
• Acts as a pneumatic splint to the pharyngeal airway and prevents alveolar collapse at end-expiration
• Increases functional residual capacity (FRC), improves V/Q matching, reduces intrapulmonary shunting
• Hemodynamic effects: reduces left ventricular preload and afterload (beneficial in cardiogenic pulmonary edema)
• Delivered via nasal mask, full face mask, or helmet (the latter used during COVID-19)
• Traditional use: OSA management; now used in acute cardiogenic pulmonary edema and postoperative atelectasis

• Fischer's Mastery of Surgery, p. 275-276

ii. Bilevel Positive Airway Pressure (BiPAP / BPAP)

• Delivers two independently adjustable pressures:
  • IPAP (Inspiratory Positive Airway Pressure): higher pressure during inspiration, augments tidal volume, reduces work of breathing, assists CO2 clearance
  • EPAP (Expiratory Positive Airway Pressure): lower pressure during expiration, equivalent to PEEP, maintains alveolar recruitment, improves oxygenation
• The difference between IPAP and EPAP (IPAP - EPAP = pressure support) determines the degree of ventilatory assist
• Supports both CO2 clearance AND oxygenation simultaneously
• Newer devices include a backup respiratory rate for patients with central apnea
• Initial settings: IPAP 12-15 cm H2O, EPAP 5 cm H2O; titrate based on clinical response
• BiPAP is the proprietary name (Philips Respironics) for bilevel PAP; the generic terms are BPAP or bilevel NPPV

• ROSEN's Emergency Medicine, p. 2613-2615
• Roberts and Hedges', p. 230-231

iii. High-Flow Nasal Cannula Oxygen (HFNC / HFNCO2)

1. Flow rates up to 60 L/min closely match patients' peak inspiratory flow demand, reducing entrainment of room air and delivering a reliable, high FiO2
2. Washes out anatomical dead space, replacing it with oxygen-enriched gas
3. FiO2 and flow rate can be titrated independently
4. Generates a small amount of PEEP (1-3 cm H2O)
5. Heated and humidified gas is better tolerated
6. Large nasal prongs occlude the nares, reducing ambient air entrainment during closed-mouth breathing

• ROSEN's Emergency Medicine, p. 1044-1045
• Fischer's Mastery of Surgery, p. 276

iv. Pressure Support Ventilation via Mask (PSV/NPPV)

• In many systems, NPPV delivered in spontaneous mode is functionally equivalent to PSV applied non-invasively
• The ventilator delivers a set inspiratory pressure every time the patient initiates a breath; inspiratory flow and inspiratory time are patient-mediated
• Many devices also offer volume-targeted modes (volume-assured pressure support, VAPS), which automatically adjust pressure to deliver a target tidal volume - useful for neuromuscular disease and OHS

v. Interfaces for NIV Delivery

3. Physiological Effects of NIV

Pulmonary Effects

• Increases alveolar recruitment and size, improving the area for gas exchange
• Improves V/Q matching
• IPAP offloads respiratory muscles and overcomes intrinsic PEEP (iPEEP) in COPD/asthma
• Changes breathing pattern: decreases respiratory rate, allows larger tidal volumes, improving alveolar ventilation

Hemodynamic Effects

• Increases intrathoracic pressure
• Reduces systemic venous return (decreases right ventricular preload)
• Increases pulmonary vascular resistance (increases RV afterload)
• Decreases left ventricular transmural pressure (reduces LV afterload) - particularly beneficial in cardiogenic pulmonary edema
• These effects may be detrimental in patients with right heart failure, hypovolemia, or distributive shock
• Fishman's Pulmonary Diseases and Disorders, p. 2622

4. Indications for NIV

Strongly Supported (Level A Evidence)

Well-Supported

• Hypercapnic respiratory failure with respiratory acidosis and increased work of breathing
• Immunosuppressed patients with acute hypoxemic respiratory failure - avoids ET intubation complications (especially ventilator-associated pneumonia, which is often fatal in this group)
• Obesity hypoventilation syndrome (OHS) exacerbations
• Postoperative respiratory failure - both prophylactic (obese patients after abdominal/thoracic surgery) and therapeutic
• Neuromuscular disease with chronic ventilatory failure (home NIV)
• Chest wall deformity (e.g., kyphoscoliosis) with chronic hypercapnia
• DNI/DNR patients - NIV as a ceiling of care or for palliative symptom relief

