Differences between remaining ability and loss of capacity in maximum aerobic impairment.

In the evaluation of exercise intolerance of patients with respiratory diseases the American Medical Association (AMA) and the American Thoracic Society (ATS) have proposed similar classification for rating aerobic impairment using maximum oxygen uptake (VO2max) normalized for total body weight (ml min-1 kg-1). However, subjects with the same VO2max weight-corrected values may have considerably different losses of aerobic performance (VO2max expressed as % predicted). We have proposed a new, specific method for rating loss of aerobic capacity (VO2max, % predicted) and we have compared the two classifications in a prospective study involving 75 silicotic claimants. Logistic regression analysis showed that the disagreement between rating systems (higher dysfunction by the AMA/ATS classification) was associated with age > 50 years (P < 0.005) and overweight (P = 0.04). Interestingly, clinical (dyspnea score) and spirometric (FEV1) normality were only associated with the VO2max, % predicted, normal values (P < 0.01); therefore, in older and obese subjects the AMA/ATS classification tended to overestimate the aerobic dysfunction. We conclude that in the evaluation of aerobic impairment in patients with respiratory diseases, the loss of aerobic capacity (VO2max, % predicted) should be used instead of the traditional method (remaining aerobic ability, VO2max, in ml min-1 kg-1).

Differences between remaining ability and loss of capacity in maximum aerobic impairment

In the evaluation of exercise intolerance of patients with respiratory diseases the American Medical Association (AMA) and the American Thoracic Society (ATS) have proposed similar classifications for rating aerobic impairment using maximum oxygen uptake (VO2max) normalized for total body weight (ml min(-1) kg(-1)).However, subjects with the same VO2max weight-corrected values may have considerably different losses of aerobic performance (VO2max expressed as percent predicted). We have proposed a new, specific method for rating loss of aerobic capacity (VO2max, percent predicted) and we have compared the two classifications in a prospective study involving 75 silicotic claimants. Logistic regression analysis showed that the disagreement between rating systems (higher dysfunction by the AMA/ATS classification) was associated with age>50 years (P<0.005) and overweight (P=0.04). Interestingly, clinical (dyspnea score) and spirometric (FEV(1)) normality were only associated with VO2max, percent predicted, normal values (P<0.01); therefore, in older and obese subjects the AMA/ATS classification tended to overestimate the aerobic dysfunction. We conclude that in the evaluation of aerobic impairment in patients with respiratory diseases, the loss of aerobic capacity (VO2max, percent predicted) should be used instead of the traditional method (remaining aerobic ability, VO2max, in ml min(-1) Kg(-1)).

Inaccuracy of noninvasive estimates of VD/VT in clinical exercise testing.

To evaluate the accuracy of noninvasive estimates of VD/VT in clinical exercise testing, we compared measurements of standard VD/VT with estimates based either on end-tidal CO2 (VD/VTET) or a published estimate of arterial PCO2 (VD/VTest) at peak exercise in 68 patients. Using regression analysis, we identified highly significant differences (p < 0.001) between each method and VD/VTstand across a broad range of observed VD/VT. Assuming a normal exercise VD/VT < or = 0.30, estimate methods were specific but were insensitive (50 percent for VD/VTET and 57 percent for VD/VTest) for identifying patients with abnormal gas exchange during exercise. Separate analysis of subgroups based on resting pulmonary function did not identify any group for which either method was acceptable. Our analysis showed that errors in estimating PaCO2, which are amplified by the Bohr equation when calculating VD/VT, are responsible for the inaccuracies of each noninvasive method. We conclude that noninvasive estimates of PaCO2 cannot replace measured arterial PCO2 for calculation of VD/VT during exercise.

Community-acquired pneumonia in adults.

