Regional differences in gas exchange in the lung of erect man

Ventilatory changes following head-up tilt and standing in healthy subjects

Passive tilting increases ventilation in healthy subjects; however, controversy surrounds the proposed mechanism. This study is aimed to evaluate the possible mechanism for changes to ventilation following passive head-up tilt (HUT) and active standing by comparison of a range of ventilatory, metabolic and mechanical parameters. Ventilatory parameters (V (T), V (E), V (E)/VO(2), V (E)/VCO(2), f and PetCO(2)), functional residual capacity (FRC), respiratory mechanics with impulse oscillometry; oxygen consumption (VO(2)) and carbon dioxide production (VCO(2)) were measured in 20 healthy male subjects whilst supine, following HUT to 70 degrees and unsupported standing. Data were analysed using a linear mixed model. HUT to 70 degrees from supine increased minute ventilation (V (E)) (P<0.001), tidal volume (V (T)) (P=0.001), ventilatory equivalent for O(2) (V (E)/VO(2)) (P=0.020) and the ventilatory equivalent for CO(2) (V (E)/VCO(2)) (P<0.001) with no change in f (P=0.488). HUT also increased FRC (P<0.001) and respiratory system reactance (X5Hz) (P<0.001) with reduced respiratory system resistance (R5Hz) (P=0.004) and end-tidal carbon dioxide (PetCO(2)) (P<0.001) compared to supine. Standing increased V (E) (P<0.001), V (T) (P<0.001) and V (E)/VCO(2) (P=0.020) with no change in respiratory rate (f) (P=0.065), V (E)/VO(2) (P=0.543). Similar changes in FRC (P<0.001), R5Hz (P=0.013), X5Hz (P<0.001) and PetCO(2) (P<0.001) compared to HUT were found. In contrast to HUT, standing increased VO(2) (P=0.002) and VCO(2) (P=0.048). The greater increase in V (E) in standing compared to HUT appears to be related to increased VO(2) and VCO(2) associated with increased muscle activity in the unsupported standing position. This has implications for exercise prescription and rehabilitation of critically ill patients who have reduced cardiovascular and respiratory reserve.

The Lung in Space

The lung is exquisitely sensitive to gravity, which induces gradients in ventilation, blood flow, and gas exchange. Studies of lungs in microgravity provide a means of elucidating the effects of gravity. They suggest a mechanism by which gravity serves to match ventilation to perfusion, making for a more efficient lung than anticipated. Despite predictions, lungs do not become edematous, and there is no disruption to, gas exchange in microgravity. Sleep disturbances in microgravity are not a result of respiratory-related events; obstructive sleep apnea is caused principally by the gravitational effects on the upper airways. In microgravity, lungs may be at greater risk to the effects of inhaled aerosols.

Tidal volume, cardiac output and functional residual capacity determine end-tidal CO2 transient during standing up in humans

In man assuming the upright position, end-tidal P(CO(2)) (P(ETCO(2))) decreases. With the rising interest in cerebral autoregulation during posture change, which is known to be affected by P(ETCO(2)), we sought to determine the factors leading to hypocapnia during standing up from the supine position. To study the contribution of an increase in tidal volume (V(T)) and breathing frequency, a decrease in stroke volume (SV), a ventilation-perfusion (V/Q) gradient and an increase in functional residual capacity (FRC) to hypocapnia in the standing position, we developed a mathematical model of the lung to follow breath-to-breath variations in P(ETCO(2)). A gravity-induced apical-to-basal V/Q gradient in the lung was modelled using nine lung segments. We tested the model using an eight-subject data set with measurements of V(T), pulmonary O(2) uptake and breath-to-breath lumped SV. On average, the P(ETCO(2)) decreased from 40 mmHg to 36 mmHg after 150 s standing. Results show that the model is able to track breath-to-breath P(ETCO(2)) variations (r(2)= 0.74, P P 0.05). Model parameter sensitivity analysis demonstrates that the decrease in P(ETCO(2)) during standing is due primarily to increased V(T), and transiently to decreased SV and increased FRC; a slight gravity-induced V/Q mismatch also contributes to the hypocapnia. The influence of cardiac output on hypocapnia in the standing position was verified in experiments on human subjects, where first breathing alone, and then breathing, FRC and V/Q were controlled.

Physiological evaluation of a new quantitative SPECT method measuring regional ventilation and perfusion

We have developed a new quantitative single-photon-emission computed tomography (SPECT) method that uses (113m)In-labeled albumin macroaggregates and Technegas ((99m)Tc) to estimate the distributions of regional ventilation and perfusion for the whole lung. The multiple inert-gas elimination technique (MIGET) and whole lung respiratory gas exchange were used as physiological evaluations of the SPECT method. Regional ventilation and perfusion were estimated by SPECT in nine healthy volunteers during awake, spontaneous breathing. Radiotracers were administered with subjects sitting upright, and SPECT images were acquired with subjects supine. Whole lung gas exchange of MIGET gases and arterial Po(2) and Pco(2) gases was predicted from estimates of regional ventilation and perfusion. We found a good agreement between measured and SPECT-predicted exchange of MIGET and respiratory gases. Correlations (r(2)) between SPECT-predicted and measured inert-gas excretions and retentions were 0.99. The method offers a new tool for measuring regional ventilation and perfusion in humans.

