Protective Lung Ventilation

OVERVIEW

Protective lung ventilation is the current standard of care for mechanical ventilation. It is synonymous with low tidal volume ventilation (4-8 mL/kg) and often includes permissive hypercapnia

• 6 mL/kg predicted body weight (not actual body weight) is most commonly quoted as this was the intervention arm of the practice defining ARDSNet ARMA trial and is physiologically normal for a healthy person
• Note: PBW in men is calculated as 50 (45 in women) + 0.91 (Height cm −152.4)

RATIONALE

• low tidal volume ventilation reduces ventilator-associated lung injury (VALI)
  • volutrauma (hyperinflation and shearing injury)
  • barotrauma (alveolar rupture and pneumothorax)
  • biotrauma (release of inflammatory mediators)
• hypercapnia may also have directly beneficial effects in ARDS
• clear evidence for benefit in ARDS in animals and humans
• growing evidence of benefit in patients without ARDS, as all mechanically ventilated patients are at risk of VALI

BEFORE THE ARDSNet ARMA TRIAL

Bendixen in NEJM 1963

• showed that higher tidal volumes (VT) during anesthesia (18 patients undergoing laparotomy) resulted in less atelectasis, less acidosis, and improved oxygenation compared to lower VT
• for the next 50 years (!) anesthesiology textbooks continue to maintain that VT should be between 10–15 mL/kg for these benefits

ARDS described by Asbaugh 1967

• early studies in ARDS patients described ventilation with a variety of VTs ranging from 10 to 24 mL/kg
• potential harms of high tidal volumes increasingly recognised in the 1970s and 1980s
• 1992 — Hickling introduces the concept of permissive hypercapnia
• 1996 — a large survey of intensivists’ ventilation practices for patients with ARDS showed most respondents reported using VTs >10 ml/ kg
• 1998 — Amato et al publish as a small RCT suggests mortality benefit from low tidal volume ventilation

EVIDENCE IN ARDS

• Following the Amato 1998 study, three other controlled ‘protective’ ventilation trials failed to find a benefit, purported to be due to study design issues
• This led to the ARDSnet ARMA trial, the practice-defining study that established protective lung ventilation being the standard of care

ARDSNet. Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute respiratory distress syndrome. The Acute Respiratory Distress Syndrome Network. N Engl J Med. 2000 May 4;342(18):1301-8. PubMed PMID: 10793162. [Free Full Text] (aka the ARMA trial)

• MC RCT, 75 ICUs in 10 cities
• n = 861 adult patients with ALI/ARDS: bilateral infiltrates, P/F ratio < 300 in the first 36 hrs
• comparison: traditional TV 12mL/kg (plateau pressures of < 50cmH2O) versus protective TV 6mL/kg (plateau pressure of < 30cmH2O)
• inclusion criteria:
  • intubated and ventilated, ALI or ARDS
• exclusion criteria:
  • > 36 hours of ventilation, < 18 years, pregnancy, increased ICP, severe neuromuscular disease, sickle cell, severe chronic lung disease, obesity, large burns, severe chronic liver disease, vasculitis with diffuse alveolar haemorrhage, history of bone marrow or lung transplantation
• outcomes in favour of low TV versus high TV ventilation:
  • in hospital mortality (31.0 percent vs. 39.8 percent, P=0.007)
  • significant reduction in duration of MV (mean [±SD], 12±11 vs. 10±11; P=0.007)
• Commentary and criticisms:
  • good baseline balance (except higher minute ventilation in low TV group)
  • good separation between treatment arms
  • low TV group received high PEEP to maintain oxygenation (a co-intervention) — this led to the ALVEOLI study which found no mortality benefit for high PEEP levels
  • terminated early after the fourth interim analysis
  • perhaps more of a study of plateau pressures that tidal volumes per se, as Pplat max was strictly adhered to
  • used a well defined PEEP protocol
  • critics have questioned the ethics of this studied, arguing that standard practice at the time was not TV 12 mL/kg for all comers and that this led to practice misalignment

