Control of breathing in patients with severe hypothyroidism

Severe hypoxemia after extubation secondary to myxedema coma: a case report

Myxedema coma is a rare and life-threatening endocrine emergency characterized by abnormalities in multiple organ systems. A 32-year-old woman with prolonged undiagnosed severe hypothyroidism was referred to our hospital owing to lower abdominal pain and menopause for more than 3 months. She underwent exploratory laparotomy and induced abortion under general anesthesia, and developed severe hypoxemia after extubation. She was diagnosed with myxedema coma, and was subsequently discharged with a good prognosis following treatment. This case suggests that myxedema coma should be considered a potential etiology of peri-operative hypoxemia. The findings in this case emphasize the importance of anesthesiologists' comprehensive understanding of myxedema coma. Prompt diagnosis followed by treatment is essential to reduce the mortality rate associated with this condition.

Results of CPAP Titration and Short-Term Adherence Rates in Patients with Obesity Hypoventilation Syndrome and Mild/Moderate Obstructive Sleep Apnea

PURPOSE

No study has assessed the titration success of CPAP therapy in patients with obesity hypoventilation syndrome (OHS) and an apnea-hypopnea index (AHI) <30 event/h. This study aimed to assess the titration success of CPAP therapy under polysomnography and subsequent short-term adherence (1 month) in patients with OHS and an AHI <30 event/h.

METHODS

Consecutive OHS patients with an AHI <30 events/h between 2010 and 2019 were included (n=54). All OHS patients were first started on CPAP during the therapeutic sleep-study. If the therapeutic-study showed that the SpO2 remained < 90% for 20% of the total sleep time, a second therapeutic study was arranged with bi-level PAP (BPAP). Thirty patients agreed to participate in the 1-month follow-up adherence study. We applied the American-Thoracic-Society criteria for PAP adherence.

RESULTS

The mean age was 54.8±14.6 years, and the mean BMI was 45.9±12.2 kg/m². Successful titration on CPAP was attained in 36 (66.7%) patients, and 18 (33.3%) required BPAP. Patients who failed the CPAP trial had a significantly higher PaCO2 and bicarbonate, a more restrictive respiratory pattern on spirometry, and a significantly higher time with SpO2<90% (mins) during sleep. The only independent correlate of CPAP-titration success on the multivariable regression analysis was the desaturation index (OR: 1.33 [1.033-1.712]). More than 80% of the participants were using CPAP therapy after one-month with no differences in adherence between the CPAP and BPAP groups.

CONCLUSIONS

The current results suggest that CPAP therapy could be an acceptable alternative therapy to BPAP in patients with OHS without severe OSA.

Obstructive sleep apnea in hypothyroidism

Introduction

Obstructive sleep apnea (OSA) and hypothyroidism are closely linked as hypothyroidism has been shown to lead to the development of OSA through multiple mechanisms. With the changing lifestyle patterns worldwide and increased prevalence of obesity, the burden of OSA has substantially increased. The association of OSA with hypothyroidism is essential to establish. If identified early, treatment of OSA and associated hypothyroidism can be done timely to minimize the potential harmful complications of OSA on all aspects of the patient's health.

Aims

This study was done to find out the prevalence of OSA in hypothyroidism patients.

Setting and Designs

It was a cross-sectional study, done over a period of 1 year in a tertiary care hospital.

Materials and Methods

A total of 100 hypothyroidism patients were enrolled after taking written consent. All patients were subjected to STOP-Bang questionnaire and patient falling in intermediate-high risk (score3-8), were taken for overnight polysomnography to confirm the diagnosis of OSA (AHI ≥5).

Statistical Analysis Used

The Statistical Package for the Social Sciences version 21.0 statistical analysis software.

