The efficiency of ventilation during voluntary hyperpnea: studies in normal subjects and in dyspneic patients with either chronic pulmonary emphysema or obesity

The Inflammatory Profile of Obesity and the Role on Pulmonary Bacterial and Viral Infections

Obesity is a globally increasing health problem, entailing diverse comorbidities such as infectious diseases. An obese weight status has marked effects on lung function that can be attributed to mechanical dysfunctions. Moreover, the alterations of adipocyte-derived signal mediators strongly influence the regulation of inflammation, resulting in chronic low-grade inflammation. Our review summarizes the known effects regarding pulmonary bacterial and viral infections. For this, we discuss model systems that allow mechanistic investigation of the interplay between obesity and lung infections. Overall, obesity gives rise to a higher susceptibility to infectious pathogens, but the pathogenetic process is not clearly defined. Whereas, viral infections often show a more severe course in obese patients, the same patients seem to have a survival benefit during bacterial infections. In particular, we summarize the main mechanical impairments in the pulmonary tract caused by obesity. Moreover, we outline the main secretory changes within the expanded adipose tissue mass, resulting in chronic low-grade inflammation. Finally, we connect these altered host factors to the influence of obesity on the development of lung infection by summarizing observations from clinical and experimental data.

Phenomenological and Thermodynamic Model of Gas Exchanges in the Placenta during Pregnancy: A Case Study of Intoxication of Carbon Monoxide

The present work simulates the transport of oxygen, carbon dioxide, and carbon monoxide between a fetus's circulatory system and the mother's. The organ responsible for this exchange is the placenta. Carbon monoxide is a common air pollutant, and it impacts the physiological conditions even in low concentration. The impacts of carbon monoxide are especially dangerous for pregnant women, fetuses, and newborn babies. A model of carbon monoxide transport, from the literature, is modified to simulate a pregnant woman (original model was a male), therefore changing some parameters to express the adjusted respiratory system. It was considered the gas exchange in the placenta, to evaluate the concentration of these different gases in the fetus arterial and venous blood. Three methods of the exergy analysis are implemented for both mother and fetus respiratory systems, aiming at the comparison with the respiratory system of a male adult. The destroyed exergy of the literature did not have the same trend as the models proposed in this article, taking into consideration the hemoglobin reactions. In contrast, the entropy generation associated only with the diffusion transport phenomena was one order of magnitude lower than the other methods. The placenta destroyed exergy rate is significantly higher compared to the irreversibilities of the mother's respiratory system. One possible explanation is the fact that the placenta has other physiological functions than gas transportation.

The Effect of Carbon Monoxide on the Exergy Behavior of the Lungs

The present work evaluates the impact of carbon monoxide (CO) inhalation on the human lung’s exergy behavior by considering different levels of intoxication and amounts of hemoglobin. Its impact is significant because CO is one of the most common air pollutants in cities and an increase in destroyed exergy may be correlated with lifespan reduction or the malfunctioning of certain human organs. An evaluation of the severity of intoxication as a function of city altitude may intensify the hazard associated with carbon monoxide. A computational model of human lungs obtained from the literature was used to calculate the concentrations of oxygen (O₂), carbon monoxide (CO), and carbon dioxide (CO₂) in the respiratory system. With the purpose of better evaluating the different levels of CO intoxication and hemoglobin concentration (which is a function of acclimatization time and some pathologies, such as anemia), a model calculating exergy efficiency for the lungs was proposed. From this model, it was possible to conclude that a higher level of intoxication is associated with lower exergy efficiency values. When associated with carbon monoxide intoxication, higher hemoglobin levels also result in lower efficiency. Eventually, a comparison between previous studies and the current study was carried out, regarding the method employed to calculate the exergy destroyed in the lungs, considering not only gas transport, but also hemoglobin concentration and its reaction with the gases from a second law perspective.

Effect of weight loss on operational lung volumes and oxygen cost of breathing in obese women

BACKGROUND

The effects of moderate weight loss on operational lung volumes during exercise and the oxygen (O2) cost of breathing are unknown in obese women but could have important implications regarding exercise endurance.

METHODS

In 29 obese women (33±8 years, 97±14 kg, body mass index: 36±4 kg m(-2), body fat: 45.6±4.5%; means±s.d.), body composition, fat distribution (by magnetic resonance imaging), pulmonary function, operational lung volumes during exercise and the O2 cost of breathing during eucapnic voluntary hyperpnea (([Vdot ]O2) vs ([Vdot ]E) slope) were studied before and after a 12-week diet and resistance exercise weight loss program.

RESULTS

Participants lost 7.5±3.1 kg or ≈8% of body weight (P<0.001), but fat distribution remained unchanged. After weight loss, lung volume subdivisions at rest were increased (P<0.05) and were moderately associated (P<0.05) with changes in weight. End-expiratory lung volume (percentage of total lung capacity) increased at rest and during constant load exercise (P<0.05). O2 cost of breathing was reduced by 16% (2.52±1.02-2.11±0.72 ml l(-1); P=0.003). As a result, O2 uptake of the respiratory muscles ([Vdot ]O2Resp), estimated as the product of O2 cost of breathing and exercise ([Vdot ]E) during cycling at 60 W, was significantly reduced by 27±31 ml (P<0.001), accounting for 46% of the reduction in total body ([Vdot ]O2) during cycling at 60 W.

