Regional Behavior of Airspaces During Positive Pressure Reduction Assessed by Synchrotron Radiation Computed Tomography

INTRODUCTION

The mechanisms of lung inflation and deflation are only partially known. Ventilatory strategies to support lung function rely upon the idea that lung alveoli are isotropic balloons that progressively inflate or deflate and that lung pressure/volume curves derive only by the interplay of critical opening pressures, critical closing pressures, lung history, and position of alveoli inside the lung. This notion has been recently challenged by subpleural microscopy, magnetic resonance, and computed tomography (CT). Phase-contrast synchrotron radiation CT (PC-SRCT) can yield in vivo images at resolutions higher than conventional CT.

OBJECTIVES

We aimed to assess the numerosity (ASden) and the extension of the surface of airspaces (ASext) in healthy conditions at different volumes, during stepwise lung deflation, in concentric regions of the lung.

METHODS

The study was conducted in seven anesthetized New Zealand rabbits. They underwent PC-SRCT scans (resolution of 47.7 μm) of the lung at five decreasing positive end expiratory pressure (PEEP) levels of 12, 9, 6, 3, and 0 cmH2O during end-expiratory holds. Three concentric regions of interest (ROIs) of the lung were studied: subpleural, mantellar, and core. The images were enhanced by phase contrast algorithms. ASden and ASext were computed by using the Image Processing Toolbox for MatLab. Statistical tests were used to assess any significant difference determined by PEEP or ROI on ASden and ASext.

RESULTS

When reducing PEEP, in each ROI the ASden significantly decreased. Conversely, ASext variation was not significant except for the core ROI. In the latter, the angular coefficient of the regression line was significantly low.

CONCLUSION

The main mechanism behind the decrease in lung volume at PEEP reduction is derecruitment. In our study involving lung regions laying on isogravitational planes and thus equally influenced by gravitational forces, airspace numerosity and extension of surface depend on the local mechanical properties of the lung.

Optimal PEEP during one-lung ventilation with capnothorax: An experimental study

BACKGROUND

One-lung ventilation (OLV) with induced capnothorax carries the risk of severely impaired ventilation and circulation. Optimal PEEP may mitigate the physiological perturbations during these conditions.

METHODS

Right-sided OLV with capnothorax (16 cm H₂ O) on the left side was initiated in eight anesthetized, muscle-relaxed piglets. A recruitment maneuver and a decremental PEEP titration from PEEP 20 cm H₂ O to zero end-expiratory pressure (ZEEP) was performed. Regional ventilation and perfusion were studied with electrical impedance tomography and computer tomography of the chest was used. End-expiratory lung volume and hemodynamics were recorded and.

RESULTS

PaO₂ peaked at PEEP 12 cm H₂ O (49 ± 14 kPa) and decreased to 11 ± 5 kPa at ZEEP (P < 0.001). PaCO₂ was 9.5 ± 1.3 kPa at 20 cm H₂ O PEEP and did not change when PEEP step-wise was reduced to 12 cm H₂ O PaCO_2. At lower PEEP, PaCO₂ increased markedly. The ventilatory driving pressure was lowest at PEEP 14 cm H₂ O (19.6 ± 5.8 cm H₂ O) and increased to 38.3 ± 6.1 cm H₂ O at ZEEP (P < 0.001). When reducing PEEP below 12-14 cm H₂ O ventilation shifted from the dependent to the nondependent regions of the ventilated lung (P = 0.003), and perfusion shifted from the ventilated to the nonventilated lung (P = 0.02).

CONCLUSION

Optimal PEEP was 12-18 cm H₂ O and probably relates to capnothorax insufflation pressure. With suboptimal PEEP, ventilation/perfusion mismatch in the ventilated lung and redistribution of blood flow to the nonventilated lung occurred.

Capnography reflects ventilation/perfusion distribution in a model of acute lung injury

BACKGROUND

Changes in the shape of the capnogram may reflect changes in lung physiology. We studied the effect of different ventilation/perfusion ratios (V/Q) induced by positive end-expiratory pressures (PEEP) and lung recruitment on phase III slope (S(III)) of volumetric capnograms.

METHODS

Seven lung-lavaged pigs received volume control ventilation at tidal volumes of 6 ml/kg. After a lung recruitment maneuver, open-lung PEEP (OL-PEEP) was defined at 2 cmH(2)O above the PEEP at the onset of lung collapse as identified by the maximum respiratory compliance during a decremental PEEP trial. Thereafter, six distinct PEEP levels either at OL-PEEP, 4 cmH(2)O above or below this level were applied in a random order, either with or without a prior lung recruitment maneuver. Ventilation-perfusion distribution (using multiple inert gas elimination technique), hemodynamics, blood gases and volumetric capnography data were recorded at the end of each condition (minute 40).

RESULTS

S (III) showed the lowest value whenever lung recruitment and OL-PEEP were jointly applied and was associated with the lowest dispersion of ventilation and perfusion (Disp(R-E)), the lowest ratio of alveolar dead space to alveolar tidal volume (VD(alv)/VT(alv)) and the lowest difference between arterial and end-tidal pCO(2) (Pa-ETCO(2)). Spearman's rank correlations between S(III) and Disp(R-E) showed a ρ=0.85 with 95% CI for ρ (Fisher's Z-transformation) of 0.74-0.91, P<0.0001.

CONCLUSION

In this experimental model of lung injury, changes in the phase III slope of the capnograms were directly correlated with the degree of ventilation/perfusion dispersion.

Atelectasis observed by computerized tomography after Caesarean section

BACKGROUND

Atelectasis after either vaginal or Caesarean delivery has not been adequately quantified. This study addresses the hypothesis that atelectasis may be worse in women who undergo Caesarean section when compared with vaginal delivery under regional anaesthesia.

METHODS

Twenty healthy non-smoking women submitted to a chest computed tomography (CT) 2 h after delivery in a University Hospital, who had experienced vaginal delivery (n=10) under combined spinal-epidural analgesia or a Caesarean section (n=10) under spinal anaesthesia, were evaluated. The percentage cross-sectional area of atelectasis in dependent lung regions were measured from the CT images obtained at cross-section of the xiphoid process and the top of the diaphragm.

RESULTS

The percentage cross-sectional area of atelectasis was 3.95% in the vaginal delivery group and 14.1% in the Caesarean group (P<0.001, Mann-Whitney rank sum test).

CONCLUSIONS

These results suggested that pulmonary atelectasis is greater after Caesarean section delivery under spinal anaesthesia than after vaginal delivery with combined spinal-epidural analgesia.

[Nursing, a quality service? The patient answers]

Being quality an updated concept in several productive sectors, we have tried to study this concept in nursing service environment at the patient's point of view. For reaching this goal, a questionnaire has been made upon which 45 Hospital de Clínicas de Porto Alegre interned patients have produced their opinions. It has been shown that generally speaking, the nursing service has been satisfactory although there is some critic from some clients and improvement could be worked out.