Alveolar hypoventilation syndrome. Studies of ventilatory control in patients selected for diaphragm pacing

The syndrome of nocturnal hypoventilation and/or apnea without major intrinsic lung disease is of greater surgical interest since the introduction of therapeutic diaphragm pacing. Of our 29 patients successfully managed by diaphragm pacing, we describe 17 in whom detailed studies of the ventilatory responses to hypoxia are available.The diagnosis was established by the following criteria: (1) an increase in arterial carbon dioxide tension (PaCO₂) (> 45 mm Hg) and a depression in the arterial oxygen tension (PaO₂) (< 75 mm Hg) at least during sleep; (2) respiratory arrests and/or apneic episodes during sleep; (3) near normal tests of ventilatory capacity; and (4) diminished ventilatory response to carbon dioxide breathing.New information on the ventilatory responses to hypoxia was obtained by a nonsteady state closed-circuit rebreathing method. The following were compared: (1) hypoxia with carbon dioxide variably absorbed so as to maintain base line PaCO₂ (“normo”-capnic hypoxia); (2) 50 per cent oxygen with carbon dioxide accumulation (hypercapnia hyperoxia); and (3) carbon dioxide accumulation in room air (hypercapnic hypoxia). The data indicate (1) 15 patients showed no measurable ventilatory response to “normo”-capnic hypoxia and two patients showed blunted responses; (2) seven patients showed no ventilatory responses to carbon dioxide whereas 10 patients showed suboptimal increases; (3) combined hypoxia and hypercapnia caused a suboptimal ventilatory response in four patients. These same four patients showed blunted as opposed to absent ventilatory responses to carbon dioxide. (4) Five patients showed no ventilatory responses to all combinations.These 17 patients together with three others not so extensively studied have been managed by nocturnal diaphragm pacing for periods of four months to nearly eight years.We conclude (1) ablation or severe blunting of the ventilatory responses to hypoxia is a frequent and previously underestimated component of this disorder; and (2) diaphragm pacing is an appropriate form of therapy.     

Quantitative assessment of pulmonary hypertension in patients with tricuspid regurgitation using continuous wave Doppler ultrasound

Doppler ultrasound examination was performed in 69 patients with a variety of cardiopulmonary disorders who were undergoing bedside right heart catheterization. Patients were classified into two groups on the basis of hemodynamic findings. Group I consisted of 20 patients whose pulmonary artery systolic pressure was less than 35 mm Hg and Group II consisted of 49 patients whose pulmonary artery systolic pressure was 35 mm Hg or greater. Tricuspid regurgitation was detected by Doppler ultrasound in 2 of 20 Group I patients and 39 of 49 Group II patients (p less than 0.001). Twenty-six of 27 patients with pulmonary artery systolic pressure greater than 50 mm Hg had Doppler evidence of tricuspid regurgitation. In patients with tricuspid regurgitation, continuous wave Doppler ultrasound was used to measure the velocity of the regurgitant jet, and by applying the Bernoulli equation, the peak pressure gradient between the right ventricle and right atrium was calculated. There was a close correlation between the Doppler gradient and the pulmonary artery systolic pressure measured by cardiac catheterization (r = 0.97, standard error of the estimate = 4.9 mm Hg). Estimating the right atrial pressure clinically and adding it to the Doppler-determined right ventricular to right atrial pressure gradient was not necessary to achieve accurate results. These findings indicate that tricuspid regurgitation can be identified by Doppler ultrasound in a large proportion of patients with pulmonary hypertension, especially when the pulmonary artery pressure exceeds 50 mm Hg. Calculation of the right ventricular to right atrial pressure gradient in these patients provides an accurate noninvasive estimate of pulmonary artery systolic pressure.     