CONCLUSIONS

Moderate weight loss yields important improvements in respiratory function at rest and during submaximal exercise in otherwise healthy obese women. These changes in breathing load could have positive effects on the exercise endurance and adherence to physical activity.

Nutrition and care considerations in the overweight and obese population within the critical care setting

Nutrition and care considerations in the overweight and obese population within the critical care setting are multifaceted. Patients requiring critical care have specialized care management needs that often times challenge health care providers. When patients are obese, this further complicates the physiologic aspects of healing, thus creating challenges to meeting both the nutritional needs of the individual and hampering treatment. This article reviews the care considerations, physiology of bariatric patients, and challenges of providing safe and quality care, including current evidence-based practice strategies developed to provide optimal support for obese patients during hospitalization and within the critical care setting.

Environmental perturbations: Obesity

Obesity currently affects about one-third of the U.S. population, while another one-third is overweight. The importance of obesity for certain conditions such as heart disease and type 2 diabetes is well appreciated. The effects of obesity on the respiratory system have received less attention and are the subject of this article. Obesity alters the static mechanical properties of the respiratory system leading to a reduction in the functional residual capacity (FRC) and the expiratory reserve volume (ERV). There is substantial variability in the effects of obesity on FRC and ERV, at least some of which is related to the location rather than the total mass of adipose tissue. Obesity also results in airflow obstruction, which is only partially attributable to breathing at low lung volume, and can also promote airway hyperresponsiveness and asthma. Hypoxemia is common is obesity and correlates well with FRC, as well as with measures of abdominal obesity. However, obese subjects are usually eucapnic, indicating that hypoventilation is not a common cause of their hypoxemia. Instead, hypoxemia results from ventilation-perfusion mismatch caused by closure of dependent airways at FRC. Many obese subjects complain of dyspnea either at rest or during exertion, and the dyspnea score also correlates with reductions in FRC and ERV. Weight reduction should be encouraged in any symptomatic obese individual, since virtually all of the respiratory complications of obesity improve with even moderate weight loss.

Respiratory function and the obesity paradox

PURPOSE OF REVIEW

Obese individuals have impaired respiratory function relative to their normal-weight counterparts. Despite these negative effects, obesity is paradoxically associated with better survival in individuals with chronic obstructive pulmonary disease (COPD). The purpose of this review is to describe this 'obesity paradox', to discuss the effects of obesity on respiratory function, and to speculate as to whether obesity-related alterations in respiratory mechanics can influence the natural history of COPD.

RECENT FINDINGS

Given the known negative effects of obesity on respiratory physiology, it is reasonable to predict that obese COPD patients would be more likely to experience greater dyspnea and exercise intolerance relative to COPD patients of normal weight. However, recent evidence suggests that obese COPD patients have similar or better dyspnea scores during exercise and do not have diminished exercise capacity. These observations may be attributable to the fact that obese COPD patients have reduced operating lung volumes and higher inspiratory capacity to total lung capacity ratios than their lean COPD counterparts.

SUMMARY

Obese patients with COPD do not appear to be at a disadvantage during exercise relative to lean COPD patients. Obesity may be associated with improved survival in COPD but specific mechanisms for this paradox remain to be elucidated.

Cigarette smoking is associated with subclinical parenchymal lung disease: the Multi-Ethnic Study of Atherosclerosis (MESA)-lung study

RATIONALE

Cigarette smoking is a risk factor for diffuse parenchymal lung disease. Risk factors for subclinical parenchymal lung disease have not been described.

OBJECTIVES

To determine if cigarette smoking is associated with subclinical parenchymal lung disease, as measured by spirometric restriction and regions of high attenuation on computed tomography (CT) imaging.

METHODS

We examined 2,563 adults without airflow obstruction or clinical cardiovascular disease in the Multi-Ethnic Study of Atherosclerosis, a population-based cohort sampled from six communities in the United States. Cumulative and current cigarette smoking were assessed by pack-years and urine cotinine, respectively. Spirometric restriction was defined as a forced vital capacity less than the lower limit of normal. High attenuation areas on the lung fields of cardiac CT scans were defined as regions having an attenuation between -600 and -250 Hounsfield units, reflecting ground-glass and reticular abnormalities. Generalized additive models were used to adjust for age, gender, race/ethnicity, smoking status, anthropometrics, center, and CT scan parameters.

MEASUREMENTS AND MAIN RESULTS

The prevalence of spirometric restriction was 10.0% (95% confidence interval [CI], 8.9-11.2%) and increased relatively by 8% (95% CI, 3-12%) for each 10 cigarette pack-years in multivariate analysis. The median volume of high attenuation areas was 119 cm(3) (interquartile range, 100-143 cm(3)). The volume of high attenuation areas increased by 1.6 cm(3) (95% CI, 0.9-2.4 cm(3)) for each 10 cigarette pack-years in multivariate analysis.

