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To determine whether the severity of thallium-201 scintigraphic defects within hours of acute myocardial infarction (MI) could be used to predict subsequent mortality, thallium-201 imaging was performed within 15 hours of the onset of symptoms in 42 patients with acute MI. Patients with pulmonary edema or shock were excluded. The extent of perfusion defect was determined in three views (anterior and 40/sup 0/ and 60/sup 0/ left anterior oblique) by both objective computer-assisted and subjective methods, and expressed as a summed defect score. Mortality for the patient group as a whole was 17% in hospital, 24% at 6 months, and 33% at last follow-up (average 9 months). Using the objective method, a high thallium defect score (7.0 or greater, corresponding to at least a moderate reduction of activity involving 40% of the left ventricle in two views) identified a subgroup of 13 patients in which mortality was 46% in hospital, 62% at 6 months, and 92% at last follow-up. Corresponding values for the 29 patients with lower objective defect scores were 3%, 7% and 7%, respectively (all p =1000 IU/1 and moderate (vs one or mild) left ventricular failure were also associated with mortality. However, a high thallium defect score was significantly more predictive than any of these variables. Stepwise, multivariate analysis showed that the thallium score alone was a better predictor than the best combination of these clinical variables, and no variable added to the predictiveness of the high defect score. These results suggest that thallium-201 scintigraphy may provide an accurate, rapid, noninvasive method for separating high-risk and low-risk subgroups of hemodynamically stable patients.« less

The accuracy of two-dimensional echocardiographic left ventricular volume measurement in an isolated heart preparation was tested using Simpson's reconstruction of progressively fewer short-axis cross sections of known location. Echocardiographic images from five ejecting hearts submerged in a special tank were obtained under conditions designed for maximal accuracy of echocardiographic volume assessment. Echocardiographic determinations of 52 volumes at various times throughout the cardiac cycle were compared, by least-squares linear regression, with simultaneous direct-volume measurements by volumetric chamber (range 9.4-44.8 ml). Echocardiographic and direct measurements correlated well for all numbers of cross sections from 1-19 (r = 0.84-0.97); however, variability of direct volume predicted from a given echocardiographic measurement increased nonlinearly as the number of cross sections per heart decreased, and was especially large when three or fewer cross sections were used (SEE = 4.6-7 ml). The accuracy of echocardiographic measures was compared for each number of cross sections per heart, varying from one to 19; accuracy was defined as the mean absolute difference between echocardiographic and direct measurements of volume, ejection fraction, and maximal rate of ejection. The accuracy of echocardiographic measurements was significantly reduced with fewer than four cross sections per heart for ventricular volume, three cross sections for ejection fraction, and five cross sections for maximal rate of ejection. In light of what appears to be required for accurate echocardiographic volume measurement in this controlled, ejecting, noninfarcted, in vitro preparation, additional cross sections may be required in intact animals and human subjects, especially in those with diseases that cause ventricular asymmetry or regional dysfunction.

Many studies have delineated the changes in the elastic properties of arterial tissue as a function of age. Despite the fact that viscoelasticity is a prominent feature of these tissues, there is little information or characterization of age-associated changes in viscoelastic properties, over a wide range of smooth muscle activation, particularly in nonhuman tissue where atherosclerosis is not a confounding factor. In the present study, using small sinusoidal length perturbations, we determined the dynamic stiffness properties across a wide range of lengths (stretch ratios from 100 to 135%) and frequencies (from 0.25 to 35 Hz) in strips excised from ascending and descending aortas from six young (2 to 4-year-old) and 12 senescent (10- to 13-year-old) beagles. Studies were performed with the smooth muscle fully activated with calcium and norepinephrine, as well as fully inactivated with cyanide, iodoacetate, and dinitrophenol. There was a cubic nonlinear dependence of stiffness modulus on length only in senescent tissue and, surprisingly, little frequency dependence in tissue of either age. Compared to the young aortas, the three-dimensional surface representing the dependence of stiffness modulus on length and frequency from both the ascending and descending regions of aged aortas was displaced higher on the stiffness axis both with the muscle fully activated and inactivated. This age difference was accentuated at longer lengths. The phase lag between force and length was greater in the young vs. the old strips only in the activated, ascending aortic tissue. We found no age differences in the content of elastin, collagen, or in the collagen/elastin ratio, to account for these mechanical property differences.(ABSTRACT TRUNCATED AT 250 WORDS)