Variance of ventilation during exercise

Expired gas concentrations were measured during a multibreath washin of He in one female and seven male subjects at rest (seated) and during cycle exercise at work rates of 70-210 W. In a computational model, the ventilation distribution was represented as a log-normal distribution with standard deviation (sigmaV); values of sigmaV were obtained by fitting the output of the model to the data. At rest, sigmaV was 0.89 +/- 0.18; during exercise, sigmaV was 0.60 +/- 0.13, independent of the level of exercise. These values for the width of the functional ventilation distribution at the scale of the acinus are approximately two times larger than those obtained from anatomic measurements in animals at a scale of 1 cm3. The values for sigmaV, together with data from the literature on the width of the functional ventilation-perfusion distribution, show that ventilation and perfusion are highly correlated at rest, in agreement with anatomic data. The structural sources of nonuniform ventilation and perfusion and of the correlation between them are unknown.

Fractal nature of regional ventilation distribution

High-resolution measurements of pulmonary perfusion reveal substantial spatial heterogeneity that is fractally distributed. This observation led to the hypothesis that the vascular tree is the principal determinant of regional blood flow. Recent studies using aerosol deposition show similar ventilation heterogeneity that is closely correlated with perfusion. We hypothesize that ventilation has fractal characteristics similar to blood flow. We measured regional ventilation and perfusion with aerosolized and injected fluorescent microspheres in six anesthetized, mechanically ventilated pigs in both prone and supine postures. Adjacent regions were clustered into progressively larger groups. Coefficients of variation were calculated for each cluster size to determine fractal dimensions. At the smallest size lung piece, local ventilation and perfusion are highly correlated, with no significant difference between ventilation and perfusion heterogeneity. On average, the fractal dimension of ventilation is 1.16 in the prone posture and 1. 09 in the supine posture. Ventilation has fractal properties similar to perfusion. Efficient gas exchange is preserved, despite ventilation and perfusion heterogeneity, through close correlation. One potential explanation is the similar geometry of bronchial and vascular structures.

Regional pulmonary blood flow during partial liquid ventilation in normal and acute oleic acid-induced lung-injured piglets

OBJECTIVE

To determine the spatial distribution of pulmonary blood flow in three groups of piglets: partial liquid ventilation in normal piglets, partial liquid ventilation during acute lung injury, and conventional gas ventilation during acute lung injury.

DESIGN

Prospective randomized study.

SETTING

A university medical school laboratory approved for animal research.

SUBJECTS

Neonatal piglets.

INTERVENTIONS

Regional pulmonary blood flow was studied in 21 piglets in the supine position randomized to three different groups: a normal group that received partial liquid ventilation (Normal-PLV) and two acute lung injury groups that received an oleic acid-induced lung injury: partial liquid ventilation during acute lung injury (OA-PLV) and conventional gas ventilation during acute lung injury (OA-Control). Acute lung injury was induced by infusing oleic acid (0.15 mL/kg iv) over 30 mins. Partial liquid ventilation was instituted with perflubron (LiquiVent, 30 mL/kg) after 30 mins in the Normal-PLV and OA-PLV groups.

MEASUREMENTS AND MAIN RESULTS

Arterial and venous blood gases, hemodynamics, and pulmonary mechanics were measured every 15 mins throughout the hour-long study. Pulmonary blood flow was assessed by fluorescent microsphere technique at baseline and after 30, 45, and 60 mins. In the Normal-PLV piglets, pulmonary blood flow decreased from baseline (before injury or partial liquid ventilation) in the most dependent areas of the lung (F ratio = 3.227; p < .001). In the OA-PLV piglets, pulmonary blood flow was preserved over time throughout the lung (F ratio = 1.079; p = .38). In the OA-Control piglets, pulmonary blood flow decreased in the most dependent areas of the lung and increased from baseline in less dependent slices over time (F ratio = 2.48; p = .003).

CONCLUSIONS

The spatial distribution of regional pulmonary blood flow is preserved during partial liquid ventilation compared with gas ventilation in oleic acid-induced lung injury.

Vascular structure determines pulmonary blood flow distribution

Scientific knowledge develops through the evolution of new concepts. This process is usually driven by new methodologies that provide observations not previously available. Understanding of pulmonary blood flow determinants advanced significantly in the 1960s and is now changing rapidly again, because of increased spatial resolution of regional pulmonary blood flow measurements.

A computer model of gas exchange during cardiopulmonary resuscitation

This paper presents a computer model of gas exchange during cardiopulmonary resuscitation (CPR) that permits independent adjustment of inspired air content (16% O2 and 4.5% CO2 present in mouth-to-mouth (MTM) ventilation or ambient air), shunt, deadspace, diffusion impairment, cardiac output, and ventilation. The model contains 15500 acini, each with its own blood supply. Gas exchange occurs at each perfused and ventilated acinus. Arterial P(O2) and P(CO2) are calculated from the summed arterial blood flow using standard formulae. The model and simulations show that MTM ventilation provides inadequate oxygenation when the victim is at high altitude or has diffusion impairment. They also show that analysis of inspired and expired gas concentrations to measure gas exchange primarily measures wash in and wash out of gas when cardiac output is low and that this explains the negative oxygen consumption and carbon dioxide production measured in a previous study.

