Initiation of high-frequency oscillatory ventilation and its effects upon cerebral circulation in pigs: an experimental study.

BACKGROUND: Current practice at high-frequency oscillatory ventilation (HFOV) initiation is a stepwise increase of the constant applied airway pressure to achieve lung recruitment. We hypothesized that HFOV would lead to more adverse cerebral haemodynamics than does pressure controlled ventilation (PCV) in the presence of experimental intracranial hypertension (IH) and acute lung injury (ALI) in pigs with similar mean airway pressure settings. METHODS: In 12 anesthetized pigs (24-27 kg) with IH and ALI, mean airway pressure (P(mean)) was increased (to 20, 25, 30 cm H(2)O every 30 min), either with HFOV or with PCV. The order of the two ventilatory modes (cross-over) was randomized. Mean arterial pressure (MAP), intracranial pressure (ICP), cerebral perfusion pressure (CPP), cerebral blood flow (CBF) (fluorescent microspheres), cerebral metabolism, transpulmonary pressures (P(T)), and blood gases were determined at each P(mean) setting. Our end-points of interest related to the cerebral circulation were ICP, CPP and CBF. RESULTS: CBF and cerebral metabolism were unaffected but there were no differences between the values for HFOV and PCV. ICP increased slightly (HFOV median +1 mm Hg, P<0.05; PCV median +2 mm Hg, P<0.05). At P(mean) setting of 30 cm H(2)O, CPP decreased during HFOV (median -13 mm Hg, P<0.05) and PCV (median -17 mm Hg, P<0.05) paralleled by a decrease of MAP (HFOV median -11 mm Hg, P<0.05; PCV median -13 mm Hg, P<0.05). P(T) increased (HFOV median +8 cm H(2)O, P<0.05; PCV median +8 cm H(2)O, P<0.05). Oxygenation improved and normocapnia maintained by HFOV and PCV. There were no differences between both ventilatory modes. CONCLUSIONS: In animals with elevated ICP and ALI, both ventilatory modes had effects upon cerebral haemodynamics. The effects upon cerebral haemodynamics were dependent of the P(T) level without differences between both ventilatory modes at similar P(mean) settings. HFOV seems to be a possible alternative ventilatory strategy when MAP deterioration can be avoided.

[Quantification of atelectases in artificial respiration: spiral-CT versus dynamic single-slice CT]. / Quantifizierung von Atelektasen bei kontrollierter Beatmung: Spiral-CT versus dynamische Einzelschicht-CT.

PURPOSE: Dynamic CT (dCT) allows visualization and quantification of ventilated lung and atelectases with high temporal resolution during continuous ventilation. This study compares a quantitative image analysis in a subcarinal single slice dCT series versus a whole lung spiral-CT, in order to analyze, whether the distribution of atelectasis of a single dCT series is representative for the whole lung. MATERIALS AND METHODS: dCT in sliding windows technique (slice thickness 1 mm, temporal increment 100 ms) was performed in 8 healthy pigs 3 cm caudal to the carina during continuous mechanical ventilation. Subsequently, a spiral-CT of the whole lung (slice thickness 2 mm; pitch 1.5; increment 2 mm) was acquired during inspiratory breath hold (airway pressure 20 mbar). Lung segmentation and planimetry of predefined density ranges were achieved using a dedicated software tool in both data-sets. Thus, the fractions of the following functional lung compartments were averaged over time: hyperinflated lung (- 1024 to - 910 HE), normal ventilated lung -900 to -300 HE) and atelectasis (-300 to +200 HE). RESULTS: Quantitative analysis of dCT-series during continuous respiration correlated with the density analysis in spiral-CT as follows: hyperinflated lung r = 0.56; normal ventilated lung r = 0.83 and atelectases r = 0.84. Analysis of spiral-CT showed the following distribution of functional lung compartments: hyperinflated lung 3.1% normal ventilated lung 77.9% and atelectasis 19.0%. In dCT, hyperinflated lung represented 6.4%, normal ventilated lung 65.2% and atelectasis 28.4% of total the lung area. CONCLUSION: The results of our study demonstrate that dCT allows monitoring of atelectasis formation in response to different ventilatory strategies. However, a deviation between dCT and spiral-CT has to be taken into account. In subcarinal dCT series, hyperinflated lung areas and atelectases were overestimated due to a craniocaudal gradient of atelectases, whereas normal ventilated lung was underestimated.