CONCLUSIONS

Smoking may cause subclinical parenchymal lung disease detectable by spirometry and CT imaging, even among a generally healthy cohort.

Effects on breathing of focal acidosis at multiple medullary raphe sites in awake goats

To gain insight into why there are chemoreceptors at widespread sites in the brain, mircrotubules were chronically implanted at two or three sites in the medullary raphe nuclei of adult goats (n = 7). After >2 wk, microdialysis (MD) probes were inserted into the microtubules to create focal acidosis (FA) in the awake state using mock cerebral spinal fluid (mCSF) equilibrated with 6.4% (pH = 7.3), 50% (pH = 6.5), or 80% CO(2) (pH = 6.3), where MD with 50 and 80% CO(2) reduces tissue pH by 0.1 and 0.18 pH unit, respectively. There were no changes in all measured variables with MD with 6.4% at single or multiple raphe sites (P > 0.05). During FA at single raphe sites, only 80% CO(2) elicited physiological changes as inspiratory flow was 16.9% above (P < 0.05) control. However, FA with 50 and 80% CO(2) at multiple sites increased (P < 0.05) inspiratory flow by 18.4 and 30.1%, respectively, where 80% CO(2) also increased (P < 0.05) tidal volume, heart rate, CO(2) production, and O(2) consumption. FA with 80% CO(2) at multiple raphe sites also led to hyperventilation (-2 mmHg), indicating that FA had effects on breathing independent of an increased metabolic rate. We believe these findings suggest that the large ventilatory response to a global respiratory brain acidosis reflects the cumulative effect of stimulation at widespread chemoreceptor sites rather than a large stimulation at a single site. Additionally, focal acidification of raphe chemoreceptors appears to activate an established thermogenic response needed to offset the increased heat loss associated with the CO(2) hyperpnea.

Pulmonary complications of obesity

Obesity can profoundly alter pulmonary function and diminish exercise capacity by its adverse effects on respiratory mechanics, resistance within the respiratory system, respiratory muscle function, lung volumes, work and energy cost of breathing, control of breathing, and gas exchange. Weight loss can reverse many of the alterations of pulmonary function produced by obesity. Obesity places the patient at risk of aspiration pneumonia, pulmonary thromboembolism, and respiratory failure. It is the most common precipitating factor for obstructive sleep apnea and is a requirement for the obesity hypoventilation syndrome, both of which are associated with substantial morbidity and increased mortality. There are numerous medical and surgical therapies for obstructive sleep apnea and obesity hypoventilation. Weight reduction in the obese is among the most effective of these measures.

The impact of morbid obesity on oxygen cost of breathing (VO(2RESP)) at rest

Oxygen consumption dedicated to respiratory work (V O(2RESP)) during quiet breathing is small in normal patients. In the morbidly obese, at high minute ventilations, VO(2RESP) is greater than in normal patients, but VO(2RESP) during quiet breathing in these patients is not known. We postulated that such patients have increased VO(2RESP) at rest which may predispose them to respiratory failure when additional respiratory workloads are imposed. We measured baseline VO(2) in morbidly obese patients immediately prior to gastric bypass surgery and again after intubation, mechanical ventilation, and paralysis, and compared their change in VO(2) to nonobese patients scheduled for elective abdominal surgery. Baseline VO(2) was higher in the obese patients compared with control patients (354.6 versus 221.4 ml/min; p = 0.0001) and the change in VO(2) from spontaneous breathing to mechanical ventilation was significant in the obese patients (354.6 versus 297.2 ml/min; p = 0.0002) but not the control patients (221.4 versus 219.8 ml/min; p = 0.86). We conclude that morbidly obese patients dedicate a disproportionately high percentage of total VO(2) to conduct respiratory work, even during quiet breathing. This relative inefficiency suggests a decreased ventilatory reserve and a predisposition to respiratory failure in the setting of even mild pulmonary or systemic insults.

Reliability of a new device to assess the oxygen consumption of human respiratory muscles

PURPOSE

This study tests the reliability of a new device for assessing the oxygen consumption of the respiratory muscles (VO2 resp.).

METHODS

Fourteen healthy male volunteers participated in the study. The device consists of an expandable external ventilatory dead space created with pieces of plastic tubing and a spirometer filled with 100% oxygen. It also incorporates a carbon dioxide absorber. Total VO2 (VO2 tot.) was recorded from the spirometric closed circuit and ventilation (V(E)), from the spirometer tracing. For each subject the test procedure was carried out in duplicate (T1 and T2) after an overnight fast. The dead space was increased at a constant rate of 260 mL every 90 s, and VO2 tot. and V(E) increased progressively. Because log VO2 tot. was linearly related to V(E), we calculated the slope value (log VO2-V(E)) and the Y-intercept (VE = 0) of the semilog regression representing, respectively, VO2 resp. and metabolic VO2 (VO2 met.).