Results

Out of 100 patients, who underwent polysomnography, 74 (74%) cases had OSA (AHI ≥5). Out of total 74 OSA cases, 29 (39.2%) cases had mild OSA (AHI 5-14), 15 (20.3%) cases had moderate OSA (AHI 15-30), and 30 (40.5%) cases had severe OSA. The age of the study population ranged between 24 and 78 years and the mean age was 58.28 ± 11.22 years. The mean age of the patients in the OSA group (59.27 ± 11.17 years) was higher than the non-OSA group (55.46 ± 11.09 years). Majority (64%) of our cases were male, and the proportion of males was found to be higher than females in both the groups (OSA/non-OSA). The body mass index (BMI) of the OSA group was found to be statistically higher as compared to that of the non-OSA group (P = 0.040). The BMI was found to be higher in severe OSA, but it was not statistically significant (P = 0.128). The mean value of FT4 was lower and thyroid-stimulating hormone (TSH) was higher in the OSA group as compared with the non-OSA group. However, no significant association was found between FT4 and TSH values in both the groups. Out of 100 cases, 41 patients were treatment naïve with mean TSH value of 13.1 ± 7 and 59 were on treatment with mean TSH of 8.3 ± 4. Treatment-naïve patients had a statistically higher number (85.3%) of OSA cases in comparison to patients on treatment (66.1) (P = 0.030).

Conclusions

Prevalence of OSA is quite high in hypothyroidism. Patients with hypothyroidism should be screened for OSA for early diagnosis, especially in individuals with higher BMI. Treatment of hypothyroidism reduces the prevalence of OSA.

Prevalence of Hypothyroidism in a Large Sample of Patients with Obesity Hypoventilation Syndrome

PURPOSE

Data on hypothyroidism in patients with obesity hypoventilation syndrome (OHS) are scarce. This study assessed the prevalence of hypothyroidism among a large group of patients with OHS.

PATIENTS AND METHODS

This was a prospective observational study of 308 consecutive patients with OHS seen between January 2002 and December 2018. Serum thyroid-stimulating hormone (TSH) and free-thyroxine (FT4) levels were measured in all patients. The OHS patients were compared with 445 patients with obstructive sleep apnoea (OSA) matched for age, sex, and body mass index (BMI).

RESULTS

The OHS patients had a mean age of 55.1 ± 13.8 years and a BMI of 43.9 ± 14.8 kg/m²; apnoea hypopnea index was ≥30 events/hr in 222 (72%). Clinical hypothyroidism was diagnosed in 58 (18.8%) of the OHS patients; only two cases (0.6%) were diagnosed in the sleep disorders clinic (newly diagnosed cases). Subclinical hypothyroidism was diagnosed in 19 (6.2%) of the OHS patients based on elevated TSH and normal FT4 levels; all cases were newly diagnosed. A logistic regression model identified female sex as the only predictor of clinical hypothyroidism in OHS patients (odds ratio: 2.801 [1.386-5.662], p = 0.004). There was no significant difference in clinical hypothyroidism prevalence between the OHS and OSA patients; however, subclinical hypothyroidism was more common in OHS than in OSA patients (6.2% vs 2.9%, respectively, p = 0.03).

CONCLUSION

Clinical hypothyroidism was prevalent among patients with OHS; however, newly diagnosed cases of clinical hypothyroidism were relatively low. Female sex was the only predictor of clinical hypothyroidism.

Is Obesity Hypoventilation Syndrome A Postmenopausal Disorder?

Previous studies have assessed the role of gender and menopause in Obstructive Sleep Apnea (OSA). It is well known that menopause is a major risk factor for OSA. However, analogous studies on obesity Hypoventilation Syndrome (OHS) are limited. Recent studies have suggested that OHS is more prevalent in postmenopausal women. Moreover, women with OHS seem to have excess comorbidities, including hypothyroidism, hypertension, pulmonary hypertension, and diabetes mellitus, compared to men. In the present perspective, we discuss recent data on the prevalence and comorbidities associated with OHS in women, as well as the use of noninvasive ventilation in women with OHS, and try to answer the question, “Is OHS a disorder of postmenopausal women?”

