A model of fluctuating alveolar gas exchange during the respiratory cycle

Monitoring Expired CO2 Kinetics to Individualize Lung-Protective Ventilation in Patients With the Acute Respiratory Distress Syndrome

Mechanical ventilation (MV) is a lifesaving supportive intervention in the management of acute respiratory distress syndrome (ARDS), buying time while the primary precipitating cause is being corrected. However, MV can contribute to a worsening of the primary lung injury, known as ventilation-induced lung injury (VILI), which could have an important impact on outcome. The ARDS lung is characterized by diffuse and heterogeneous lung damage and is particularly prone to suffer the consequences of an excessive mechanical stress imposed by higher airway pressures and volumes during MV. Of major concern is cyclic overdistension, affecting those lung segments receiving a proportionally higher tidal volume in an overall reduced lung volume. Theoretically, healthier lung regions are submitted to a larger stress and cyclic deformation and thus at high risk for developing VILI. Clinicians have difficulties in detecting VILI, particularly cyclic overdistension at the bedside, since routine monitoring of gas exchange and lung mechanics are relatively insensitive to this mechanism of VILI. Expired CO₂ kinetics integrates relevant pathophysiological information of high interest for monitoring. CO₂ is produced by cell metabolism in large daily quantities. After diffusing to tissue capillaries, CO₂ is transported first by the venous and then by pulmonary circulation to the lung. Thereafter diffusing from capillaries to lung alveoli, it is finally convectively transported by lung ventilation for its elimination to the atmosphere. Modern readily clinically available sensor technology integrates information related to pulmonary ventilation, perfusion, and gas exchange from the single analysis of expired CO₂ kinetics measured at the airway opening. Current volumetric capnography (VCap), the representation of the volume of expired CO₂ in one single breath, informs about pulmonary perfusion, end-expiratory lung volume, dead space, and pulmonary ventilation inhomogeneities, all intimately related to cyclic overdistension during MV. Additionally, the recently described capnodynamic method provides the possibility to continuously measure the end-expiratory lung volume and effective pulmonary blood flow. All this information is accessed non-invasively and breath-by-breath helping clinicians to personalize ventilatory settings at the bedside and minimize overdistension and cyclic deformation of lung tissue.

Relationship between retinal vessel tortuosity and oxygenation in sickle cell retinopathy

Background

Reduced retinal vascular oxygen (O₂) content causes tissue hypoxia and may lead to development of vision-threatening pathologies. Since increased vessel tortuosity is an early sign for some hypoxia-implicated retinopathies, we investigated a relationship between retinal vascular O₂ content and vessel tortuosity indices.

Methods

Dual wavelength retinal oximetry using a commercially available scanning laser ophthalmoscope was performed in both eyes of 12 healthy (NC) and 12 sickle cell retinopathy (SCR) subjects. Images were analyzed to quantify retinal arterial and venous O₂ content and determine vessel tortuosity index (VTI) and vessel inflection index (VII) in circumpapillary regions. Linear mixed model analysis was used to determine the effect of disease on vascular O₂ content, VTI and VII, and relate vascular O₂ content with VTI and VII. Models accounted for vessel type, fellow eyes, age and mean arterial pressure.

Results

Retinal arterial and venous O₂ content were lower in SCR (O_2A = 11 ± 4 mLO₂/dL, O_2V = 7 ± 2 mLO₂/dL) compared to NC (O_2A = 18 ± 3 mLO₂/dL, O_2V = 13 ± 3 mLO₂/dL) subjects (p < 0.001). As expected, O₂ content was higher in arteries (15 ± 5 mLO₂/dL) than veins (10 ± 4 mLO₂/dL) (p < 0.001), but not different between eyes (OD: 12 ± 5 mLO₂/dL; OS:13 ± 5 mLO₂/dL) (p = 0.3). VTI was not significantly different between SCR (0.18 ± 0.07) and NC (0.15 ± 0.04) subjects, or between arteries (0.18 ± 0.07) and veins (0.16 ± 0.04), or between eyes (OD: 0.18 ± 0.07, OS:0.17 ± 0.05) (p ≥ 0.06). VII was significantly higher in SCR (10 ± 2) compared to NC subjects (8 ± 1) (p = 0.003). VII was also higher in veins (9 ± 2) compared to arteries (8 ± 5) (p = 0.04), but not different between eyes (OD: 9 ± 2; OS: 9 ± 2) (p = 0.2). There was an inverse linear relationship between vascular O₂ (13 ± 5 mLO₂/dL) content and VII (9 ± 2) (β = -0.5; p = 0.02).

