MAXIMAL DIFFUSING CAPACITY OF THE LUNG FOR CARBON MONOXIDE

Diffusion capacity in children: what happens with exercise?

There is comparatively little data on diffusion capacity in children during exercise. With the advent of improved technology, there is an increasing interest in exercise testing of children in order to predict the evolution of lung disease. In addition to the standard measure of exercise capacity, the VO(2max), interest is evolving in the consequences of alterations in diffusion capacity which may be unmasked with exercise. This review will consider what is known about diffusion capacity with exercise in children with well documented lung disease in the form of cystic fibrosis, healthy controls and swimmers as elite athletes with the largest lung volumes.

Red cell distribution and the recruitment of pulmonary diffusing capacity

The distribution of red blood cells in alveolar capillaries is typically nonuniform, as shown by intravital microscopy and in alveolar tissue fixed in situ. To determine the effects of red cell distribution on pulmonary diffusive gas transport, we computed the uptake of CO across a two-dimensional geometric capillary model containing a variable number of red blood cells. Red blood cells are spaced uniformly, randomly, or clustered without overlap within the capillary. Total CO diffusing capacity (DLCO) and membrane diffusing capacity (DmCO) are calculated by a finite-element method. Results show that distribution of red blood cells at a fixed hematocrit greatly affects capillary CO uptake. At any given average capillary red cell density, the uniform distribution of red blood cells yields the highest DmCO and DLCO, whereas the clustered distribution yields the lowest values. Random nonuniform distribution of red blood cells within a single capillary segment reduces diffusive CO uptake by up to 30%. Nonuniform distribution of red blood cells among separate capillary segments can reduce diffusive CO uptake by >50%. This analysis demonstrates that pulmonary microvascular recruitment for gas exchange does not depend solely on the number of patent capillaries or the hematocrit; simple redistribution of red blood cells within capillaries can potentially account for 50% of the observed physiological recruitment of DLCO from rest to exercise.

Cardiac responses to maximal upright cycle exercise in healthy boys and men

Previous investigations have indicated that children demonstrate a lower cardiac output at a given oxygen uptake during exercise compared with adults. This study compared cardiac responses with maximal upright cycle exercise in 15 boys (mean age 10.9 yr) and 16 men (mean age 30.7 yr) to determine whether this observation reflects differences in size or age-related influences on myocardial function. Stroke volume, aortic peak velocity, and systolic ejection time were measured to peak exercise in all subjects using Doppler ultrasound techniques. No significant differences were observed in resting, submaximal, or peak mean values for these variables relative to body size between the boys and men. Average values for peak stroke index, cardiac index, and peak aortic velocity were 59 (+/-11) mL.m-2, 11.33 (+/-2.32) L.min-1.m-2, and 152 (+/-30) cm.s-1, respectively, for the boys. Respective values for the men were 61 (+/-14) mL.m-2, 11.08 (+/-2.52) L.min-1.m-2, and 144 (+/-24) cm.s-1 (P > 0.05). This study failed to demonstrate evidence of impaired cardiac responses to maximal exercise in prepubertal boys compared with that in adult males.

Recruitment of diffusing capacity with exercise in patients after pneumonectomy

Patients after pneumonectomy are severely limited upon exercise, but impairments in gas exchange are generally mild. One potential explanation of this observation is the existence of functional reserves of diffusing capacity (DLCO), which may be recruited during exercise, predominantly by increasing pulmonary blood flow (Qc). After pneumonectomy, DLCO reserves are recruited even at rest. To investigate if the pattern of recruitment of DLCO is altered and if reserves of DLCO are exhausted during exercise after pneumonectomy, DLCO, lung volume, and cardiac output were measured by the rebreathing method at rest and at multiple levels of steady-state exercise in eight subjects after pneumonectomy and in eight age- and sex-matched nonsmoking normal subjects. In patients after pneumonectomy, the slopes of increase in DLCO [ml.(min.mm Hg)-1.m-2] with respect to QC [ml.min-1.m-2] were normal (0.91 +/- 0.09 x 10(-3) in the pneumonectomy group, 1.16 +/- 0.12 x 10(-3) in the control group, mean +/- SE, p less than 0.05). Thus, the pattern of DLCO recruitment was not significantly affected by pneumonectomy. The ratio of DLCO/Qc fell more rapidly during exercise in patients after pneumonectomy, but the lowest value of the ratio achieved was relatively normal in all except one patient. Declines in arterial O2 saturation at exercise were mild and insufficient to explain the exercise limitation except in the patient whose DLCO/Qc fell below normal. There was no evidence that an upper limit of recruitment was approached. We conclude that the normal ability to recruit DLCO during exercise after pneumonectomy constitutes an important compensatory feature that prevents significant arterial O2 desaturation. In most patients, exercise is limited by a reduced maximal stroke index before reserves of diffusing capacity are exhausted.