RESULTS

When compared with values in the literature, these values did not differ from those recorded in subjects of a similar age group. The VO2 resp. and VO2 met. calculated in T1 and T2 were not different (VO2 resp. = 0.0066 +/- 0.0005 for T1 vs 0.0067 +/- 0.0005 log mL x min(-1)/L x min(-1) for T2 and VO2 met. = 269.3 +/- 28.6 for T1 vs 281.9 +/- 24.1 mL x min(-1) for T2). The coefficients of variation were: 25% at T1 and 23% at T2 for VO2 resp. and 34% at T1 and 29% at T2 for VO2 met. Moreover, significant correlations (r = 0.96, P < 0.001 for VO2 resp., r = 0.95, P < 0.001 for VO2 met.), high coefficients of determination (r2 = 0.92 for VO2 resp., r2 = 0.90 for VO2 met.) and negligible SEE (0.0005 for VO2 resp., 0.2 mL x min(-1) for VO2 met.) were found between the two tests. When we plotted the mean values of VO2 resp. and VO2 met. measured at T1 and T2 against their respective differences, more than 95% of the slight differences ranged between the limits defined by mean value +/- 2 SD, reflecting the small discrepancy between duplicate measurements.

CONCLUSION

The results confirm that the test performed with this device is useful and reliable for assessing the VO2 resp. in healthy subjects.

Breathing efficiency during inspiratory threshold loading in patients with chronic obstructive pulmonary disease

Patients with chronic obstructive pulmonary disease (COPD) demonstrate an increased oxygen cost of breathing. It is as yet unclear whether this is related to a decreased breathing efficiency. The aim of the present study was to compare breathing efficiency in 16 patients with COPD (11 men, five women) and 16 healthy elderly subjects (seven men, nine women), and to investigate a possible relationship between breathing efficiency and resting energy expenditure (REE). REE was measured using a ventilated hood system. Breathing efficiency was assessed by measuring oxygen consumption (V'O2), mean inspiratory mouth pressure (MIP) and flow during breathing at rest and subsequently during breathing against an inspiratory threshold (40% of maximal inspiratory pressure). During loaded breathing there was a significant increase in V'O2, MIP, and external work of breathing compared with unloaded breathing in both groups. As intended, ventilation did not increase significantly during the breathing efficiency test in the patients with COPD. The breathing efficiency (median, range) of the patients with COPD was similar (3.7%, 1.4-8.7%) to that of the healthy elderly subjects (3.2%, 1.7-8.3%). Breathing efficiency was not correlated with REE in either group. In the present study, in which dynamic hyperinflation was probably prevented, no difference in breathing efficiency was found between healthy elderly subjects and COPD patients when breathing against an external inspiratory threshold. Furthermore, breathing efficiency was not related to REE in both groups.

Oxygen cost of ventilation in the resting horse

The purpose of this study was to develop a technique to measure the oxygen cost of ventilation and the values of ventilatory parameters in seven normal horses rebreathing carbon dioxide (CO2). All the horses responded to increased inspiratory levels of CO2 by increasing their tidal volume (VT) and frequency of breathing (Vf). The mean (SE) oxygen cost litre-1 of ventilation, measured at rates of ventilation greater than 200 litres min-1 was 1.7 (0.04) ml litre-1, similar to that of normal human subjects ventilating submaximally. It was concluded that the CO2 rebreathing test is a practical, non-invasive means of measuring the oxygen cost of breathing and the ventilatory response to CO2 in horses.

Intravenously administered atrial natriuretic factor in patients with COPD. Effects on ventilation-perfusion relationships and pulmonary hemodynamics

The potent pulmonary vasodilating property of atrial natriuretic factor (ANF) may alter gas exchange in patients with COPD. We examined the hemodynamic and gas exchange responses to intravenous infusion of ANF (0.01 and 0.03 ng/min/kg body weight) in eight stable patients with COPD studied during spontaneous breathing, using the inert gas elimination technique. When compared with baseline, ANF infusion was associated with a dose-dependent decrease in pulmonary artery pressure (from 27.3 +/- 2.5 to 23.9 +/- 1.8 and 20.2 +/- 1.7 mm Hg, respectively) and a dose-dependent increase in blood flow perfusing poorly ventilated and unventilated units (VA/Q < 0.1: from 5.80 +/- 2.05 to 7.25 +/- 2.5 and 12.0 +/- 5.4 percent of total blood flow, respectively; p = 0.02). However, PaO2 remained unchanged (70.2 +/- 3.6, 68.1 +/- 3.8 65.4 +/- 3.5 mm Hg, respectively) because of a significant increase in minute ventilation (VE) from 8.6 +/- 0.8 to 9.6 +/- 0.8 and 10.3 +/- 0.7 L/min (p < 0.002). Six additional COPD patients receiving intravenously administered ANF at the same dosages were studied during controlled mechanical ventilation using right heart catheterization. In these patients, pulmonary vasodilation was associated with a significant increase in venous admixture (from 12.7 +/- 2.4 to 14.4 +/- 2.9 and 17.5 +/- 3.5 percent of total blood flow, respectively; p < 0.02), and a dose-dependent reduction in arterial PO2 (from 117 +/- 17 to 110 +/- 15 and 96.4 +/- 8.8 mm Hg, respectively; p < 0.05). The present results show that ANF infusion is associated with alterations in the VA/Q relationship in patients with COPD. However, a decrease in arterial oxygenation may be prevented by an increase in VE.