An overnight increase in CO2 predicts mortality in sleep disordered breathing

Background and objective: Sleep disordered breathing (SDB) is highly prevalent but under‐recognized. Evidence is accumulating for its role as a predictor of mortality independent of cardiovascular risk factors. The role of hypercapnia in SDB is not known systematically, and between 11 and 43% of patients with SDB develop chronic hypercapnia. Hypercapnia predicts mortality in other respiratory conditions. The role of hypercapnia in independently predicting mortality in patients with SDB after assessing for the presence of airways disease and obesity was investigated. Methods: The records of 396 consecutive patients were examined retrospectively. Univariate and multivariate analyses were performed using Cox proportional hazards regression to determine the association between gas exchange and polysomnography (PSG) variables and all‐cause mortality, adjusted for potential confounders, including age, gender and presence of co‐morbidity and airways disease. Results: The mean age of our patient population was 55 ± 15 years, and the mean body mass index (BMI) was 30.6 ± 6.2 kg/m². Out of the 322 patient population, 258 were men. After 10 years, 25% had died, following a maximum follow‐up of 16.7 years. Mortality among patients with SDB was predicted by a difference between evening and morning PaCO₂ (ΔPaCO₂) of ≥7 mm Hg and evening hypoxaemia (PaO₂ < 65 mm Hg), independent of age and the presence of co‐morbidity and airways disease. There was a significant correlation between minimum oxygen saturation (SpO₂) and ΔPaCO₂ ≥ 7 mm Hg (P = 0.002) and evening PaO₂ < 65 mm Hg (P < 0.001). Conclusions: An overnight increase in CO₂ and evening hypoxaemia are independent mortality predictors in SDB. A low minimum SpO₂ identifies patients in whom morning and evening arterial blood gases are beneficial.     

Assessment and management of severe asthma

Clinical and functional features shown by fifty-eight patients during seventy-six admissions to the hospital with severe asthma are reported. Thoracic overinflation, sinus tachycardia, pulsus paradoxus and electrocardiographic abnormalities were common. Arterial oxygen tension (PaO₂) was invariably reduced, but elevation of arterial carbon dioxide tension (PaCO₂) above 45 mm Hg was less common than reduction below 40 mm Hg. Dead space to tidal volume ratio was considerably elevated when measurement was possible.     

Therapeutic embolization of the pulmonary artery in pulmonary arteriovenous fistula

A 35 year old white man was admitted in January 1976 for further treatment of pulmonary A-V fistula. In 1975, he had dyspnea, cyanosis and clubbing; pulmonary angiogram showed a fistula in the lower lobe of the left lung and one in the upper lobe of the right lung; the arterial oxygen tension (PaO₂) was 54 mm Hg and the calculated intrapulmonary shunt was 25 per cent of the cardiac output; a lobectomy of the left lung was performed. On this admission, dyspnea, cyanosis and clubbing were still present: the PaO₂ was 67 mm Hg and the intrapulmonary shunt 14 per cent. Since it was possible to show angiographically that, by balloon inflation during Swan-Ganz catheterization of the upper lobe of the right lung, the single pulmonary A-V fistula was completely obliterated with correction of the intrapulmonary shunt, mechanical occlusion was attempted as an alternative to further surgery. Through a No. 7 French catheter inserted via a femoral vein into the pulmonary artery of the upper lobe of the right lung and with the help of repeated selective angiography, six mechanical occluding devices (each consisting of a 5 cm segment of steel guidewire with 3 cm wool tails) were placed into the vessels feeding the fistula. Twenty minutes after placement of the last two coils, the fistula was almost totally occluded on angiogram; the PaO₂ was 79 mm Hg and the intrapulmonary shunt was 7 per cent; over the following three weeks, without further manipulation, these returned to normal (91 mm Hg and 4 per cent, respectively). No complications have been noted. The patient has been symptom free during 12 months of close follow-up; results of oxygen studies have remained normal; on chest film the six coils have not changed position. The procedure described here, to our knowledge performed for the first time for correction of pulmonary A-V fistula, should be considered in preference to surgical resection in selected patients with this condition.     