Adaptability of the pulmonary system to changing metabolic requirements

The conventional view of the healthy pulmonary system during exercise is of a very precise and mechanically efficient homeostatic regulator of ventilation and gas exchange occurring within the reserves of a near ideal architecture of the lung and chest wall. These regulatory and architectural limits may be exceeded in the healthy pulmonary system when extremely high levels of metabolic demand are needed. For example, arterial hypoxemia will often occur at exercise intensities demanding greater than 25 liters/min cardiac output. This may be due to inadequate red cell transit time in the pulmonary capillary bed whose blood volume has been maximally recruited, thereby resulting in alveolar-end-capillary oxygen disequilibrium. At these extreme levels of exercise the hyperventilatory response may be minimal (and clearly inadequate in terms of alveolar oxygenation) despite substantial and progressive metabolic acidosis or hypoxemia or both. This evidence of compromised ventilatory response and inadequate gas exchange in the highly fit human suggests that the pulmonary system may not be reasonably designed or adaptable (with long-term physical training) to the extreme demands imposed on gas transport by a truly adapted cardiovascular system.

Regional pulmonary clearance of inhaled C15O and C15O2 in man at rest and during exercise

The regional clearance of radioactive carbon monoxide (C15O) and carbon dioxide (C15O2), after a single maximal inspiration, was investigated in six normal subjects seated on a bicycle, at rest and during exercise at 50 W. The clearances were measured sequentially during breath-holding by four pairs of scintillation counters vertically aligned over the right lung. The clearance rate (k) for C15O and C15O2 increased from apex to base and from rest to exercise. On exercise, the apex base gradient (calculated over 11 cm vertical distance) for C15O decreased by 52% because of a larger increase in the upper zones but the C15O2 gradient did not change. Thus the increase in blood flow (approximately equal to kC15O2) on exercise was accompanied by recruitment and/or distension of pulmonary capillary blood volume (approximately equal to kC15O2), most marked in the upper regions. The ratio of the clearance (C15O/C15O2) decreased in the lower zones on exercise by 25% without significant change in the upper zone. This ratio reflects the product of capillary mean transit times and peripheral vascular and extravascular volumes. Since the latter increase on exercise, capillary transit times must have shortened considerably at a moderate level of exercise at all levels in the lung.

Exercise testing in pulmonary sarcoidosis

The change in transfer coefficient (Kco) with increasing heart rate during exercise was studied in 25 normal subjects and in 21 patients with pulmonary sarcoidosis. The slope of the Kco response against heart rate was found to be 0·0053 mmol min⁻¹ kPa⁻¹ l⁻¹ per beat in the normal group but in many of the patients was two standard deviations or more below this normal slope, even when their routine function tests were normal. This response of Kco to exercise is a more sensitive index of changed function than more routine function tests in pulmonary sarcoidosis.

What is clinical smoke poisoning?

In this 13-year study, 51 patients were admitted with the primary diagnosis of "smoke poisoning" "carbon monoxide (CO) poisoning" or "respiratory burns." Forty patients (78%) had diagnosis of smoke poisoning with minor or no skin burns. The study indicated that clinical diagnosis of CO poisoning cannot be made reliably without carboxyhemoglobin (COHg) determination and that smoke poisoning patients often had CO poisoning. Seventeen of 19 smoke poisoning patients (89%) had CO poisoning above COHb levels of 15% saturation. Carbon monoxide was successfully removed from the blood by improving alveolar ventilation and oxygen concentration. However, there were 2 smoke poisoning deaths as the result of gaseous chemical injury. There was a correlation coefficient of 0.87 between initial COHg levels and patients' hospital days primarily determined by patients' pulmonary complications. Since CO is non-irritating, COHb levels may be used as an additional indicator of suspected pulmonary injury by noxious combustion gases.

Alterations in distribution of blood flow to the lung's diffusion surfaces during exercise

We measured simultaneously, by single breath methods, pulmonary capillary blood flow (Q(c)), carbon monoxide diffusing capacity (DL(CO)), and isotopic oxygen ((18)O(18)O) diffusing capacity (DL(18) (O2)) in five normal males during conditions of rest and moderate exercise at mixed venous O(2) tensions (PO(2) 33-44 mm Hg). During moderate exercise at a work load of 100 W. pulmonary capillary blood flow increased from 6.9+/-1.5 to 12.9+/-3.4 min(-1) and DL(18) (O2) increased from 25+/-4 to 43+/-3 ml.min(-1).mm Hg(-1), whereas DL(CO) showed no significant change (45+/-5 to 49+/-10 ml.min(-1).mm Hg(-1)). DL(18) (O2) increased proportionally to Q(c) (r = 0.74), where DL(CO) did not (r = 0.08). The greater increase in DL(18) (O2) during exercise can be explained by a more homogeneous diffusion/perfusion (DL(O2)/Q(c)) distribution in the individual respiratory exchange units during exercise. This improved distribution of DL(O2)/Q(c) acts to help prevent an increase in alveolar-arterial O(2) tension difference from developing despite the decrease in pulmonary erythrocyte transit times that occur during exercise. The insignificant rise in DL(CO) with exercise under these hypoxic breathholding conditions may result from pulmonary vasomotor responses to short-term hypoxia or from relative insensitivity of DL(CO) to moderate levels of exercise.