Oxygen consumption of respiratory muscles in patients with COPD

We measured the oxygen consumption (VO2) of respiratory muscles in 8 COPD patients and 12 age-matched healthy subjects using a closed circuit device which allows a continuous increase in external dead space and is equipped with a 9-L Collins spirometer. Furthermore, we measured simultaneously mouth occlusion pressure at 0.1 s of inspiration (P0.1), minute ventilation (VE), and other ventilatory parameters during the measurement of total VO2 (VO2 tot). We found that the logarithm of VO2tot (logVO2tot) had a good correlation with VE in both groups. The mean slope of the regression line of logVO2tot and VE (delta logVO2tot/delta VE) of COPD patients was significantly higher than that of normal subjects (p < 0.001). However, the mean Y-intercept (metabolic VO2[VO2met]) of the regression lines did not differ between the two groups. The P0.1 in COPD patients was higher than that in normal subjects at the corresponding dead space loading. However, the VE did not differ between the two groups except for at rest and the first 1 min after dead space loading. These results suggest that the VO2 of respiratory muscles in patients with COPD is higher at given ventilation compared with that in age-matched normal subjects and that this increased VO2 partly may be due to an augmented ventilatory drive.

Respiratory muscles and ventilatory failure: 1993 perspective

Some conditions that predispose to ventilatory failure increase the work of breathing (chronic obstructive pulmonary disease [COPD], obesity, kyphoscoliosis), whereas others cause severe respiratory muscle weakness. Specific reasons for muscle weakness include critical illness (electrolyte imbalance, acidemia, shock, sepsis), chronic illness (poor nutrition, cachexia), and neuromuscular diseases. Inspiratory muscle weakness from mechanical disadvantage to the diaphragm is characteristic of asthma and COPD. The increased work of breathing combined with muscle weakness increases the pressure needed to inspire a breath and decreases maximal inspiratory pressure. When this pressure exceeds 0.4, dyspnea and inspiratory muscle fatigue ensue. One way to lower this pressure and avert fatigue is to lower the tidal volume. Ventilatory drive is high, not low, in ventilatory failure. Concomitant shortening of inspiration and breath duration cause the small tidal volume and increased respiratory rate. Gas exchange is compromised by ventilation/perfusion imbalance, and the ratio of dead space to tidal volume is also increased by rapid, shallow breathing. Reduction in tidal volume minimizes dyspnea, but the small tidal volume is inadequate for gas exchange. Acute treatment of respiratory muscle failure involves respiratory muscle rest through mechanical ventilation and removal of noxious influences (infection, metabolic disarray), whereas chronic treatment involves rebuilding the contractile apparatus by nutritional repletion and training.

Oxygen consumption and ventilation during normal labor

Oxygen consumption (VO2) and minute ventilation (VE) were measured breath-by-breath for 10 min periods in the third trimester of pregnancy in 16 healthy women. These measurements were repeated during the first stage of labor in eight of the women. The 10-min mean VO2 was 3.56 ml/kg/min (+/- 0.82 SD) at term and 4.28 ml/kg/min (+/- 0.93) during labor, for an average increase of 23 percent (+/- 28 percent, p = 0.04) from third trimester to labor. The mean VE was 0.15 L/kg/min (+/- 0.03) at term and increased significantly (p = 0.05) to 0.24 L/kg/min (+/- 0.11) during labor for an average increase in VE of 65 percent (+/- 78 percent). Peak VO2 and VE occurred during contractions with five-breath average peak VO2 being 86 percent (+/- 53%) above the 10-min mean value at term and VE increasing 167 percent (+/- 154 percent) from third trimester to peak values during labor. These data may be useful in identifying patients at risk for developing respiratory insufficiency during labor. We propose an algorithm for approaching the obstetric patient with respiratory disease.

Chronic lung disease in the sleep apnea syndrome

Several well controlled epidemiologic and hemodynamic studies suggest that about 20% of sleep apnea syndrome (SAS) patients will have chronic obstructive pulmonary disease (COPD), and the majority of these patients (with combined diseases) will have pulmonary hypertension. Indeed it has been suggested that only patients with underlying hypoxemia, such as that from COPD, will develop right heart failure in the OSA setting. Experience shows that apnea/COPD patients will have severe hypersomnolence associated with the OSA, cough and dyspnea with the airways disease, and edema and plethora related to chronic hypoxemia. Many patients present with respiratory failure and are diagnosed at the time of initial intubation and mechanical ventilation. Episodic nocturnal hypoxemia may be worsened by a steeper rate of desaturation due to lower alveolar and blood oxygen stores, and longer apneas perhaps contributed to by depressed chemosensitivity. Daytime hypoxemia may also add to the severe hemodynamic disturbances. Since COPD cannot be cured, aggressive treatment of SAS is critical. Past studies have shown that tracheostomy or nasal CPAP in this setting not only leads to resolution of episodic nocturnal desaturation but may lead to rapid improvement in daytime oxygenation in many patients. Pulmonary hypertension and other measures of cardiopulmonary function improve when apnea is cured. Elimination of the SAS may disclose nonapneic REM related desaturation that could require supplemental oxygen therapy in addition to tracheostomy or nasal CPAP. Pulmonary function testing in SAS patients with smoking histories, followed by aggressive treatment of SAS, is recommended.

