Adult respiratory distress syndrome profiles by computed tomography

The acute respiratory distress syndrome

ARDS is a complex response of the lung to direct (inhalational) and indirect (hematogenous) insults. It is easy to be pessimistic about the benefit of the approaches we have described, but there is evidence that overall survival has improved in recent years. To maintain this progress, new therapies for ARDS must be rigorously evaluated, and their routine use should be recommended only after careful scrutiny of the evidence. Such a course will eliminate the unnecessary risks and costs often associated with unproved therapies.

The effect of positive end-expiratory pressure on regional pulmonary perfusion during acute lung injury

PURPOSE

To determine whether positive end-expiratory pressure (PEEP) would change the altered regional pulmonary perfusion pattern caused by oleic acid (OA)-induced acute lung injury was the aim of this study. Our hypothesis was that fixed intravascular obstruction would not be affected by PEEP, leaving the perfusion pattern unchanged if this was the principal cause of perfusion redistribution after lung injury.

METHODS

Regional measurements of pulmonary perfusion and lung water concentration (LWC) were obtained both before and after lung injury and before and after PEEP (10 cm H2O) with the nuclear medicine imaging technique of positron emission tomography after injections of H2 15O in 7 supine anesthetized dogs. Four animals were also treated with meclofenamate to enhance perfusion redistribution after acute lung injury.

RESULTS

The application of PEEP before lung injury did not significantly affect the regional perfusion pattern. After administration of OA, LWC increased and fractional flow (without PEEP) decreased to the most edematous lung regions, consistent with perfusion redistribution. With the reapplication of PEEP, the perfusion pattern reverted toward the baseline pattern. The magnitude of this effect was roughly proportional to the effect of PEEP on regional inflation (r = .53), as evidenced by changes in LWC.

CONCLUSIONS

The reversibility with PEEP of the perfusion pattern caused by acute lung injury suggests that fixed intravascular obstruction is not the principal determinant of perfusion redistribution after OA-induced injury.

The value of portable chest roentgenography in adult respiratory distress syndrome. Comparison with computed tomography

In 17 patients with adult respiratory distress syndrome, we used data derived from computed tomographic (CT) scan densitometric analysis to validate the value of portable chest roentgenograms in objectively estimating the amount of pulmonary edema. Chest roentgenograms and CT scans were taken in the same ventilatory conditions (apnea at 10 cm H2O of positive end-expiratory pressure [PEEP]); blood gas samples and hemodynamic parameters were collected at the same time. Roentgenographic analysis was undertaken by independent observers using two standardized scoring systems proposed in the literature. CT scan analysis was performed using the CT number frequency distribution and the gas lung volume (measured by helium dilution technique) to estimate quantitatively the lung density, the lung weight, and the percentage of normally aerated and nonaerated tissue. Knowing the mean CT number of the pulmonary parenchyma in a group of normal subjects, we also inferred the ideal lung weight expected in the study population and computed the excess tissue mass as the difference between actual and ideal lung weight. Both the roentgenographic scoring systems showed direct correlation with the pulmonary impairment as detected by CT scan densitometric analysis (CT number, percentage of nonaerated tissue, lung weight, and excess tissue mass; p less than 0.01) and inverse relation with the percentage of normally aerated tissue (p less than 0.01). We also found a relationship between roentgenographic scores and the impairment in gas exchange as detected by shunt fraction (p less than 0.05). We conclude that standardized reading of portable chest roentgenograms by means of scoring tables is a valuable tool in estimating the amount of pulmonary edema in a patient with adult respiratory distress syndrome.

Tidal volume reduction in ARDS. Effect on cardiac output and arterial oxygenation

During continuous positive pressure ventilation (CPPV), mean airway pressure and lung volume will be influenced both by the tidal volume (VT) employed and the amount of positive end-expiratory pressure (PEEP). The effect of varying levels of CPPV on PaO2 and cardiac output (Q) has been previously assessed by adjusting the level of PEEP at constant VT. This study examined the influence of a 200-ml reduction in VT, at a constant PEEP of 15 cm H2O, on the PaO2 and Q of 21 patients with adult respiratory distress syndrome (ARDS). The relationship between change in Q and change in total respiratory system compliance (Cst) after VT reduction was also examined. VT reduction from 14.1 +/- 0.8 ml/kg to 11.2 +/- 0.9 ml/kg yielded an increase in Q (+ 15 +/- 12 percent, p less than 0.01) without a significant change in PaO2 (-6.3 +/- 15.0 mm Hg, p = 0.08). Cst increased with VT reduction (+ 3.1 +/- 1.8 ml/cm H2O). There was only a modest correlation (r = +0.42, p = 0.06) between delta Q percent and delta Cst following VT reduction. VT reduction at high level PEEP may yield a significant improvement in Q and net O2 delivery, but the degree of hemodynamic improvement is variable and is not reliably predicted noninvasively by measurement of Cst.

Hollow fiber hemodialyzers for partial respiratory support

Acute respiratory failure is still characterized by a high mortality rate, in spite of the development of ingenious treatment modalities involving mechanical ventilation and extracorporeal respiratory support. The coexistence of renal failure further complicates the outlook for respiratory failure patients. In this paper the development of the blood oxygenator and current aspects of its application are described. The concept of applying dialysis membranes for partial respiratory support is presented. An experimental animal model which was performed with A-V cannulation using dialysis membranes for partial respiratory support and simultaneous dialysis is analyzed. It is concluded that dialysis membranes are efficient for blood gas exchange and that their use in the management of the above-mentioned types of critically ill patients is promising.

The prone position in ARDS patients. A clinical study

The gas exchange and hemodynamics were evaluated before, during, and after a two-hour period of prone position in 13 moderate-severe ARDS patients. Lung computerized tomography was obtained in both the supine and prone positions in two of these patients. Average arterial oxygenation improved after prone positioning (p less than 0.01). A PaO2 improvement of at least 10 mm Hg after 30 minutes of prone position was used as a criterion to discriminate between responders and nonresponders to the postural change. Eight patients met the "responders" group criterion, and in the five nonresponder patients, the PaO2 did not change significantly throughout the study. Computerized tomograms in the prone position showed disappearance of posterobasal densities and appearance of new densities in the anterior regions, in both patients studied. One of these was a responder, the other a nonresponder. A brief test period in prone position is indicated in ARDS patients to identify those who may benefit from this postural treatment. The definite mechanism of the arterial oxygenation improvement observed remains to be clarified.

Assessment of lung water distribution by nuclear magnetic resonance. A new method for quantifying and monitoring experimental lung injury

We have developed a new analytical method that uses nuclear magnetic resonance (NMR) imaging data to quantify lung water content and distribution. This new method generates a distribution of lung water density in which the fraction of voxels corresponding to a given water density is plotted on the vertical axis as a function of water density on the horizontal axis, thereby complementing the spatial information provided by the NMR image. We obtained reproducible lung water distribution data at comparable lung volumes in normal excised lungs and in intact living rats. In normal excised unperfused rat lungs, the distribution varied with the degree of inflation, but the changes were small compared with those associated with lung edema. The lung water density distribution changed markedly after induction of lung edema by intrabronchial saline instillation, intravenous oleic acid injection, and rapid intravenous saline infusion. Lung water density distribution data were well correlated (correlation coefficient = 0.948 for the excised lungs and 0.823 for the intact living rats) with gravimetric lung water measurements. The new analytical method is noninvasive, provides easily repeatable measurements, and is as sensitive as the gravimetric technique to lung water changes.