Indicator dilution measurements of extravascular water in the lungs

Multiple indicator dilution studies of the pulmonary circulation were carried out in conscious, resting and exercising, and anesthetized dogs under conditions where there was no pulmonary edema. Labeled red cells, water, and albumin were injected together into the pulmonary artery, and effluent dilution patterns were obtained from the descending thoracic aorta. The product of the mean transit time differences between labeled water and red cells, and the pulmonary water flow was used to estimate extravascular parenchymatous water; and this was expressed as a proportion of the water content of the blood-drained lung at postmortem examination. These estimates of the proportional water content were found to increase with flow, and to approach an asymptotic value. Reconsideration of the flow patterns in capillaries, however, led to the postulate that extravascular water should be calculated, utilizing as the appropriate vascular reference a substance that uniformly labels the water in red cells and plasma, and which is confined to the circulation, rather than a tracer that only labels red cells. The mean transit time of this substance is approximated by the sum of the mean transit times of labeled red cells and albumin, each weighted according to the proportion of the water content of blood present in that phase. The values for lung water content so computed also increased with flow, and appeared to approach an asymptote that corresponded to approximately two-thirds of the wet lung weight. The estimated values for the water space after pentobarbital anesthesia corresponded to the lower values obtained in the resting conscious animals. When the anesthetized animals were also bled, the estimated water space was disproportionately large, in relation to the previous values. These experimental results support the hypothesis that dilutional estimates of the lung water space reflect pulmonary capillary filling; that this filling increases with exercise; and that a relative increase in filling also occurs as part of the response to hemorrhage.

Farmer's lung. A clinical, radiographic, functional, and serological correlation of acute and chronic stages

In assessing patients suffering from farmer's lung, the acute stage must be distinguished from the chronic stage of the disease. The conspicuous radiographic signs in the acute farmer's lung episode and the often dramatic clearing make an important contribution to the diagnosis. The radiographic changes in chronic farmer's lung are not specific and cover a wide range of appearances. Even minor nodular changes are significant. Farmer's lung, acute and chronic, is not a disease predominantly characterized by a defect in gas exchange. During the acute illness the reduction in diffusing capacity is often accompanied by a decrease in lung volumes; the pulmonary function profile of the chronic stage is variable. In only a relatively small proportion of chronic farmer's lung patients does a defect in gas exchange predominate, and in some it may be manifest only during exercise. Airway obstruction is a feature of chronic farmer's lung. In chronic farmer's lung patients discrepancies between the severity of complaints and results of pulmonary function tests are not infrequent. In some patients with considerable disability conventional pulmonary function studies may demonstrate little or no impairment of the functions measured. In patients suffering from an acute farmer's lung episode, serological tests should be positive, possibly in high titre. In the chronic stage of the disease the chance of finding positive serology in a patient diminishes with the length of time elapsed since the last acute episode. The period of serological transition appears to be the third year.

The pulmonary capillary bed in various forms of pulmonary hypertension

Pulmonary diffusing capacity for CO (D L ), membrane diffusing capacity (D M ), and pulmonary capillary blood volume (V c ) were measured in 44 patients with various forms and severity of pulmonary vascular disease, and these measurements were compared with hemodynamic data obtained at cardiac catheterization, with clinical disability, and, in 27 patients, with the results of corrective cardiac surgery. In 11 of these patients, D L , D M , and V c were remeasured up to 8 months after cardiac surgery.

In general, the diffusing characteristics and pressure-volume relationships in this larger group confirmed the previously reported findings in mitral valve disease. 1 D M was impaired in almost all cases regardless of etiology or severity. Most of the 11 patients restudied showed no change in D L , D M , or V c up to 8 months after surgery. Reduction in V c correlated well both with clinical disability and with surgical results.

Two patients with primary pulmonary hypertension and three of the four patients with equally severe pulmonary hypertension due to mitral valve disease had high V c relative to pulmonary vascular resistance. If the measurements of V c are not falsely high, then it appears that the capillary bed makes relatively and absolutely less contribution to the total rise in pulmonary vascular resistance in these patients. This could be explained by intense precapillary vasoconstriction in the presence of a relatively normal pulmonary capillary bed.

Physiological alterations in the pulmonary capillary bed at rest and during exercise. The effect of body position and trimethaphan camphorsulfonate

The reactivity of the pulmonary capillary bed during exercise as estimated by change in the pulmonary diffusing capacity (D l co ) has been shown to be dependent upon at least two separate mechanisms. The initial (0 to 10 second) D l co rise with exercise appears to be volume-pressure dependent and may be altered by mechanisms influencing these factors in the lungs. The later elevation of D l co with exercise was demonstrated to be primarily independent of the initial rise and uninfluenced by factors affecting peripheral venous return, that is body position and ganglionic blockade.