Airway insufflation: physiologic effects on acute and chronic gas exchange in humans

Reduction in dead space through conventional tracheostomy has been used to treat patients with chronic CO2 retention. The insufflation of air directly into the trachea by transtracheal catheter (airway insufflation, AI) provides reductions in dead space as great or greater than those of tracheostomy. The physiologic effects of AI on gas exchange have not been adequately studied because instillation of gases into the trachea contaminates minute ventilation (VL), dead space volume (VD), tidal volume (VT), and other indices of gas exchange, as measured by usual technics. We overcame this problem by devising special methods of measuring inspired and expired ventilation, alveolar and dead space ventilation, and VT and VD by using pneumotachographic timing of inspiration and expiration so that true inspired and expired ventilation were calculated. We studied 5 patients with chronic CO2 retention from either COPD, scoliosis, or muscular dystrophy (annual average PaCO2 = 45 to 75 mm Hg) during 75 min of AI with serial gas exchange and arterial blood gas measurements. AI at about 5 L/min of room air through the trachea in 5 patients reduced VL by 18% (from 7.91 to 6.48 L/min), VT by 25% (from 450 to 338 ml), and VD by 37% (from 223 to 141 ml), while not affecting PaCO2 (from 51.8 to 48.2 mm Hg) or PaO2 (from 65.1 to 63.4 mm Hg). In 2 patients, AI administered continuously for 4 to 12 months (as 30 to 50% O2) maintained PaCO2 as well as or better than breathing enriched O2 from a tracheal collar via an open tracheostomy.(ABSTRACT TRUNCATED AT 250 WORDS)

Oxygen consumption of the respiratory muscles in normal and in malnourished patients with chronic obstructive pulmonary disease

Patients with severe chronic obstructive pulmonary disease (COPD) commonly experience weight loss. An increased energy expenditure for respiration might explain the increased caloric requirements and weight loss seen in this patient population. We measured the oxygen cost of augmenting ventilation (O2 cost) using an open circuit technique with dead-space stimulation of ventilation in nine normally nourished (greater than 90% ideal body weight) and in 10 malnourished (less than 90% ideal body weight) patients with COPD as well as in seven normal control subjects. O2 cost was significantly elevated in the malnourished patients with COPD (4.28 +/- 0.98 ml O2/L ventilation) relative to the normally nourished group (2.61 +/- 1.07) and the normal control subjects (1.23 +/- 0.51) (p less than 0.001). The measured resting energy expenditure (REEmeas) was also increased compared with predicted values (REEpred) in the malnourished population (REEmeas/REEpred = 94.57 +/- 6.21% for control subjects, 105.5 +/- 19.66% for normally nourished patients with COPD, and 119.4 +/- 11.69% for malnourished patients with COPD) (p less than 0.005). The malnourished population was characterized by a greater degree of hyperinflation (RV/TLC = 0.55 +/- 0.09 for normally nourished versus 0.69 +/- 0.06 for malnourished patients) and inspiratory muscle weakness (PImax = 51 +/- 16.5 for the normally nourished and 34 +/- 12.2 for the malnourished population). We conclude that malnourished patients with COPD are characterized by a relative increase in resting energy requirements and, specifically, increased energy requirements for augmenting ventilation. This increase in energy requirements may result from the increased mechanical work load associated with severe COPD and/or a reduced ventilatory muscle efficiency.

Oxygen uptake during weaning from mechanical ventilation

Total body oxygen uptake (VO2) increases during the transition from machine-assisted ventilation to spontaneous breathing. Since the volume of oxygen consumed by the respiratory muscles must contribute to the increase in VO2 (delta VO2), we explored whether delta VO2 and/or measurements of respiratory power output (Wresp) provide clinically useful information in the evaluation of disease state and weaning decisions in patients with respiratory failure. We determined the metabolic, ventilatory, and hemodynamic responses of ten patients during weaning from controlled mechanical ventilation, and compared delta VO2 and Wresp of patients without overt heart-lung disease (group 1) to that of patients with significant cardiopulmonary dysfunction and ventilator-dependent respiratory failure (group 2). We reasoned that for delta VO2 to be clinically useful, individual values must either clearly differ between groups, must be higher in patients with heart-lung disease, and/or correlate with weaning outcome and independent measurements of respiratory work. The VO2 increased in nine of ten patients. The differences between the groups in the values of delta VO2 (27 ml/min and 49 ml/min) and respiratory power (9.38 J/min and 11.99 J/min) were not significant. delta VO2 and Wresp were not correlated (r = 0.2), and neither predicted weaning outcome. We conclude that the sensitivity and specificity of delta VO2 and Wresp appear insufficient for evaluation of disease state and weaning decisions in individual patients.