High-frequency partial liquid ventilation in respiratory distress syndrome: hemodynamics and gas exchange

Partial liquid ventilation using conventional ventilatory schemes improves lung function in animal models of respiratory failure. We examined the feasibility of high-frequency partial liquid ventilation in the preterm lamb with respiratory distress syndrome and evaluated its effect on pulmonary and systemic hemodynamics. Seventeen lambs were studied in three groups: high-frequency gas ventilation (Gas group), high-frequency partial liquid ventilation (Liquid group), and high-frequency partial liquid ventilation with hypoxia-hypercarbia (Liquid-Hypoxia group). High-frequency partial liquid ventilation increased oxygenation compared with high-frequency gas ventilation over 5 h (arterial oxygen tension 253 +/- 21.3 vs. 17 +/- 1.8 Torr; P < 0.001). Pulmonary vascular resistance decreased 78% (P < 0.001), pulmonary blood flow increased fivefold (P < 0.001), and aortic pressure was maintained (P < 0.01) in the Liquid group, in contrast to progressive hypoxemia, hypercarbia, and shock in the Gas group. Central venous pressure did not change. The Liquid-Hypoxia group was similar to the Gas group. We conclude that high-frequency partial liquid ventilation improves gas exchange and stabilizes pulmonary and systemic hemodynamics compared with high-frequency gas ventilation. The stabilization appears to be due in large part to improvement in gas exchange.

Ventilation inhomogeneities and mixed venous blood N2 in multibreath N2 washout

The single path model of airway gas transport, with and without a distributed blood source term, was used to simulate multiple-breath N2 washout by breathing pure O2 in two lung models: a single-region lung model (SRLM) which produces series inhomogeneity, and a seven-region lung model (7RLM) incorporating both series and parallel inhomogeneities. Normalized phase III slopes (Sn) from N2 expirograms were computed for each breath and compared with published human experimental data obtained under similar conditions. The 7RLM predicts well the trend of experimental Sn N2 changes and is superior to the SRLM in the first part (the unsteady state), implying that this part of the curve is mostly due to convective mixing of the seven parallel flow streams. In the quasi-steady state, the 7RLM is not obviously superior to the SRLM. Functional residual capacity and pulmonary perfusion are shown to strongly affect the number of breaths required to reach the quasi-steady state. The anatomical dimensions that appear to be critical in SRLM are not as important in the 7RLM.

Distribution of pulmonary blood flow and total lung water during partial liquid ventilation in acute lung injury

BACKGROUND

Gas exchange is improved during partial liquid ventilation (PLV) with perfluorocarbon in animal models of acute lung injury. The mechanisms are not fully defined. We hypothesize that redistribution of pulmonary blood flow (PBF) along with redistribution of, and decrease in, total lung water (TLW) during PLV may improve oxygenation.

METHODS

We characterized PBF and TLW in anesthetized adult dogs by using positron emission tomography with H2(15)O. Measurements of gas exchange, PBF, and TLW were made before and after acute lung injury was induced with intravenous oleic acid. The same measurements were made during PLV (with 30 ml/kg perfluorocarbon) and compared with gas ventilated (GV) controls.

RESULTS

Oxygenation was significantly improved during PLV. PBF redistributed from the dependent zone of the lung to the nondependent zones, thus potentially improving ventilation/perfusion relationships. However, a similar pattern of PBF redistribution was observed during GV such that there was no significant difference between groups. TLW redistributed in a similar pattern during PLV. By quantitative measurements, PLV ameliorated the continued accumulation of TLW compared with GV animals.

CONCLUSIONS

We conclude that PBF and TLW redistribution and attenuation of increases in TLW may contribute to the improvement in gas exchange during PLV in the setting of acute lung injury.

Evaluation of estimates of alveolar gas exchange by using a tidally ventilated nonhomogenous lung model

The purpose of this study was to evaluate algorithms for estimating O2 and CO2 transfer at the pulmonary capillaries by use of a nine-compartment tidally ventilated lung model that incorporated inhomogeneities in ventilation-to-volume and ventilation-to-perfusion ratios. Breath-to-breath O2 and CO2 exchange at the capillary level and at the mouth were simulated by using realistic cyclical breathing patterns to drive the model, derived from 40-min recordings in six resting subjects. The SD of the breath-by-breath gas exchange at the mouth around the value at the pulmonary capillaries was 59.7 +/- 25.5% for O2 and 22.3 +/- 10.4% for CO2. Algorithms including corrections for changes in alveolar volume and for changes in alveolar gas composition improved the estimates of pulmonary exchange, reducing the SD to 20.8 +/- 10.4% for O2 and 15.2 +/- 5.8% for CO2. The remaining imprecision of the estimates arose almost entirely from using end-tidal measurements to estimate the breath-to-breath changes in end-expiratory alveolar gas concentration. The results led us to suggest an alternative method that does not use changes in end-tidal partial pressures as explicit estimates of the changes in alveolar gas concentration. The proposed method yielded significant improvements in estimation for the model data of this study.

