Role of the carotid bodies in breath-holding

Breath-holding as a novel approach to risk stratification in COVID-19

BACKGROUND

Despite considerable progress, it remains unclear why some patients admitted for COVID-19 develop adverse outcomes while others recover spontaneously. Clues may lie with the predisposition to hypoxemia or unexpected absence of dyspnea ('silent hypoxemia') in some patients who later develop respiratory failure. Using a recently-validated breath-holding technique, we sought to test the hypothesis that gas exchange and ventilatory control deficits observed at admission are associated with subsequent adverse COVID-19 outcomes (composite primary outcome: non-invasive ventilatory support, intensive care admission, or death).

METHODS

Patients with COVID-19 (N = 50) performed breath-holds to obtain measurements reflecting the predisposition to oxygen desaturation (mean desaturation after 20-s) and reduced chemosensitivity to hypoxic-hypercapnia (including maximal breath-hold duration). Associations with the primary composite outcome were modeled adjusting for baseline oxygen saturation, obesity, sex, age, and prior cardiovascular disease. Healthy controls (N = 23) provided a normative comparison.

RESULTS

The adverse composite outcome (observed in N = 11/50) was associated with breath-holding measures at admission (likelihood ratio test, p = 0.020); specifically, greater mean desaturation (12-fold greater odds of adverse composite outcome with 4% compared with 2% desaturation, p = 0.002) and greater maximal breath-holding duration (2.7-fold greater odds per 10-s increase, p = 0.036). COVID-19 patients who did not develop the adverse composite outcome had similar mean desaturation to healthy controls.

CONCLUSIONS

Breath-holding offers a novel method to identify patients with high risk of respiratory failure in COVID-19. Greater breath-hold induced desaturation (gas exchange deficit) and greater breath-holding tolerance (ventilatory control deficit) may be independent harbingers of progression to severe disease.

Prior oxygenation, but not chemoreflex responsiveness, determines breath-hold duration during voluntary apnea

Central and peripheral respiratory chemoreceptors are stimulated during voluntary breath holding due to chemostimuli (i.e., hypoxia and hypercapnia) accumulating at the metabolic rate. We hypothesized that voluntary breath-hold duration (BHD) would be (a) positively related to the initial pressure of inspired oxygen prior to breath holding, and (b) negatively correlated with respiratory chemoreflex responsiveness. In 16 healthy participants, voluntary breath holds were performed under three conditions: hyperoxia (following five normal tidal breaths of 100% O2 ), normoxia (breathing room air), and hypoxia (following ~30-min of 13.5%-14% inspired O2 ). In addition, the hypoxic ventilatory response (HVR) was tested and steady-state chemoreflex drive (SS-CD) was calculated in room air and during steady-state hypoxia. We found that (a) voluntary BHD was positively related to initial oxygen status in a dose-dependent fashion, (b) the HVR was not correlated with BHD in any oxygen condition, and (c) SS-CD magnitude was not correlated with BHD in normoxia or hypoxia. Although chemoreceptors are likely stimulated during breath holding, they appear to contribute less to BHD compared to other factors such as volitional drive or lung volume.

Physiologic Effects of Oxygen Supplementation During Exercise in Chronic Obstructive Pulmonary Disease

Supplemental long-term oxygen therapy (LTOT) is a well-established therapy that improves mortality in patients with chronic obstructive pulmonary disease (COPD) with resting hypoxemia. In the large number of patients with COPD who do not have severe resting hypoxemia but who desaturate with exercise, the clinical benefits that can be obtained by supplemental O₂ therapy during exercise is an area of interest and active research. A summary of current evidence for benefits of supplemental O₂ therapy and a review of physiologic mechanisms underlying published observations are reviewed in this article.

The Logic of Carotid Body Connectivity to the Brain

The carotid body has emerged as a therapeutic target for cardio-respiratory-metabolic diseases. With the expansive functions of the chemoreflex, we sought mechanisms to explain differential control of individual responses. We purport a remarkable correlation between phenotype of a chemosensory unit (glomus cell-sensory afferent) with a distinct component of the reflex response. This logic could permit differential modulation of distinct chemoreflex responses, a strategy ideal for therapeutic exploitation.

