THE DISTRIBUTION OF BLOOD AND GAS WITHIN THE LUNGS MEASURED BY SCANNING AFTER ADMINISTRATION OF 133XE

Ventilation/Perfusion Relationships and Gas Exchange: Measurement Approaches

Ventilation-perfusion ( V ˙ A / Q ˙ ) matching, the regional matching of the flow of fresh gas to flow of deoxygenated capillary blood, is the most important mechanism affecting the efficiency of pulmonary gas exchange. This article discusses the measurement of V ˙ A / Q ˙ matching with three broad classes of techniques: (i) those based in gas exchange, such as the multiple inert gas elimination technique (MIGET); (ii) those derived from imaging techniques such as single-photon emission computed tomography (SPECT), positron emission tomography (PET), magnetic resonance imaging (MRI), computed tomography (CT), and electrical impedance tomography (EIT); and (iii) fluorescent and radiolabeled microspheres. The focus is on the physiological basis of these techniques that provide quantitative information for research purposes rather than qualitative measurements that are used clinically. The fundamental equations of pulmonary gas exchange are first reviewed to lay the foundation for the gas exchange techniques and some of the imaging applications. The physiological considerations for each of the techniques along with advantages and disadvantages are briefly discussed. © 2020 American Physiological Society. Compr Physiol 10:1155-1205, 2020.

Methodology for ventilation/perfusion SPECT

Ventilation/perfusion single-photon emission computed tomography (V/Q SPECT) is the scintigraphic technique of choice for the diagnosis of pulmonary embolism and many other disorders that affect lung function. Data from recent ventilation studies show that the theoretic advantages of Technegas over radiolabeled liquid aerosols are not restricted to the presence of obstructive lung disease. Radiolabeled macroaggregated human albumin is the imaging agent of choice for perfusion scintigraphy. An optimal combination of nuclide activities and acquisition times for ventilation and perfusion, collimators, and imaging matrix yields an adequate V/Q SPECT study in approximately 20 minutes of imaging time. The recommended protocol based on the patient remaining in an unchanged position during the initial ventilation study and the perfusion study allows presentation of matching ventilation and perfusion slices in all projections as well as in rotating volume images based upon maximum intensity projections. Probabilistic interpretation of V/Q SPECT should be replaced by a holistic interpretation strategy on the basis of all relevant information about the patient and all ventilation/perfusion patterns. PE is diagnosed when there is more than one subsegment showing a V/Q mismatch representing an anatomic lung unit. Apart from pulmonary embolism, other pathologies should be identified and reported, for example, obstructive disease, heart failure, and pneumonia. Pitfalls exist both with respect to imaging technique and scan interpretation.

Distribution of blood flow and ventilation in the lung: gravity is not the only factor

Current textbooks in anaesthesia describe how gravity affects the regional distribution of ventilation and blood flow in the lung, in terms of vertical gradients of pleural pressure and pulmonary vascular pressures. This concept fails to explain some of the clinical features of disturbed lung function. Evidence now suggests that gravity has a less important role in the variation of regional distribution than structural features of the airways and blood vessels. We review more recent studies that used a variety of methods: external radioactive counters, measurements using inhaled and injected particles, and computer tomography scans. These give a higher spatial resolution of regional blood flow and ventilation. The matching between ventilation and blood flow in these small units of lung is considered; the effects of microgravity, increased gravity, and different postures are reviewed, and the application of these findings to conditions such as acute lung injury is discussed. Down to the scale of the acinus, there is considerable heterogeneity in the distribution of both ventilation and blood flow. However, the matching of blood flow with ventilation is well maintained and may result from a common pattern of asymmetric branching of the airways and blood vessels. Disruption of this pattern may explain impaired gas exchange after acute lung injury and explain how the prone position improves gas exchange.

Differences in regional wash-in and wash-out time constants for xenon-CT ventilation studies

Xenon-enhanced computed tomography (Xe-CT) has been used to measure regional ventilation by determining the wash-in (WI) and wash-out (WO) rates of stable Xe. We tested the common assumption that WI and WO rates are equal by measuring WO-WI in different anatomic lung regions of six anesthetized, supine sheep scanned using multi-detector-row computed tomography (MDCT). We further investigated the effect of tidal volume, image gating (end-expiratory EE versus end-inspiratory EI), local perfusion, and inspired Xe concentration on this phenomenon.