Conclusions

The findings augment knowledge of relationship between retinal vascular oxygenation and morphological changes and potentially contribute to identifying biomarkers for assessment of retinal hypoxia due to SCR and other retinopathies.

Arterial to end-tidal Pco2 difference during exercise in normoxia and severe acute hypoxia: importance of blood temperature correction

Negative arterial to end-tidal Pco₂ differences ((a-ET)Pco₂) have been reported in normoxia. To determine the influence of blood temperature on (a-ET)Pco₂, 11 volunteers (21 ± 2 years) performed incremental exercise to exhaustion in normoxia (Nx, P_Io₂: 143 mmHg) and hypoxia (Hyp, P_Io₂: 73 mmHg), while arterial blood gases and temperature (ABT) were simultaneously measured together with end-tidal Pco₂ (P_ETco₂). After accounting for blood temperature, the (a-ET) Pco₂ was reduced (in absolute values) from −4.2 ± 1.6 to −1.1 ± 1.5 mmHg in normoxia and from −1.7 ± 1.6 to 0.9 ± 0.9 mmHg in hypoxia (both P < 0.05). The temperature corrected (a-ET)Pco₂ was linearly related with absolute and relative exercise intensity, VO₂, VCO₂, and respiratory rate (RR) in normoxia and hypoxia (R²: 0.52–0.59). Exercise CO₂ production and P_ETco₂ values were lower in hypoxia than normoxia, likely explaining the greater (less negative) (a-ET)Pco₂ difference in hypoxia than normoxia (P < 0.05). At near-maximal exercise intensity the (a-ET)Pco₂ lies close to 0 mmHg, that is, the mean P_aco₂ and the mean P_ETco₂ are similar. The mean exercise (a-ET)Pco₂ difference is closely related to the mean A-aDO₂ difference (r = 0.90, P < 0.001), as would be expected if similar mechanisms perturb the gas exchange of O₂ and CO₂ during exercise. In summary, most of the negative (a-ET)Pco₂ values observed in previous studies are due to lack of correction of P_aco₂ for blood temperature. The absolute magnitude of the (a-ET)Pco₂ difference is lower during exercise in hypoxia than normoxia.

Application of Mathematical Modeling for Simulation and Analysis of Hypoplastic Left Heart Syndrome (HLHS) in Pre- and Postsurgery Conditions

This paper is concerned with the mathematical modeling of a severe and common congenital defect called hypoplastic left heart syndrome (HLHS). Surgical approaches are utilized for palliating this heart condition; however, a brain white matter injury called periventricular leukomalacia (PVL) occurs with high prevalence at or around the time of surgery, the exact cause of which is not known presently. Our main goal in this paper is to study the hemodynamic conditions under which HLHS physiology may lead to the occurrence of PVL. A lumped parameter model of the HLHS circulation has been developed integrating diffusion modeling of oxygen and carbon dioxide concentrations in order to study hemodynamic variables such as pressure, flow, and blood gas concentration. Results presented include calculations of blood pressures and flow rates in different parts of the circulation. Simulations also show changes in the ratio of pulmonary to systemic blood flow rates when the sizes of the patent ductus arteriosus and atrial septal defect are varied. These changes lead to unbalanced blood circulations and, when combined with low oxygen and carbon dioxide concentrations in arteries, result in poor oxygen delivery to the brain. We stipulate that PVL occurs as a consequence.

Volumetric capnography: the time has come

PURPOSE OF REVIEW

Volumetric capnography (VCap) measures the kinetics of carbon dioxide (CO2) elimination on a breath-by-breath basis. A volumetric capnogram contains extensive physiological information about metabolic production, circulatory transport and CO2 elimination within the lungs. VCap is also the best clinical tool to measure dead spaces allowing a detailed analysis of the functional components of each tidal volume, thereby providing clinically useful hints about the lung's efficiency of gas exchange. Difficulties in its bedside measurement, oversimplifications of its interpretation along with prevailing misconceptions regarding dead space analysis have, however, limited its adoption as a routine tool for monitoring mechanically ventilated patients.