Effects of vasodilators on pulmonary hemodynamics and gas exchange in left ventricular failure

Nitroprusside (NP) has been shown to improve left ventricular function in patients with congestive heart failure, but despite an increased cardiac output and decreased pulmonary capillary pressure, arterial oxygen tension (P_aO₂) may fall. In order to determine the mechanism of this hypoxemia, and to determine if similar effects occur with non-parenteral vasodilators, hemodynamic, respiratory, and blood gas responses to NP, hydralazine (H), and hydralazine combined with isiosorbide dinitrate (H+N) were studied in 10 patients with left ventricular failure. At the dosages used, all three drug regimens increased cardiac output equivalently, but pulmonary vascular responses differed. NP and H+N decreased mean pulmonary artery pressure, pulmonary wedge pressure, and pulmonary arteriolar resistance, while H did not. NP decreased P_aO₂ by 10.4 mm. Hg (p < .01) and H+N decreased it by 5.3 mm. Hg (p < .06) while H did not alter P_aO₂. Arteriolar-alveolar oxygen gradient increased with NP (150 ± 39 per cent, p < .01) and with H+N (73 ± 23 per cent, p < .01) but not H alone (51 ± 16 per cent). Similarly, per cent change in venous admixture increased on NP (28.7 ± 3.3 to 38.5 ± 3.1 per cent, p < .01) and H+N (28.1 ± 3.3 to 36.8 ± 3.5 per cent, p < .01) but not H alone (28.1 ± 3.3 to 31.5 ± 4.1 per cent). There was no increase in arterial carbon dioxide tension or change in pulmonary function studies with any of the drugs. Due to the increase in cardiac output, oxygen delivery index (cardiac output times arterial oxygen content) increased with each regimen despite the changes in P_aO₂. Changes in arteriolar-alveolar oxygen gradient correlate with the changes in pulmonary arteriolar resistance. Thus vasodilators which have prominent pulmonary vascular effects can decrease P_aO₂ in patients with congestive heart failure, and this effect is most likely due to increasing ventilation-perfusion inequities.     

Hemodynamic characteristics of hypertension after coronary bypass surgery and effects of the converting enzyme inhibitor.

In 13 patients with hypertension after coronary bypass surgery, the underlying hemodynamic mechanism was an increased systemic vascular resistance. To elucidate the role of the renin-angiotensin system in left ventricular function, plasma renin activity and hemodynamic measurements were performed during the peak of the hypertensive episode and 15 to 30 minutes after the administration of the converting enzyme inhibitor SQ20881 (1 mg/kg body weight). Eight of the 13 patients responded to the inhibitor; in these patients control plasma renin activity was 8.78 ± 2.3 ng/ml per hour, and mean arterial pressure decreased markedly from 123 ± 6 to 101 ± 5 mm Hg (P < 0.005), due to a decrease in systemic vascular resistance from 22 ± 2 to 14 ± 1 units (P < 0.005). The decrease in systemic vascular resistance induced by the inhibitor correlated with the control plasma renin activity (r = 0.825, P < 0.05). In the responders, administration of the inhibitor also increased cardiac output from 5.26 ± 0.36 to 7.39 ± 0.61 liters/min (P < 0.005) because of an increase in stroke volume from 61 ± 3 to 78 ± 5 ml/beat (P < 0.005), although left ventricular filling pressure decreased from 8.5 ± 1 to 6.0 ± 1 mm Hg (P < 0.005). In the remaining five patients (nonresponders to the inhibitor), control plasma renin activity was 1.8 ± 0.3 ng/ml per hour (P < 0.05 compared with the responders), but the control hemodynamic data did not differ from those of the responders. In the nonresponders the small decrease in mean arterial pressure (from 128 ± 5 to 123 ± 4 mm Hg, P < 0.025) was due to a decrease in stroke volume and cardiac output, but systemic vascular resistance did not change. As in the responders, administration of the inhibitor decreased left ventricular filling pressure in this group from 8 ± 1 to 6 ± 0.8 mm Hg (P < 0.025).These results indicate that the renin-angiotensin system is often involved in postcoronary artery bypass hypertension. In patients with increased renin, blockade of angiotensin II improved left ventricular function by reducing afterload rather than preload, thus indicating that angiotensin II acts on the heart and veins.     