Acute shifts in the pressure-volume relationship of the left ventricle occur under several conditions. One potential mechanism for this phenomenon is ventricular interdependence, that is, the influence of right ventricular filling on the left ventricle. This mechanism was studied in an isolated cross-circulated ejecting canine heart preparation with balloons sewn into both right and left ventricles and connected to volumetric chambers allowing continuous and accurate simultaneous measurement of right and left ventricular pressures and volume. Sets of left ventricular pressure-volume loops were obtained under a variety of right ventricular volumes. Experiments were repeated with and without the pericardium. Both diastole and systole were studied. We show that in the pericardium-free heart working over a normal range of volumes, the right ventricle exerts little influence on left ventricular pressure. With the pericardium present, there is a small but significant effect of right ventricular volume on left ventricular pressure during systole and diastole. Over a wider range of volumes imposed in the arrested heart, the right ventricle does influence left ventricular pressure even without the pericardium. Thus we have demonstrated that ventricular interdependence is not likely to lead to large acute pressure-volume shifts, either during diastole or systole, except in the presence of considerably above normal right ventricular volumes.

The extent of abnormality in early thallium-201 and gated cardiac blood pool scintigrams has been reported to be useful for predicting mortality in patients with acute myocardial infarction (AMI). To compare the two techniques, 91 patients admitted consecutively with evident or strongly suspected AMI underwent both imaging studies within 15 hours of the onset of symptoms. Patients with pulmonary edema or shock were excluded. AMI developed in 84% of patients, and 6-month mortality for the entire group was 16%. A thallium defect score of 7.0 or greater (corresponding to at least a moderate reduction of activity involving 40% of the left ventricular circumference) identified a subgroup of 14 patients with 64% 6-month mortality rate. Similarly, a left ventricular ejection fraction of 35% or less identified a high-risk subgroup of 10 patients with a 6-month mortality of 60%. Mortality in the remaining patients was 8% for thallium score less than 7 and 11% for ejection fraction greater than 35%. The mortality rate was highest among patients who had concordant high-risk scintigrams (five of six, 83%), lowest in those with concordant low-risk studies (five of 64, 8%) and intermediate in those with discordant results (four of 11, 36%). Of a number of clinical variables, only the appearance of Q waves, peak creatine kinase greater than 1000 IU/I, and history of infarction were significantly associated with mortality. High-risk thallium or blood pool scintigraphic results were significantly more predictive and a thallium score of 7 or greater was more sensitive for detecting nonsurvivors than ejection fraction 35% or less at a similar level of specificity. Stepwise multiple logistic analysis showed that the thallium score was the best predictor of mortality, but that appearance of Q waves and ejection fraction were additive. Using these three variables, 11 patients were calculated to have a 50% or greater chance of dying and eight (73%) actually died, compared with six of 70 (9%) with a calculated chance of death of less than 50%. These results in a prospectively identified and consecutive group of patients support the value of early thallium and blood pool scintigraphy for separating high- and low-risk subgroups of hemodynamically stable infarct patients.

Long-term follow-up was obtained on 138 patients who participated in a prospective, randomized study comparing two weeks with three weeks of hospitalization following uncomplicated acute myocardial infarction. Follow-up information was available on 123 (89%) of all randomized patients. The mean follow-up period was 35 months for those patients who died and 99 months for those who survived. No differences were found between the two groups with respect to survival, cardiac-related deaths, frequency or severity of angina pectoris, subsequent myocardial infarction, incidence of congestive heart failure, number of ventricular aneurysms, or subsequent medical therapy. A significantly greater number of survivors in both groups stopped smoking and had a normal initial heart size than those who died. This long-term follow-up study further supports the conclusions of earlier short-term studies that two weeks of hospitalization is safe in patients with uncomplicated acute myocardial infarction.