RESULTS

WO time constant was greater than WI in all lung regions, with the greatest differences observed in dependent base regions. WO-WI time constant difference was greater during EE imaging, smaller tidal volumes, and with higher Xe concentrations. Regional perfusion did not correlate with WI-WO. We conclude that Xe-WI rate can be significantly different from the WO rate, and the data suggest that this effect may be due to a combination of anatomic and fluid mechanical factors such as Rayleigh-Taylor instabilities set up at interfaces between two gases of different densities.

Gravity is an important but secondary determinant of regional pulmonary blood flow in upright primates

Original studies leading to the gravitational model of pulmonary blood flow and contemporary studies showing gravity-independent perfusion differ in the recent use of laboratory animals instead of humans. We explored the distribution of pulmonary blood flow in baboons because their anatomy, serial distribution of vascular resistances, and hemodynamic responses to hypoxia are similar to those of humans. Four baboons were anesthetized with ketamine, intubated, and mechanically ventilated. Different colors of fluorescent microspheres were given intravenously while the animals were in the supine, prone, upright (repeated), and head-down (repeated) postures. The animals were killed, and their lungs were excised, dried, and diced into approximately 2-cm3 pieces with the spatial coordinates recorded for each piece. Regional blood flow was determined for each posture from the fluorescent signals of each piece. Perfusion heterogeneity was greatest in the upright posture and least when prone. Using multiple-stepwise regression, we estimate that 7, 5, and 25% of perfusion heterogeneity is due to gravity in the supine, prone, and upright postures, respectively. Although important, gravity is not the predominant determinant of pulmonary perfusion heterogeneity in upright primates. Because of anatomic similarities, the same may be true for humans.

Effects of posture on the distribution of pulmonary ventilation and perfusion in children and adults

In the adult the distributions of ventilation and of perfusion show the same directional dependence on gravity. In children, however, the distribution of ventilation in response to gravity is the reverse of that seen in adults. The aim of the current study was to determine whether perfusion showed the same reversal in children or followed the adult pattern. Distribution of perfusion was measured with intravenous technetium-99m macroaggregated albumin and distribution of ventilation with inhaled krypton-81m. Eighteen children and seven adults were studied; they had been referred for lung scanning for various respiratory problems. The effect of gravity was examined by giving aliquots of macroaggregated albumin and 81mKr by inhalation to the subject in the supine and the lateral decubitus position. Counts in the dependent lung were compared with those in the upper lung. The dependent lung in the lateral decubitus position received more of the total perfusion than it did in the supine position in seven children with a normal chest radiograph (mean 7.0%, range 4.8-10.9% more) and in 11 children with an abnormal radiograph (mean 3.4%(0.1-10.0%)). Ventilation, however, changed in the opposite direction, falling by 7.1% (-3.2% to -12.8%) in five children with a normal chest radiograph and 11.2% (-2.8% to -19.3%) in eight children with an abnormal radiograph. Fractional V/fractional Q (an index of the ventilation:perfusion ratio) decreased in the dependent lung in the children when they moved from the supine to the decubitus position. The same directional change was recorded in adults, but it was significantly less than in the children, irrespective of whether the chest radiograph was abnormal. In children and adults with various respiratory problems the effect of posture on the distribution of perfusion is similar.

Scintigraphic evaluation of regional pulmonary ventilation

Radioactive gases are useful for evaluating pulmonary ventilation because they allow assessment of regional lung function in a physiologic, noninvasive manner. The most widely used radioactive gases are Xe-133, Xe-127, and Kr-81m. These gases provide visual and/or quantitative assessments of regional ventilation or ventilation-perfusion matching in obstructive airways disease, and aid the scintigraphic diagnosis of pulmonary embolism. The precision of ventilation scintigraphy is limited by detector resolution, signal distortion at depth, poor counting statistics, and respiratory motion.

