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The popularity of the maximum expiratory flow-volume curve (FVC) is in part due to the effort independence of expiratory flow. Of interest are expiratory flow rates at specific lung volumes, usually 50 and 25 percent of vital capacity (VC); Vmax50 and Vmax25, which make accurate assessment of lung volumes essential. Changes in lung volume during the test are due to both the volume of gas expired and the volume change due to gas compression (Vcomp). In normal subjects, Vcomp is small but may be considerable in those with airflow obstruction. When the FVC is measured in a plethysmograph (FVCp), both expired volume and Vcomp are measured. When the volume of the FVC is derived from gas expired at the mouth (FVCm), Vcomp is not considered and differences in Vmax25 or Vmax50 may occur. The magnitude of these errors was assessed in 30 children and young adults: nine normal subjects, ten with cystic fibrosis (CF) and 11 with asthma. For Vmax50, use of FVCm instead of FVCp resulted in an error of 8 +/- 7 percent (mean +/- 1 SD) in the normal subjects compared to 32 +/- 23 in those with CF (p less than 0.01) and 24 +/- 18 for those with asthma (p less than 0.05). For Vmax25, the errors were similar. These errors were not predictable from FEV1 or RV/TLC but were related to a combination of expiratory effort, the shape of the FVCp, and the absolute volume of gas that was being compressed (p less than 0.0001). These findings suggest that expiratory flows in the FVCm are not effort-independent in the face of significant airflow obstruction and that comparisons of values derived from an FVCp with those from an FVCm may not be valid.
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To evaluate tissue surface pH as an indicator of the acid base status of arterial blood during periods of reduced blood flow, we examined the relationship between tissue surface pH, arterial pH, arterial lactic acid levels, and base deficit in fifteen spontaneously breathing anesthetized cats. Blood flow was reduced by hemorrhage to 50% of control blood pressure values (Fig. 3) or by infusing norepinephrine (10 mcg/kg/min intravenously) for one hour (Fig. 2). During these procedures, tissue surface pH was inversely related to arterial base deficit (r = - 0.665, p less than 0.02) and lactic acid (r = 0.822, p less than 0.001) but not related to blood pH (Fig. 4). The errors resulting from measuring tissue surface pH continuously for 2 1/2 to 8 hours were examined by comparing the in vivo recalibration drift of the electrodes. The pH and reference electrodes drifted less than 0.1 pH units during this time. However, when examined in separate experiments for recalibration drift in vitro, we observed up to 0.67 pH units drift after 24 hours of reference electrode contamination with blood, but only 0.02 pH units drift in the pH electrode after this period. Since tissue surface pH is linearly correlated with the blood concentration of lactic acid or the base deficit during periods of acute blood flow reduction, tissue surface pH monitoring may be helpful for following the severity of the resulting anaerobic metabolism in neonates at risk for reduced peripheral perfusion or hypoxia. However, improvements in the reference electrode will be necessary to make long term tissue surface pH monitoring reliable if there is a reasonable risk of contamination of the reference electrode with blood.
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Three types of error in coding birth weight to computer tapes are described: 1) confusion of ounces with pounds, 2) mistaken reading of one pound as eleven pounds, and 3) errors in placement of the decimal. All will allocate low birth weight infants to high birth weight categories. Examination of the reported gestational age of the infant or of the reported cause of death may allow these errors to be detected.
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OBJECTIVE: The purpose of this study was to describe the CT features of infradiaphragmatic air that may develop in patients after mechanically assisted ventilation, its location, its pathway of transdiaphragmatic dissection, and its extension into the abdomen. MATERIALS AND METHODS: We retrospectively evaluated six consecutive adult patients with pneumomediastinum associated with positive end-expiratory pressure therapy who developed intraabdominal air and were imaged with CT in our institution-between 1993 and 1995. Abdominal CT examinations were reviewed and correlated with the clinical findings, follow-up examinations, and exploratory laparotomies in four patients. RESULTS: In four patients, air present in the anterior mediastinum (endothoracic fascia) was seen to extend into the anterior abdominal wall within the extraperitoneal space. In a fifth patient, the air was located extraperitoneally and intraperitoneally. In the remaining patient, air was present exclusively in the peritoneal cavity. In only two patients did we detect small amounts of air in the posterior retroperitoneum. In five patients, we also detected subcutaneous emphysema and/or air dissection into the muscle planes of the anterolateral abdominal wall. CONCLUSION: In patients on mechanically assisted ventilation, anterior mediastinal air can dissect through the diaphragm into the anterior abdominal extraperitoneal space. This anterior pathway of infradiaphragmatic extension of air can be erroneously diagnosed as intraperitoneal air, which may lead to unnecessary exploratory laparotomies. Also, anterior mediastinal air can enter the peritoneal cavity, particularly in patients with a history of median sternotomy.
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A consecutive series of 62 patients who reported tinnitus at the time of their first attendance at a neuro-otology clinic were studied. Loudness matches were obtained both at the frequency of the tinnitus and at 1 kHz. These matches were expressed in dB HL, dB SL and in units derived from individualised loudness functions (personal loudness units; PLUs). Self-reports of the loudness of tinnitus at the time of loudness matching were obtained on five different scales. Moderate correlations were found between self-reported loudness and some of the scales by which the loudness match was expressed. When subjects who had some difficulty with the test procedures were excluded, the correlations between PLU expressions of the matched loudness and certain of the self-report scales were found to be markedly improved. Correlations of traditional expressions of matched loudness with self-report improved to a limited extent. It was concluded that: measurement error can appreciably reduce the maximum correlation between the best self-report measure and loudness match measures; PLU conversions of matched loudness data produce the highest correlations with self-report measures of loudness, and explicitly labelled self-report scales (Guttman and adjectival) produce better correlations with loudness match values than other self-report scales.
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