Current concepts in the pathogenesis of the obesity-hypoventilation syndrome. Mechanical and circulatory factors

The pathogenesis of carbon dioxide retention associated with obesity, the obesity hypoventilation syndrome (OHS), remains obscure. In an attempt to Identify factors which might Initiate or contribute to this syndrome, we reviewed respiratory and circulatory function in two groups of obese subjects: those who were not hypercapnic (simple obesity) and those who were (OHS).Obese subjects in both groups display reduction of lung and chest wall compliance, normal airway resistance, closure of peripheral lung units and increased energy cost of breathing. These abnormalities are more severe in those who hypoventilate, especially the reduction In compliance. Respiratory muscle efficiency is reduced in both groups. Inspiratory muscle strength of patients with OHS is 60 to 70 per cent of normal. In OHS arterial carbon dioxide tension (PaCO₂), vital capacity and maximum voluntary ventilation improve significantly with weight toss, whereas in simple obesity there Is little change in these factors with weight loss.In both groups the major circulatory findings are increased total and pulmonary blood volume, with preservation of a normal ratio between the two; and good perfusion but marked underventilation of dependent regions of the lung. These changes are more pronounced In OHS. Left ventricular end diastolic pressure is elevated in some patients, but the rise is not confined to those with OHS. In OHS alveolar hypoxia and acidemia produce pulmonary arterial vasoconstriction and pulmonary arterial hypertension. As a consequence pulmonary artery pressure exceeds left ventricular pressure at the end of diastole.We suggest that excessive reduction of chest wall compliance and inspiratory muscle weakness interact with the circulatory abnormalities already present in simple obesity to generate carbon dioxide retention. The contribution of altered central nervous system function to this process remains controversial.     

Tetralogy of Fallot in adults. A report on 147 patients.

Presented here is the clinical and hemodynamic profile of 147 patients, above the age of 18 with tetralogy of Fallot. Cardiac catheterization and selective cineangiocardiography were performed in all. Infundibular pulmonary stenosis, a subaortic large infracristal ventricular septal defect, mitral-aortic fibrous continuity and equal pressures in both the ventricles and aorta were considered mandatory for the diagnosis of tetralogy of Fallot. Cardiac enlargement was seen in 25.8 per cent of the patients, and 15.6 per cent were in congestive cardiac failure; 9.5 per cent had systemic hypertension, and aortic regurgitation was present in 6.7 per cent. A reticular pattern in the lung fields due to bronchial collaterals was seen in 23.1 per cent. The incidence of right aortic arch (19.9 per cent), absent left pulmonary artery (2.8 per cent), absent right pulmonary artery (0.7 per cent) and dextrocardia (1.4 per cent) is brought out. The right atrial mean pressure was increased in 4.8 per cent and a prominent "a" wave greater than 10 mm Hg was present in 10.9 per cent. The right ventricular end-diastolic pressure was increased in 23.8 per cent and the left ventricular end-diastolic pressure in 25.9 per cent of the patients.     

Left ventricular gallop sound and acute myocardial infarction

A ventricular gallop sound, pulmonary râles, and cardiogenic shock have important prognostic implications in patients with acute myocardial infarction. The purpose of this study was to examine the relationship between VGS and death in myocardial infarction. A VGS was audible in 63 of 156 patients (40 per cent) with proved myocardial infarction admitted to a Myocardial Infarction Research Unit during a 24 month period. Fifty-three patients were studied hemodynamically, and 25 of 27 patients with an S₃ gallop demonstrated an abnormally elevated pulmonary artery enddiastolic pressure greater than 12 mm. Hg. Hospital deaths occurred in 40 per cent of patients with a VGS versus 15 per cent of those without this auscultatory finding. Seventy-five per cent of patients in cardiogenic shock died, but a VGS was not significantly associated with death in this group. Although in all patients a VGS was associated with increased mortality rate, deaths were not due to cardiogenic shock. This study confirms the need for hemodynamic monitoring in patients with acute myocardial infarction who exhibit a VGS as evidence of left ventricular power failure. Treatment should be based upon both clinical observations and continued evaluation of the hemodynamic alterations associated with left ventricular power failure.     