The effect of physiological manoeuvres on the absorption of inhaled nedocromil sodium

In a previous study we showed, in both asthmatic patients and in healthy subjects, a marked increase in plasma concentration of nedocromil immediately following an exercise challenge with associated FEV1 measurements. To identify which component of the exercise challenge is responsible, we have now studied the effect of various manoeuvres on plasma nedocromil concentration in eight healthy subjects after inhalation of 1 ml nedocromil solution (1% w/v) via a Wright nebuliser. Each patient was dosed on six occasions, separated by at least 3 days. Between 15 and 23 min after dosing one of the following manoeuvres was performed: control (no manoeuvre); steady exercise for 8 min, a series of FEV1 measurements, exercise plus FEV1 measurements, three Valsalva manoeuvres and hyperventilation for 3 min. Mean plasma drug concentrations under control conditions were similar at 15 and 23 min after dosing. However, there were significant increases in plasma drug concentration following exercise, FEV1 manoeuvres and exercise plus FEV1 manoeuvre. There were no significant changes in plasma drug concentration following Valsalva manoeuvres and hyperventilation. The results suggest that certain manoeuvres increase the absorption of nedocromil sodium, probably as a consequence of an increase in lung volume.

Local pulmonary blood flow: control and gas exchange

We studied the local response of the pulmonary vasculature to combined changes in alveolar PO2 and PCO2 in the right apical lobe (RAL) of six conscious sheep. That lobe inspired an O2-CO2-N2 mixture adjusted to produce one of 12 alveolar gas compositions: end-tidal PCO2 (PETCO2) of 40, 50, and 60 Torr, each coupled with end-tidal PO2 (PETO2) of 100, 75, 50, and 25 Torr. In addition, at each of the four PETO2, the inspired CO2 was set to 0 and PETCO2 was allowed to vary as RAL perfusion changed. The remainder of the lung, which served as control (CL) inspired air. Fraction of the total pulmonary blood flow going to the RAL (%QRAL) was obtained by comparing the methane elimination from the RAL to that of the whole lung, and expressed as a percentage of that fraction at PETCO2 = 40, PETO2 = 100. Cardiac output, pulmonary vascular pressures, and CL gas tensions were unaffected or only minimally affected by changes in RAL gas composition. A drop in PO2 from 100 to 50 Torr decreased local blood flow by 60% in normocapnia and by 66% at a PCO2 of 60. At all levels of oxygenation, an increase in PCO2 from 40 to 60 reduced QRAL by nearly 50%. With these stimulus-response data, we developed a model of gas exchange, which takes into account the effects of test segment size on blood flow diversion. This model predicts that: (1) when the ventilation to one compartment of a two compartment lung is progressively decreased, PAO2 remains above 60 Torr for up to 60% reductions in alveolar ventilation, irrespective of compartment size; (2) the decrease in PAO2 that occurs at altitude is accompanied by a drop in PACO2 that limits the decrease in conductance and minimizes the pulmonary hypertension; and (3) as we stand, local blood flow control by the alveolar gas tensions halves the alveolar-arterial PO2 and PCO2 differences imposed by gravity.

Single-breath, room-air method for measuring closing volume (phase 4) in the normal human lung

The purpose of this study was to evaluate a new method to measure closing volume (CV). This new method does not require oxygen or inert gases to be inhaled to obtain the onset of phase 4. Because there are regional differences in the concentrations of the resident alveolar gases (O2, CO2, and N2), there should be an abrupt change in the concentration of these gases at the terminal portion of a prolonged expired vital capacity (VC) that marks the onset of phase 4. Nine normal healthy subjects, 30 to 65 years of age, inspired room air from residual volume (to mimic the maneuver of the standard single breath N2 (SBN2) washout test) to total lung capacity. During the expiration (flow constant at 250 ml.s-1) following a 10-s breath hold at total lung capacity, the exhaled gas was analyzed with a mass spectrometer for fractions of O2, CO2, and N2. Although the onset of phase 4 can be shown as the change in concentration of any of the three alveolar resident gases, oxygen was selected because (1) it demonstrates a greater apex to base concentration gradient than that found with CO2 and N2, and (2) a clear identification of the onset of phase 4 (minimum value of O2 fraction). With this method, the mean +/- SEM of CV was 16.8 +/- 1.52 percent (CV x 100/VC). No significant difference was found among the room air method, SBN2 method, and the helium bolus technique.

Cyclic perfusion of the lung by dense gas breathing may reduce the (A-a)DO2

The decrease in the alveolar-arterial O2 difference (A-a)Do2 in dense atmosphere could be the result of cyclic lung perfusion due to large swing in pleural pressure during breathing cycles /4/. Using a mathematical model of the lung to calculate (A-a)Do2, a slight decrease was demonstrated in (A-a)Do2 as perfusion changed from steady to cyclic. The present study incorporates variable vertical partitioning of perfusion in this model, as a function of instantaneous total blood flow. This demonstrated a reduction of the Po2 difference between the apex and the base of the lung, and a reduction of (A-a)Do2, when perfusion was assumed to be cyclic, of 4.1 to 5.0 torr when blood flow was continuous throughout the breath, and of 5.0 to 5.7 torr when perfusion was assumed to be pulsatile. The results agree with experimental findings: reduction of (A-a)Do2 when breathing dense gas and improved apical perfusion in pulsatile blood flow. Calculation of the spatial VA/Q inequality suggests there is no correlation with (A-a)Do2 reduction. The decrease in (A-a)Do2 is attributed to the mixing of lung capillary blood flows having different O2 saturations.