Although the frequency of community-acquired pneumonia caused by Streptococcus pneumoniae continues to be high, studies show that Mycoplasma pneumoniae, Chlamydia pneumoniae, or Legionella pneumophila are the etiologic agents in 20% to 40% of community-acquired pneumonia in adults. The clinical presentation of pneumonia caused by these organisms may be indistinguishable from pneumonia due to S pneumoniae. Separation of cases of pneumonia due to S pneumoniae as typical and that caused by M pneumoniae, C pneumoniae, or L pneumophila as atypical is unwarranted and unhelpful in planning therapy. As many as 35% to 50% of patients do not have an etiologic agent identified. Community-acquired pneumonia can have high morbidity and mortality in patients who are older, have underlying lung disease, diabetes mellitus, or other comorbid conditions, or who have decreased immune function regardless of the specific etiologic agent. In choosing appropriate empiric antimicrobial therapy in hosts who are not immunocompromised, erythromycin and other macrolide antibiotics have the advantage of being effective against a wide range of pathogens likely to be encountered, including S pneumoniae, M pneumoniae, and L pneumophila, and of having some benefit against C pneumoniae. In other patients, the selection of antibiotic therapy can be based on age, clinical suspicion, epidemiologic data, and laboratory test results. Antimicrobial therapy can be directed at specific organisms when and if they are identified.

Exercise testing in the evaluation of impairment and disability.

Integrative cardiopulmonary exercise testing has evolved from a specialized research laboratory tool into a valuable clinical test that is especially useful for assessment of symptoms of exertional dyspnea and exercise intolerance. There is increasingly convincing evidence that evaluation and quantitation of impairment are enhanced by use of exercise testing. In particular, exercise testing has the advantages of objective determination of abnormal exercise tolerance, increased sensitivity for subtle pulmonary gas exchange abnormalities, the ability in many instances to identify unsuspected or unanticipated non-pulmonary causes of impairment, and a useful quantitation of impairment. Although exercise testing is especially valuable in those with mild-to-moderate lung disease in whom questions about the presence of occupational disease and its contribution to impairment are raised, other important questions can be addressed with these methods (Table 10). Therefore there is a high likelihood that the logic of assessing work capacity while the subject performs work will become increasingly clear.

Ventilator management of severe asthma.

The Scientific Board of the California Medical Association presents the following inventory of items of progress in internal medicine. Each item, in the judgment of a panel of knowledgeable physicians, has recently become reasonably firmly established, both as to scientific fact and important clinical significance. The items are presented in simple epitome, and an authoritative reference, both to the item itself and to the subject as a whole, is generally given for those who may be unfamiliar with a particular item. The purpose is to assist busy practitioners, students, researchers, or scholars to stay abreast of these items of progress in internal medicine that have recently achieved a substantial degree of authoritative acceptance, whether in their own field of special interest or another.The items of progress listed below were selected by the Advisory Panel to the Section on Internal Medicine of the California Medical Association, and the summaries were prepared under its direction.

A method for increasing confidence in respiratory gas exchange measurements in mechanically ventilated patients.

Measurement of O2 uptake (VO2) and CO2 output (VCO2) with automated instruments can be used to optimize nutritional management of critically ill patients. However, these measurements may be made infrequently because of calibration problems or suspected inaccuracies, especially when the patient is given supplemental oxygen. A simple method is described for periodic testing of automated gas exchange measurements. While a test lung is mechanically ventilated an accurately known mixture of CO2 and nitrogen is introduced into the inspired gas, simulating VO2 by dilution and VCO2 by addition of CO2. The ratio of VCO2:VO2 (respiratory gas exchange ratio, R) in the "expired gas" should be [FGCO2/(1-FGCO2)] x [(1-FIO2)/FIO2], where FGCO2 is the fraction of CO2 in the diluting gas and FIO2 is the fraction of O2 in the inspired gas. R is independent of the flow rate of the diluting gas or the rate of ventilation of the test lung. Using a mixing chamber-mass spectrometer, we found that R calculated from measurement of CO2 and O2 concentrations in the simulated mixed expired gas closely matched the predicted R for FIO2 = 0.21-0.50. On the other hand, when an automated gas exchange measurement device was tested, R was sometimes excessively high, especially for FIO2 greater than 0.35. This method, using a single diluting gas and without precision flowmeters, may be useful for periodic testing of respiratory gas exchange instruments in the intensive care unit. If a discrepancy is found between measured and predicted R, measurements should not be relied upon until further calibration or repairs can be effected.

Selection criteria for exercise training in pulmonary rehabilitation.