Regional lung function in asbestos workers

Regional lung function was measured using radioactive xenon-133 in a group of normal subjects and in two groups of asbestos workers. When compared with the normal group, patients with pulmonary asbestosis showed impaired ventilation of the lower zones. Subjects with calcified pleural plaques without radiological evidence of lung parenchymal fibrosis did not show this abnormality.

Regional lung function in ankylosing spondylitis

Xenon-133 was used to study regional pulmonary function in nine patients with chest cage rigidity due to ankylosing spondylitis. In comparison with normal subjects, the patients showed an overall diminution in lung volume and the proportion of inhaled xenon reaching the lung apices was reduced but the distribution of injected xenon was normal. The possible relationship between these findings and apical lung disease in ankylosing spondylitis is mentioned.

Effect of papain-induced emphysema on the distrubtion of pleural surface pressure

Distribution of transpulmonary pressure (Ptp) at FRC, during spontaneous breathing, and during inflation of the relaxed respiratory system was studied in the supine, lateral and head-up postures after papain had been injected intratracheally into dogs. Functional and morphological changes of the lung resembling those of panlobular emphysema occurred in papain-treated dogs. In all postures the relationship between lung height and Ptp at FRC was steeper and shifted to the left of that obtaining for normal dogs. During artifical inflation changes of Ptp were larger in the dependent than in the upper lung and the vertical gradient of Ptp eventually disappeared, as observed in normal dogs. During spontaneous breathing changes of Ptp were uniform in the horizontal postures, but larger over the upper regions in the head-up posture; whereas they are uniform in all postures in normal dogs. From local Ptp values and lung P-V curve, distribution of specific lung volumes at FRC and ventilation during spontaneous breathing were assessed for both normal and papain-treated head-up dogs: the results agreed with those obtained using radioactive gases in normal and in elderly man or emphysematous subjects with no apparently localized lesions.

The measurement of regional ventilation during tidal breathing: a comparison of two methods in healthy subjects, and patients with chronic obstructive lung disease

Regional ventilation has been measured in 17 healthy volunteers, and 24 patients with chronic obstructive lung disease during tidal breathing using 133-Xe. The wash-in and wash-out of 133-Xe were recorded by a gamma camera interfaced to a small digital computer. Regional ventilation was calculated as the distribution of tidal volume per unit lung volume-a measure of relative ventilation--and from the wash-out curves as the fractional exchange of air per second. Determination of the regional fractional exchange of air showed a significant difference between the patients with chronic obstructive lung disease and normal subjects for all regions. The distribution of tidal volume per unit lung volume did not effect such a clear separation. Significant correlations were found between the whole-lung fractional exchange of air in the patients with chronic obstructive lung disease and their FEV1 r equal 0-70, MMFR r equal 0-70, FVC r equal 0-56 and FEV1/FVC r equal 0-57. It is suggested that measurement of regional ventilation as the fractional exchange of air is more realistic than methods that determine relative ventilation or only make use of the early part of the wash-out 133-Xe.

Regional lung function after pneumonectomy

Hall, D. R. (1974).Thorax, 29, 425-431. Regional lung function after pneumonectomy. Regional lung function was studied with xenon-133 in 10 patients with bronchial carcinoma before and again three to six months after pneumonectomy. It was found that ventilation and perfusion were often considerably reduced in the tumour-bearing lung and that the greater part of the cardiac output had apparently been diverted to the unaffected lung before operation. The regional distribution of ventilation and perfusion in the lung remaining after surgery was essentially unchanged from that measured in the same lung preoperatively and no different from the pattern seen in 10 normal volunteers.