RECENT FINDINGS

Improvements in CO2 measuring technologies and more advanced algorithms for faster and more accurate analysis of volumetric capnograms have increased our physiological understanding and thus the clinical usefulness of VCap. The recently validated VCap-based method for estimating alveolar partial pressure of CO2 provided a breakthrough for a fully noninvasive breath-by-breath measurement of physiological dead space.

SUMMARY

Recent advances in VCap and our improved understanding of its clinical implications may help in overcoming the known limitations and reluctances to include expired CO2 kinetics and dead space analysis in routine bedside monitoring. It is about time to start using this powerful monitoring tool to support decision making in the intensive care environment.

An algorithm to improve the estimation accuracy of a non-invasive method for cardiac output measurement based on prolonged expiration

Cardiac output (CO) monitoring is important in the hemodynamic management of critically ill patients. In a previous study, a novel non-invasive technique for CO monitoring based on prolonged expiration was proposed. The novel method showed good agreement with thermodilution on stable mechanically ventilated patients; unstable patients were excluded. The aim of this study is to improve the outcome of the above mentioned method on hemodynamic unstable patients, requiring vasoactive medications, and showing marked cardiogenic oscillations on waveforms related to expired gases. This prospective study has been carried out on three cardiac surgery patients; eighteen CO measurements were performed on each patient, and these values were compared with data obtained by thermodilution. The designed and tested algorithm allowed to reach a good agreement between CO measured by our method and by thermodilution (e.g., the mean percentage differences were 4%, 11% and 3%). Even though further validation is necessary, the results are quite promising and the adopted solution appears to allow the suitability of the prolonged expiration method also on unstable patients.

Non-invasive estimation of cardiac output in mechanically ventilated patients: a prolonged expiration method

A non-invasive method for the estimation of cardiac output in mechanically ventilated patients is described. The method is based on prolonged expiration, and relies on measurement of gas concentrations and flow rate. A pneumatic system, with an ad hoc designed orifice resistance, has been made and experimentally characterized to adapt the breathing circuit to this application. Cardiac output is calculated using two algorithms and the results are compared with the ones obtained by thermodilution. To this purpose, we prospectively enrolled twenty mechanically ventilated patients, who had undergone cardiac surgery, and both algorithms show good correlation with thermodilution (R > 0.8). The application of the first algorithm gave mean cardiac output values slightly lower than those obtained by thermodilution (-6%), while the application of the second algorithm gave higher values (+30%). Difference standard deviations between paired measurements is 0.72 L min(-1) for the first algorithm and 1.07 L min(-1) for the second one. Standard deviation obtained by the application of the first algorithm is slightly lower than those relative to other minimally invasive techniques. Through prolonged expiration, and standardization and automation of the procedure on mechanically ventilated patients, the proposed system allows to obtain a non-invasive estimation of cardiac output.

Paradigm shift for the alcohol breath test

The alcohol breath test (ABT) has been used for quantification of ethyl alcohol in individuals suspected of driving under the influence for more than 50 years. In this time, there has been little change in the concepts underlying this single breath test. The old model, which assumes that end-exhaled breath alcohol concentration is closely related to alveolar air alcohol concentration, is no longer acceptable. This paper reviews experimental research and mathematical modeling which has evaluated the pulmonary exchange processes for ethyl alcohol. Studies have shown that alcohol exchanges dynamically with the airway tissue both during inspiration and expiration. The airway tissue interaction makes it impossible to deliver air with alveolar alcohol concentration to the mouth. It is concluded that the ABT is dependent on physiological factors that need to be assessed for accurate testing.

Mechanical ventilator for delivery of ¹⁷O₂ in brief pulses

The ¹⁷O nucleus has been used recently by several groups for magnetic resonance (MR) imaging of cerebral metabolism. Inhalational delivery of ¹⁷O₂ in very brief pulses could, in theory, have significant advantages for determination of the cerebral metabolic rate for oxygen (CMRO₂) with MR imaging. Mechanical ventilators, however, are not typically capable of creating step changes in gas concentration at the airway. We designed a ventilator for large animal and human studies that provides mechanical ventilation to a subject inside an MR scanner through 25 feet of small-bore connecting tubing, and tested its capabilities using helium as a surrogate for ¹⁷O₂. After switching the source gas from oxygen to helium, the 0-90% response time for helium concentration changes at the airway was 2.4 seconds. The capability for creating rapid step changes in gas concentration at the airway in large animal and human studies should facilitate the experimental testing of the delivery ¹⁷O₂ in brief pulses, and its potential use in imaging CMRO₂.