The Influence of Age on Interaction between Breath-Holding Test and Single-Breath Carbon Dioxide Test

Introduction. The aim of the study was to compare the breath-holding test and single-breath carbon dioxide test in evaluation of the peripheral chemoreflex sensitivity to carbon dioxide in healthy subjects of different age. Methods. The study involved 47 healthy volunteers between ages of 25 and 85 years. All participants were divided into 4 groups according to age: 25 to 44 years (n = 14), 45 to 60 years (n = 13), 60 to 75 years (n = 12), and older than 75 years (n = 8). Breath-holding test was performed in the morning before breakfast. The single-breath carbon dioxide (SB-CO2) test was performed the following day. Results. No correlation was found between age and duration of breath-holding (r = 0.13) and between age and peripheral chemoreflex sensitivity to CO2 (r = 0.07). In all age groups there were no significant differences in the mean values from the breath-holding test and peripheral chemoreflex sensitivity tests. In all groups there was a strong significant inverse correlation between breath-holding test and SB-CO2 test. Conclusion. A breath-holding test reflects the sensitivity of the peripheral chemoreflex to carbon dioxide in healthy elderly humans. Increasing age alone does not alter the peripheral ventilatory response to hypercapnia.

An insulin based model to explain changes and interactions in human breath-holding

Until now oxygen was thought to be the leading factor of hypoxic conditions. Whereas now it appears that insulin is the key regulator of hypoxic conditions. Insulin seems to regulate the redox state of the organism and to determine the breakpoint of human breath-holding. This new hypoxia-insulin hypotheses might have major clinical relevance. Besides the clinical relevance, this hypothesis could explain, for the first time, why the training of the diaphragm, among other factors, results in an increase in breath-holding performance. Elite freedivers/apnea divers are able to reach static breath-holding times to over 6 min. Untrained persons exhibit an unpleasant feeling after more or less a minute. Breath-holding is stopped at the breakpoint. The partial oxygen pressure as well as the carbon dioxide pressure failed to directly influence the breakpoint in earlier studies. The factors that contribute to the breakpoint are still under debate. Under hypoxic conditions the organism needs more glucose, because it changes from the oxygen consuming pentose phosphate (36 ATP/glucose molecule) to the anaerobic glycolytic pathway (2ATP/glucose molecule). Hence insulin, as it promotes the absorption of glucose, is set in the center of interest regarding hypoxic conditions. This paper provides an insulin based model that could explain the changes and interactions in human breath-holding. The correlation between hypoxia and reactive oxygen species (ROS) and their influence on the sympathetic nerve system and hypoxia-inducible factor 1 alpha (HIF-1α) is dealt with. It reviews as well the direct interrelation of HIF-1α and insulin. The depression of insulin secretion through the vagus nerve activation via inspiration is discussed. Furthermore the paper describes the action of insulin on the carotid bodies and the diaphragm and therefore a possible role in respiration pattern. Freedivers that go over the breakpoint of breath-holding could exhibit seizures and thus the effect of insulin, blood glucose levels and corticosteroids in hippocampal seizures is highlighted.

Quality of life in lung cancer patients: impact of baseline clinical profile and respiratory status

As cure is attainable in very few cases of lung cancer, the imperative issue is to make quality of life (QOL) as good as possible as part of the palliative care package. The aim of this paper was to evaluate the baseline QOL of lung cancer patients and observe its association with various clinical parameters and overall respiratory status. A total of 101 patients were administered the European Organization for Research and Treatment of Cancer core quality of life (EORTC QLQ-C30, version 3) questionnaire. Clinical profile and measures of respiratory status, including spirometry, measures of dyspnoea, and 6-min walk test, were recorded. Higher Karnofsky Performance Status (KPS) significantly correlated with better global health status (P < 0.001) and healthy level of functioning (P < 0.001). The cumulative symptom burden was significantly associated with global QOL (P = 0.01) and physical, role and cognitive function scales (P < 0.05). All dyspnoea measures negatively correlated with global QOL and functioning scales. Spirometric indices showed a positive correlation with all functional scales (P < 0.05) except social. In conclusion, lung cancer patients have unsatisfactory QOL, with the global health status and physical functions being most affected. Number of symptoms, KPS, dyspnoea and spirometry significantly affect QOL.