Accidental hypothermia in an alcoholic population

Thirty-nine cases of accidental hypothermia are reviewed. Data indicate that mortality varies with the presence of underlying disease rather than with the degree of hypothermia or the methods of rewarming. In 31 patients with hypothermia alone (average temperature 85 °F) mortality was 6.25 per cent. In eight patients with hypothermia and another primary condition (average temperature 84 °F) mortality was 75 per cent.Intractable cardiac arrhythmia has been reported as the primary cause of death in hypothermia. In these patients, death during hypothermia resulted from pulmonary complications. Ventricular arrhythmias, when they occurred, were responsive to routine measures such as electrical cardioversion and myocardial suppressant drugs. Methods of treatment are discussed.     

Hemodynamic effects of nifedipine versus hydralazine in primary pulmonary hypertension

The acute hemodynamic effects of both sublingual nifedipine (N) and intravenous hydralazine (Hy) were studied in 5 patients with primary pulmonary hypertension to ascertain whether the capacity for pulmonary vasodilatation was generalized or drug-specific, and to determine which of the 2 agents had preferential pulmonary vasodilatory effects. For the group as a whole, neither N nor Hy produced changes in heart rate, mean pulmonary capillary wedge or right atrial pressures. Both N and Hy reduced mean systemic arterial pressure (before N 90 ± 8 mm Hg, after N 76 ± 7 mm Hg, p < 0.01; before Hy 92 ± 11 mm Hg, after Hy 68 ± 8 mm Hg, p < 0.05), and decreased systemic vascular resistance (before N 1,558 ± 645 dynes s cm^?5, after N 1,192 ± 430 dynes s cm^?5, p < 0.05; before Hy 1,700 ± 415 dynes s cm^?5, after Hy 957 ± 285 dynes s cm^?5, p < 0.05). In addition, N administration resulted in an increased cardiac output (before N 4.5 ± 2.0 liters/min, after N 4.8 ± 2.0 liters/min, p < 0.01); Hy administration was associated with a more varied effect on cardiac output (before Hy 4.0 ± 1.0 liters/min, after Hy 5.3 ± 1.8 liters/min, p < 0.10, difference not significant [NS]). Although for the group neither agent decreased mean pulmonary artery pressure (PAP) (before N 51 ± 13 mm Hg, after N 44 ± 13 mm Hg, NS; before Hy 50 ± 15 mm Hg, after Hy 51 ± 15 mm Hg, NS) or pulmonary vascular resistance (before N 873 ± 458 dynes s cm^?5, after N 680 ± 450 dynes s cm^?5, NS; before Hy 945 ± 454 dynes s cm^?5, after Hy 715 ± 309 dynes s cm^?5, NS), 4 of 5 patients had a decrease in PAP after N and 1 had no change, and only 1 of 5 patients had a decreased PAP after Hy administration. Moreover, compared with the changes after Hy administration, PAP declined after N administration (δPAP after Hy 0.2 ± 9 mm Hg, after N ?7 ± 9 mm Hg, p < 0.05). The PA diastolic to mean PA wedge pressure gradient tended to decrease after N administration (after N ?5.0 ± 10 mm Hg, after Hy +4.7 ± 7 mm Hg, NS), suggesting more pulmonary vasodilatation after N administration. Moreover, the ratio of pulmonary to systemic vascular resistances was unchanged after N but increased after Hy administration (before N 0.55 ± 0.2, after N 0.53 ± 0.2, NS; before Hy 0.55 ± 0.2, after Hy 0.74 ± 0.3, p < 0.02), indicating the more balanced vasodilatory effect of N. Two patients were treated chronically with Hy but had intolerable adverse effects; 1 was subsequently treated successfully with N. A third patient had pulmonary edema (presumably neurogenic) 30 minutes after Hy administration; this patient later died. Another patient has symptomatically improved with chronic N therapy. Thus, N appears to be a more specific pulmonary arterial vasodilator than Hy in acute drug testing; in this small group of patients with primary pulmonary hypertension, N appears to be more efficacious when administered chronically.     