The implications of arterial Po2 oscillations for conventional arterial blood gas analysis

In a surfactant-depletion model of lung injury, tidal recruitment of atelectasis and changes in shunt fraction lead to large Pao2 oscillations. We investigated the effect of these oscillations on conventional arterial blood gas (ABG) results using different sampling techniques in ventilated rabbits. In each rabbit, 5 different ventilator settings were studied, 2 before saline lavage injury and 3 after lavage injury. Ventilator settings were altered according to 5 different goals for the amplitude and mean value of brachiocephalic Pao2 oscillations, as guided by a fast responding intraarterial probe. ABG collection was timed to obtain the sample at the peak or trough of the Pao2 oscillations, or over several respiratory cycles. Before lung injury, oscillations were small and sample timing did not influence Pao2. After saline lavage, when Po2 fluctuations measured by the indwelling arterial Po2 probe confirmed tidal recruitment, Pao2 by ABG was significantly higher at peak (295 +/- 130 mm Hg) compared with trough (74 +/- 15 mm Hg) or mean (125 +/- 75 mm Hg). In early, mild lung injury after saline lavage, Pao2 can vary markedly during the respiratory cycle. When atelectasis is recruited with each breath, interpretation of changes in shunt fraction, based on conventional ABG analysis, should account for potentially large respiratory variations in arterial Po2.

Simplified models for gas exchange in the human lungs

This paper presents a hierarchy of models with increasing complexity for gas exchange in the human lungs. The models span from a single compartment, inflexible lung to a single compartment, flexible lung with pulmonary gas exchange. It is shown how the models are related to well-known models in the literature. A long-term purpose of this work is to study nonlinear phenomena seen in the cardio-respiratory system (for example, synchronization between ventilation rate and heart rate, and Cheyne-Stokes respiration). The models developed in this paper can be regarded as the controlled system (plant) and provide a mathematical framework to link between "molecular-level", and "systems-level" models. It is shown how changes in molecular level affect the alveolar partial pressure. Two assumptions that have previously been made are re-examined: (1) the hidden assumption that the air flow through the mouth is equal to the rate of volume change in the lungs, and, (2) the assumption that the process of oxygen binding to hemoglobin is near equilibrium. Conditions under which these assumptions are valid are studied. All the parameters in the models, except two, are physiologically realistic. Numerical results are consistent with published experimental observations.

Hyperpolarized helium-3 MR imaging of pulmonary function

Recent advances in HP MR imaging contrast agents have led to novel tests of pulmonary function. Many of these tests show promise in the clinical arena.

Determination of regional VA/Q by hyperpolarized 3He MRI

Alveolar ventilation/perfusion ratio (VA/Q) is a key parameter in functional imaging of the lung. Herein, regional VA/Q was calculated from regional values of alveolar partial pressure of oxygen (PAO2) measured by hyperpolarized 3He gas MRI (HP 3He MRI). Yorkshire pigs (n = 7, mean weight = 25 kg) were paralyzed and maintained under isoflurane anesthesia. Animals were placed into a birdcage coil, then transferred to the bore of a 1.5 T MRI unit. Prior to imaging, animals were manually ventilated with room air for 5 min, then a 3He gas mixture was administered during breathhold and imaging performed. PAO2 was measured based on the decay rate of 3He signal. Subjects' blood gas concentrations were measured and these values and PAO2 values entered into a system of four equations with four unknowns. Calculated VA/Q values were analyzed by preparing frequency distributions for the entire lung and compared to VA/Q frequency distributions previously established in the literature as normal using other diagnostic techniques. Distributions were consistent with those in the literature, indicating that HP 3He MRI may be an accurate, quantitative, noninvasive, and nonradioactive method for acquiring VA/Q for small regions of the lung.