To determine whether nifedipine reduces pulmonary artery pressure and pulmonary vascular resistance index during rest and exercise in patients with hypoxic pulmonary hypertension, we studied 6 clinically stable patients using a randomized, double-blind, crossover design. While patients were hypoxic, nifedipine significantly lowered mean pulmonary artery pressure during rest from (mean +/- SEM) 38 +/- 2 mmHg with placebo to 35 +/- 3 mmHg with nifedipine (p less than 0.01) and during exercise from 63 +/- 4 mmHg with placebo to 51 +/- 3 with nifedipine (p less than 0.01). During hypoxia nifedipine reduced pulmonary vascular resistance index during rest by 27% from 7.84 +/- 0.5 units with placebo to 5.71 +/- 0.6 units with nifedipine (p less than 0.02) and during exercise by 44% from 7.84 +/- 1 units with placebo to 4.37 +/- 1 units with nifedipine (p less than 0.001). Nifedipine when added to low flow oxygen reduced pulmonary vascular resistance index during rest by 16% from 6.15 +/- 0.8 units with oxygen to 5.14 +/- 0.5 units with oxygen plus nifedipine (p less than 0.007) and during exercise by 27% from 5.9 +/- 0.9 units with oxygen to 4.3 +/- 0.7 units with oxygen plus nifedipine (p less than 0.005). On room air nifedipine decreased PaO2 during rest by only 4 +/- 1 mmHg and did not decrease exercise PaO2. During oxygen therapy nifedipine decreased PaO2 during rest by 12 +/- 4 mmHg and during exercise by 8 +/- 3 mmHg. Nifedipine therapy, however, substantially increased oxygen delivery during rest and exercise.(ABSTRACT TRUNCATED AT 250 WORDS)

To determine the effects of different anesthesias on the performance of the arterial baro-reflex, the open-loop characteristic of the carotid sinus reflex was analyzed in 24 rabbits under anesthesia with pentobarbital (30 mg/kg), urethan (800 mg/kg), alpha-chloralose (80 mg/kg), or a mixture of alpha-chloralose (40 mg/kg) and urethan (0.4 g/kg). For each rabbit and anesthesia, mean systemic arterial pressure and heart rate were measured as carotid sinus pressure was changed in 10-mmHg steps between 40 and 150 mmHg. This set of measurements was repeated four times at 1-h intervals. A logistic function curve was fitted to the carotid sinus pressure-arterial pressure relationship. The parameters of this curve were then analyzed to delineate the specific effects of the anesthesias on the relationship. The main finding was that the response range and the slope parameters under alpha-chloralose anesthesia were significantly smaller than those obtained under the other anesthesias. Propylene glycol, used as the solvent for chloralose, did not affect the reflex control of arterial pressure or heart rate. The reflex under chloralose-urethan anesthesia showed characteristics similar to those under urethan anesthesia. We conclude that although alpha-chloralose has traditionally been used in the dog to obtain strong reflex responses, it weakens the reflex control of arterial pressure in the rabbit.

To understand more fully the role of the arterial baroreceptor reflex on systemic and pulmonary vascular resistance, we studied the influence of the carotid sinus baroreceptor reflex control system on the entire systemic and pulmonary arterial pressure-flow relationships. Ten pentobarbital-anesthetized dogs, whose carotid sinuses were isolated, were used in a preparation in which the right and left hearts were bypassed to control systemic and pulmonary blood flows. At intrasinus pressures of 50, 125, and 200 mm Hg, systemic and pulmonary arterial pressures were measured in response to step changes in systemic and pulmonary blood flows from 0 up to 200 ml/min per kg. The systemic arterial pressure-flow relationship exhibited a marked nonlinearity, especially at either high or low flows. A third-order polynomial fit was found to represent the steady state systemic arterial pressure-flow relationship best. Blood flow autoregulation was reflected as a secondary change in systemic arterial pressure at constant flow approximately 15 seconds after the initial response. Blood flow autoregulation was seen in the entire systemic vascular bed over the entire range of flows studied. The degree of autoregulatory significantly contributed to the shape of the systemic arterial pressure-flow relationship. The steady state arterial pressure-flow relationship shifted upward and toward the pressure axis, increasing the calculated incremental resistance and total peripheral resistance as intrasinus pressure was decreased. The systemic zero-flow arterial pressure was found to increase with decreases in intrasinus pressure. The pulmonary arterial pressure-flow relationship was found to be linear in the range of flows studied from 25 up to 200 ml/min per kg.(ABSTRACT TRUNCATED AT 250 WORDS)