ARDSNet VENTILATION STRATEGY

• This is described in detail in ARDSnet Ventilation Strategy
• note that the tidal volume could be adjusted so long as the plateau pressure was <30 cmH20
  • avoiding excessive plateau pressures may be as important as low tidal volumes

EVIDENCE IN NON-ARDS PATIENTS

Serpa Neto et al (2012) performed a meta-analysis of 20 studies (n=2822) that showed patients receiving lower tidal volumes had:

• decreased lung injury development (risk ratio [RR], 0.33; 95% CI, 0.23 to 0.47; I2, 0%; NNT = 11)
• decreased mortality (RR, 0.64; 95% CI, 0.46 to 0.89; I2, 0%; NNT=23)

The IMPROVE study, NEJM 2013

• MC RCT
• n=400 abdominal surgery patients
• non-protective versus protective lung ventilation
• RR =0.4 for a composite of major pulmonary and extrapulmonary complications occurring within the first 7 days after surgery for protective lung ventilation

A number of studies support the protective lung ventilation approach in settings such as:

• multiple transfusions
• trauma
• sepsis
• high-risk surgery (including cardiac surgery)

CONCLUSION

• Protective lung ventilation should be used for mechanically ventilated patients with ARDS and those with risk factors for developing ARDS
• IT is also a reasonable default ventilation strategy in all mechanically ventilated patients
• set a tidal volume of 6mL/kg based on predicted body weight (PBW) and target plateau pressures <30 cmH20
• It can be combined with an open lung approach in moderate-severe ARDS (i.e. higher PEEP and recruitment manoeuvres)
• the ventilation strategy should be modified in patients with obstructive lung disease to prevent dynamic hyperinflation (COPD and asthma)

References and Links

LITFL

• CCC — Open Lung Approach to Ventilation
• CCC — Ventilator Associated Lung Injury (VALI)
• CCC — Volutrauma

FOAM and web resources

• EMCrit Lecture – Dominating the Vent: Part I

Journal articles

• ARDSNet. Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute respiratory distress syndrome. The Acute Respiratory Distress Syndrome Network. N Engl J Med. 2000 May 4;342(18):1301-8. PMID: 10793162.
• Futier E, Constantin JM, Paugam-Burtz C, Pascal J, Eurin M, Neuschwander A, Marret E, Beaussier M, Gutton C, Lefrant JY, Allaouchiche B, Verzilli D, Leone M, De Jong A, Bazin JE, Pereira B, Jaber S; IMPROVE Study Group. A trial of intraoperative low-tidal-volume ventilation in abdominal surgery. N Engl J Med. 2013 Aug 1;369(5):428-37. PMID: 23902482.
• Lipes J, Bojmehrani A, Lellouche F. Low Tidal Volume Ventilation in Patients without Acute Respiratory Distress Syndrome: A Paradigm Shift in Mechanical Ventilation. Crit Care Res Pract. 2012;2012:416862. PMC3318889.
• Moloney ED, Griffiths MJ. Protective ventilation of patients with acute respiratory distress syndrome. Br J Anaesth. 2004 Feb;92(2):261-70. PMID: 14722180. [Free Full Text]
• Petrucci N, De Feo C. Lung protective ventilation strategy for the acute respiratory distress syndrome. Cochrane Database Syst Rev. 2013 Feb 28;2:CD003844. PMID: 23450544.
• Schultz MJ, Haitsma JJ, Slutsky AS, Gajic O. What tidal volumes should be used in patients without acute lung injury? Anesthesiology. 2007 Jun;106(6):1226-31. PMID: 17525599. [Free Full Text]
• Serpa Neto A, Cardoso SO, Manetta JA, Pereira VG, Espósito DC, Pasqualucci Mde O, Damasceno MC, Schultz MJ. Association between use of lung-protective ventilation with lower tidal volumes and clinical outcomes among patients without acute respiratory distress syndrome: a meta-analysis. JAMA. 2012 Oct 24;308(16):1651-9. PMID: 23093163.