Dale T. Matsuoka

Effects of aging on cardiovascular responses to parasympathetic withdrawal

OBJECTIVES The study was done to determine whether the effects of parasympathetic withdrawal on heart rate, blood pressure (BP), and systolic and diastolic function are altered with normal aging. BACKGROUND Cardiac responses to beta-adrenergic sympathetic stimulation decline with aging as does the heart rate response to parasympathetic withdrawal, but the extent to which other responses to parasympathetic withdrawal decrease is less clear. METHODS Heart rate, BP, systolic function, and diastolic filling responses to parasympathetic withdrawal induced by atropine (0.02 mg/kg) were compared in 50 healthy subjects, 28 older (ages 65 to 80 years, mean 70 years; 18 females all on estrogen) and 22 young (age 18 to 32 years, mean 26 years; 12 females) subjects, using radionuclide angiography. RESULTS Parasympathetic withdrawal in the older group caused less of an increase in heart rate (+33 vs. +48 beats/min), cardiac index (+0.6 vs. +1.5 l/m(2)), systolic blood pressure (-1 vs. +7 mm Hg), and early diastolic filling rate (+1.7 vs. +2.4 end-diastolic volumes/s) (all p < or = 0.01). At similar declines in the diastolic filling period, end-diastolic volume index (EDVI) fell substantially more in the older group (-11.6 vs. -2.4 ml/m(2), p < 0.001). The only gender difference was in diastolic filling rate, which was similar in the young males and females, but significantly less in older males than in older females. CONCLUSIONS The responses to parasympathetic withdrawal as well as sympathetic stimulation decline with aging, and both contribute to the reduced cardiovascular responses to stress with advancing age.

Efficacy of thyroid blockade on thyroid radioiodine uptake in 123I-mIBG imaging.

Although iodinated radiopharmaceuticals usually contain a small quantity of unbound iodine, it is difficult to establish the degree to which thyroid activity on scintigraphic images reflects uptake of free radioiodine. The objective of the present study was to examine the effectiveness of thyroid blockade in subjects undergoing 123I-meta-iodobenzylguanidine (mIBG) imaging and to estimate the relative contribution of bound and unbound radioiodine to imaging findings. Methods: All subjects were participants in prospective trials of 123I-mIBG cardiac imaging in which pretreatment with thyroid blockade was optional unless locally required. In a pilot project, 15 subjects (6 blocked) had thyroid uptake measured at 4 h using a probe system. Fifteen-minute (early) and 4-h (late) anterior planar chest images that included the thyroid region were visually scored for thyroid uptake (scale of 0–4) in another group of 152 subjects (98 blocked). Quantitative analysis based on thyroid regions of interest was performed on anterior planar images from a further sample of 669 subjects (442 blocked). For all 3 investigations, quantitative comparisons of thyroid uptake were made between the blocked and nonblocked subjects. Results: There was no statistical difference between probe uptake of the 6 blocked and 9 nonblocked subjects. However, in the second series, mean visual score on the late images was significantly lower for blocked than nonblocked subjects (P < 0.001). In the region-of-interest analyses, net thyroid counts were significantly higher on the late images of nonblocked subjects (P < 0.0001), and compared with early images, 87% of subjects who received blockade showed decreased or unchanged counts whereas 75% of nonblocked subjects had increased net thyroid activity. In nonblocked subjects, an estimated 79% of thyroid counts on late images could be attributed to unbound 123I. Conclusion: On the basis of 3 different methods for assessing thyroid uptake of 123I, use of thyroid blockade pretreatment in 123I-mIBG imaging prevents increase of thyroid activity over time because of uptake of unbound 123I. In most subjects, there is a low level of 123I-mIBG thyroid activity that probably represents specific uptake in sympathetic nerve terminals.

Radionuclide ventriculographic quantitation of left ventricular dimensions. Comparison to echocardiography.

Left ventricular internal cavity dimensions (LVID) were determined from radionuclide ventriculographic (RNVG) studies using a spatial calibration algorithm and visually defined edges and were compared to the results from two-dimensional echocardiography. Routine clinical cases were used with no additional views and no attenuation or scatter correction. In an initial set of 21 patients, mean RNVG LVlD was 5.7 k 1.1 (mean k 1 S.D.) cm compared to 5.3 k 1.0 cm for echocardiography. In a prospective validation of the regression equations derived in the initial set of patients, regression-corrected RNVG results were within 5 mm of those determined echocardiographically in 18/22 patients and the mean LVlD values were the same. Quantitative estimation of LVlD by RNVG is simple, rapid, and reproducible. Systematic overestimation of dimensions compared to echocardiography can be corrected using a regression equation.

Comparison of scintigraphic renal differentials using radiolabeled orthoiodohippurate and glucoheptonate

Differential renal blood flow and parenchymal mass were compared in 50 pairs of scintigraphic studies performed on 44 patients. Background corrected total kidney counts were determined between 1 to 2 minutes following injection of 1-123 or 1-131 orthoiodohippurate (OIH), and at 1 to 2.5 hours after injection of Tc-99m glucoheptonate (GHA). The OIH and GHA differentials agreed within 5% in 36 studies (72%). When the OIH differentials were symmetric (45% to 55% in each kidney), agreement with the GHA results was complete (17 out of 17). By comparison, 42% (14 out of 33) of studies with asymmetric OIH differentials had a >5% disparity from the GHA split function. Serum creatinine was elevated at the time of 9 out of 14 studies (64%) with OIH/GHA disagreement and 18 out of 36 studies (50%) with OIH/GHA agreement. Of the 14 studies with disparate OIH/GHA differentials, the GHA differential was more symmetric in 10 (71%). When different scinitigraphic measures of differential renal flow and parenchymal mass disagree, this is most likely due to the effects of asymmetric renal impairment rather than to those of global renal dysfunction.