Respiratory Manifestations of Hypothyroidism: A Systematic Review

BACKGROUND

Hypothyroidism has been associated with increased pulmonary morbidity and overall mortality. A systematic review was conducted to identify the prevalence and underlying mechanisms of respiratory problems among patients with thyroid insufficiency.

METHODS

PubMed and EMBASE databases were searched for relevant literature from January 1950 through January 2015 with the following study eligibility criteria: English-language publications; adult subclinical or overt hypothyroid patients; intervention, observational, or retrospective studies; and respiratory manifestations. The Preferred Reporting Items for Systematic reviews and Meta-Analyses statement was followed, and Cochrane's risk of bias tool was used.

RESULTS

A total of 1699 papers were screened by two independent authors for relevant titles. Of 109 relevant abstracts, 28 papers underwent full-text analyses, of which 22 were included in the review. Possible mechanisms explaining respiratory problems at multiple physiological levels were identified, such as the ventilator control system, diaphragmatic muscle function, pulmonary gas exchange, goiter caused upper airway obstruction, decreased capacity for energy transduction, and reduced glycolytic activity. Obstructive sleep apnea syndrome was found among 30% of newly diagnosed patients with overt hypothyroidism, and demonstrated reversibility following treatment. The evidence for or against a direct effect on pulmonary function was ambiguous. However, each of the above-mentioned areas was only dealt with in a limited number of studies. Therefore, it is not possible to draw any strong conclusions on any of these themes. Moreover, most studies were hampered by considerable risk of bias due for example to small numbers of patients, lack of control groups, randomization and blinding, and differences in body mass index, sex, and age between subjects and controls.

CONCLUSION

Mechanistic data linking hypothyroidism and respiratory function are at best limited. This area of research is therefore open for retesting hypotheses, using appropriate study designs and methods.

Gender differences in patients with obesity hypoventilation syndrome

The role of gender and menopause in obstructive sleep apnoea is well known; however, no study has reported the impact of gender on the clinical presentation and the nocturnal respiratory events in patients with obesity hypoventilation syndrome. Therefore, this study prospectively evaluated differences in the clinical characteristics of women and men with obesity hypoventilation syndrome in a large cohort of patients with obstructive sleep apnoea. During the study period, a total of 1973 patients were referred to the sleep clinic with clinical suspicion of obstructive sleep apnoea. All patients underwent overnight polysomnography, during which time spirometry, arterial blood samples and thyroid tests were routinely obtained. Among 1973 consecutive patients, 1693 (617 women) were diagnosed with obstructive sleep apnoea, among whom 144 suffered from obesity hypoventilation syndrome (96 women). The prevalence of obesity hypoventilation syndrome among women and men was 15.6% and 4.5%, respectively (P < 0.001). Women with obesity hypoventilation syndrome were significantly older than men with obesity hypoventilation syndrome (61.5 ± 11.9 years versus 49.1 ± 12.5 years, P < 0.001). Although there were no significant differences between genders regarding symptoms, body mass index, spirometric data or daytime PaCO2 , women with obesity hypoventilation syndrome suffered significantly more from hypertension, diabetes and hypothyroidism. The prevalence of obesity hypoventilation syndrome was higher in post-menopausal (21%) compared with pre-menopausal (5.3%) women (P < 0001). HCO3 and duration of SpO2 <90% were the only independent predictors of obesity hypoventilation syndrome. In conclusion, this study reported that among subjects referred to the sleep disorders clinic for evaluation of obstructive sleep apnoea, obesity hypoventilation syndrome is more prevalent in women than men, and that women with obesity hypoventilation syndrome suffer from significantly more co-morbidities. Post-menopausal women with obstructive sleep apnoea have the highest prevalence of obesity hypoventilation syndrome.