Efficiency of work of inspiratory muscles in standing and in exercising dogs

The objective of this study was to elucidate the resistive, elastic, and total inspiratory muscle work and mainly the efficiency of the inspiratory muscle work in still standing and in exercising dogs. In 180 experiments with nine mongrel dogs, respiratory and circulatory functions were recorded at rest and during a 1.5-mph walk on a treadmill belt which was inclined by 9 degrees. Each dog had been trained and surgically prepared before the series of experiments started. An electronic pressure transducer together with a fluid-filled catheter was attached to the ribs at the midthoracic level. Airflow was monitored with a pneumotachygraph attached to a face mask. Aortic flow was monitored with an electromagnetic flow transducer. Catheters in the atria, the ventricles, and the pulmonary and systemic vessels were used to determine hemodynamic parameters and blood gas levels. All recorded and all derived functions were evaluated by a dedicated computer. At rest, resistive inspiratory work (0.98 kg cm) was smaller than elastic work (1.36 kg cm). Elastic work did not change with exercise but resistive work increased to 1.46 kg.cm. The increase of total work from 2.33 kg.cm at rest to 2.79 kg.cm during exercise did not reach significance. Inspiratory power increased with exercise. Estimated diaphragmatic oxygen consumption increased from 2.23 ml/min at rest to 3.64 ml/min during exercise. It was 1.6 percent of the total oxygen consumption at rest and 1.7 percent of the total oxygen consumption during exercise. The estimated efficiency of the work of inspiratory muscles was 15.8 percent at rest and 25.8 percent during exercise. These values did not vary substantially between different animals.

Metabolic and respiratory changes during weaning from mechanical ventilation

Weaning from mechanical ventilation is a procedure performed daily in intensive care units. This study sought to determine whether among postoperative patients there were any differences in the changes in oxygen consumption (VO2) and carbon dioxide production (VCO2) between those patients in whom mechanical ventilation was successfully discontinued and those in whom it was continued or reinstituted. A stepwise reduction in mandatory breaths (from 10 to 12 to 4 to 6), followed by a period of continuous positive airway pressure (CPAP), was the weaning method. In the group of patients (N = 18) who were successfully weaned, VO2 and VCO2 increased 10 +/- 8 (SD) percent and 10 +/- 9 percent, respectively, while VE decreased 9 +/- 8 percent and PaCO2 was unchanged when values at an IMV of 10 to 12 were compared with those on CPAP. In the group (N = 17) who were not successfully weaned, VO2 and VCO2 increased 8 +/- 10 percent and 6 +/- 9 percent, respectively, while PaCO2 rose (37.9 +/- 4 to 42.5 +/- 2.9) significantly (p less than 0.02). There was a significantly greater decrease (15 +/- 3 percent) in VE than in the other group. Changes in VO2 or VCO2 did not aid in predicting which patients would be successfully weaned.

Measuring the oxygen cost of breathing in normal adults and patients with cystic fibrosis

It has been suggested that the oxygen consumption of the respiratory muscles (VO2 resp) may play a role in limiting exercise performance in both healthy subjects and those with chronic airflow limitation (CAL). In order to measure VO2 resp reproducibly at both rest and on exercise, ventilation (VE) and total oxygen consumption (VO2) in 3 normal subjects and in 3 patients with cystic fibrosis were measured while breathing air and again when VE was stimulated by the addition of CO2 to the inspired gas. Since external work was the same it was assumed that any changes in VO2 would be due to the increase in VO2 resp during stimulated breathing allowing for the calculation of VO2 resp. The oxygen cost of breathing was higher in the patients with the increasing ventilation of exercise. These values were reproducible on repetitive measurements. It is concluded that the method employed is applicable in normal subjects and in patients with CAL, and that the O2 cost of breathing is higher in patients with CAL. The O2 cost of breathing increases as VE increases but even during exercise the VO2 resp is only a small fraction of the total VO2 and is unlikely to be a determining factor limiting exercise performance in either normal subjects or those with CAL.

Oxygen cost of breathing. Changes dependent upon mode of mechanical ventilation

We describe a patient with respiratory failure who demonstrated marked increases in O2 consumption (VO2) when breathing with synchronized intermittent mandatory mechanical ventilation (SIMV). When the mode of ventilation was changed to facilitate inspiratory gas flow (pressure-support) during spontaneous breathing, O2 consumption decreased 27 percent. Several important factors contributing to the increased O2 cost of breathing in patients requiring mechanical ventilation are reviewed, including the high internal resistance of demand-flow SIMV systems.