Radiology compared with xenon-133 scanning and bronchoscopic lobar sampling as methods for assessing regional lung function in patients with emphysema

Regional lung function was assessed by radiographic methods, by regional function studies using xenon-133 scans, and by lobar sampling with a mass spectrometer flow-meter at bronchoscopy in 12 patients who subsequently had bullae resected at operation. The information given by these three methods of regional assessment was subsequently compared with the findings at operation. When only one lobe was abnormal on the radiographs, these alone were adequate to locate the major site of the emphysema and the regional tests gave relatively little extra information. The xenon scan was sometimes helpful in assessing the state of the remaining lung, but this information could be deduced from the radiographs and overall lung function tests, especially the carbon monoxide transfer and mechanical measurements. Bronchoscopic sampling was helpful in determining whether the affected lobe was acting as a ventilated dead-space. When more than one lobe was affected the regional function tests supplemented the radiographs in defining the site of bullous change as well as locating dead space. Xenon scans, although widely employed for such preoperative assessments, added little to the topographical information obtained by careful radiology. The combination of radiology, lobar sampling, and overall function tests is recommended for assessing which emphysematous patients are likely to benefit from surgery.

Regional lung function in patients with increased unilateral transradiancy of the lung in the presence of cardiac disease

Regional lung function has been studied with xenon-133 in two patients with unilateral transradiancy of the lung and coincidental cardiac disease. One patient had severe mitral stenosis with pulmonary venous and pulmonary arterial hypertension, the other had a secundum type of atrial septal defect with a pulmonary blood flow which was increased to 25 litres per minute. In both instances the overall perfusion of the transradiant lung was diminished but the regional distribution of blood flow was the same within both the `normal' and the transradiant lung. In the patient with mitral stenosis there was an abnormal increase in upper zone perfusion resulting in a reversal of the normal pattern of pulmonary blood flow while in the patient with an atrial septal defect the blood flow difference between the upper and lower zones was decreased and thus the normal vertical gradient was less. These studies suggest that the hypoplastic pulmonary arteries in the transradiant lung reacted in the same way to alterations in pressure and flow as the vessels in the other lung.

Regional lung function in scoliosis

Radioactive xenon-133 was used to study the pattern of regional lung function in 35 scoliotic patients and 10 normal subjects in the sitting posture. The scoliotic curves were classified into three anatomical sites: high, if the apical vertebra was located between Th1 and Th5; mid, between Th6 and Th10; and low, between Th11 and L4; only the primary curve was considered and in 70% of the patients this measured more than 60°.

The mean pattern of perfusion in patients with low and mid curves was not significantly different from that of the normals, nor was there any significant difference between the convex and concave lungs in these two groups. Patients with high curves showed three distinct patterns: [List: see text]

The mean pattern of ventilation in the three groups of scoliotic patients was not significantly different from that of the normals although there was wide individual variation. The only patients to show a significantly abnormal ventilation/perfusion ratio were the high group.

Regional lung volumes calculated from the equilibration scan indicated that in a majority of scoliotics the lung on the convex side of the curvature was larger than that on the concave side.

Our results suggest that both the anatomical site and the angle of the scoliotic curvature have an effect on regional lung function but that this effect falls equally on the convex and concave lungs.

Distribution of pulmonary blood flow after myocardial ischemia and infarction

Regional distribution of pulmonary blood flow and ventilation was determined with the 133 xenon technic in the erect position at the bedside in 15 patients an average of 6 days after uncomplicated myocardial infarction (MI) and in five patients with severe angina in the same coronary care unit. Follow-up studies were repeated within 3 to 25 weeks on six of the patients with MI.

There was marked reduction in perfusion to the lung base after myocardial infarction. Patients with severe angina showed some underperfusion of the lower lung zones, but to a much less degree than those with acute MI. The pattern of pulmonary perfusion reverted toward that seen in angina in the follow-up studies of patients with MI. Distribution of ventilation was normal in all patients.

The results of the study suggest that there are probably chronic changes in the pulmonary vasculature of patients with arteriosclerotic heart disease which lead to redistribution of pulmonary blood flow toward the apex, and that the marked under-perfusion of the lung base demonstrated following acute myocardial infarction reflects an acute increase in the pulmonary venous and interstitial pressures most likely due to occult left ventricular failure.

Regional lung function in patients with obstructive lung diseases

Regional lung function was measured, using radioactive xenon-133, in a group of normal subjects and in three carefully defined groups of patients with obstructive lung disease. When compared with the normal subjects, patients in the emphysematous group showed a relative reduction of ventilation and perfusion to the upper zones, while patients having chronic bronchitis without cardiac or respiratory failure showed a predominantly lower zone defect. In the group of patients with chronic bronchitis with cardiac and respiratory failure no consistent pattern was found.