In adult female hamsters hypothyroidism stimulates D1 receptor-mediated breathing without altering D1 receptor expression

Hypothyroidism affects cardiopulmonary regulation and function of dopaminergic receptors. Here we evaluated effects of 5 months of hypothyroidism on dopamine D1 receptor modulation of breathing in female hamsters using a D1 receptor antagonist SCH 23390. Euthyroid hamsters (EH) served as controls. Results indicated that hypothyroid female hamsters (HH) exhibited decreased body weights and minute ventilation (VE) following hypoxia due to decreased frequency of breathing (F). Moreover, SCH 23390 administration in HH increased VE by increasing tidal volume during exposure to air, hypoxia and following hypoxia. Relative to vehicle, SCH 23390 treatment decreased body temperature and hypoxic VE responsiveness in both groups. In EH, SCH 23390 decreased F in air, hypoxia and post hypoxia, and VE during hypoxia trended to decrease (P=0.053). Finally, expression of D1 receptor protein was not different between the two groups in any region evaluated. Thus, hypothyroidism in older female hamsters affected D1 receptor modulation of ventilation differently relative to euthyroid animals, but not expression of D1 receptors.

Impact of overt and subclinical hypothyroidism on exercise tolerance: a systematic review

PURPOSE

This systematic review describes the state of the art of the impact of hypothyroidism on exercise tolerance and physical performance capacity in untreated and treated patients with hypothyroidism.

METHOD

A systematic computer-aided search was conducted using biomedical databases. Relevant studies in English, German, and Dutch, published from the earliest date of each database up to December 2012, were identified.

RESULTS

Out of 116 studies, a total of 38 studies with 1,379 patients fulfilled the inclusion criteria. These studies emphasize the multifactorial causes of exercise intolerance in untreated patients by the impact of limitations in different functional systems, with cardiovascular, cardiopulmonary, musculoskeletal, neuromuscular, and cellular metabolic systems acting in concert. Moreover, the studies affirm that exercise intolerance in patients is not always reversible during adequate hormone replacement therapy. As a consequence, despite a defined euthyroid status, there remains a significant group of treated patients with persistent complaints related to exercise intolerance who are suffering from limitations in daily and sport activities, as well as an impaired quality of life. An explanation for this phenomenon is lacking. Only 2 studies investigated the effects of a physical training program, and they showed inconsistent effects on the performance capacity in untreated patients with subclinical hypothyroidism.

CONCLUSIONS

A limited body of knowledge exists concerning exercise tolerance in treated patients with hypothyroidism, and there is an insufficient amount of quantitative studies on the effects of a physical training program. To enhance exercise and sports participation for this specific group, more research in this forgotten area is warranted.

Respiratory failure associated with hypoventilation in a patient with severe hypothyroidism

A 70-year-old Japanese man was admitted to hospital because of decreased consciousness due to type II respiratory failure. Severe hypothyroidism was diagnosed and considered to be associated with hypoventilation due to respiratory muscle dysfunction and sleep apnea syndrome. His status was improved partially by replacement of thyroid hormone. Despite maintaining a euthyroid state, improvement of respiratory muscle dysfunction was incomplete.

CO(2) rebreathing: an undergraduate laboratory to study the chemical control of breathing

The Read CO2 rebreathing method (Read DJ. A clinical method for assessing the ventilatory response to carbon dioxide. Australas Ann Med 16: 20-32, 1967) provides a simple and reproducible approach for studying the chemical control of breathing. It has been widely used since the modifications made by Duffin and coworkers. Our use of a rebreathing laboratory to challenge undergraduate science students to investigate the control of breathing provided 8 yr of student-generated data for comparison with the literature. Students (age: 19-22 yr, Research Ethics Board approval) rebreathed from a bag containing 5% CO2 and 95% O2 (to suppress the peripheral chemoreflex to hypoxia). Rebreathing was performed, and ventilation measured, after hyperventilation to deplete tissue CO2 stores and enable the detection of the central chemoreflex threshold. We analyzed 43 data sets, of which 10 were rejected for technical reasons. The mean threshold and ventilatory sensitivity to CO2 were 43.3 ± 3.8 mmHg and 4.60 ± 3.04 l·min(-1)·mmHg(-1) (means ± SD), respectively. Threshold values were normally distributed, whereas sensitivity was skewed to the left. Both mean values agreed well with those in the literature. We conclude that the modified rebreathing protocol is a robust method for undergraduate investigation of the chemical control of breathing.