Less Well-Established / Conditional

• Acute severe asthma - short trials reasonable with close monitoring and low threshold to intubate
• Acute hypoxemic, non-hypercapnic respiratory failure (pneumonia, ARDS) - benefit uncertain; NIV failure in moderate-severe ARDS is associated with high mortality
• Chest trauma / flail chest - possible benefit in reducing intubation rates
• Post-extubation in selected patients (prevention of re-intubation)

1. Exacerbation of COPD
2. Exacerbation of congestive heart failure and cardiogenic pulmonary edema
3. Exacerbation of asthma
4. Immunocompromised patients with hypoxemic respiratory failure
5. Hypoxemic respiratory failure (general)
6. Do-not-resuscitate / do-not-intubate advance directives

5. Contraindications and Limitations of NIV

Absolute Contraindications

• Inability to protect the airway (severe encephalopathy)
• High aspiration risk (vomiting, severe GI bleeding)
• Difficulty clearing secretions
• Facial trauma/surgery
• Upper airway obstruction
• Significant hemodynamic instability

Limitations of NIV

• Patient intolerance - claustrophobia, discomfort from tight mask, air leak - the most common cause of NIV failure
• Inability to cooperate - agitation, delirium, poor GCS
• Secretion management - NIV does not assist with airway suctioning; patients must clear secretions themselves (BiPAP is especially difficult in patients with copious secretions, as expectoration is nearly impossible)
• Risk of aspiration - no protective endotracheal cuff; upper airway defenses, though partially intact, are imperfect
• Neuromuscular blockade is a contraindication, as NIV (both CPAP and BiPAP) requires an open glottis

• Delayed intubation risk - delayed decision to intubate in deteriorating patients is associated with worse outcomes than early intubation; NIV failure in ARDS patients who subsequently required intubation had markedly higher mortality
• pH < 7.25 / severe acidosis - NIV is less effective; most authorities recommend proceeding to invasive MV
• NIV failure - determinants include pH <7.30 on admission, marked mental status alteration, high comorbidity burden, and high severity scores
• Monitoring intensity required - NIV requires close nursing supervision; ABG should be checked within 1-2 hours of initiation
• Interface problems - air leaks reduce efficacy; pressure sores develop over nasal bridge with prolonged use
• Gastric distension - swallowed air can cause abdominal distension
• Murray & Nadel's Respiratory Medicine, p. 3197
• ROSEN's Emergency Medicine, p. 2606

6. Applications of NIV

6.1 Acute Exacerbation of COPD (AECOPD)

• Bilevel IPAP offloads respiratory muscles and overcomes iPEEP (auto-PEEP)
• Reduces respiratory rate and allows more effective lung emptying
• Increases tidal volume and improves alveolar ventilation
• Reduces work of breathing

• Reduces mortality, intubation rates, and hospital/ICU length of stay
• ERS/ATS guidelines strongly recommend BPAP for ARF from COPD with pH ≤7.35
• NIV must be started early - late initiation after medical treatment failure eliminates the benefit
• If pH <7.20, invasive MV is generally recommended

6.2 Acute Cardiogenic Pulmonary Edema (ACPE)

• Prevents alveolar collapse at end-expiration
• Forces fluid from alveolar into interstitial space by increasing hydrostatic pressure
• Improves oxygenation and gas exchange
• Reduces LV preload and afterload - particularly valuable in the failing heart
• Decreases work of breathing rapidly

• Meta-analyses demonstrate NIV (usually CPAP) significantly reduces intubation rate and improves mortality in ACPE
• No clear outcome difference between CPAP and bilevel for CPE
• An early concern about increased AMI rates with bilevel NIV has been refuted by subsequent trials

6.3 Hypoxemic Non-hypercapnic Respiratory Failure

• Evidence is less robust than for COPD or CPE
• Most benefit in patients with pulmonary edema (cardiogenic or non-cardiogenic) or pneumonia
• For moderate-severe ARDS (PaO2/FiO2 < 200), NIV failure rate is high; invasive MV is generally preferred
• NIV failure in ARDS patients who then require intubation is associated with markedly worse mortality

6.4 Acute Severe Asthma

• Bilevel NIV increasingly used but evidence remains limited
• Data from large retrospective series show NIV used in >40% of ventilated asthma patients, with failure rate (requiring intubation) of 4.7%
• In-hospital mortality: NIV 2.3% vs invasive MV 14.5%
• A short trial of BPAP is reasonable, particularly in those not responding to standard medical therapy
• Recommendation: maintain a low threshold to intubate if no improvement