Breath-holding and its breakpoint

This article reviews the basic properties of breath-holding in humans and the possible causes of the breath at breakpoint. The simplest objective measure of breath-holding is its duration, but even this is highly variable. Breath-holding is a voluntary act, but normal subjects appear unable to breath-hold to unconsciousness. A powerful involuntary mechanism normally overrides voluntary breath-holding and causes the breath that defines the breakpoint. The occurrence of the breakpoint breath does not appear to be caused solely by a mechanism involving lung or chest shrinkage, partial pressures of blood gases or the carotid arterial chemoreceptors. This is despite the well-known properties of breath-hold duration being prolonged by large lung inflations, hyperoxia and hypocapnia and being shortened by the converse manoeuvres and by increased metabolic rate. Breath-holding has, however, two much less well-known but important properties. First, the central respiratory rhythm appears to continue throughout breath-holding. Humans cannot therefore stop their central respiratory rhythm voluntarily. Instead, they merely suppress expression of their central respiratory rhythm and voluntarily 'hold' the chest at a chosen volume, possibly assisted by some tonic diaphragm activity. Second, breath-hold duration is prolonged by bilateral paralysis of the phrenic or vagus nerves. Possibly the contribution to the breakpoint from stimulation of diaphragm muscle chemoreceptors is greater than has previously been considered. At present there is no simple explanation for the breakpoint that encompasses all these properties.

Denervation of carotid baro- and chemoreceptors in humans

Experimental denervation in animals has shown that carotid baro- and chemoreceptors play an eminent role in maintaining blood pressure and blood gas homeostasis. Denervation of carotid sinus baro- and chemoreceptors in humans may occur as a complication of invasive interventions on the neck or after experimental surgical treatment in asthma. In this topical review, the short- and long-term effects of carotid baro- and chemoreceptor denervation on the control of circulation and ventilation in humans are discussed. Carotid baroreceptor denervation in humans causes a persistent decrease in vagal and sympathetic baroreflex sensitivity and an increase in blood pressure variability; however, carotid denervation does not lead to chronic hypertension. Therefore, although carotid baroreceptors contribute to short-term blood pressure control, other receptors are able to maintain normal chronic blood pressure levels in the absence of carotid baroreceptors. Conversely, carotid chemoreceptor denervation leads to permanent abolition of normocapnic ventilatory responses to hypoxia and reduced ventilatory responses to hypercapnia.

Dose-response effect of oxygen on hyperinflation and exercise endurance in nonhypoxaemic COPD patients

Dynamic hyperinflation contributes to exertional breathlessness and reduced exercise tolerance in chronic obstructive pulmonary disease (COPD) patients. This study examined whether oxygen supplementation results in a dose-dependent decrease in hyperinflation associated with functional and symptomatic improvement. Ten severe COPD patients without clinically significant oxygen (O2) desaturation during exercise, and seven healthy subjects, performed five exercise tests at 75% of maximally tolerated work rate. Inspired oxygen fraction (FI,O2) was varied (0.21, 0.3, 0.5, 0.75 and 1.0) among tests in a randomized order. Ventilation (V'E) was measured, and end-inspiratory (EILV) and end-expiratory (EELV) lung volume changes were assessed from inspiratory capacity manoeuvres. In the patients, compared to room air, endurance time increased with FI,O2=0.3 (mean+/-SEM 92+/-20%) and increased further with FI,O2=0.5 (157+/-30%). At isotime, compared to room air, there were significant reductions in dyspnoea score, EELV, EILV, V'E and respiratory frequency (fR) with FI,O2=0.3. Improved endurance time negatively correlated with change in EELV (r=0.48, p=0.002) and EILV (r=0.43, p=0.005). The dyspnoea rating decrease correlated with fR decrease. In healthy subjects, smaller V'E and fR decreases were observed at FI,O2=0.5, accompanied by more modestly increased endurance. Oxygen supplementation during exercise induced dose-dependent improvement in endurance and symptom perception in nonhypoxaemic chronic obstructive pulmonary disease patients, which may be partly related to decreased hyperinflation and slower breathing pattern. This effect is maximized at an inspired oxygen fraction of 0.5.

