Sensitivity and specificity of echocardiographic evidence of tamponade: implications for ventricular interdependence and pulsus paradoxus.

The reported sensitivity of the echocardiographic finding of right atrial collapse for the diagnosis of tamponade ranges from 50% to100%; specificities have ranged from 33% to 100%. Its sensitivity in identifying right ventricular collapse ranges from 48% to 100% whereas the specificity ranges from 72% to 100%. Collapse of either the right atrium or right ventricle is not reliable except in cases where the risk of tamponade is high, consistent with Bayes' theorem. If the patient has hypotension, tachycardia, dyspnea, increased venous pressure, and a pericardial effusion, the diagnosis of tamponade will likely be sustained. To explain pulsus paradoxus, most echocardiographic reports have invoked Dornhorst's theory that inspiratory filling of the right ventricle actively collapses the left ventricle by successfully competing for a fixed total pericardial space ("ventricular interdependence"). However, the pericardial space is not fixed in tamponade but increases with inspiration, and the right heart is much more likely to collapse than the left, given their relative thickness. Pulsus paradoxus depends on the inspiratory surge to the right heart, exaggerated by the small stroke volume of both ventricles induced by tamponade, and vascular coupling between the pulmonary and systemic beds, with a transit time of one to two heart beats.

Thermal stress in sudden infant death: Is there an ambiguity with the rebreathing hypothesis?

OBJECTIVE: To assess the role of thermal stress in the cause of sudden infant death syndrome (SIDS), and to compare risk factors with those of rebreathing. METHODOLOGY: Analysis of publications concerning the epidemiology and physiology of thermal stress in SIDS. RESULTS: A strong association between thermal regulation and ventilatory control was found, specifically for prolonged apnea. Infections, excessive room heat and insulation, and prone sleeping produce significantly increased odds ratios for SIDS. Although some of the risk factors for rebreathing could be explained by the effects of thermal stress, several factors for thermal stress could not reasonably be explained by the rebreathing hypothesis. CONCLUSIONS: Although the risk of thermal stress is widely accepted abroad, it has received relatively little attention in the United States. The incidence of SIDS in the United States can likely be further reduced by educating the public against the dangers of overheating, as an integral part of the back-to-sleep campaign.

Large ductal flow may cause high velocity in the descending aorta without coarctation: improved diagnosis using the continuity equation.

Large ductal flow will increase the flow through the aortic isthmus and may falsely suggest coarctation of the aorta when the Bernoulli equation is applied. The continuity equation (A1 x V1 = A2 x V2) does not depend on unwarranted assumptions, and its use should avoid misdiagnosis.

The black infant's susceptibility to sudden infant death syndrome and respiratory infection in late infancy.

Between 2 and 11 months of age, the risk of sudden infant death syndrome (SIDS) declines more slowly in black infants than in infants of other races. This phenomenon might also be a feature of certain non-SIDS causes of death. Identifying these causes may through analogy provide support for the theory that SIDS is a disease of the central nervous system, an unusual consequence of respiratory infection, or a form of suffocation. We used logistic regression analysis on details of infant deaths in the United States, 1985-1991, to examine the difference between the rates of decline with increasing age in the mortality rates of black infants and infants of other races. We defined slower rate of decline in black infants as a positive difference. The magnitude and direction (positive) of the difference for deaths due to respiratory infection were similar to those for SIDS. It is unlikely that this difference in the rates of decline for respiratory infection can be explained by diagnostic cross-misclassification between respiratory infection and SIDS. SIDS appears to be a disease of the respiratory system caused by infection that affects that system's control centers.

The effect of the weekend on the risk of sudden infant death syndrome.

