Low Tidal Volumes
Parameter Lung-Protective Strategy Traditional Strategy Inflation Volume 5 – 10 mL/kg 10 – 15 mL/kg Mechanical Sighs none 15 – 30 mL/kg at 6 – 12/hr End-Inspiratory Peak Pressure Plateau < 35 cm H2O Peak < 50 cm H2O PEEP 5 – 15 cm H2O Only when needed to keep FiO2 < 0.60 ABG Hypercapnea allowed, pH 7.20 – 7.44 pH 7.36 – 7.44
Because inflation is preferentially distributed to working tissue, PPV tends to over-distend normal lung, which is exaggerated when larger volumes are used. This can lead to alveolar rupture (interstitial emphysema, pneumomediastinum, pneumothorax), and leaky capillaries (pulmonary edema).
The lung-protective strategy was shown to reduce mortality in ARDS patients by 9% [NEJM 342: 1301, 2000], but has since been advocated for all patients in respiratory failure, not just those in ARDS [Crit Care Med 32: 1817, 2004]
Low Tidal Volumes in the Operating Room
Most of the data on low tidal volumes is based on critically ill patients in the ICU. However, a study from Mayo Clinic suggested that intraoperative tidal volumes are related (positive correlation) to the risk of post-operative ALI [Gajic O, et al. Crit Care Med 32: 1817, 2004]. This hypothesis is further supported by Lellouche et al.’s retrospective analysis of tidal volumes on presentation to the ICU following cardiac surgery, which showed that tidal volumes > 10 cc/kg were a risk factor for prolonged ICU stay and organ failure [Lellouche et al. Anesthesiology 116: 1072, 2012]
Daily Spontaneous Breathing Trials
Hyperventilation in Head Trauma
Hyperventilation in Head Trauma Patients:
- Compensatory mechanisms (changes in lactate, bicarbonate) normalize CBF within 4-6 hrs of initiation of hyperventilation [Andrews’ text].
- Highly controversial [Chest 127: 1812, 2005], with the 2007 Cochrane Database Review concluding that there is inadequate data to assess whether benefit or harm exists [Roberts, I; Schierhout, G: Cochrane Review 2007]. The Brain Trauma Foundation recommends against chronic hyperventilation [J Neurotrauma 17: 537, 2000]
- Dacey studied 13 trauma patients and found that if CPP is kept @ 70 mm Hg, hyperventilation to 25 mm Hg (as opposed to 30 mm Hg) for up to 5 days does not change CMRO2 or CBF [J Neurosurg 96: 103, 2002]. Another study of 8 patients showed that pCO2 could be safely dropped by 10 mm Mg for 30 minutes (20% of the time CBF dropped significantly, however the lactate/pyruvate ratio and glutamate levels did not change [Acta Neurochir Suppl 75: 45, 1999]). Other studies suggest that hyperventilation is in fact detrimental – one showed that while CPP improves, CBF dropped [Crit Care Med 30: 1950, 2002] . Another of 34 patients suggested that pCO2 27-32 mm Hg can result in harmful local reductions of cerebral perfusion that cannot be detected by assessing SjvO2 [J Neurosurg 96: 97, 2002]. A study of 30 TBI patients showed that hyperventilation from 36 to 29 mm Hg reduced cerebral blood flow (p<.001) and increased oxygen extraction ratio and ischemic brain volume (17+/-22 vs. 88+/-66 mL; p<.0001) [Crit Care Med 35: 568, 2007]
PEEP in the Neurosurgical Patient
PEEP in the neurosurgical patient:
Four studies from the 1970’s-80’s suggested that PEEP has a minimal, clinically insignificant effect on ICP [Int Care Med 31: 323, 2005]. A study of twenty comatose TBI patients showed that increasing PEEP from 0-5 to 5-10 or 10-15 mm Hg trended to improve ICP and CPP [J Trauma 53: 488, 2002]. Another study showed that when PEEP caused alveolar recruitment, ICP was unaffected, but when PEEP lead to hyperventilation, ICP increased [Int Care Med 31: 373, 1005]. A study of SAH patients showed that PEEP of 12 mm Hg did not affect MAP in ALI/ARDS patients but did lower MAP in patients with no lung pathology [J Trauma 58: 571, 2005]. Summary: Use of PEEP 1) only if needed 2) does not necessarily affect ICP 3) evaluate its effect on compliance and hyperventilation to gauge safety 4) 5 mm Hg may be safe for all.