Hypothyroidism attenuates SCH 23390-mediated depression of breathing and decreases D1 receptor expression in carotid bodies, PVN and striatum of hamsters

Hypothyroidism can lead to depressed breathing. We determined if propylthiouracil (PTU)-induced hypothyroidismin hamsters (HH) altered dopamine D1 receptor expression, D1 receptor-modulated ventilation, and ventilatory chemoreflex activation by hypoxia or hypercapnia. Hypothyroidism was induced by administering 0.04% PTU in drinking water for 3 months. Ventilation was evaluated following saline or 0.25mg/kg SCH 23390,a D1 receptor antagonist, while awake hamsters breathed normoxic (21% O(2) in N(2)), hypoxic (10% O(2)in N(2)) and hypercapnic (5% CO(2) in O(2))air. Relative to euthyroid hamsters (EH), HH exhibited decreased D1 receptor protein levels in carotid bodies, striatum, and hypothalamic paraventricular nucleus, but not in the nucleus tractus solitarius. Relative to EH, HH exhibited lower ventilation during exposure to normoxia, hypoxia, or hypercapnia, but comparable ventilatory responsiveness to chemoreflex activation. SCH23390 decreased ventilation of EH hamsters exposed to normoxia, hypoxia, and hypercapnia. In HH SCH23390 increased ventilation during baseline normoxia and did not affect ventilation during exposure to hypoxia and hypercapnia, resulting in reduced ventilatory responsivess to chemoreflex activation by hypoxia and hypercapnia. Furthermore, in HH D1 receptor protein levels are decreased in several brain regions and within the carotid bodies. Moreover, D1 receptor-modulation of breathing at rest and during gas exposures were depressed in EH but not HH.

Reversibility of fibrotic appearance of lungs with thyroxine replacement therapy in patients with severe hypothyroidism

OBJECTIVE

To present 2 cases of hypothyroidism with hypoxia associated with computed tomographic (CT) features suggestive of pulmonary fibrosis that resolved with correction of the hypothyroidism.

METHODS

Clinical case histories are described, comparative radiologic pulmonary images before and after treatment are provided, and the pertinent literature regarding possible pathologic mechanisms is reviewed.

RESULTS

Our first patient, a 68-year-old woman, presented with symptomatic severe hypothyroidism associated with respiratory failure. A CT scan of her lungs showed appearances suggestive of pulmonary fibrosis. Replacement therapy with levothyroxine led to correction of hypoxia and radiologic abnormalities. Our second patient, a 26-year-old man, presented with symptoms suggestive of obstructive sleep apnea that persisted despite use of positive pressure ventilation. Biochemical evaluation revealed severe hypothyroidism, and a CT scan disclosed pulmonary appearances consistent with fibrosis. His symptoms and radiologic abnormalities also improved after correction of hypothyroidism with levothyroxine therapy.

CONCLUSION

Radiologic pulmonary abnormalities suggestive of fibrotic disease are associated with severe hypothyroidism. Invasive investigations such as lung biopsy should be deferred until the clinical and radiologic responses to thyroxine replacement therapy have been assessed.

Thyroid status affects 5-HT2A receptor modulation of breathing before, during, and following exposure of hamsters to acute intermittent hypoxia

