Effect of inspiratory flow rate on regional distribution of inspired gas

Changes in rib cage shape during quiet breathing, hyperventilation and single inspirations

We measured the relative changes in upper and lower rib cage volume (delta RCU and delta RCL, respectively) using an induction coil plethysmograph (Respitrace) in seven normal seated subjects during relaxed passive expiration from total lung capacity, during quiet breathing, rapid breathing at 60 breaths/min (RB), during exercise-induced hyperventilation (EH) and during single fast inspirations. The plot of the RCU vs RCL was slightly curvilinear during relaxation from total lung capacity in all subjects. However, the inferred changes in rib cage shape were similar during RB and EH to those observed during quiet breathing and the relaxation manoeuvres. In contrast, single rapid inspirations were associated with marked and variable changes in rib cage shape, being most prominent in the first part of the breath. Our results suggest that during cyclic breathing respiratory muscle activity is so co-ordinated that the pattern of rib cage shape change is similar to that observed during relaxation. In contrast, single rapid inspirations are associated with markedly different and variable shape changes of the rib cage, presumably due to different patterns of inspiratory muscle recruitment. The results are consistent with the observation that during tidal breathing regional ventilation distribution is flow independent.

Differential ventilation in acute bilateral lung disease. Influence on gas exchange and central haemodynamics

Eight patients with acute respiratory failure (ARF) due to diffuse and rather uniform lung disease were intubated with a double-lumen bronchial tube and ventilated in the lateral decubital position by two synchronized ventilators. Ventilation of each lung was individually adjusted to match the expected regional blood flow (differential ventilation). When ventilation with equal volumes (i.e. 50% of tidal volume to each lung) was performed, a 19% reduction of venous admixture (P less than 0.001) and a 22% increment in arterial oxygen tension (P less than 0.001) were seen. Comcomitantly, the cardiac output increased by 17% (P less than 0.001), to which a reduced pulmonary vascular resistance may have contributed. The net result was a 14% increment of the oxygen availability (P less than 0.001). An attempt to go further, giving 2/3 of the tidal ventilation to the dependent lung, was made on six of the patients. However, this ventilatory pattern did not further improve the gas exchange and also had detrimental effects on the haemodynamics. It is concluded that differential ventilation with equal tidal volumes in the lateral position can substantially improve gas exchange and central haemodynamics in patients with ARF due to diffuse lung disease.

Effects of unequal pressure swings and different waveforms on distribution of ventilation: a non-linear model simulation

In an attempt to understand the role of unequal pleural pressure swings and of different waveforms of pleural pressure variation in the distribution of ventilation during cyclic breathing, a mathematical model simulation was performed. The computer model which incorporates non-linear resistances and compliances as well as sinusoidal, square, and triangular waveforms of pleural pressure variations indicates that the distribution of ventilation is insensitive to the waveform of the pleural pressure. The distribution is also little changed by the depth of breathing (amplitude), but it is affected significantly by the pattern of different pressures over the regions of the model. For sinusoidal, triangular, and low amplitude square wave pleural pressures with equal amplitudes on both compartments, air was distributed preferentially to the lower compartment under the influence of the static pressure difference. With unequal amplitudes, more air flowed to the compartment experiencing the larger pressure swing. This was virtually independent of the waveform and of the amplitudes of the pleural pressure variation. Comparison of the present results with a constant flow model reveals that the overall distribution of tidal air during cyclic breathing is very different from the results obtained in constant rate inspiration experiments or in bolus distribution experiments. New experiments performed under cyclic breathing conditions are thus indicated.

Effect of inspiratory flow rate on the esophageal pressure gradient in upright humans

We measured pressures in 2 lung regions in 5 seated subjects with 2 esophageal balloons placed 7.2 +/- 0.6 cm (mean +/- SE) apart in the mid-thorax in order to obtain the esophageal pressure gradient (EPG). Pressure differences between the lower thoracic balloon and a balloon placed in the stomach were obtained to measure transdiaphragmatic pressure (pdi). Inspiratory maneuvers were made from functional residual capacity (FRC) at either low inspiratory flow rates (slow VI) (mean slow VI: 0.22 +/- 0.05 L/sec) or at rapid flow rates (fast VI) (2.15 +/- 0.15 L/s). At FRC the mean EPG was 0.19 +/- 0.04 cm H2O/cm. The mean EPG was consistently lower at 500 cc above FRC during fast VI (0.14 +/- 0.03 cm H2O/cm) than during slow VI (0.28 +/- 0.05 cm H2O/cm) (P less than 0.001, paired 't' test). Fast VI results in amplification of pressure in the lower as compared to the upper region wih reduction in EPG. Diaphragmatic contraction appears to influence alterations in EPG during fast VI.