While exercise training appears to have no effect on resting respiratory function, and its effect on ventilation/perfusion relationships is uncertain, it can significantly reduce the rate of lactic acid production, carbon dioxide generated from buffering of acid and the hydrogen ion stimulus to breathe during exercise. We had two objectives in this study: 1) to determine if patients who might benefit from exercise training could be selected based on resting respiratory function measurements; 2) to determine if the work rate at which the metabolic acidosis starts to develop could be reliably determined, non-invasively, by a simple modification of the recently described V-slope method of Beaver et al. Patients with severe obstructive lung disease, all of whom experienced exertional dyspnoea, underwent incremental exercise testing to determine if they could exercise to a level causing metabolic acidosis. About two thirds of the patients with severe airflow obstruction developed a significant metabolic acidosis (arterial standard HCO3- decrease of more than 2 mEq.l-1 after two minutes recovery following an incremental exercise test to maximum). The oxygen uptake (VO2) at which the metabolic acidosis (directly measured) and that of which the increase in CO2 in the expired air attributable to buffering (V-slope method), were in close agreement. There was no significant correlation between the magnitude of the exercise metabolic acidosis and the forced expiratory volume in one second (FEV1) or the diffusing capacity for carbon monoxide (DLCO). Thus, it is necessary to perform exercise testing in order to select patients for exercise training, based on the benefits accrued from reducing the exercise metabolic acidosis.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of altering the proportion of dietary fat and carbohydrate on exercise gas exchange in normal subjects.

A low proportion of dietary calories as carbohydrate has been suggested for patients with chronic obstructive pulmonary disease, because oxidation of carbohydrate (CHO) compared to fat results in greater CO2 production (VCO2) and, at the same arterial PCO2 (PaCO2), higher alveolar and minute ventilation (VE) and increased dyspnea. We hypothesized that a low CHO-high fat diet, although reducing VCO2 and VE at rest, might result in only a small change in VCO2 and VE during exercise. Eight healthy volunteers were randomized to receive for 24 h either isocaloric diets containing 10% or 70% of total calories from CHO (remainder of nonprotein calories from fat). Measurements of VCO2, VE, and respiratory gas exchange ratio (R) were made at rest and during constant work rate cycle exercise below the anaerobic threshold. Five to seven days later, the alternate diet was given and the studies were repeated. At rest, mean VCO2 and R were significantly lower after the low CHO diet compared to the high CHO diet. Mean resting VE was less but not significantly (high CHO 9.6 [0.7] versus low CHO 8.7 [0.8] L/min, mean [SEM]). During exercise, mean VCO2 and R were significantly less after the low CHO diet, but mean VE was only slightly smaller and not significantly different between diets (high CHO 25.4 [1.1] versus low CHO 24.0 [1.0] L/min). The increase in VCO2 from rest to exercise was relatively independent of the substrate mix recently consumed, suggesting that the exercising muscles use stored muscle glycogen as substrate during short bouts of low-intensity exercise despite changes in substrate utilization by nonmuscle tissues at rest.(ABSTRACT TRUNCATED AT 250 WORDS)

Metabolic acidosis during exercise in patients with chronic obstructive pulmonary disease. Use of the V-slope method for anaerobic threshold determination.

Patients with chronic obstructive pulmonary disease (COPD) usually have limited exercise tolerance owing to low ventilatory capacity. Because metabolic acidosis induced by exercise increases ventilatory drive, decreasing the hydrogen ion stimulus may improve exercise capacity. However, in those with mechanical limitation to ventilation or chemoreceptor insensitivity, identifying metabolic acidosis may be difficult using gas exchange methods that depend on the ventilatory response to the acidosis. We compared a modification of a gas exchange method (V-slope) for determining the lactate (anaerobic) threshold (AT), which is independent of ventilatory response with a method using the change in blood standard bicarbonate (HCO3-) level in COPD and normal subjects during cycle incremental exercise. In 43 normal subjects, the VO2 at which metabolic acidosis was identified using the two method correlated (r = 0.75), although mean values differed. In 22 patients with moderately severe to severe COPD, eight who had a change in standard HCO3- less than 2.0 mEq/L between rest and 2 min of recovery from exercise (group 1) were contrasted with 14 whose blood standard HCO3- fell by greater than 2.5 mEq/L (group 2). Mean VC was higher and FEV1/VC was lower in group 2, but mean FEV1, maximal voluntary ventilation, and diffusing capacity for carbon monoxide were not different. The degree of obstruction did not correlate strongly with the degree of exercise metabolic acidosis. The AT determined by the V-slope method was compared with that from standard HCO3-; good correlation between these methods was found (r = 0.98), although mean values were different. The V-slope method predicted metabolic acidosis in 10/14 who had a fall in HCO3- more than 2.5 mEq/L. A significant proportion of patients with COPD seem to develop metabolic acidosis during exercise. The V-slope gas exchange method may be useful in selecting those patients with COPD who develop exercise metabolic acidosis and might therefore benefit from exercise training.