Effects of dihydrocodeine on chemosensitivity and exercise tolerance in patients with chronic heart failure

OBJECTIVES

We sought to test the hypothesis that suppression of chemosensitivity (respiratory response to arterial blood gases) with dihydrocodeine may improve dyspnea and exercise tolerance in patients with chronic heart failure.

BACKGROUND

Exertional dyspnea is a common limiting symptom in patients with chronic heart failure. The mechanisms underlying this symptom are not fully understood but may be related to increased ventilation caused, in part, by the augmentation of chemosensitivity. Suppression of chemosensitivity with mild opiates may thus improve this symptom as well as exercise tolerance.

METHODS

Twelve men with chronic heart failure (mean [+/-SE] age 65.5 +/- 1.5 years, range 58 to 75; left ventricular ejection fraction 21.3 +/- 3.0%, range 8 to 39) received placebo or dihydrocodeine (1 mg/kg body weight) on two separate days in a randomized, double-blind design. One hour later, hypoxic and hypercapnic chemosensitivities were assessed using the transient inhalations of pure nitrogen and the rebreathing of 7% carbon dioxide in 93% oxygen, followed by treadmill cardiopulmonary exercise testing. The symptoms of dyspnea and fatigue during the exercise test were assessed using a modified Borg scale from 0 to 10.

RESULTS

There was a significant fall in hypoxic and hypercapnic chemosensitivities with dihydrocodeine administration compared with placebo (0.447 +/- 0.096 vs. 0.746 +/- 0.104 liter/min per percent arterial oxygen saturation, p = 0.005; 2,480 +/- 0.234 vs. 2.966 +/- 0.283 liter/min per mm Hg, p = 0.01, respectively). Exercise duration was prolonged from 455 +/- 27 s on placebo to 512 +/- 27 s (p = 0.001) with dihydrocodeine, and peak oxygen consumption increased from 18.0 +/- 0.6 to 19.7 +/- 0.6 ml/kg per min (p = 0.002). The ventilatory response to exercise, characterized by the regression slope relating minute ventilation to carbon dioxide output, decreased from 34.19 +/- 2.35 to 30.85 +/- 1.91 (p = 0.01). With dihydrocodeine administration, the change in the modified Borg score for dyspnea was -0.80 (p = 0.003) at 6 min and -0.33 (p = 0.52) at peak exercise, whereas that for fatigue did not change significantly. Arterial oxygen saturation was maintained during exercise despite dihydrocodeine administration (99.3% at rest vs. 98.9% at peak exercise, p = 0.21).

CONCLUSIONS

Augmented chemosensitivity is important in the pathophysiology of chronic heart failure. Its suppression with dihydrocodeine was associated with a reduction of exercise ventilation, an improvement in exercise tolerance and a decrease in breathlessness. Pharmacologic modulation of chemosensitivity may benefit patients with chronic heart failure and merits further investigation.