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.

Distribution of inspired gas to each lung in anesthetized human subjects

The distribution of ventilation in man during halothane anesthesia was studied in a two-compartment lung model in which each lung was ventilated separately by means of a double-lumen tracheal tube. Eight subjects were studied prior to scheduled surgery. Tidal volume distribution was even between the lungs in the supine position (horizontal distribution) as was distribution of dynamic lung compliance, resistance and dead space. The vertical distribution was assessed when the patient was in the left lateral position. Dependent dynamic lung compliance and dead space were lower and lung resistance was higher than in the non-dependent lung. These factors favoured a non-dependent lung ventilation and, moreover, caused a re-distribution from dependent to non-dependent lung during an end-inspiratory pause (EIP), thus increasing the inhomogeneity of ventilation. The application of a positive end-expiratory pressure (PEEP) of 10 cmH2O improved dependent ventilation and abolished redistribution between the lungs. In conclusion, uneven distribution of dynamic lung compliance and lung resistance causes inhomogeneous ventilation distribution, favouring the non-dependent lung. An EIP enhances and a PEEP reduces the inhomogeneity of ventilation.

Cranio-caudal rib cage distortion with increasing inspiratory airflow in man

Changes in anteroposterior and transverse diameters of the chest wall, in cranial and caudal esophageal pressure (delta Pes), and diaphragm (Adi) and cranial intercostal muscle activity (Aic) were measured in supine and seated subjects during relaxation and full inspirations from FRC at constant slow or high flow rates. No distortion of the rib cage occurred during relaxation of slow inspirations. During fast inspirations cranial part of rib cage expanded relatively more than caudal one because of greater Aic for any level of Adi. Rib cage distortion during fast inspirations, as well as the occurring between seated and supine posture, was directionally consistent with flow or posture related redistribution of apicobasal lung expansion, suggesting that changes in rib cage shape affect regional lung expansion also in man. During relaxation or slow inspirations cranio-caudal difference in delta Pes (delta Pc-c) was essentially nil over all the inspiratory capacity. During fast inspirations delta Pc-c was negative (seated posture) or nil (supine posture) for small lung volume increments, positive for large volume increments. Delta Pc-c dependence on lung volume is compatible with the response of a model consisting of two parallel compartments with different degree of expansion at FRC and equal volume dependent mechanical properties.

A statistical description of the human tracheobronchial tree geometry

Most physiological studies which made use of lung geometry have utilized average deterministic models of the tracheobronchial tree geometry, such as Weibel's Model A (1963). However, as shown by morphometric studies, it is well known that there are significant inter-subject and intra-subject variabilities in the structural components of the human lung. Hence, inherent inaccuracies exist when deterministic dimensions for lung geometry are used. In this paper, a statistical description of the lung geometry is presented. Using Weibel's Model A as the underlying average model, probability distributions for the lengths and the diameters of airways and for the number and volume of alveoli are proposed based on morphometric data. As a check for consistency, the probability distribution of the functional residual capacity is derived from those associated with airways and alveoli and it is compared with reported data. Results of this comparison are favorable, suggesting that the statistical description presented herein represents a self-consistent model for lung geometry which can be used for studies of problems related to pulmonary physiology.

Studies on intra-pulmonary gas distribution in the extremely obese. Influence of anaesthesia and artificial ventilation with and without positive end-expiratory pressure

Intrapulmonary gas distribution was studied in 10 extremely obese patients: (1) during spontaneous breathing awake; (2) during anaesthesia with controlled ventilation and zero end-expiratory pressure (ZEEP), and (3) as under (2) but with a positive end-expiratory pressure of approximately 15 cmH2O (PEEP). Gas distribution was assessed quantitatively by means of a multiple-breath nitrogen wash-out technique and subsequent fractional analysis, which permitted the calculation of nitrogen wash-out delay (NWOD). Gas distribution was also analyzed by means of a single-breath nitrogen wash-out in order to determine the slope of the "alveolar plateau." Gas distribution was within normal limits during spontaneous breathing, judged from multiple-breath as well as single-breath wash-out. With anaesthesia and ZEEP, NWOD was higher, indicating less efficient gas mixing, and the slope of the "alveolar plateau" was twice as steep as during spontaneous breathing. With PEEP, distribution of inspired gas improved (lowered NWOD and flatter slope). Theoretical considerations and clinical experiments led to the conclusion that uneven distribution in the anaesthetized obese is caused both by regional differences in the pulmonary time constants (as in obstructive lung disease) and by airway closure.