Seven-day individual weighed dietary intakes and anthropometric measurements were determined in 123 children, 2 to 19 yr of age, from 26 poor families in Lima: each included one child who had been malnourished, six were adopting families. Heights and weights were converted to "ages" based on Boston reference data and local data, then to "quotients" as percentages of actual ages. Mineral and vitamin intakes were expressed as amounts per day and per 1000 kcal, calories and protein as percentages of FAO/WHO recommendations for age and height age and of modified recommendations based on size of Peruvian children. Regression analyses identified common sources of nutrients and greater dependence of intakes on body size than on age. Analysis of covariance for sex identified different nutrient-growth correlations. Polynomial regression analysis identified percentage protein from animal sources and percentage fat calories as having significant quadratic as well as linear correlations with achieved growth. In males, multiple regression analysis identified percentage protein from animal sources and beta-carotene intakes as strongly associated with achieved height and weight, and percentage fat calories as strongly associated with weight quotient/height quotient ratios. In females the correlations were not as strong, possibly because a significant percentage had reached the menarche some time before the survey and were probably no longer growing. Nevertheless, calorie intake, as a percentage of the recommendation for height age, was prominent in the regressions for height quotient and percentage fat calories in that for weight quotient. The very strong association of animal protein intake with male height, despite seemingly generous total protein intakes, is difficult to reconcile with current recommendations. The possible role of vitamin A (as beta-carotene) has potentially important implications for food policies.

A prospective randomized clinical trial of intravenous nitroglycerin administered for 48 hours following acute infarction was undertaken to determine whether clinical improvement and/or preservation of ischemic myocardium could be demonstrated. One hundred four patients were randomized to receive either nitroglycerin or placebo infusion. Nitroglycerin infusion was titrated to lower mean arterial pressure 10% using non-invasive monitoring. Early nitroglycerin treatment, defined as treatment initiated less than 10 hours after symptom onset, resulted in a lower incidence of new congestive heart failure, myocardial infarct extension, and/or early death from pump failure (15%) compared to late nitroglycerin treatment (50%, p = 0.008) or early placebo treatment (48%, p = 0.01). Improvement in ejection fraction of greater than or equal to 10% was demonstrated in 7 of 20 (35%) early nitroglycerin treated patients compared to 6, 11 and 0% of patients in the three other subgroups, respectively (p = 0.004). Similarly, thallium-201 perfusion scintigrams showed a greater than 75% improvement in defect score in 23 (48%) patients receiving nitroglycerin within 10 hours compared to 14%, 33% and 0% respectively, in the remaining subgroups (p = 0.035). However, before routine administration of nitroglycerin can be recommended for all patients with acute infarction, larger clinical trials will be required.

Interaction of carotid sinus and aortic arch reflex control of total peripheral resistance was studied in eight dogs anesthetized with sodium pentobarbital and placed on constant flow cardiac bypass. Carotid sinus and aortic arch baroreceptor areas were isolated and separately perfused at controlled pressures. Combinations of carotid sinus and aortic arch pressures were delivered at random in steps of 25 mm Hg over the 50-225 mm Hg pressure range, and systemic arterial pressure was measured. Changes in arterial pressure reflected changes in total peripheral resistance. A multiple linear regression showed that both carotid sinus and aortic arch pressures exhibited a sigmoidal relationship with arterial pressure. Independent of carotid and aortic baroreceptor pressures, arterial pressure was found to be a periodic function of time (period = 2 hours) in all dogs. The average carotid sinus reflex open loop gain was found to be 0.231 +/- 0.092, while average aortic arch open loop gain was 0.141 +/- 0.088. The gain of either the carotid sinus or aortic arch reflex was not influenced by the absolute pressure level of the other receptor area. In a separate series of experiments performed in the same dogs, we tested the hypothesis that a nonlinear temporal summation of the reflex control of total peripheral resistance might exist when the inputs to carotid and aortic baroreceptors are changed simultaneously. With both inputs held at the region of maximum gain, 25 mm Hg step changes were imposed first on carotid sinus pressure, then on aortic arch pressure, and then on both simultaneously. A temporal inhibition of the two reflexes showed that simultaneous excitation of both receptors resulted in a smaller reflex response than the sum of individual responses.

This study extends our previous observation on patients with a tetralogy of Fallot, operated on by the late Dr; Alfred Blalock and his associates between 1945 and 1951, from a 15 year follow-up to 20 years and assesses the final status of these patients 20 to 28 years after their first operation. This study is mainly concerned with the 432 patients known to be alive at the beginning of the 15th postoperative year. At the beginning of the 20th year, 376 patients were alive, 24 had died, 32 had been lost to follow-up. Review of the final status of those 432 patients showed that 169 had no further cardiac surgery after their initial operation, 36 had further palliative surgery, and 227 had total correction. Thirty-seven percent of the first group, and 79.3% of those with total correction were doing well. These two groups, however, are not comparable. Approximately 250 patients have married; 161 have one or more children. Thirty-five percent have graduated from college and 68.7% are earning substantial incomes. The high scholastic achievement of many of these patients is strong evidence that low oxygen saturation of arterial blood is not a prime cause of mental retardation. The occupations of the patients indicate that the quality of their lives is extremely good and that a cardiac handicap in childhood does not preclude success in adult life. Approximately 69% of these patients have repaid in taxes the cost to society of their rehabilitation.