Studies on circulatory response to hypoxia in the denervated transplanted human heart

The relative importance of centrally mediated reflex responses versus the release of circulating humoral substances in the heart's response to acute hypoxia was studied in the transplanted human heart with its residual innervated recipient atrium and denervated donor atrium. The sinus node rates of both the donor and recipient atria were compared by monitoring P wave activity on the standard electrocardiogram in six patients during control and hypoxic conditions. Breathing 13 per cent oxygen for 10 minutes resulted in an average oxygen saturation of 75 per cent and an oxygen tension (pO₂) of 40 mm Hg, and was associated with a 16 per cent (p 0&;#x030C;.05) increase in the recipient sinus node rate, but with no change in the donor sinus node rate. Pretreatment with atropine abolishes this increase in the innervated recipient sinus node. Breathing 10 per cent oxygen resulted in an oxygen saturation of 55 per cent and a pO₂ of 25 mm Hg, with an associated 10 per cent increase in both donor and recipient sinus node rate. These studies indicate that changes in heart rate during mild degrees of hypoxemia are mediated by a central reflex mechanism which leads to the release of normal vagal parasympathetic tone. The increase in heart rate that occurs in both the innervated and denervated sinus node rates during more severe degrees of hypoxia is probably due to the release of circulating humoral substances, since reinnervation has not been documented in the human cardiac allograft.     

Effects of the converting enzyme inhibitor (SQ 20881) on the pulmonary circulation in man.

The effects of the converting enzyme inhibitor (SQ 20881) on the pulmonary circulation were investigated in 13 patients in whom systemic hypertension developed following coronary artery bypass surgery. Pulmonary vascular resistance was decreased by the inhibitor, from 128 ± 19 to 92 ± 20 dynes sec cm^?5 (or by 30 ± 7 per cent; P < 0.005), and this resulted in a decrease in mean pulmonary artery pressure from 17 ± 1 to 13 ± 1 mm Hg (or by 23 ± 3 per cent, P < 0.005). Consequently, right ventricular work was decreased by the inhibitor by 30 per cent (P < 0.01), despite an increase in cardiac output (increase in stroke volume) by 16 ± 6 per cent (P < 01). This increase occurred despite a 13 ± 3 per cent decrease in right ventricular filling pressure. The changes in pulmonary vascular resistance correlated with the pretreatment plasma renin activity (r = 0.74, P < 0.01), as did the decrease in mean pulmonary artery pressure (R = 0.82, P < 0.001), but neither change was related to the decrease in left ventricular filling pressure nor to changes in cardiac output or mean arterial pressure.These results indicate that blockade of the formation of angiotensin II by the converting enzyme inhibitor results in reductions in pulmonary vascular resistance and pulmonary artery pressure which are unrelated to alterations in left ventricular function. Thus, angiotensin inhibition may have therapeutic value in various clinical states characterized by pulmonary hypertension—especially if renin levels are high.     

The effect of heroin and multiple drug abuse on the electrocardiogram

The electrocardiograms (ECG) of 75 asymptomatic individuals admitted to a methadone treatment program were analyzed to determine the effect of street heroin and other drugs of abuse. All patients were free of cardiac, renal, or pulmonary disease and had no laboratory abnormalities. Two groups of patients were studied. In Group I there were 34 patients on heroin only which was taken within 24 hours of the ECG. Abnormalities were noted in 55 per cent; QT_c prolongation was found in 19 per cent, and significant bradyarrhythmias were found in 19 per cent. In Group II there were 41 patients on methadone with multiple drug abuse; changes were found in 66 per cent. QT_c prolongation was noted in 34 per cent, prominent U waves in 32 per cent, and bradyarrhythmias were found in 32 per cent.It is now a well-known fact that the sudden death syndrome in addicts may not be a pharmacologic or dosage-related phenomenon. The existence of conduction, depolarization, and repolarization abnormalities, as well as bradyarrhythmias in a significant number of drug-dependent individuals may play a role in the production and facilitation of lethal arrhythmias and may be the mechanism of the acute fatal reaction.     