133Xenon washout patterns during diaphragmatic breathing. Studies in normal subjects and patients with chronic obstructive pulmonary disease

Studies of the washout of radioactive 133xenon were performed in six normal subjects and six patients with chronic obstructive pulmonary disease during normal and diaphragmatic breathing. Subjects were unable to change the distribution of ventilation with diaphragmatic breathing. In all normal subjects and in three of the six subjects with chronic obstructive pulmonary disease, overall washout improved with diaphragmatic breathing. It is suggested that this change was related to the slower, deeper tidal volumes used by these subjects during diaphragmatic breathing.

Pulmonary gas exchange in dogs ventilated with mixtures of oxygen with various inert gases

To study the influence of physical properties of the respired gas on alveolar gas exchange, alveolar-arterial partial pressure differences for O2 and CO2 were measured in anesthetized dogs that were artificially ventilated with gas mixtures of O2 in N2, He, Ar or SF6. In both hyposia and normoxia alveolar-arterial PO2 differences had the tendency to increase slightly in the sequence of the respired inert gases SF6 less than Ar less than N2 less than He, while arterial-alveolar PCO2 differences remained practically unchanged. Pulmonary diffusing capacity for CO(DCO), determined by the single breath technique, revealed no significant differences between the four gas mixtures used. The possible mechanisms underlying these results are discussed in connection with the physical properties of the respired gas mixtures.

Frequency dependence of regional lung clearance of 133Xe in normal men

We measured the one-half time of 133Xe washout from the whole lung and from 9 horizontal lung regions in 6 normal men during normal breathing and at 50 breaths/min. The diaphragm was located with a contour map prior to the selection of the 9 horizontal regions. Artifacts caused by the tissue solubility of 133Xe were reduced by expressing regional clearance relative to whole lung clearance. During normal breathing there was a progressive increase in the relative clearance of 133Xe from the lung apex to the base when each region was compared to the lung as a whole. At 50 breaths/min, relative clearance increased from the lung apex only for about 18.5 to 20 cm. At that point, the relative clearance rate of the horizontal regions began to fall and at the lung base was slower than that of the whole lung. These results are consistent with the concept that dependent lung regions have longer time constants than regions at the top of the lung.

Influence of diaphragmatic contraction and expiratory flow on the pattern of lung emptying

Transdiaphragmatic pressure (Pdi) and expiratory flow (V) were monitored during vital capacity single breath N2 washouts in 7 seated subjects. Transient increases in V were produced (1) actively, by subjects increasing mouth pressure while expiring through a constant resistance of (2) passively, by the operator transiently decreasing the resistance. Voluntary contraction of the diaphragm (increased Pdi) was achieved when abdominal muscles were tensed while maintaining V constant. In 5 subjects a transient increase in Pdi of 25-150 cm H2O consistently produced a transient increase in expired N2 concentration of 1.80 +/- 0.06% (Mean +/- 1 SE); in 1 subject N2 concentration decreased by 0.8% to 2.7% N2, and in one subject the alveolar plateau was uninfluenced by changes in Pdi. Passive increases in V up to 21/sec had no effect on FEN2 in any of the subjects. Active increase in V changed FEN2 only when associated with increases in Pdi. Qualitatively similar results were obtained during helium (He) bolus washouts. However, whereas diaphragmatic contraction, maintained throughout expiration, had no measurable influence on the N2 washout, it changed the slope of the He alveolar plateau in 6 out of 7 subjects. We conclude that in normal subjects the alveolar N2 plateau is relatively insensitive to flow variations up to 21/sec. The fluctuations in FEN2 observed when the expiratory flow is varied are due to concomittant changes in Pdi. We propose that diaphragmatic contraction changes the pattern of lung emptying by altering the vertical gradient of pleural pressure.