Hypoxic ventilatory response predicts the extent of maximal breath-holds in man

To understand the factors influencing breath-holding performance, we tested whether the hypoxic (HVR) and hypercapnic ventilatory responses (HCVR) were predictors of the extent of maximal breath-holds as measured by breath-hold duration, the lowest oxyhemoglobin saturation (SpO2min), lowest calculated PaO2 (PaO2min) and highest end-tidal PCO2 (PETCO2max) reached. Steady state isocapnic HVR and hyperoxic HCVR were measured in 17 human volunteers. Breath-holds were made at total lung capacity (TLC), at TLC following hyperventilation, at functional residual capacity, and at TLC with FIO2 = 0.15. SpO2 was measured continuously by pulse oximetry, and alveolar gas was measured at the end of breath-holds by mass spectrometry. PaO2min was calculated from SpO2min and PETCO2max. HVR was a significant predictor of both SpO2min and PaO2min. HVR and forced vital capacity were predictors of breath-hold duration by multiple linear regression. HCVR had no significant predictive value. We conclude that HVR, but not HCVR, is a significant predictor of breath-holding performance.

Oxygen may improve dyspnea and endurance in patients with chronic obstructive pulmonary disease and only mild hypoxemia

Oxygen (O2) has been reported to improve exercise tolerance in some patients with chronic obstructive pulmonary disease (COPD) despite only mild resting hypoxemia (PaO2 greater than 60 mm Hg). To confirm these prior studies and evaluate potential mechanisms of benefit, we measured dyspnea scores by numeric rating scale during cycle ergometry endurance testing and correlated the severity of dyspnea with right ventricular systolic pressure (RVSP) measured by Doppler echocardiography during a separate supine incremental exercise test. Both sets of exercise were performed according to a randomized double-blind crossover protocol in which patients breathed compressed air or 40% O2. We studied 12 patients with severe COPD (FEV1 0.89 +/- 0.09 L [mean +/- SEM], FEV1/FVC 37 +/- 2%, DLCO 9.8 +/- 1.5 ml/min/mm Hg[47% of predicted], PaO2 71 +/- 2.6 mm Hg). With endurance testing on compressed air, PaO2 did not change significantly in the group as whole (postexercise PaO2 63 +/- 5.1 mm Hg, p = NS), but did fall to less than 55 mm Hg in four patients from this group. Duration of exercise increased on 40% O2 from 10.3 +/- 1.6 to 14.2 +/- 1.5 min (p = 0.005), and the rise in dyspnea scores was delayed. Oxygen delayed the rise in RVSP with incremental exercise in all patients and lowered the mean RVSP at maximum exercise from 71 +/- 8 to 64 +/- 7 mm Hg (p less than 0.03). Improvement in duration of exercise correlated with decrease in dyspnea (r2 = 0.66, p = 0.001) but not with decreases in heart rate, minute ventilation, or RVSP.(ABSTRACT TRUNCATED AT 250 WORDS)

Physiologic changes following bilateral carotid-body resection in patients with chronic obstructive pulmonary disease

In an attempt to establish the characteristic pattern of change in (1) indices of pulmonary function, (2) arterial blood gas, and (3) arterial acid-base status which result from bilateral carotid body resection (BCBR), we analyzed the results of 146 consecutive patients with severe chronic obstructive pulmonary disease who had undergone BCBR at a single hospital site. In addition, we analyzed the results of incremental exercise testing in 21 similar patients, performed at a separate hospital. On average, TLC, FRC, and RV all decreased after surgery, whereas FEV1 increased. Although the data were highly variable, PaO2 decreased on average in proportion to the increase in PaCO2. The smaller group exercised to a higher metabolic rate after surgery; maximum ventilation, however, was unchanged. This led to a further fall in PaO2 and increase in PaCO2. We conclude that individual patient responses to BCBR are highly variable, but the procedure typically results in a small further hypoxemia and hypercapnia with no further impairment to pulmonary function.