Cardiovascular effects of obesity and hypertension

Although often coexisting in the same patient, obesity and essential hypertension exert disparate cardiovascular effects. An excess of adipose tissue augments cardiac output, stroke volume, and left ventricular filling pressure, expands intravascular volume, and lowers total peripheral resistance. In contrast, essential hypertension in a non-obese patient is associated with a contracted intravascular volume, high total peripheral resistance, and normal cardiac output, but increased left ventricular stroke work due to high afterload. Left ventricular adaptation will consist of eccentric hypertrophy in the obese (irrespective of arterial pressure) and concentric hypertrophy in the non-obese hypertension patient. The combination of obesity and hypertension burdens the heart with high preload and high afterload, thereby greatly enhancing the risk of congestive heart failure. Peripheral resistance and intravascular volume may be normal in mildly hypertension obese patients because of the mutually antagonising effects of the increase in arterial pressure and the increase in body weight. The fall in arterial pressure associated with weight loss seems to be caused by a decrease in adrenergic activity which leads to a fall in cardiac output without change in vascular resistance. Obesity hypertension may be the result of an inappropriately raised cardiac output in the presence of a relatively restricted arterial capacity due to the low vascularity of adipose tissue. In morbid obesity increased blood viscosity may contribute to the raised arterial pressure.

Sleep and respiration of rats during hypoxia

1. The effects of hypoxia on slow-wave sleep (SWS) and of SWS on respiratory responses to hypoxia were investigated on rats provided with chronically implanted cortical electrodes. 2. During the daytime (5-7 hr periods) the proportion of time spent in SWS was 45% (S.E. +/- 1.0%) when the rats breathed air. Exposure to 10% O2 (equivalent to 18,000 ft.) reduced this proportion to 27% (S.E. +/- 2.5%). During hypoxia the intensity of e.e.g. activity in SWS (mean, rectified slow-wave voltage) rarely equalled the normal values characteristic of the same rats in fully developed SWS breathing air. The normal pattern of 5-15 min episodes of SWS was changed by hypoxia to a series of brief (2-3 min) incompletely developed episodes. 3. Addition of CO2 to inspired gas failed to prevent the reduction of SWS during hypoxia. CO2 in normal O2 did not alter sleep significantly. The effects of hypoxia on sleep therefore depend upon changes in O2 pressure rather than upon changes in CO2. 4. The effect of SWS on respiration of rats breathing air was to decrease frequency and minute volume by 10-20%. In hypoxia, however, the frequency increased markedly when the animals entered SWS ; minute volume was not significantly changed. It follows that stimulation of breathing by hypoxia is greater during SWS than during wakefulness. 5. The anomalous increase of respiratory frequency when hypoxic rats entered SWS was abolished by addition of CO2 to the hypoxic gas mixture. 6. Steady-state gaseous metabolism (Vo2 Vco2) was decreased 18 +/- 3% during hypoxia and was increased 31 +/- 4% during exposure to 5% CO2. The implications of these changes for interpretation of respiratory responses to O2 and CO2 are discussed.

The relationship of respiratory failure to the oxygen consumption of, lactate production by, and distribution of blood flow among respiratory muscles during increasing inspiratory resistance

An animal model was developed to determine if blood flow to the respiratory muscles limits oxygen delivery and thus work output during inspiratory resistance. With incremental increases in the rate of work of breathing to 15 times the resting level, blood flow to the diaphragm rose exponentially 26-fold. Blood flow to other inspiratory and a few expiratory muscles increased to a much smaller extent, often only at the greater work loads. Cardiac output and blood pressure did not change. Arterial-venous oxygen content difference across the diaphragm became maximal at low work rates and thereafter all increases in oxygen delivery during higher work rates were accomplished by increments in blood flow. Oxygen consumption of the respiratory musculature calculated by blood flow times oxygen extraction increased exponentially with increasing work of breathing and was less than the increase in total body oxygen consumption at each work load. Hypoxemia and respiratory acidosis occurred when the animals inspired through the highest resistance; blood flow and oxygen consumption were even higher than that observed during previous resistances and there was no evidence of a shift to anaerobic metabolsim in blood lactate and pyruvate levels. Respiratory failure did not appear to be a consequence of insufficient blood flow in this model.

The distribution of blood flow, oxygen consumption, and work output among the respiratory muscles during unobstructed hyperventilation

An animal model was developed to describe respiratory muscle work output, blood flow, and oxygen consumption during mechanical ventilation, resting spontaneous ventilation, and the increased unobstructed ventilatory efforts induced by CO2 rebreathing. Almost all of the work of breathing was inspiratory work at all ventilatory levels; thus, only blood flows to the diaphragm and external intercostals increased in the transition from mechanical to spontaneous ventilation, and they further increased linearly as ventilatory work was incrementally augmented ninefold by CO2 rebreathing. No other muscles of inspiration manifest increased blood flows. A small amount of expiratory work was measured at high ventilatory volumes during which two expiratory muscles (transverse abdominal and intercostals) had moderate increases in blood flow. Blood pressure did not change, but cardiac output doubled. Arterial-venous oxygen content difference across the diaphragm increased progressively, so oxygen delivery was augmented by both increased blood flow and increased oxygen extraction at all work loads. Oxygen consumption increased linearly as work of breathing increased, so efficiency did not change significantly. The mean efficiency of the respiratory muscles was 15.5%. These results differ significantly from the patterns previously observed by us during increased work of breathing induced by inspiratory resistance, suggesting a different distribution of work load among the various muscles of respiration, a different fractionation of oxygen delivery between blood flow and oxygen extraction, and a higher efficiency when shortening, not tension development, of the muscle is increased.