OBJECTIVE: The risk of sudden infant death syndrome (SIDS) is associated strongly with socioeconomic status. However, many infants who live in one socioeconomic environment, with its attendant level of risk of SIDS over the weekend, often are exposed to a different level of risk during the work week (because of day care for the infant). If the association between SIDS and socioeconomic status acts through the quality of supervision of the infant, then there could be an immediate change in the level of risk as the infant moves from home to outside care to home again. In this scenario, infants of economically disadvantaged parents would have a higher risk of SIDS over the weekend than they do during the week. On the other hand, infants of economically advantaged parents would be at lower risk over the weekend. Therefore, the relative risk of SIDS associated with the weekend (risk over the weekend vs risk during the work week) should be found to decrease as the number of years of maternal education (a surrogate for socioeconomic status) increases. Testing this prediction is the objective of the study. METHODOLOGY: Instances of SIDS in the postneonatal period (28-364 days) among the cohort of all infants born in the United States between January 1989 and December 1991 were analyzed. The number 798.0, taken from the International Classification of Diseases, was used to identify 14 996 cases of SIDS. Deaths among hospital patients were distinguished from all other deaths. The latter were divided into four categories: 1) death occurred in the emergency department; 2) the infant was dead on arrival at the emergency department; 3) death occurred at a residence; and 4) death occurred at some other place. Maternal education was divided into four categories: <12, 12, 13 to 15, and >/=16 years. The weekend ratio was defined as the ratio of SIDS cases on Saturday and Sunday (times 5) and Monday through Friday (times 2). The predicted trend in this ratio by maternal education was tested by applying a chi(2) test-for-trend. RESULTS: The overall weekend ratio was 1.00, indicating that the risk of SIDS was no higher over the weekend than it was Monday through Friday. However, for infants of mothers with <12 years of education, the ratio was 1. 13. For infants of mothers with >/=16 years of education, it was 0. 55. The trend in the ratio as maternal education increased (1.13, 0. 99, 0.86, and 0.55) was highly significant (chi(2) = 74.2; 1 degree of freedom). Each of the four ratios, with the exception of 0.99, was significantly different from 1.00 (z = 3.74, 2.45, and 6.09, respectively). The ratios for infants of mothers with 13 to 15 and >/=16 years of education also were significantly different from each other (z = 4.57). For all causes of death combined (including the relatively small number of SIDS cases) among hospital inpatients, there was no significant trend in the weekend ratio as the level of maternal education increased. However, among deaths not attributable to SIDS or accidents occurring outside the hospital, there was a slight but significant declining trend (chi(2) = 8.4; 1 degree of freedom) The risk of an accidental death was highest over the weekend for all four maternal education categories. On an average working day, the risk of SIDS among offspring of mothers with <12 years of education was found to be 3.9 times greater than that among offspring of mothers with >/=16 years of education. At the weekend, the relative risk increased to 7.9. A plot of the weekend ratio against single years of maternal education revealed a unimodal distribution with a peak at 11 years. CONCLUSIONS: First, the results of the study are consistent with the level of risk of SIDS, changing promptly toward the risk level obtained in the baby's new environment. Variability in the observation of unusual respiratory events seems the most likely explanation. It is unlikely that confounding factors played a role in the results for tertiary-educated mothers

The role of ductal constriction in transient stenosis of the left pulmonary artery.

To confirm the hypothesis that narrowing of the left pulmonary artery is produced by ductal closure, Doppler velocities in the pulmonary arteries from 20 premature infants with a patent duct were compared to velocities from 20 with a closed duct. In infants with a patent duct, the mean Doppler velocity in the main, right, and left pulmonary arteries were, respectively, 86 cm/sec +/- 5 standard error (SE), 120 +/- 7, and 125 +/- 8. With closed duct, the comparable velocities were 102 cm/sec +/- 7, 129 +/- 8, and 190 +/- 11. The only significant difference was the higher velocity in the left pulmonary artery associated with a closed duct (p = 0.00003). This physiologic origin of left-branch stenosis associated with ductal constriction should be considered when a murmur is detected after administration of indomethacin (in addition to possible persistent ductal flow), and as an explanation for some murmurs in the normal newborn. No anatomic or physiologic support was found for the theories that murmurs in the neonate were due to ductal flow, or to acute angulation of the branch pulmonary arteries.

The seasonal distribution of infant deaths by age: a comparison of sudden infant death syndrome and other causes of death.

OBJECTIVE: To examine the possibility that among deaths in infancy the increase in the winter/summer ratio with increasing age is not peculiar to sudden infant death syndrome (SIDS). METHODOLOGY: Details of the winter (December-February)/summer (June-August) ratio among deaths in neonates (< 28 days) and post neonates dying in the United States of America between 1979 and 1990 were abstracted from published statistics. The primary causes of death were classified according to the ninth Revision of the International Classification of Diseases. RESULTS: For every non-traumatic cause of death including SIDS, the winter/summer ratio was higher among postneonates than neonates. This was not seen for deaths due to trauma. Cases of SIDS and deaths due to infection had the highest ratios in both age categories. Causes of death occurring predominantly in the neonatal period (e.g. anencephaly) had the lowest overall ratios. CONCLUSIONS: Neither the greater number of SIDS cases in the winter, nor the increasing winter/summer ratio with increasing age is unique to SIDS.

Are bedding and rebreathing suffocation a cause of SIDS?