The BIO 14.6 hamster (dystrophic), animal model of limb girdle muscular dystrophy, exhibits low plasma triiodothyronine levels, muscle weakness, and decreased breathing. After exposure to acute intermittent bouts of hypoxia, dystrophic hamsters depress ventilation relative to baseline resulting in ventilatory long-term depression (LTD). Control hamsters may increase ventilation relative to baseline resulting in ventilatory long-term facilitation (LTF). Serotonin (5-HT) receptors, especially the 5-HT(2A) subtype, are involved in the development of LTF. The purpose of this study was to evaluate the role of 5-HT(2A) receptors in ventilatory and metabolic responses before, during, and following intermittent hypoxia in eleven euthyroid, nine dystrophic, and eleven propylthiouracil (PTU)-induced hypothyroid male hamsters. Animals received subcutaneous injections of vehicle or 0.5 mg/kg MDL (5-HT(2A) receptor antagonist). Plethysmography was used to evaluate ventilatory responses of the three groups to air, five bouts of 5 min of 10% oxygen, each interspersed with 5 min of air, followed by 60 min of exposure to air. CO(2) production was measured using the flow-through method. Vehicle-treated dystrophic and PTU-treated hamsters exhibited LTD. MDL decreased body temperature in all groups. After MDL treatment, the euthyroid group exhibited LTD. MDL treatment in the dystrophic, but not in the PTU-treated hamsters, maintained tidal volume, but did not reverse LTD. CO(2) production was increased in the euthyroid group with MDL treatment. Thus, 5-HT(2A) receptors affect body temperature, ventilation, and metabolism in hamsters. The differential responses noted in this study may be in part dependent on thyroid hormone status.

Assessing Respiratory Drive and Central Motor Pathway in Humans: Clinical Implications

Feedback from sensory elements as well as projection from higher Central Nervous System structures modify the level and pattern of motor outflow to the respiratory muscles and hence ventilation. In this review we describe the different methods to evaluate the degree to which higher centers determine the level and pattern of ventilation and coordinate use of the respiratory muscles in healthy humans and in patients with a number of respiratory disorders.

Management of myxedematous respiratory failure: review of ventilation and weaning principles

Respiratory failure in myxedema is a complex medical emergency and may require prolonged ventilatory assistance. Appropriate management of this medical problem requires an understanding of its effects on the central nervous system and peripheral neuromusculoskeletal system. Weaning of these patients can be very protracted and requires a diligent multidisciplinary approach. Because of its infrequency, ventilatory management of severe hypothyroidism has not been studied in a controlled fashion. The first part of this review discusses the mechanisms of respiratory failure in myxedema. The second part explores strategies in mechanical ventilation and weaning of myxedematous patients.

Acute respiratory alkalosis associated with low minute ventilation in a patient with severe hypothyroidism

PURPOSE

Patients with severe hypothyroidism present unique challenges to anesthesiologists and demonstrate much increased perioperative risks. Overall, they display increased sensitivity to anesthetics, higher incidence of perioperative cardiovascular morbidity, increased risks for postoperative ventilatory failure and other physiological derangements. The previously described physiological basis for the increased incidence of postoperative ventilatory failure in hypothyroid patients includes decreased central and peripheral ventilatory responses to hypercarbia and hypoxia, muscle weakness, depressed central respiratory drive, and resultant alveolar hypoventilation. These ventilatory failures are associated most frequently with severe hypoxia and carbon dioxide (CO2) retention. The purpose of this clinical report is to discuss an interesting and unique anesthetic presentation of a patient with severe hypothyroidism.

CLINICAL FEATURES

We describe an unique presentation of ventilatory failure in a 58 yr old man with severe hypothyroidism. He had exceedingly low perioperative respiratory rate (3-4 bpm) and minute ventilation volume, and at the same time developed primary acute respiratory alkalosis and associated hypocarbia (P(ET)CO2 approximately 320-22 mmHg).

CONCLUSION

Our patient's ventilatory failure was based on unacceptably low minute ventilation and respiratory rate that was unable to sustain adequate oxygenation. His profoundly lowered basal metabolic rate and decreased CO2 production, resulting probably from severe hypothyroidism, may have resulted in development of acute respiratory alkalosis in spite of concurrently diminished minute ventilation.

Control of breathing in a subset of patients with systemic lupus erythematosus

BACKGROUND

Inspiratory muscle weakness and abnormalities in breathing pattern and in respiratory drive have been reported in patients with multisystem disorders. In patients with systemic lupus erythematosus (SLE), data on respiratory muscle strength and control of breathing are scarce.