Quantitated scintillation scanning for the measurement of lung perfusion

A method using quantitated lung scans is described for the accurate proportioning of lung perfusion. The results obtained by this method have been compared directly with those obtained from bronchospirometry, and a good correlation has been obtained (r = + 0·93).

The measurement of ventilation-perfusion relationships using a multiple crystal rectilinear scanner

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The effect of positive centrifugal acceleration upon the distribution of ventilation and perfusion within the human lung, and its relation to pulmonary arterial and intraoesophageal pressures

The influence of gravity on the distributions of ventilation and blood flow, as demonstrated by the effects of posture, is discussed and explained on the theoretical basis of gravitational gradients of pressure within the lung tissue and blood vessels. Positive acceleration steepens these gradients so that measurement of regional ventilation and perfusion in subjects riding on a human centrifuge allows the theory to be extended. Ventilation and blood flow were measured using radioactive xenon and scanning the lung. In addition, pulmonary arterial pressures were monitored during acceleration, and a hydrostatic indifference plane was demonstrated lying 5 cm below the hilum. At this level, the pulmonary arterial pressure averaged 22.2 cm water (16-3 mmHg) systolic, 9.3 cm water (6.8 mmHg) diastolic, and had a mean pressure of 14-5 cm water (10.7 mmHg). It was unaffected by accelerations of up to three times normal gravity (3 g). A gradient in intraoesophageal pressure was demonstrated by the use of a double balloon lying in the lower oesophagus, and this gradient, believed to be related to the density of the lung, was found to be proportional to the applied acceleration. It averaged 0-37 cm water/cm per g. At 3 g with the subjects seated erect, the base of the lung was better ventilated than the apex (in terms of ventilation per unit alveolar volume) in the ratio of 2*6 to 1; the corresponding ratio at 1 g was 1-8 to 1. Ventilation increased linearly with distance down the lung a t all levels of acceleration investigated. At 3 g, the upper 14 cm of the vertical height of the lung were without perfusion, and perfusion increased linearly with distance down the remaining lung three times as fast as it did a t 1 g. At 1 g, only the uppermost 4*5 cm of the lung were without perfusion. The unperfused lung represented 45 % of the total ventilated volume a t 3 g and 13 % of the total at 1 g. At 3 g, the pulmonary blood flow at the lung base was 3*3 times the average value for the whole lung, whereas at 1 g this excess was only 1*8 times. These results are discussed in terms of an interstitial pressure gradient in the lung of the order of 0-3 cm water/cm per g along the gravitational axis, and an intravascular (hydrostatic) pressure gradient of 1 cm water/cm per g.

Regional and overall inequality of ventilation and blood flow in patients with chronic airflow obstruction

Measurements of the topographical distribution of pulmonary blood flow and ventilation have been made in 26 subjects with chronic irreversible obstructive lung disease using a radioactive xenon scanning technique. The distribution of blood flow was found to range between a normal gradient and the reverse of normal. Differences between left and right lungs were present in patients with associated bronchiectasis, previous lobectomy, and obvious radiological emphysema but not in most of those subjects with no obvious destructive changes. Ventilation was commonly decreased towards the base of the lung, that is the reverse of the normal pattern. Acetylcholine altered the blood flow distribution pattern in three out of nine subjects and this may represent vasodilatation in hypoxic regions of the lung. Repeatability of the scanning technique was found to be 10% (one standard deviation) for blood flow measurements and 11% for single-breath ventilation measurements. Examination of the expired xenon plateaux following a single inhalation of xenon in air consistently revealed a fall in count rate with expired volume, indicating the presence of poorly ventilated alveoli. However, there was a poor correlation between the degree of ventilatory inequality revealed by external counting on the one hand and the slope of the expired plateau on the other. The shape of the expired xenon plateau following xenon injection was usually horizontal and correlated poorly with the topographical distribution of blood flow. This is evidence that the extent of the inequality of ventilation-perfusion ratios at the alveolar level in patients with chronic lung disease cannot be obtained by external counters.

Surgical treatment of emphysema

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