Variable radiomorphologic data of high altitude pulmonary edema. Features from 60 patients

The purpose of the study was to collect radiomorphologic data of a large population of subjects with high altitude pulmonary edema. A blinded retrospective analysis of 60 patients severe enough to warrant hospital admission is reported. Immediately after rescue to low altitude, the severity of HAPE was graded using a quadrant-based scoring system (0-4 each quadrant). Its distribution and the morphologic features were noted. HAPE was more severe in the base, and specifically, the right lower quadrant, as compared to the other quadrants. It was often located both centrally and peripherally (60 percent) and in 92 percent was characterized by air space disease of homogeneous (n = 40) rather than patchy distribution (n = 15). In recurrent HAPE (n = 13), radiomorphologic data were as variable as among different HAPE patients. We conclude that HAPE does not have one common radiomorphologic condition. Based on the literature, earlier experience, and follow-up observations, we hypothesize that it may start patchy and peripheral, supporting the concept of uneven vasoconstriction with overperfusion and/or permeability leak. Later on, such as in the severe cases studied, it becomes homogeneous. Recurrent episodes generally do not show an identical distribution of HAPE, suggesting that structural abnormalities are not involved in the pathogenesis of HAPE.

A combined parallel and series distribution model of inspired inert gases

The distribution within the lungs of inspired gas has been demonstrated to be uneven by the technique of the external counting of inspired radioactive gas (Milic-Emili et al. (1966) J. Appl. Physiol. 21: 749-759). The contribution of this regional distribution to the slope of the alveolar plateau observed at the lips from an inspired gas marker has been debated, particularly the part played by incomplete diffusive mixing (Sikand et al. (1966) J. Appl. Physiol. 21: 1331-1337). We have repeated the experiments of Sikand, obtaining similar results, by inspiring 1.9 L of 79% argon and 21% oxygen from functional residual capacity, with a subsequent expiration to residual volume, after various breath-holding times. The opposing views of the above authors (parallel versus series inhomogeneities) are here used to develop a model in which the lung is divided into seven regions from apex to base, each region being allocated a compliance curve (polynomial equation of third order) after that of Milic-Emili. Each model region then receives a volume of inspired gas according to its compliance and its regional dead space. This dead space has been allocated on the basis of increasing path lengths of inspired gas from the apex to the base. Beyond the front of this dead space, the mixing of gas is taken to be exponential with respect to expired volume and a curve is then allocated to this alveolar region. The model thus contains both parallel (inter-regional) and series (intra-regional) components. Following a simulated expiration of these seven regions, the model expired curve so obtained is in close agreement with the experimental data, both in respect of shape and of the quantity of tracer contained within it in the range of 0.75-4.5 L of expired gas. We therefore conclude that inter-regional factors are the principal determinant of the last 2.5 L of the expired gas tracer curve and that intra-regional components play a significant role in the first 1.25 L. The model is also applicable to any other inhaled inert gas.

Alterations in ventilatory pattern and ratio of dead-space to tidal volume

The effects of alterations in ventilatory pattern on the simultaneously measured physiologic and anatomic dead-spaces (VDphys and VDan, respectively) and the dead-space to tidal volume ratio (VD-VT) were studied in 17 healthy normal subjects (13 men, four women, ages 21 to 36 years). There were no significant changes in VDan with increases in respiratory frequency (f) or tidal volume (VT). The VDphys increased (mean change +0.153 L, p less than 0.05) with increase in VT (mean increase +0.84 L, p less than 0.01), but did not alter significantly with a twofold increase in f, at control VT. Increase in VT significantly reduced VD/VT (mean change -10.4 percent, p less than 0.05), but increase in f, at control VT, did not significantly alter VD/VT. These results indicate that in normal subjects, increase in VT alters ventilation/perfusion matching in the lungs, whereas an increase in f, at constant VT, has no effect on ventilation/perfusion matching. Increases in VD/VT cannot, therefore, be ascribed to alterations in ventilatory pattern where either VT, or f, or both are increased.

Understanding the meaning of the shunt fraction calculation

The pulmonary shunt fraction (Qs/Qt) is frequently calculated in critically ill patients to monitor the effectiveness of pulmonary oxygenation. The breathing of pure oxygen often results in higher calculated Qs/Qt values that have been attributed to the development of atelectasis, ventilation-perfusion imbalance, or both. To interpret properly the changes in calculated Qs/Qt that occur when the inspired oxygen fraction is altered, the changes produced in all the variables affecting Qs/Qt must be known. This tutorial presents an in-depth analysis of the four variables affecting the calculation of Qs/Qt: VO2 (oxygen uptake), Qt (cardiac output), Cc'O2 (oxygen content in pulmonary end capillaries), and CvO2 (oxygen content in mixed venous blood). These variables are related according to the following equation, which is derived by combining the Fick and the classic shunt equations: Qs/Qt = 1 - [(VO2/Qt)/(Cc'O2 - CvO2)]. Three-dimensional surface representations relating these variables are also presented to help the reader understand the effects of these variables on the calculated Qs/Qt.