Effects of respiratory rate, plateau pressure, and positive end-expiratory pressure on PaO2 oscillations after saline lavage

One of the proposed mechanisms of ventilator-associated lung injury is cyclic recruitment of atelectasis. Collapse of dependent lung regions with every breath should lead to large oscillations in PaO2 as shunt varies throughout the respiratory cycle. We placed a fluorescence-quenching PO2 probe in the brachiocephalic artery of six anesthetized rabbits after saline lavage. Using pressure-controlled ventilation with oxygen, ventilator settings were varied in random order over three levels of positive end-expiratory pressure (PEEP), respiratory rate (RR), and plateau pressure minus PEEP (Delta). Dependence of the amplitude of PaO2 oscillations on PEEP, RR, and Delta was modeled by multiple linear regression. Before lavage, arterial PO2 oscillations varied from 3 to 22 mm Hg. After lavage, arterial PO2 oscillations varied from 5 to 439 mm Hg. Response surfaces showed markedly nonlinear dependence of amplitude on PEEP, RR, and Delta. The large PaO2 oscillations observed provide evidence for cyclic recruitment in this model of lung injury. The important effect of RR on the magnitude of PaO2 oscillations suggests that the static behavior of atelectasis cannot be accurately extrapolated to predict dynamic behavior at realistic breathing frequencies.

Within-breath arterial PO2 oscillations in an experimental model of acute respiratory distress syndrome

Tidal ventilation causes within-breath oscillations in alveolar oxygen concentration, with an amplitude which depends on the prevailing ventilator settings. These alveolar oxygen oscillations are transmitted to arterial oxygen tension, PaO2, but with an amplitude which now depends upon the magnitude of venous admixture or true shunt, QS/QT. We investigated the effect of positive end-expiratory pressure (PEEP) on the amplitude of the PaO2 oscillations, using an atelectasis model of shunt. Blood PaO2 was measured on-line with an intravascular PaO2 sensor, which had a 2-4 s response time (10-90%). The magnitude of the time-varying PaO2 oscillation was titrated against applied PEEP while tidal volume, respiratory rate and inspired oxygen concentration were kept constant. The amplitude of the PaO2 oscillation, delta PaO2, and the mean PaO2 value varied with the level of PEEP applied. At zero PEEP, both the amplitude and the mean were at their lowest values. As PEEP was increased to 1.5 kPa, both delta PaO2 and the mean PaO2 increased to a maximum. Thereafter, the mean PaO2 increased but delta PaO2 decreased. Clear oscillations of PaO2 were seen even at the lowest mean PaO2, 9.5 kPa. Conventional respiratory models of venous admixture predict that these PaO2 oscillations will be reduced by the steep part of the oxyhaemoglobin dissociation curve if a constant pulmonary shunt exists throughout the whole respiratory cycle. The facts that the PaO2 oscillations occurred at all mean PaO2 values and that their amplitude increased with increasing PEEP suggest that QS/QT, in the atelectasis model, varies between end-expiration and end-inspiration, having a much lower value during inspiration than during expiration.

Airway mechanics, gas exchange, and blood flow in a nonlinear model of the normal human lung

A model integrating airway/lung mechanics, pulmonary blood flow, and gas exchange for a normal human subject executing the forced vital capacity (FVC) maneuver is presented. It requires as input the intrapleural pressure measured during the maneuver. Selected model-generated output variables are compared against measured data (flow at the mouth, change in lung volume, and expired O2 and CO2 concentrations at the mouth). A nonlinear parameter-estimation algorithm is employed to vary selected sensitive model parameters to obtain reasonable least squares fits to the data. This study indicates that 1) all three components of the respiratory model are necessary to characterize the FVC maneuver; 2) changes in pulmonary blood flow rate are associated with changes in alveolar and intrapleural pressures and affect gas exchange and the time course of expired gas concentrations; and 3) a collapsible midairway segment must be included to match airflow during a forced expiration. Model simulations suggest that the resistances to airflow offered by the collapsible segment and the small airways are significant throughout forced expiration; their combined effect is needed to adequately match the inspiratory and expiratory flow-volume loops. Despite the limitations of this lumped single-compartment model, a remarkable agreement with airflow and expired gas concentration measurements is obtained for normal subjects. Furthermore, the model provides insight into the important dynamic interactions between ventilation and perfusion during the FVC maneuver.

The alcohol breath test--a review

The alcohol breath test (ABT) is evaluated for variability in response to changes in physiological parameters. The ABT was originally developed in the 1950s, at a time when understanding of pulmonary physiology was quite limited. Over the past decade, physiological studies have shown that alcohol is exchanged entirely within the conducting airways via diffusion from the bronchial circulation. This is in sharp contrast to the old idea that alcohol exchanges in the alveoli in a manner similar to the lower solubility respiratory gases (O2 and CO2). The airway alcohol exchange process is diffusion (airway tissue) and perfusion (bronchial circulation) limited. The dynamics of airway alcohol exchange results in a positively sloped exhaled alveolar plateau that contributes to considerable breathing pattern-dependent variation in measured breath alcohol concentration measurements.