Bilateral carotid body resection for the relief of dyspnea in severe chronic obstructive pulmonary disease. Physiologic and clinical observations in three patients

For many patients with advanced chronic airflow limitation (COPD) the treatment of dyspnea remains inadequate despite medications, rehabilitation programs, and supplemental oxygen. Bilateral carotid body resection (BCBR) is a controversial operation which has been reported anecdotally to relieve dyspnea in such patients, but its risks and long-term effects are not known. We studied pulmonary function and the ventilatory response to exercise of three severely dyspneic COPD patients who had chosen independently and without our knowledge to undergo this operation. All three patients reported improvement in dyspnea following BCBR despite the absence of improvement in their severe airflow limitation (mean FEV1 = 0.71 L before and 0.67 L after BCBR). The three patients died 6, 18 and 36 months after the removal of their carotid bodies, still convinced of the efficacy of their surgery. Their reported relief of dyspnea was associated with substantial decreases in minute ventilation and deterioration in arterial blood gases. Arterial blood gases worsened both at rest (PO2 fell from 57 to 45 mm Hg; PCO2 rose from 45 to 57 mm Hg) and during identical steady state exercise (at peak exercise, PO2 fell from 46 to 37 mm Hg and PCO2 rose from 50 to 61 mm Hg) postoperatively. Total minute ventilation decreased postoperatively both at rest (-3.4 L/min, -25 percent) and with exercise (-9.4 L/min, -39 percent) primarily because of decreases in respiratory rate (from 21 to 16 breaths/min at rest and from 25 to 18 breaths/min with exercise), and this was associated with decreases in both oxygen uptake (-26 percent) and carbon dioxide production (-22 percent) for the same external exercise workload. Whether the reported improvement in dyspnea was due to decrease in ventilation resulting from decrease in respiratory drive, a surgical placebo effect or some other unestablished effect of removal of the carotid bodies deserves further study.

Effects of peripheral and central chemoreflex activation on the isopnoeic rating of breathing in exercising humans

1. Respiratory sensation during exercise is generally considered to be related to respiratory mechanical factors which may be manifest as an abnormal relationship between the force applied to the lungs and chest wall and the resulting motion (if any); that is, a 'length-tension' inappropriateness (Campbell & Howell, 1963). This suggests that there should be a direct correlation between ventilation (VE) and the associated intensity of the perceived sensation, such that the sensation associated with a particular level of VE should remain essentially constant regardless of the source of respiratory stimulation. 2. In order to establish whether certain respiratory stimuli might be 'dyspnoeagenic' (i.e. capable of evoking an intensity of respiratory sensation out of proportion to their influence on VE), we investigated the influence of both peripheral chemoreflex activation (induced by isocapnic hypoxia) and central chemoreflex activation (induced by hypercapnic hyperoxia) on the intensity of respiratory sensation in seven healthy adults during moderate cycle ergometer exercise (i.e. below the lactate threshold, theta 1ac). 3. In each test, an 'isopnoea' was established for which a particular level of VE was sustained over a prolonged period (approximately 30 min) while the proportional contributions to the ventilatory drive from either exercise and the peripheral chemoreflex or from exercise and the central chemoreflex were slowly altered to new stable levels, without the subject's knowledge, VE, tidal volume, inspiratory and expiratory durations, mean inspiratory flow, and end-tidal PCO2 and PO2 (PET,CO2, PET,O2) were monitored breath-by-breath. The intensity of respiratory sensation was rated with a visual analogue scale. 4. Isopnoeic ratings of respiratory sensation were systematically greater for peripheral chemoreflex activation by isocapnic hypoxia during exercise at 50% theta 1ac (for which the degree of peripheral chemoreflex activation, estimated by hyperoxic transition or 'Dejours' testing, averaged approximately 23% of the total VE), compared to 90% theta 1ac during isocapnic hyperoxia. Ratings during exercise at 50% theta 1ac for central chemoreflex activation by hypercapnic hyperoxia were not systematically different from 90% theta 1ac during isocapnic hyperoxia, however. 5. As VE was stable throughout each isopnoea and the MVV (maximum voluntary ventilation) was uninfluenced by the test condition, the dyspnoea index (VE x 100/MVV) was not affected. Breathing pattern was also unaffected. 6. We conclude that in normal subjects exercising moderately, activation of the peripheral chemoreceptors by isocapnic hypoxia evokes an intensity of respiratory sensation which is out of proportion to that evoked by an isopnoeic stimulation of the central chemoreceptors with hypercapnic hyperoxia at the same level of exercise.(ABSTRACT TRUNCATED AT 400 WORDS)