Hypoxic pulmonary vasoconstriction and pulmonary gas exchange in normal man

Blood gases, hemodynamics and ventilation were measured in 7 healthy volunteers at baseline while breathing room air (FIO2 0.21), during hypoxia (FIO2 0.125, 15 min) and after nifedipine 20 mg sublingually at FIO2 0.21 (45 min) and at FIO2 0.125 (15 min). Distributions of ventilation-perfusion ratios (VA/Q) were determined, using the multiple inert gas elimination technique, at baseline, during hypoxia, and again during hypoxia after nifedipine intake. Hypoxia was associated with an average increase in pulmonary vascular resistances by 104%, which was partially inhibited by nifedipine. The inert gas data showed a mild deterioration in the distribution of VA/Q ratios during hypoxia. However, when blood flow and ventilation were constrained to the baseline normoxic values in the distributions recovered during hypoxia ('normalization procedure') a slight improvement in VA/Q matching could be evidenced, which was blunted during hypoxia after nifedipine. This was interpreted as the functional effect of hypoxic pulmonary vasoconstriction (HPV). Using the 'normalized' distributions, we computed the relationship between the decrease in compartmental blood flow that occurred during hypoxia and the corresponding alveolar PO2, and calculated the gain due to HPV feedback using equations of the control theory. The contribution of HPV to the stability of compartmental VA/Q was greatest for alveolar PO2 values around 60 mm Hg, but at best the feedback had only a moderate efficiency.

Virulence for guinea pigs of tubercle bacilli isolated from the sputum of participants in the BCG trial, Chingleput District, South India

This study, conducted in Madras, India and in Madison, Wisconsin, USA, was concerned with the virulence of isolates of Mycobacterium tuberculosis obtained from the sputum of individuals living in the Chingleput district of south India. The following results were obtained. 1. The findings of Mitchison with respect to the predominance of low virulence for guinea pigs among isolates from persons living Madras, were confirmed on isolates from the sputum of residents of the Chingleput district. 2. A high correlation was found between the log10 number of tubercle bacilli recovered from the spleen of guinea pigs infected intramuscularly with 1.0 mg of tubercle bacilli and the root index of virulence. 3. A high correlation was found between the log10 number of tubercle bacilli recovered from the spleen of guinea pigs infected intramuscularly with 1.0 mg of tubercle bacilli and the number recovered from the spleen of guinea pigs infected by the respiratory route with 5-10 tubercle bacilli. 4. Relatively low correlations were found between RIV and the susceptibility of isolates to thiophene-2 carboxylic acid hydrazide or to hydrogen peroxide.

The validity of different methods of analysis of 133Xe washout curves for the determination of lung function

The influence of different mathematical methods on the analysis of 133Xe washout data, used in the assessment of lung function in the pig, were investigated. When the data were fitted using a bi-exponential or a bi-exponential plus a constant type equation, neither the two clearance rate constants nor the overall 50% clearance time were found totally appropriate for the study of lung function. A new two compartmental model for the in vivo clearance of 133Xe is proposed. The model assumes the whole blood as one compartment and the lung, including the pulmonary blood, as the second compartment. This suggests that the curvature of the xenon clearance curve is the result of recording the summation of the activities from the alveoli and the pulmonary blood and not, as previously described, due to the existence of two different sub-populations of alveoli.

Gas exchange in tidally ventilated and non-steadily perfused lung model

We studied the effect of cyclic lung perfusion - fast cycle in synchrony with heart beats and slow cycle in synchrony with ventilation - on gas exchange in a lung model. There was almost no effect in the fast cycle. In a homogeneous single-lung unit, arterial PO2 increased, and the (A - a)DO2 decreased (by approximately 0.5 Torr), as the amplitude of the slow cyclic lung perfusion (TIP) increased. The calculated (A - a)DO2 and (a - A)DCO2 were negative. Maximal PaO2 was found when peak lung perfusion was delayed with respect to ventilation by 0.2 of a cycle. In a non-homogeneous nine-unit lung, cyclic lung perfusion caused an increase in PaO2 and a decrease in (A - a)DO2 by 2 Torr as compared to steady perfusion. No apparent negative (A - a)DO2 was found, but apparent negative (a - A)DCO2 was calculated at no pulmonary shunt and also with 5% shunt. The correlation of cyclic lung perfusion to the reduced (A - a)DO2 in dense-gas breathing - where large swings of pleural pressure are expected - and its effect on the diffusion capacity of the lung are discussed. Non-steady perfusion of the lung as caused by ventilatory movements expanded our understanding of gas exchange and shed some light on a few controversial experimental findings, such as the negative (a - A)DCO2, the decreased (A - a)DO2 while breathing dense gas, and the effects of gas density on diffusion capacity of the lung.