6.5 Immunosuppressed Patients

• Invasive MV carries extremely high risk of ventilator-associated pneumonia (VAP), which is often fatal
• NIV keeps upper airway defenses intact, minimizes VAP risk
• Associated with lower ET intubation rate, shorter ICU stay, lower ICU mortality

6.6 Obesity Hypoventilation Syndrome (OHS)

• Bilevel NIV (with backup rate) or CPAP
• Acute exacerbations can be managed with NIV with similar efficacy as COPD exacerbations
• Nocturnal NIV forms the cornerstone of chronic management

6.7 Neuromuscular Disease and Chest Wall Deformity

• Volume-targeted modes (VAPS) particularly useful for ensuring adequate ventilation
• Home NIV in conditions like ALS, post-polio syndrome, severe scoliosis
• Reduces hospitalizations and improves quality of life and survival

6.8 Postoperative Respiratory Applications

• Large reductions in FRC
• Transient diaphragmatic dysfunction
• Atelectasis, hypoxemia, hypercapnia

• Early CPAP for ≥6 hours after thoracic/major abdominal surgery significantly reduces pulmonary complications and reintubation
• Particularly effective in obese postoperative patients - improves oxygenation and prevents atelectasis
• Helmet CPAP is widely used in the immediate post-anesthesia care period

• In one large multicenter RCT (293 patients with ARF after abdominal surgery), NIV vs standard oxygen therapy: intubation rate reduced from 46% to 33%; health care-associated infections reduced from 49% to 31%
• Airway pressures should be kept at the lowest effective level given the risk of disrupting surgical anastomoses
• Murray & Nadel's Respiratory Medicine, p. 3198-3199

6.9 Weaning and Extubation

• Extubation to NIV in selected patients can accelerate the weaning process
• A meta-analysis of 16 trials (n=994, predominantly COPD) found extubation to NIV vs continued invasive weaning significantly decreased mortality
• Particularly useful in patients with advanced age, underlying cardiac/respiratory disease, prolonged MV duration, or hypercapnia during weaning trials
• Does not benefit unselected post-extubation patients; should be used preventively in selected populations rather than as rescue for established post-extubation failure

6.10 DNI/DNR Patients

• NIV can provide respiratory support as a ceiling of care while underlying cause is treated
• In palliative setting, NIV may reduce dyspnea (use is controversial; no clear evidence)
• A time-limited trial may allow patient to survive until family arrival ("time bridge")

7. Advantages of NIV Over Conventional Mechanical Ventilation (CMV)

• Roberts and Hedges' Clinical Procedures in Emergency Medicine, p. 231

8. Predictors of NIV Success and Failure

Predictors of SUCCESS

• pH 7.25-7.35 (moderate acidosis that is responsive)
• PaCO2 < 92 mmHg
• Younger age
• Better neurological status / Ability to cooperate
• Lower APACHE score
• COPD or cardiogenic pulmonary edema as the underlying diagnosis
• Improvement in pH, RR, and PaCO2 within 1-2 hours of initiation

Predictors of FAILURE (criteria for immediate intubation)

• pH < 7.25 or worsening pH despite NIV
• Persistent tachypnea and acidemia at 30-60 minutes
• GCS impairment not attributable to CO2 narcosis
• Inability to tolerate mask
• Hemodynamic instability
• Respiratory or cardiac arrest
• Persistent inability to clear secretions

9. Practical Settings and Initiation Protocol

• Low-High approach: IPAP 10, EPAP 5, FiO2 100% - then titrate IPAP upward
• High-Low approach: IPAP 20-25, EPAP 5, FiO2 100% - then titrate downward
• Use EPAP 8 if morbid obesity or iPEEP
• Titrate FiO2 to maintain SpO2 88-92% (86-92% in COPD)
• Increase pressure support (IPAP-EPAP) if hypercapnic; increase EPAP if hypoxic

• Clinical reassessment at 30 minutes
• ABG at 1-2 hours after initiation
• Failure to improve: escalate to invasive MV

10. Summary Table

• Roberts and Hedges' Clinical Procedures in Emergency Medicine, Chapter 8
• ROSEN's Emergency Medicine, Chapter 60
• Fishman's Pulmonary Diseases and Disorders, Chapter 148
• Murray & Nadel's Textbook of Respiratory Medicine, Chapter on NIV
• Fischer's Mastery of Surgery, 8th ed.
• Harrison's Principles of Internal Medicine, 22nd ed., Chapter 313