Clinical and hemodynamic results of the Fontan operation for tricuspid atresia

Thirty patients aged 3 months to 26 years (median 9 years) with tricuspid atresia or severe tricuspid stenosis underwent a modified Fontan operation at Children's Hospital, Boston, between 1973 and 1980. Thirty-six palliative operations had been performed previously in 20 patients. Preoperative hemodynamic measurements included: mean pulmonary arterial pressure 6 to 18 mm Hg (median 9); pulmonary vascular resistance 0.3 to 2.2 Woods units/m² (median 1.5) and left ventricular end-diastolic pressure 3 to 14 mm Hg (median 7).There were five hospital deaths (17 percent), including two in the last 24 operations. The 25 survivors have been followed up for 4 to 85 months (median 23) with no late deaths. Twenty patients are in New York Heart Association class I and four in class II; one infant was not classified. Two patients have required reoperation, at, respectively, 71 and 26 months postoperatively; the former for late-developing conduit obstruction and residual bidirectional shunting; and the latter for a residual right to left shunt at atrial level. Early postoperative monitoring of hemodynamics in the intensive care unit revealed that a mean right atrial pressure of 17 mm Hg or greater, without obstruction of the anastomosis, was invariably associated with serious morbidity or mortality.Fifteen patients underwent elective cardiac catheterization 9 to 21 months (median 13) postoperatively. The mean right atrial pressure ranged between 5 and 12 mm Hg (median 8) and mean pulmonary arterial pressure between 4 and 11 mm Hg (median 7). The cardiac index (measured in 12 patients) was 1.9 to 4.0 liters/min per m² (median 3.2). The left ventricular ejection fraction measured in 11 patients did not change significantly postoperatively, remaining normal in 10. Three patients had a residual right to left shunt at atrial level while the arterial oxygen saturation in the remaining 12 patients was 93 to 97 percent. Thus the modified Fontan operation can be performed with excellent clinical and hemodynamic results in selected patients with tricuspid atresia.     

Peripheral neuropathy presenting with respiratory insufficiency as the primary complaint. Problem of recognizing alveolar hypoventilation due to neuromuscular disorders

A 57 year old farmer, initially believed to have hypoventilation secondary to medullary respiratory insensitivity, died with a peripheral neuropathy and marked involvement of the phrenic nerves. Peripheral neuropathy has not previously been reported to present in this manner. Routine pulmonary function tests that would help to distinguish patients with hypoventilation due to neuromuscular disorders from patients with hypoventilation due to diseases of the lung parenchyma and depression of the medullary respiratory centers were investigated. Five subjects with severe neuromuscular disease were studied (arterial carbon dioxide tension [pCO₂]48 to 69 mm Hg, vital capacity [VC] 13 to 79 per cent of predicted and 1 second forced expiratory volume [FEV₁] 76 to 96 per cent of VC). The mean ratio of maximum mid-inspiratory flow (MMIF) to maximum mid-expiratory flow (MMEF) was 0.79. In age-matched control subjects this ratio was 1.41. In addition to observing the ratio of MMIF to MMEF other effective clinical screening procedures to distinguish patients with hypoventilation secondary to neuromuscular disorders from patients with medullary respiratory insensitivity include (1) identification of weakness of the muscles used for ventilation by measuring the static maximum inspiratory and expiratory airway pressures, (2) determination of ability to lower the arterial pCO₂ with voluntary hyperventilation, and (3) comparison of the maximum breathing capacity to the minute ventilation while breathing 7.5 per cent carbon dioxide for 3 minutes. The latter two measurements permit assessment of central hypoventilation in the presence of intrinsic lung disease.     

How to be less invasive

Unless cardiac output is reduced, alveolar ventilation can be monitored without arterial blood gas analysis by estimating arterial carbon dioxide tension (PaO₂)from rebreathing measurement of mixed venous carbon dioxide tension (Pv̄CO₂) (PaCO₂ = 0.8 Pv̄CO₂). If cardiac output is reduced, the Pv̄CO₂ - PaCO₂ difference increases, reflecting the increased venoarterial carbon dioxide (CO₂) content difference (Fick principle). A reduction in cardiac output can thus be quantified without catheterization of the central circulation by measuring both PaCO₂ and Pv̄CO₂. The significance of such a reduction in cardiac output for oxygen (O₂) delivery to tissues is determined by calculation of mixed venous O₂ saturation (Sv̄O₂), using the inter-relationship of CO₂ production to O₂ consumption. With normal arterial oxygenation (SaO₂) and hemoglobin concentration, PaCO₂ < 0.8 Pv̄CO₂ - 12 implies a reduction in cardiac output such that Sv̄O₂ is less than 33 per cent, which indicates inadequate O₂ delivery to tissues. Ear oximetry and rebreathing measurement of Pv̄CO₂ are simple, noninvasive, bedside techniques.     