End-tidal carbon dioxide tension and temperature changes after coronary artery bypass surgery

Variations in end-tidal carbon dioxide partial pressure (PETCO2) and temperature were measured for six hours following coronary artery bypass surgery in twenty patients. In the recovery room, the patients were mechanically ventilated with a tidal volume of 12 ml X kg-1. Arterial blood gases were drawn every two hours, and the respiratory frequency was adjusted to maintain arterial carbon dioxide pressure (PaCO2) in the range of 30-45 mmHg. Naso-pharyngeal temperature was recorded every 30 minutes, and PETCO2 was measured continuously. The mean difference between temperature-corrected arterial and end-tidal CO2 pressure measurements was 3.2 mmHg (SD = 2.8; r = 0.963). This difference did not vary with time, temperature or PCO2. The largest temperature increases (mean 1.7 degree C/hour) occurred at a mean of 253 minutes after the end of surgery. End-tidal PCO2 increased markedly as temperature rose, in spite of a coincident increase in ventilation and then decreased as temperature stabilized. Large increases in CO2 production, caused by the metabolic demands during rewarming, most likely account for these changes. It is concluded that end-tidal CO2 recordings are reliable, and can help in maintaining normocarbia during the short but unstable period associated with rewarming following cardiac surgery.

Practical aspects of differential ventilation with selective peep in acute respiratory failure

Hypoxaemia in association with acute respiratory failure continues to be a severe problem in some intensive care patients. Among strategies proposed, we want to focus attention on differential ventilation with selective PEEP, administered in the lateral position. This ventilation technique has proved successful in the treatment of refractory hypoxaemia due to severe bilateral lung disease. The rationale of this concept is briefly presented in this paper, where the main emphasis is laid on the practical aspects of its clinical application. Two case reports are included as examples of our experiences.

Simultaneous O2 and CO diffusing capacity estimates from assumed lognormal VA, Q and DL distributions

O2 and CO pulmonary transfer data obtained in dogs under steady-state conditions in hypoxia by Savoy et al. (Respir. Physiol. 42: 43-59, 1980) have been submitted to reevaluation and have yielded new estimates of the lung diffusing capacity, DL. For the proposed DL computation it has been assumed that functional inhomogeneity can be considered as resulting from lognormal distributions of the VA/Q and VA/DL ratios. The standard deviation, sigma, of the VA/Q distribution is computed from the measured (PA -Pa)CO2, and the same sigma value is assumed to prevail for the VA/DL distributions. This is equivalent to assume constant DL/Q ratios in the entire lung. With the so defined distributions, DL values, called D sigma O2 and D sigma CO, were sought, for which the model calculations yielded O2 partial pressures and CO fluxes equal to those measured. Compared with DL estimates computed with conventional procedures, these results show that D sigma O2 is twice as large as DLO2 computed with ideal alveolar PO2 and that D sigma CO lies between DLCO computed with the mean alveolar PCO and that computed with the ideal alveolar PCO. The D sigma O2/D sigma CO ratio was on the average 1.2, a value which, unlike the ratios obtained with conventional DLO2 and DLCO estimates, is in good agreement with the characteristics of diffusion and of chemical association of O2 and CO with blood.

Regional lung ventilation in ankylosing spondylitis

Patients with ankylosing spondylitis (AS) sometimes develop apical lung fibrosis and cavitation. It has been suggested that one causative factor is reduced apical ventilation due to rigidity of the thoracic cage. We measured regional ventilation in 27 patients with AS and 18 normal volunteers. Twelve patients (Group A) had chest expansion greater than 2 cm, twelve (Group B) had chest expansion of 2 cm or less and three (Group C) had apical lung lesions on chest radiographs; patients in Groups A and B had no radiographic lung lesions. Ventilation per unit volume (VE/VA min-1) was calculated from 81Krm washout curves. The ratio of upper zone to lower zone ventilation (VR) was calculated. VR in Group A (0.74 +/- 0.11) and group B (0.75 +/- 0.12) was not significantly different from VR in controls (0.76 +/- 0.08). There was no significant correlation between VR and FVC, FEV1 or maximal chest expansion. In patients with apical fibrosis only the radiographically abnormal areas had reduced ventilation. Patients with AS do not underventilate the upper zones of the lungs except in the presence of radiographically visible fibrosis.

Regional differences in lung function during anaesthesia and intensive care: clinical implications

Anaesthesia and most frequently acute respiratory failure are accompanied by a lowered functional residual capacity (FRC). This lowering promotes airway closure in dependent lung units and forces ventilation to non-dependent regions. Perfusion, on the other hand, is forced towards dependent lung units. A ventilation-perfusion mismatch is created and hypoxaemia may develop. General PEEP counters airway closure, but impedes cardiac output and forces perfusion further to dependent regions. In addition, barotrauma may occur. Improved matching of ventilation and perfusion can be achieved by: (1) positioning the subject in the lateral posture; (2) ventilating each lung separately in proportion to its perfusion (differential ventilation); and (3) applying PEEP only to the dependent lung (selective PEEP). Because of less overall intrathoracic pressure and lung expansion, interference with the total lung blood flow and the danger of barotrauma should be less than with general PEEP. Improved gas exchange with a 50-100% increase in PaO2 has been observed in a limited number of patients with acute bilateral lung disease studied so far during differential ventilation and selective PEEP.