Intra-arterial and cuff blood pressure responses during incremental cycle ergometry.

Brachial intra-arterial blood pressure [systolic (AS) and diastolic (AD)] and cuff blood pressure [systolic (CS) and fourth- and fifth-phase diastolic (CD)] were simultaneously measured by a single observer in 13 middle-aged men during 1-min incremental cycle exercise. On the average, the mean AS exceeded the mean CS by 10 to 11 mm Hg, while the mean AD exceeded the average fourth and fifth CD by 5 and 13 mm Hg, respectively. During incremental exercise, AS, CS, AD, and fourth-phase CD increased, while fifth-phase CD decreased. We also measured intra-arterial blood pressure in nine young adult men smokers during 1-min incremental cycle exercise. In both groups, the average intra-arterial blood pressures increased in a relatively linear fashion from rest to maximal exercise: AS change = 74 +/- 5 mm Hg (SE) and AD change = 28 +/- 3 mm Hg for young men; AS change = 59 +/- 5 mm Hg and AD change = 12 +/- 3 mm Hg for middle-aged men. In this population of middle-aged smokers, intra-arterial mean blood pressure during exercise approximated diastolic plus 2/5 pulse pressure for intra-arterial measures or diastolic plus 1/2 pulse pressure for cuff measures rather than the traditional formula of diastolic plus 1/3 pulse pressure.

Comparison of arterial-end-tidal PCO2 difference and dead space/tidal volume ratio in respiratory failure.

End-tidal CO2 monitors are used to estimate arterial CO2 pressure (PaCO2), but appropriate use of this noninvasive method of assessing blood gases is unclear. In patients with lung disease, the end-tidal CO2 pressure (PETCO2) can differ from PaCO2 because of ventilation-perfusion (VA/Q) mismatching, and changes in PETCO2 may be seen with corresponding increase, decrease, or no change in PaCO2 depending on what happens to VA/Q mismatching. We compared the difference between PETCO2 and PaCO2 in 17 patients undergoing mechanical ventilation. Large differences were found between PaCO2 and PETCO2 in individual patients; P(a-et)CO2 correlated closely with VD/VT. Our studies confirm that PetCO2 is a poor estimate of PaCO2 in patients with respiratory failure. However, the P(a-et)CO2 may be the most appropriate use for end-tidal PCO2 monitoring. In addition, we found that the end-tidal CO2 monitor may be easily adapted for expedient measurement of VD/VT.

Diffusing capacity for carbon monoxide as a predictor of gas exchange during exercise.

In patients with pulmonary disease, the diffusing capacity for carbon monoxide has been used to predict abnormal gas exchange in the lung. However, abnormal values for arterial blood gases during exercise are likely to be the most sensitive manifestations of lung disease. We compared the single-breath diffusing capacity for carbon monoxide at rest with measurements of gas exchange during exercise, including arterial oxygen tension, the alveolar-arterial difference in oxygen tension, the arterial-end-tidal difference in carbon dioxide tension, and the dead-space/tidal-volume ratio in 276 current and former shipyard workers. Sixteen workers had a diffusing capacity for carbon monoxide below 70 percent of predicted; one or more measurements of gas exchange during exercise were abnormal in 14. In contrast, of 96 men who had abnormal gas exchange during exercise, only 14 had a diffusing capacity for carbon monoxide below 70 percent of predicted. Neither the type nor the degree of abnormality in gas exchange could be predicted from the diffusing capacity. We conclude that diffusing capacity for carbon monoxide at rest is a specific but insensitive predictor of abnormal gas exchange during exercise and that, if indicated, measurements of arterial blood gases should be obtained during exercise.