Suffocation by bedclothes became a popular diagnosis in the 1940s but gradually became replaced with the diagnostic label of Sudden Infant Death Syndrome (SIDS). In 1991 a paper purported that, instead of SIDS, pillows filled with polystyrene beads had caused death by rebreathing suffocation; this conclusion was reached on the basis of experiments with anesthetized rabbits breathing through a doll's head that was placed face down on the pillow. Because of the anesthesia, rabbits could not change their face down position. The doll's nares could not collapse, which would have resulted in rapid death due to conventional suffocation. The rabbits required up to 3 hours or more to die of hypercarbia and hypoxia. Studies in normal infants revealed that they turned from the face-down position after only 2 minutes. (The only infant who retained CO2 soon died of a fatal neurologic disorder, with central hypoventilation). Using the rabbit/doll's head and mechanical models, a wide range of bedding was indicted, including cushions, sheepskins, pillows, comforters, foam mattresses, and even simple blankets and sheets as potentially causing fatal rebreathing. Except for the use of pillows in general, as well as mattresses filled with kapok and bark, there has been no epidemiologic support for these indictments. Although normal infants are unlikely to succumb to rebreathing suffocation, infants with blunted ventilatory responsiveness and delayed arousal due to prior hypoxia were hypothesized to be at increased risk. Support for this concept was found in the pathology of the brain stem in victims of SIDS that was attributed to prior hypoxic injury. In infants who survived prolonged apnea, less than 20% have demonstrated a diminished ventilatory responsiveness to hypercarbia, but, more significantly, none had an absent response. Arousal to hypercarbia, an abnormality which is crucial to the hypothesis of rebreathing suffocation, is regularly present in normal subjects, but the threshold is higher in near-SIDS infants; however, no instances of failure to arouse have been reported in near-SIDS. If the infant is placed on his or her back or side, the issue of bedding could become moot; unfortunately, a sizable percentage of infants are still being placed prone for sleep. Instead of confusing parents with an ever-expanding list of "dangerous bedding," the message "Back to Sleep" should be emphasized.

Rate-based management of fetal supraventricular tachycardia.

We reviewed the ultrasonographic studies and the clinical course of 22 fetuses with supraventricular tachycardia to determine whether the heart rate alone could serve as a basis for conservative management. Hydrops was not encountered with heart rates under 230 beats per minute. The conditions of all 22 fetuses stabilized without invasive administration of medications. Eighteen were delivered vaginally and only four by cesarean section. No fetal or neonatal losses occurred. Regardless of the type of supraventricular tachycardia, reducing heart rate in these fetuses to levels preventing or resolving hydrops allowed term vaginal delivery, thereby reducing the substantial problems of ventilating an immature or hydropic neonate.

Negative effect of a short interpregnancy interval on birth weight following loss of an infant to sudden infant death syndrome.

It was hypothesized that a short interpregnancy interval immediately following the birth of an infant that had succumbed to sudden infant death syndrome (SIDS) (and no other cause of death) would be associated with a reduced mean birth weight in the next infant. Mothers who had given birth to two children in the state of Oregon between 1975 and 1984 and whose first child had died in infancy from either SIDS (n = 84) or some other cause (n = 305) were identified from vital records. A multiple regression analysis in which adjustment was made for possible confounding variables (including the birth weight of the deceased child) was conducted. When the firstborn child had succumbed to SIDS, the mean birth weight of the next baby was 314 g (2,978 g vs. 3,292 g, p = 0.04) lower when the interpregnancy interval was less than 6 months versus greater than 6 months. In contrast, a less-than-6-month interval had a slightly positive effect (60 g) on the mean birth weight of the next baby when the firstborn child had died due to a cause other than SIDS. These results suggest that parents who have lost a child to SIDS may wish to delay a new pregnancy for at least 6 months.

Recommendations to avoid the prone sleeping position and recent statistics for sudden infant death syndrome in the United States.

OBJECTIVE: To determine whether two recent, nonsynchronized recommendations to avoid the prone position for sleeping infants were each followed by a decline in the incidence or expected number of cases of sudden infant death syndrome (SIDS). DATA SOURCES: Data were collected from SIDS counseling programs, state vital statistics, and medical examiner records of 44 states and the city of Los Angeles, Calif. Data for a state were excluded because of either incompleteness or nonresponsiveness to request. DATA SYNTHESIS: In the 8 months following an article in the Seattle Times advising against the use of the prone position for sleeping infants, the incidence of SIDS fell by 52.0% in King County (where 32 households in every 100 receive the Seattle Times) and by 19.9% in Snohomish County (16 in 100 households). In the remaining 37 counties of Washington State (on average, < 1 in 100 households are subscribers), the incidence rose 3.4%. Examination of medical examiner records for King County and Snohomish County revealed no compensatory increase in other causes of death and no cases attributed to aspiration. In the 12 months following the initial 8 months, the number of SIDS cases in King County remained at approximately half the previous annual average (25 vs 49 cases). At the national level, the American Academy of Pediatrics' recommendation on April 15, 1992, was followed in the next 6 months by a decrease of 12.0% in the number of SIDS cases compared with the previous year. CONCLUSIONS: The results are consistent with those of intervention programs in other countries. A national campaign to inform parents of the risk of the prone position in early infancy should be given serious consideration.

Redefinition of the sudden infant death syndrome: the disadvantages.

For many years the definition of SIDS has been the sudden death of an infant that was unexpected by history and in which a thorough postmortem examination failed to demonstrate an adequate cause of death. In 1991 a report was published in this journal from a panel convened by the NICHD which recommended that the diagnosis of SIDS not be made unless a death scene investigation has been conducted. The panel recommended further exclusions from the diagnosis of SIDS of certain "unresolved" cases. We believe the changes recommended by the NICHD panel are impractical and may have a serious negative impact on SIDS research and on the surviving family members of the SIDS victims.