Mechanical power thresholds during mechanical ventilation: An experimental study.

The extent of ventilator-induced lung injury may be related to the intensity of mechanical ventilation--expressed as mechanical power. In the present study, we investigated whether there is a safe threshold, below which lung damage is absent. Three groups of six healthy pigs (29.5 ± 2.5 kg) were ventilated prone for 48 h at mechanical power of 3, 7, or 12 J/min. Strain never exceeded 1.0. PEEP was set at 4 cmH2 O. Lung volumes were measured every 12 h; respiratory, hemodynamics, and gas exchange variables every 6. End-experiment histological findings were compared with a control group of eight pigs which did not undergo mechanical ventilation. Functional residual capacity decreased by 10.4% ± 10.6% and 8.1% ± 12.1% in the 7 J and 12 J groups (p = 0.017, p < 0.001) but not in the 3 J group (+1.7% ± 17.7%, p = 0.941). In 3 J group, lung elastance, PaO2 and PaCO2 were worse compared to 7 J and 12 J groups (all p < 0.001), due to lower ventilation-perfusion ratio (0.54 ± 0.13, 1.00 ± 0.25, 1.78 ± 0.36 respectively, p < 0.001). The lung weight was lower (p < 0.001) in the controls (6.56 ± 0.90 g/kg) compared to 3, 7, and 12 J groups (12.9 ± 3.0, 16.5 ± 2.9, and 15.0 ± 4.1 g/kg, respectively). The wet-to-dry ratio was 5.38 ± 0.26 in controls, 5.73 ± 0.52 in 3 J, 5.99 ± 0.38 in 7 J, and 6.13 ± 0.59 in 12 J group (p = 0.03). Vascular congestion was more extensive in the 7 J and 12 J compared to 3 J and control groups. Mechanical ventilation (with anesthesia/paralysis) increase lung weight, and worsen lung histology, regardless of the mechanical power. Ventilating at 3 J/min led to better anatomical variables than at 7 and 12 J/min but worsened the physiological values.

Mobilizing Carbon Dioxide Stores. An Experimental Study.

Rationale: Understanding the physiology of CO2 stores mobilization is a prerequisite for intermittent extracorporeal CO2 removal (ECCO2R) in patients with chronic hypercapnia.Objectives: To describe the dynamics of CO2 stores.Methods: Fifteen pigs (61.7 ± 4.3 kg) were randomized to 48 hours of hyperventilation (group "Hyper," n = 4); 48 hours of hypoventilation (group "Hypo," n = 4); 24 hours of hypoventilation plus 24 hours of normoventilation (group "Hypo-Baseline," n = 4); or 24 hours of hypoventilation plus 24 hours of hypoventilation plus ECCO2R (group "Hypo-ECCO2R," n = 3). Forty-eight hours after randomization, the current [Formula: see text]e was reduced by 50% in every pig.Measurements and Main Results: We evaluated [Formula: see text]co2, [Formula: see text]o2, and metabolic [Formula: see text]co2 ([Formula: see text]o2 times the metabolic respiratory quotient). Changes in the CO2 stores were calculated as [Formula: see text]co2 - metabolic V̇co2. After 48 hours, the CO2 stores decreased by 0.77 ± 0.17 l kg-1 in group Hyper and increased by 0.32 ± 0.27 l kg-1 in group Hypo (P = 0.030). In group Hypo-Baseline, they increased by 0.08 ± 0.19 l kg-1, whereas in group Hypo-ECCO2R, they decreased by 0.32 ± 0.24 l kg-1 (P = 0.197). In the second 24-hour period, in groups Hypo-Baseline and Hypo-ECCO2R, the CO2 stores decreased by 0.15 ± 0.09 l kg-1 and 0.51 ± 0.06 l kg-1, respectively (P = 0.002). At the end of the experiment, the 50% reduction of [Formula: see text]e caused a PaCO2 rise of 9.3 ± 1.1, 32.0 ± 5.0, 16.9 ± 1.2, and 11.7 ± 2.0 mm Hg h-1 in groups Hyper, Hypo, Hypo-Baseline, and Hypo-ECCO2R, respectively (P < 0.001). The PaCO2 rise was inversely related to the previous CO2 stores mobilization (P < 0.001).Conclusions: CO2 from body stores can be mobilized over 48 hours without reaching a steady state. This provides a physiological rationale for intermittent ECCO2R in patients with chronic hypercapnia.