Studies conducted in adult ICUs were eligible for inclusion. Studies had to report at least one of the outcomes: mortality, length of stay, occupancy rates or ICU organisation. Studies of specific procedures, in very specific patient populations, those carried out in a single ICU or those that compared low- and high-volume days were excluded.
Four studies included all ICU patients, five included only mechanically ventilated patients, two included only patients with severe sepsis, one only patients with respiratory, gastrointestinal or neurologic diagnoses and one only post-cardiac arrest patients. Study duration was typically two to three years (range four months to 11 years). Numbers of ICUs in each study varied from 14 to 294. The definition of patient volume varied across studies. Studies commonly used the simplified acute physiology score (SAPS) or the acute physiology and chronic health evaluation (APACHE II, III or IV) to adjust for severity of illness. Two studies were from the UK.
Two reviewers independently performed the selection, with disagreements resolved by consensus.