Fractional changes in lung capillary blood volume and oxygen saturation during the cardiac cycle in rabbits

Changes in local pulmonary capillary blood volume (Vc) and oxygen saturation (S) have been difficult to measure in live animals. By utilizing the differences in absorption of light at two wavelengths (650 and 800 nm), we estimated the fractional change in Vc and S during the course of the cardiac cycle in eight anesthetized, ventilated rabbits at low and high lung volumes. Observations were made of the pattern of diffusely backscattered light, from an approximately 1-cm3 volume of lung illuminated with a point source placed on the pleural surface through a thoracotomy. At low lung volume, the fractional change in Vc was approximately 13%, the change in S was approximately 4.6%, and the mean S was close to 77%. The fluctuations in Vc and S lagged behind peak systemic blood pressure by about one-fifth and three-fifths of a cycle, respectively. At high lung volume, there were no important fluctuations in Vc or S, and the mean S was approximately 82%. These results are consistent with fluctuations in pulmonary capillary pressure and gas exchange over the cardiac cycle, and with decreasing capillary compliance with increasing lung volume.

Model analysis of intra-acinar gas exchange

A previously described multibranch-point model, incorporating branching asymmetry within an acinus, has been extended to include gas exchange at the alveolar surface. Using a transport equation for simultaneous convection and diffusion within the gas phase and independent perfusion of all nodes, we obtained steady-state solutions for the temporal and spatial distributions of O2 and CO2 tensions within an acinus during a respiratory cycle. Results for conditions corresponding to both rest and moderate exercise indicated a significant inhomogeneity of gas concentrations within a single acinus. The coefficient of variation of PACO2 at end-inspiration during exercise reached 11.3%. Despite this non-uniformity the computation of a negligible PAO2 - PAO2 difference indicated no impairment in gas exchange. The simulations are consistent with the hypothesis that in the normal lung the whole acinus acts functionally as a gas exchanging unit and ventilation-perfusion inequality has an interacinar basis.

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.

A theoretical model for studying the rate of oxygenation of blood in pulmonary capillaries

A mathematical analysis of the process of gas exchange in the lung is presented taking into account the transport mechanisms of molecular diffusion, convection and facilitated diffusion of the species due to haemoglobin. Since the rate at which blood gets oxygenated in the pulmonary capillaries is very fast, it is difficult to set up an experimental study to determine the effects of various parameters on equilibration rate. The proposed study is aimed at determining the effects of various physiological parameters on equilibration rate in pathological conditions. Among the significant results are that 1. dissolved oxygen takes longer to achieve equilibration across the pulmonary membrane and carbon dioxide attains equilibration faster, 2. the equilibration length increases with increase in blood velocity, haemoglobin concentration, calibre of pulmonary capillaries and fall in alveolar PO2, 3. the alveolar PCO2 and forward and backward reaction rates of haemoglobin with CO2 do not materially affect the equilibration rate or length. 4. At complete equilibration, by the end of the pulmonary capillary 92% of the total haemoglobin has combined with oxygen and 8% free pigment is left which is present as carbamino haemoglobin, met haemoglobin, carboxy haemoglobin etc. These results are of some importance for anaemic conditions, muscular exercise, meditation, altitude physiology, hypo-ventilation, hyperventilation, etc.

Analysis of the effects of pulsatile capillary blood flow and volume on gas exchange

Blood flow into the pulmonary capillaries and the volume of blood within the capillary bed are both pulsatile with the cardiac cycle. We have developed a quantitative model of diffusional gas exchange in the lung to investigate the effects of coupling between these two time-varying parameters on lung O2 and CO2 exchange. For normal man breathing room air at rest, the computed results agree well with previous predictions for the constant flow and volume case, and for the case of pulsatile flow alone. When coupled time-varying pulmonary capillary blood flow and volume are included, using the best data available in the literature to define these parameters, diffusional O2 exchange is improved over the cases of pulsatile flow or volume alone, and closely approximates that obtained for the hypothetical constant flow and volume case. CO2 exchanges, and O2 exchange during hypoxia, are not affected by pulsatile flow and/or volume. These results suggest that O2 exchange is efficient in the presence of coupled blood flow and blood volume pulsations as they exist in the lung capillaries, and that these conditions may be optimal for gas exchange under certain physiological (or pathological) conditions.