METHODS

We studied a subset of nine female patients with SLE with no major findings of cardiovascular, renal, or neurologic involvement, and with a normal routine chest radiograph. An age- and sex-matched normal group was also studied as a control. We evaluated lung volumes, diffusing lung properties (TLCO, TLCO/VA), maximal inspiratory (MIP) and expiratory (MEP) pressures, end-tidal carbon dioxide tension (PCO2), and breathing pattern: ventilation (VE), tidal volume (VT), inspiratory time (TI), and respiratory frequency (Rf). Neural respiratory drive, assessed in terms of mean inspiratory flow (VT/TI), mouth occlusion pressure (P0.1), and surface electromyographic activity of the diaphragm (Edi) and intercostal (Eps) muscles was also evaluated.

RESULTS

As a whole, patients exhibited mild decrease in MIP; vital capacity was slightly reduced in two patients and TLCO/VA was moderately reduced in three. During a hypercapnic rebreathing test, delta VT/delta PCO2 was lower, delta P0.1/delta PCO2 was normal, while delta Edi/delta PCO2 and delta Eps/delta PCO2 were higher in patients compared with normal control subjects. delta VT/delta PCO2 significantly related to MIP. At 60 mm Hg of PCO2 patients maintained the rapid and shallow pattern of breathing (RSB) exhibited during room-air breathing: lower VT, shorter TI, and greater Rf, with VE, VT/TI, and Edi being greater compared with the normal control subjects.

CONCLUSIONS

These data seem to indicate that in this SLE subset, mild decrease in respiratory muscle strength may accompany an increased respiratory drive, and contribute to a qualitatively abnormal ventilatory response (RSB) to carbon dioxide stimulation.

Mechanical loading and control of breathing in patients with severe chronic obstructive pulmonary disease

BACKGROUND

High neural drive to the respiratory muscles and rapid and shallow breathing are frequently observed in patients with chronic obstructive pulmonary disease (COPD), and both mechanical and chemical factors are thought to play a part. However, the interrelation between these factors and the modifications in the control of breathing are not clearly defined. The effects of an acute decrease in mechanical load by the administration of a high dose of a beta 2 agonist were studied.

METHODS

Nine spontaneously breathing patients with severe COPD took part in the study. Criteria for entry were FEV1 of < 40% of predicted and an improvement in FEV1 of < 200 ml after inhalation of 400 micrograms fenoterol. The following parameters were measured: lung volumes, tidal volume (VT), respiratory frequency (Rf), maximal pleural pressure during a sniff manoeuvre (PPLmax), pleural pressure swings (PPLsw), lung resistance (RL), RL/PPLmax ratio, and surface electromyographic activity (EMG) of diaphragm (EDI) and parasternal (EPS) muscles. Arterial oxygen saturation (SaO2), end tidal carbon dioxide pressure (PETCO2), and the electrocardiogram were also monitored. Each variable was measured under control conditions and 20 and 40 minutes after the inhalation of 800 micrograms fenoterol. In five patients the effects of placebo were also studied.

RESULTS

Fenoterol resulted in an increase in FEV1 and decrease in FRC. SaO2 did not change, while PETCO2 fell and heart rate increased. The VT increased, and Rf decreased, PPLsw fell and PPLmax increased, thus the PPLsw/PPLmax ratio fell. Both RL and RL/PPLmax also fell, and a substantial decrease in EDI and EPS was observed. Changes in PPLsw were related to changes in FEV1 and RL. Changes in VT and Rf, and EDI/TI and EPS/TI were also related to changes in PPLsw and RL/PPLmax ratio, but not to changes in FEV1. No variation was observed with placebo.

CONCLUSIONS

In patients with severe COPD a decrease in inspiratory muscle loading relative to the maximal available strength, as expressed by the RL/PPLmax and PPLsw/PPLmax ratios, appears to be the major determinant of changes in breathing pattern and inspiratory muscle activity (decrease in EMG).