We have studied the effect of the carotid sinus baroreceptor reflex on respiration in 10 vagotomized, spontaneously breathing, pentobarbital anesthetized dogs. The carotid body chemoreceptor reflex response was eliminated by surgically excluding the carotid bodies from the carotid sinus baroreceptor area. Steady state frequency, tidal volume, and minute ventilation were measured after 25 mm Hg step changes in intrasinus pressure between 50 and 200 mm Hg. Over this range, the step decreases in intrasinus pressure caused concomitant increases in mean arterial pressure from 86 to 182 mm Hg. All of the respiratory response curves were sigmoidal in shape. Decreasing intrasinus pressure from 200 to 50 mm Hg caused respiratory frequency to increase from 4.8 to 9.7/min, and tidal volume to decrease from 704 to 515 ml. The calculated total ventilation, however, increased from 3180 to 4530 ml/min. The time of inspiration decreased from 3.7 to 2.4 seconds, and the time of expiration decreased from 9.8 to 4.1 seconds. These ventilatory responses are shown to be baroreceptor reflex mediated, and not secondary to changes in arterial pressure. These findings indicate that not only does the carotid sinus baroreceptor reflex control arterial pressure, but it also simultaneously influences ventilation, through changes in both respiratory frequency and tidal volume.

Dilation of the infarct zone (infarct expansion) has been shown to be a cause of regional left ventricular dilation within days of transmural anterior myocardial infarction. Whether this process stabilizes, continues or regresses thereafter, and whether uninfarcted myocardium is affected is unknown. To explore this, two to six (mean four) serial two dimensional echocardiographic studies were obtained over a 3 to 30 month period (mean 13) in 13 patients beginning 10 to 21 days after anterior transmural myocardial infarction. With use of the papillary muscles as internal left ventricular landmarks, anterior and posterior segmental dilation in each patient was assessed from the slope of the regression line of segment length versus time. Study patients were classified into those with (seven patients) and those without (six patients) infarct expansion. Infarct expansion was defined as an anterior segment length 10 to 21 days after infarction that exceeded the upper limit of normal determined from 13 age-and sex-matched normal control subjects. Ten to 21 days after infarction, the average anterior segment length in the seven patients with expansion was 65 percent greater than that in normal control subjects. The seven patients with infarct expansion showed significant and continuing dilation during long-term observation of both the infarcted anterior (p Thus, infarct expansion occurring within 3 weeks of transmural myocardial infarction appears to be the main contributor to left ventricular dilation, and is associated with impaired functional status. Moreover, a significant increase in ventricular size due to infarct expansion appears to predict chronic progressive ventricular enlargement due to lengthening of both the infarcted zone and the uninfarcted segment.

The growth characteristics of children from four villages in northern Peru were compared with those of poor urban children in the capital city, in whom short stature but generally satisfactory weight for height relationships after infancy had been demonstrated. Height for age and weight for age fell more rapidly during infancy in the rural than in the urban children of both sexes. Rural girls caught up with the urban girls in height during childhood but did not match them in weight until late adolescence. Their weight to height ratios were consistently lower after 1 year of age, most strikingly between 2 and 5 years of age, and did not approach or match those of the urban girls until adolescence. Rural boys did not catch up with the urban boys in height or weight (differences in height were not statistically significant between 6 and 10 years, however) and their ratios remained consistently lower until late adolescence, most strikingly in early childhood and during puberty. Such urban-rural and sex differences, if typical and current, might well call for very different remedial measures at different ages in each of the populations.