Oxygen chemoreception by carotid body cells in culture

Chemoreceptors for oxygen reside within the carotid body, but it is not known which cells actually sense hypoxia and by what mechanisms they transduce this information into afferent signals in the carotid sinus nerve. We have developed systems for the growth of glomus cells of the carotid body in dissociated cell culture. Here we demonstrate that, as in vivo, these cells contain the putative neurotransmitters dopamine, serotonin, and norepinephrine. Oxygen tension regulates the rate of dopamine secretion from the glomus cells. Similar to chemically stimulated catecholamine secretion from other adrenergic cells this hypoxia-stimulated release requires extracellular calcium. These results are compatible with the suggestion that the glomus cells of the carotid body are chemoreceptor cells and that they signal hypoxia by regulated secretion of dopamine.

Respiratory responses of diabetics to hypoxia, hypercapnia, and exercise

The respiratory responses of 52 diabetics and 65 non-diabetic controls to hypoxia, hypercapnia, and exercise were studied. Twenty five per cent of the diabetics had evidence of impaired sensitivity to hypoxia or decreased ventilatory response to hypercapnia, while 7% of the diabetics who performed the exercise tests had an abnormal pattern of respiration during exercise; 33% of the diabetics who performed all three tests of respiratory reflex action had at least one abnormal test response. There was no correlation between the presence of an abnormal respiratory response and the presence of clinical diabetic complications. Abnormal respiratory reflexes could not be predicted from the results of the "routine" pulmonary function tests. The possibility that the abnormal respiratory responses were due to autonomic neuropathy is discussed.

Impaired response to hypoxia after bilateral carotid body resection for treatment of bronchial asthma

Decreased ventilatory responsiveness to isocapnic hypoxia and increased breath-holding times were measured in a 12-year-old asthmatic boy who had undergone bilateral resection of the carotid bodies. The patient had episodes of cyanosis in which he became disoriented, but he had no subjective feelings of discomfort or dyspnea.

Carotid body hypertrophy in patients with cystic fibrosis and cyanotic congenital heart disease

The carotid bodies from 71 patients ranging in age from 28 weeks' gestation to 30 years were obtained at autopsy. Patients were divided into two groups based on the presence or absence of chronic hypoxemia. There was a high correlation between the weight of individual carotid bodies in each case. Among the 12 patients with chronic hypoxemia, eight patients had carotid bodies heavier than predicted by statistical analysis. Of these eight patients, six had cystic fibrosis and two had cyanotic heart disease. Morphometric and cell population analyses of the carotid bodies from these eight patients and from those of the control population indicated that enlargement of the carotid bodies during normal or abnormal growth results from proportionate increases in lobule parenchyma and stroma. There was also an increase in the width and length of the lobules without an increase in the diameter of the cell cords or a change in the size or proportion of the chief cells. Growth and development of the carotid bodies were studied in a control group of 59 patients without chronic hypoxemia. There were no sex related differences in carotid body weights. The combined weight of the carotid bodies correlated most strongly with body weight, although there was some correlation with age and body length. A regression equation reflecting the data relating to body weight (BW) is: Combined weight of carotid bodies (in mg.) = 0.29 BW (in kg.) +3.0. Leukemic infiltrates were present in two patients with acute lymphocytic leukemia, and diffuse lymphocytic infiltration with nodule formation was present in one patient with mental retardation. Metaplastic cartilage was present in a carotid body of one patient.

Breath holding and rebreathing at low and high altitude

Breath holding and rebreathing have been carried out at sea level in lowlanders and at high altitude in acclimatized lowlanders and in highlanders. It has been shown that the values of gasping time, breath-holding time, rebreathing time and the composition of the alveolar gases at the breaking point are modified by chronic hypoxia, especially in highlanders. The modifications observed can be explained by different sensitivities to humoral stimuli of the different groups of subjects. The importance of nonhumoral factors, as studied by comparing apnea and rebreathing, seems modified at altitude, especially in highlanders, although the mechanism of this change is unknown.