Validation of a new live cell strain system: characterization of plasma membrane stress failure.

Motivated by our interest in lung deformation injury, we report on the validation of a new live cell strain system. We showed that the system maintains a cell culture environment equivalent to that provided by conventional incubators and that its strain ouput was uniform and reproducible. With this system, we defined cell deformation dose (i.e., membrane strain amplitude)-cell injury response relationships in alveolar epithelial cultures and studied the effects of temperature on them. Deformation injury occurred in the form of reversible, nonlethal plasma membrane stress failure events and was quantified as the fraction of cells with uptake and retention of fluorescein-labeled dextran (FITC-Dx). The undeformed control population showed virtually no FITC-Dx uptake at any temperature, which was also true for cells strained by 3%. However, when the membrane strain was increased to 18%, ~5% of cells experienced deformation injury at a temperature of 37 degrees C. Moreover, at that strain, a reduction in temperature to 4 degrees C resulted in a threefold increase in the number of cells with plasma membrane breaks (from 4.8 to 15.9%; P < 0.05). Cooling of cells to 4 degrees C also lowered the strain threshold at which deformation injury was first seen. That is, at a 9% substratum strain, cooling to 4 degrees C resulted in a 10-fold increase in the number of cells with FITC-Dx staining (0.7 vs. 7.5%, P < 0.05). At that temperature, A549 cells offered a 50% higher resistance to shape change (magnetic twisting cytometry measurements) than at 37 degrees C. We conclude that the strain-injury threshold of A549 cells is reduced at low temperatures, and we consider temperature effects on plasma-membrane fluidity, cytoskeletal stiffness, and lipid trafficking as responsible mechanisms.

Patient-ventilator interactions.

The focus of this chapter has been the information contained in airway pressure, volume, and flow traces from mechanically-ventilated patients. It has not been our intention to make ventilator management recommendations. Practitioners willing to make the effort to interpret patient-ventilator interactions will find it difficult to manage ventilator-dependent patients without feedback from waveform analysis in the future.

The determinants of respiratory rate during mechanical ventilation.

The independent and interactive effect of feedback related to volume, CO2, inspiratory flow, and arousal state on the regulation of respiratory rate in mechanically ventilated humans is not well characterized. We examined the rate response of eight normal volunteers during both quiet wakefulness and non-rapid-eye-movement (NREM) sleep, while mechanically ventilated through a nasal mask in an assist/control mode with a machine back-up rate of 2 breaths/min. Tidal volume (VT) was set slightly above spontaneous VT and then increased by 0.2 L every 3 min up to 1.8 L or 25 ml/kg. Either an inspiratory flow of 40 L/min or an inspiratory time of 2 s (iso-T(I)) was set, with CO2 added (F(I)CO2 > 0) or F(I)CO2 = 0. Measurements were made during both quiet wakefulness and NREM sleep. We found that as VT increased, the respiratory rate decreased; the rate decline was observed during wakefulness and sleep, and under isocapnic as well as hypocapnic conditions. Increasing inspiratory flow raised the respiratory rate during wakefulness and NREM sleep. During NREM sleep, hypocapnia resulted in wasted ventilator trigger efforts. In summary, both VT and inspiratory flow settings affect the respiratory rate, and depending on state, can affect CO2 homeostasis. Ventilator settings appropriate for wakefulness may cause ventilatory instability during sleep.

Efficacy of nocturnal nasal ventilation in stable, severe chronic obstructive pulmonary disease during a 3-month controlled trial.

OBJECTIVE: To evaluate the efficacy of nocturnal nasal ventilation (NNV) in patients with rigidly defined, severe but stable chronic obstructive pulmonary disease (COPD) and hypercapnia. DESIGN: By randomization, eligible patients were assigned to an active or a sham treatment arm. Data from these two groups were analyzed statistically. MATERIAL AND METHODS: Initially, 35 patients with severe COPD (forced expiratory volume in 1 second [FEV1] of less than 40% predicted) and daytime hypercapnia (arterial carbon dioxide tension [PaCO2] of more than 45 mm Hg) were enrolled in a 3-month NNV trial. After a minimal observation period of 6 weeks, 13 patients were judged to be clinically stable and were randomized to NNV (N = 7) or sham (N = 6) treatment, consisting of nightly use of a bilevel positive airway pressure (PAP) device set to deliver an inspiratory pressure of either 10 or 0 cm of water (H2O). The device was used in the spontaneous or timed mode and set to a minimal expiratory pressure of 2 cm H2O. Patients underwent extensive physiologic testing including polysomnography and were introduced to the bilevel PAP system during a 2.5-day hospital stay. RESULTS: The NNV and sham treatment groups were similar in mean age (71.0 versus 66.5 years), PaCO2 (54.7 versus 48.5 mm Hg), and FEV1 (0.62 versus 0.72 L). Only four of seven patients in the NNV group were still using the bilevel PAP device at the completion of the trial, as opposed to all six patients in the sham group. Only one patient had a substantial reduction in PaCO2 - from 50 mm Hg at baseline to 43 mm Hg after 3 months of NNV. He declined further NNV treatment with bilevel PAP. Sham treatment did not lower PaCO2. Lung function, nocturnal oxygen saturation, and sleep efficiency remained unchanged in both groups. CONCLUSION: Disabled but clinically stable patients with COPD and hypercapnia do not readily accept and are unlikely to benefit from NNV.

Tidal volume maintenance during weaning with pressure support.

Ventilation was measured in 31 difficult-to-wean patients while pressure support (PS) was reduced by 5 cm H2O every 20 min. Weaning had to be aborted in 14 of 31 patients (Group F) because they met predefined distress criteria. The remaining 17 patients who were able to complete the "weaning test" (Group S) had larger static respiratory compliances (Cstat = 0.08 +/- 0.02 versus 0.05 +/- 0.01 L/cm H2O, p < or = 0.05) and a lower dead space to tidal volume ratio (0.55 +/- 0.05 versus 0.64 +/- 0.06, p < or = 0.05). Group S patients had a larger tidal volume (VT) than did those of Group F at most PS settings. The groups differed with respect to VT maintenance during PS withdrawal (p < 0.01). In Group S, VT fell exponentially with machine support and stabilized at PS levels between 5 and 10 cm H2O. In contrast, Group F patients defended VT at higher PS settings but were unable to maintain VT during distress. Ventilatory response parameters such as the rapid shallow breathing index were of limited value in predicting weaning outcome and yielded receiver operator curve area values between 0.66 and 0.82 over the range of PS settings tested. We conclude that the gradual withdrawal of machine support does not facilitate the recognition of impending respiratory failure.