Acute respiratory failure in scoliosis or kyphosis: prolonged survival and treatment

Acute respiratory failure (ARF) in adults with severe thoracic spinal deformity is said to be a preterminal event with a median survival of one year. Twenty patients with ARF (mean +/- S.D., arterial oxygen tension [PaO2] 35 +/- 7 mm Hg, arterial carbon dioxide tension [PaCO2] 63 +/- 9 mm Hg, pH 7.34 +/- 0.08) due to severe scoliosis or kyphosis (spinal curve 113 +/- 28 degrees ) were seen between 1965 and 1980. All 20 survived the initial episode of ARF and during the follow-up period (median, six years) experienced 2.4 additional episodes of ARF. The age at presentation was 52 years (mean, range 13 to 78), and the cause of spinal deformity was idiopathic (seven patients), poliomyelitis (seven), tuberculosis (five), and arthrogryposis multiplex congenita (one). ARF was treated with controlled low dose oxygen by Venturi mask and intensive general measures in 13 patients and by mechanical ventilation in seven. Of the latter seven patients, ventilatory failure was treated in two with a tank respirator and a cuirass, avoiding endotracheal intubation. Outpatient management was similar to conventional therapy for chronic obstructive pulmonary disease (COPD). Severe restrictive ventilatory impairment characterized the group after recovery from the first episode of ARF: vital capacity (VC) 906 +/- 362 ml (31 percent predicted) and FEV1 589 +/- 197 ml (23 percent). During follow-up, the VC decreased by 1.5 ml/year and the FEV1 by 13.9 ml/year. The PaO2 increased by 2.6 mm Hg/year and the PaCO2 increased by 1.7 mm Hg/year. Successful management of ARF due to severe scoliosis or kyphosis is possible in the great majority of patients, and long-term survival may be expected. Unlike COPD, pulmonary function following ARF in kyphosis or scoliosis deteriorates at a slower than expected rate and, in fact, may improve with treatment over many years.     

Evaluation of transcutaneous CO2 responses following acute changes in PaCO2 in healthy subjects

Background and objective: Transcutaneous blood gas‐monitoring systems with miniaturized SpO₂ (peripheral blood oxygen saturation)/PCO₂ combined sensors (TOSCATM) have been widely used. There are no reports of the inter‐ and intra‐individual variability in transcutaneous measurements of PaCO₂ (PtcCO₂) in response to acute progressive changes in PaCO₂. This study examined inter‐ and intra‐individual variability of PtcCO₂ measurements under semi‐steady‐state conditions, and characterized the behaviour of PtcCO₂ in response to acute progressive changes in PaCO₂. Methods: Subjects breathed mixed gases through a mouthpiece connected to an automatic arterial blood gas controller. Using end‐tidal PCO₂ and PO₂ as guides, PaCO₂ was controlled to increase and/or decrease between baseline and ≥ 60 mm Hg, in a stepwise (n = 9) or progressive fashion (n = 6). Arterial blood was sampled when needed. Results: Intra‐individual correlation coefficients between PtcCO₂ and PaCO₂ were excellent in all subjects (0.971–0.989); however, the slope of the regression line varied among subjects (1.040–1.335). Bias and limits of agreement (± 2 SD from bias) between PtcCO₂ and PaCO₂ were ?1.8 mm Hg and ?7.7 to 4.1 mm Hg. Changes in PtcCO₂ in response to acute progressive changes in PaCO₂ also varied among subjects. Conclusion: The PtcCO₂ measurement system allows reliable estimation of PaCO₂ in a given subject. However, caution is needed when comparing absolute values between subjects or when acute changes in PaCO₂ occur.