In order to estimate the importance of a variety of environmental and dietary factors as determinants of growth in a group of 123 poor Peruvian urban children between 2 and 19 years old, we found it necessary to express anthropometric measurements in units that were not age- or sex-dependent. Height quotient and weight quotient for each child were calculated from height and weight ages derived from the 50th percentile of the Boston reference data for the appropriate sex. Only 5% of the children had heights above the Boston 50th percentile (height quotient greater than 100) and 18% had weights above the 50th percentile (weight quotient greater than 100), but 88% had weights that were appropriate or excessive for height (weight/height quotient greater than or equal to 1.00). Some CATch-up" gains in relative height and weight were apparent in preschool children but more impressive gains in both linear and ponderal growth, relative to the Boston data, were evident between 8 and 13.5 years in girls and 10 and 17 years in boys. When the same quotients were calculated for a much larger sample from the same socioceonomic level it seemed likely that this last peak was due to earlier puberty and sexual maturation, and that quotients derived from the Boston data would have different meanings at different ages, making them inappropriate for further statistical analysis. New quotients for the study population, derived from the larger Peruvian group, did not have sex- or age-dependent trends. Racial and regional differences in patterns of growth must be taken into account in the interpretation of anthropometric and nutritional data.

The growth status and the nutrient intakes of 123 children from 26 urban poor families in Peru were related to per capita expenditure for food. Children from six better off families were taller and heavier (p less than 0.001), with no difference in weight for height. They had significantly higher calorie and total protein intakes (as percentage of recommended) and higher intakes of animal protein, fat, calcium, carotene, riboflavin, and vitamin C. When macronutrient intakes were expressed as percentages of recommended calorie intakes, correcting for age and relative size, all of the increase in total protein intake was due to animal protein, vegetable protein remaining constant. Almost all of the increase in adequacy of total calories was due to increasing fat intakes, relatively much less to carbohydrate, and this only among the poor families. In this population, as more money becomes available to purchase food, there is an increase in animal protein and fat intakes, over an almost constant vegetable protein and carbohydrate intake. There is a simultaneous increase in consumption of fruits and vegetables, accounting for increases in the carotene and vitamin C intakes.

The exact mechanism or mechanisms by which nitroglycerin exerts its beneficial effect on pacing-induced regional myocardial ischemia has not been ellucidated previously. In an open-chest, anesthetized canine preparation a fixed, flow limiting stenosis was applied to the left anterior descending (LAD) coronary artery and heart rate was increased by atrial pacing. Mass spectrometry was used to measure myocardial oxygen (PmO2) and carbon dioxide (PmCO2) tensions. Myocardial blood flow was measured by the radioactive microsphere technique. Application of the stenosis resulted in regional decreases in PmO2 and increases in PmCO2 of greater magnitude in the subendocardial than in the subepicardial layer. Atrial pacing resulted in a further decrease in PmO2 and increase in PmCO2 as well as a reduction in subendocardial blood flow. Nitroglycerin (TNG) infusion reduced mean arterial pressure 20 mm Hg, resulting in a 14 mm Hg reduction in PmCO2 in the more ischemic subendocardial layer (P less than 0.05). Myocardial blood flow decreased in all regions; however, the magnitude of this decrease was less in the ischemic region. Addition of aortic constriction abolished both the afterload and preload lowering effects of nitroglycerin but improved ischemic zone blood flow. These data demonstrate that nitroglycerin reduces the severity of pacing-induced regional myocardial ischemia primarily by reducing the determinants of myocardial oxygen demand. We found that when these effects are counteracted, improvement in myocardial oxygen supply becomes the dominant mechanism.

Several studies published within the past 20 years have reported an increase in the incidence of HIB meningitis. We sought to investigate whether there was evidence of a recent increase in metropolitan Baltimore. Cases of HIB meningitis occurring in children aged 0-5 years residing in Baltimore City & County were identified through: (1) medical records bearing the discharge diagnosis of meningitis or sepsis and (2) laboratory reports of positive blood or CSF cultures of HIB. The survey included all 19 hospitals in Baltimore City & County, plus an additional 41 hospitals surrounding this area. The study period was Jan. 1965 - Dec. 1975. A case was included if there was (a) CSF pleocytosis and isolation of HIB from blood or (b) CSF culture was positive for HIB. These criteria were met by 253 patients. Age adjusted incidence rates were calculated per 100,000 population at risk using the Baltimore City & County census data with appropriate adjustments for changing distribution of age. There was no difference in incidence rates between blacks and whites or between males and females. In contrast to most previously published studies, we failed to document any increase in incidence of HIB meningitis. Important factors explaining this difference include the timing, geographical location and methodology of the study. *Attack rate per 100,000 at risk.