Theoretical basis of single breath gas absorption tests

Absorption of gas from alveoli is examined in a simplified model of the respiratory system during a stylized single breath consisting of constant inspiratory flow, constant expiratory flow, and breathholding. The equations describing gas behavior are general since they are based upon conservation of mass. The equations simplify considerably when gases that are not soluble in pulmonary tissue and/or blood are utilized. In a three-compartment model, diffusing capacity of the lung for carbon monoxide (DCO) will be underestimated except when both uneven distribution of lung volume and DCO are present; under most circumstances, the standard clinical 10-s method [9] is at least as accurate as any other. When pulmonary capillary blood flow (Qc) is calculated by the one point method [2] in a one-compartment lung, it is underestimated; in the three-compartment model, it is underestimated except when both uneven distribution of Qc and lung volume are present. The multiple single breath method [2] accurately measures DCO and Qc. Measurement of pulmonary tissue volume is improved by correcting the value of the intercept of acetylene absorption to the time when carbon monoxide apparently began rather than utilizing the beginning of inspiration.

Airway closure and distribution during hip arthroplasty

Airway closure, resting lung volume (FRC) and gas distribution were assessed single breath nitrogen washout during hip arthroplasty. Airway closure occurred above FRC in all the patients during enflurane anaesthesia and surgery, with no change in either FRC or airway closure when the femoral and acetabular prostheses were inserted. The slope of the nitrogen alveolar plateau, which was considered to indicate gas distribution, rose significantly from 4.1 to 5.7% N2/1 when the prostheses were inserted. By the end of surgery the slope had returned to the initial value. This finding indicates a transient effect on the airways when the prostheses are implanted.

Pressure-volume and airway closure relationships in each lung in anaesthetized man

Airway closure, functional residual capacity (FRC) and transpulmonary pressure-volume curves were assessed for each lung separately in the anaesthetized subject by means of a double lumen tracheal catheter. In the supine position airway closure occurred synchronously in the two lungs and 0.2-0.31 above FRC. The pressure- volume curves in both lungs were rather similar and critical closing pressure (CP) was approximately 3 cmH2O in each lung. In the left lateral posture, FRC was increased in the non-dependent and reduced in the dependent lung, while closing capacity (CC) remained unaltered. Airway closure was asynchronous and discontinuous between the two lungs. This was caused by the non-linear transpulmonary pressure-volume curve in the lungs, in conjunction with the vertical pleural pressure gradient. An interpulmonary "pendelluft" phenomenon was observed in the left lateral posture, increasing inhomogeneity of ventilation. It may depend on regional differences in compliance.

Comparison of lung diffusing capacity during rebreathing and during slow exhalation

In five normal sitting subjects DLCO and Qc were measured from the disappearances of a stable isotope of carbon monoxide (C18O) and of acetylene with respect to an inert and insoluble reference gas (Helium). Measurements were made during two respiratory maneuvers: (1) during rebreathing both at functional residual capacity (FRC) and near total lung capacity (TLC); and (2) during a slow exhalation at a constant rate from TLC to FRC. Changes in gas concentration were measured at the mouth during both maneuvers with a respiratory mass spectrometer. Mean DLCO was significantly higher during rebreathing near TLC (34.6 ml . min-1 . mm Hg-1) than near FRC (28.8 ml . min-1 . mm Hg). Mean DLCO measured during slow exhalation near FRC (32.7 ml . min-1 . mm Hg) was significantly higher than DLCO measured during rebreathing over the same volume range. Measurements of Qc were not significantly different between the rebreathing and slow exhalation maneuvers. Differences in DLCO between the two methods at FRC were not due to differences in Qc.

Pulmonary blood flow and tissue volume. Assessment of a non-invasive technique of measurement

Pulmonary blood flow and tissue have been measured by a rebreathing technique in which the disappearance of soluble and insoluble gases from an inhaled mixture is followed continuously using a respiratory mass spectrometer. Although measurements made with diethyl ether are more reproducible than those made with freon-22, they are inaccurate because ether (the more soluble gas) is taken up by the tissues surrounding the respiratory dead space. Experiments carried out on four normal subjects demonstrate that measurements of both pulmonary blood flow and tissue volume are influenced by the volume of gas in the lungs during the manoeuvre. Possible reasons for this are discussed and an exponential relationship between the two variables and lung volume is suggested.

Airway closure during anaesthesia, and its prevention by positive end expiratory pressure

Airway closure, functional residual capacity (FRC) and the transpulmonary pressure volume relationship of each lung were studied in the anaesthetized subject in the supine and the left lateral positions. In the supine posture, FRC was of approximately the same size in each lung as was closing capacity (CC). CC exceeded FRC in either lung. In the left lateral position, FRC was increased by 0.91 in the non-dependent lung and was reduced by 0.21 in the dependent lung, while CC was unaltered in either lung. Consequently, FRC exceeded CC in the non-dependent lung and was further lowered beneath CC in the dependent lung. Airway closure did not occur in the non-dependent lung until an average of 0.51 of gas had been expelled after the dependent lung had ceased to empty. The addition of positive end-expiratory pressure (PEEP) in the range 0.5-2 kPa, increased FRC more in the non-dependent than the dependent lung. The findings suggest that airway closure is evenly distributed in the horizontal level, while it has a discontinuous distribution between the dependent and non-dependent lung. Moreover, the increase in lung volume caused by PEEP has a distribution that is by no means ideal for the purpose of countering airway closure.