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Problems in the interpretation of blood pressure changes seen in rats fed a breakfast cereal
Life Sciences, Vol . 22, pp . 1821-1826 Printed in the U .S .A .
Pergamon Press
PROBLEMS IN THE INTERPRETATION OF BLOOD PRESSURE CHANGES SEEN IN RATS FED A BREAKFAST CEREAL Anna V . A . Resurreccion and W . O . Caster Home Economics Nutrition The University of Georgia Athens, GA 30602 (Received in final form April 10, 1978)
Summary Systolic blood pressures were measured on the same animals by direct and indirect methods . A tail cuff was used for the indirect measurements, and the direct measurements were obtained with a strain gauge after a needle was inserted into a major artery . Good agreement was found between normal values and elevated pressures produced by norepinephrine, but gross differences were observed in a nutritional experiment in which oatmeal was fed . Credibility requires that blood pressure changes be demonstrated by direct as well as by indirect measurements . In 1938, Byrom and Wilson (1) demonstrated the possibility of estimating the systolic blood pressure of the rat by an indirect method making use of a blood pressure cuff around the tail . These indirect pressure measurements have been shown (2-6) to be in good agreement with direct measurements obtained by using a cannula placed into a major artery . The indirect measurements are, however, susceptible to consistent biases related to cuff width (4), position of the cuff on the rat's tail (2), use of anesthetized or unanesthetized rats (5), the temperature of the rat (2), the correct time lag necessary for stabilization of blood pressure (2) and instrumentation (6) . It has therefore been suggested (2,4) that each laboratory should standardize it's technique of indirect measurement by comparison with a direct blood pressure method . In a recent study (7), using indirect blood pressure measurements, it was found that the feeding of certain common breakfast cereals was associated with an increase in blood pressure, and that this effect was inversely correlated with the sodium content of those cereals . The sodium concentrations ranged from 60 to 1800 ppm, and the lower the sodium content, the higher the blood pressure . The correlation between sodium and blood pressure, after 30 days of feeding, was r = -0 .68 (P < 0 .01) and at 45 days was r = -0 .79 . The highest systolic blood pressures (180 and 185 mm Hg) and the lowest sodium concentrations were observed with a diet of oatmeal . 0300-9653/78/0522-1821$02 .00/0 Copyright Q 1978 Pergamon Press
1822
Problems in Blood Pressura Measurement
Vol . 22, No . 2 0, 1978
This negative correlation between dietary sodium and systolic blood pressure, is in direct disagreement with a substantial literature (8) and is of considerable nutritional interest . Before proceeding with nutritional studies, therefore, it would seem prudent to cross-check the indirect blood pressure procedure rather carefully against a direct blood pressure method . The purpose of this study is to determine the effect of feeding oatmeal on the systolic blood pressure of the rat . Materials and Methode Weanling, male, white Sprague Dawley rats were housed individually in wire cages with raised floors, and maintained on either one of the experimental or control diets . Water and feed The three experimental diets used in were supplied ad libitum. this study were as follows : Oatmeal (Quick Quaker Oats, The Quaker Oats Co ., Chicago, IL .), Oatmeal supplemented with 6$ casein, and Oatmeal supplemented with 6$ casein and a complete mineral mixture (9) . The two control diets were a standard stock diet (Purina Laboratory Chow, Ralston Purina Co ., St . Louis, MO .) and a nutritionally defined complete diet consisting of 168 casein, 48 lactalbumin, 48 cellulose, 48 of the complete mineral mixture of Williams and Briggs (9), 4$ of a complete (10) vitamin mixture prepared in sucrose (11), 488 wheat starch, 16$ sucrose and 4$ corn oil . Indirect (tail cuff) measurement of systolic blood pressure was obtained by passing the rat's tail through a tail cuff 25 mm wide . A rubber bulb connected to a pneumatic pulse transducer was taped on the tail, distal to the cuff . Cuff pressure, cycling interval, and inflation and deflation rates were maintained constant by using a programmed electro-sphygmomanometer PE 300 . Maximum cuff pressure was 200 mm Hg unless a higher pressure was needed . Cycling interval was 30 sec and inflation and deflation Immediately after the rates were maintained at 17 .5 mm Hg/sec . last of three successive inflation and deflation cycles was completed, direct blood pressure (strain gauge) measurements were made by cannulation of the lower abdominal aorta with a 22 gauge hypodermic needle attached to a pressure transducer RP 1500 and a strain gauge coupler type 7172 . All indirect and direct measurements of blood pressure were recorded on a phyaiograph, and made use of matching transducers and related commercially available units (Narco Bio-Systems Inc ., Houston, TX) . After 36 days of feeding, the rats were anesthetized with ether, and blood pressures of the rats were determined by both the indirect and direct methods described above . The value for tail cuff systolic pressure of each rat was the average of three successive readings obtained during inflation of the tail cuff . The direct values for systolic and diastolic blood pressures were obtained by recording the strain gauge output and averaging the highest and the lowest reading, respectively, for each heart beat seen over a 10 - 20 second recording period .
Vol . 22, No . 20, 1978
Problems in Blood Pressure Measurement
1823
Results and Discussion Figure 1 shows a comparison of the systolic blood pressure measurements obtained in this laboratory by indirect (tail cuff) and direct (strain gauge) methods . The elevated blood pressures were obtained by injecting norepinephrine (Levo-arterenol, Winthrop Laboratories, New York, N .Y .) intraperitoneally, and making pressure measurements 8-15 minutes after injection . The tail cuff measurement was obtained 8-10 minutes after injection and the strain gauge measurement was obtained immediately thereafter . The highest blood pressures were obtained with norepinephrine at doses of 1-3 mg/Rg body weight . 2001 v~ E `r
TC = 37.7 " 0.62 SG
15 0
U
Q
H
100'
100
150
STRAIN
GAUGE
200 (mm Hq)
FIG~ 1 COMPARISON OF SYSTOLIC BLOOD PRESSURES ESTIMATED FROM DIRECT iSTRAIN GAUGE) AND INDIRECT NAIL CUFF) MEASUREMENTS Good agreement was observed between the two methods over a range of tail cuff values from 98 to 175 mm Hg and strain gauge measurements ranging from 110 to 200 mm Hg . A correlation coef ficient of r = 0 .83 (P G 0 .01) over this entire range of pressures was found . This correlation coefficient is at least as good as that obtained by Bunag et al . (5), who reported a 0 .76 correlation between tail cuff and âortic systolic pressure measurement in anesthetized rats . The quantitative relationship between tail cuff and strain gauge measurements was found to be : Tail Cuff = 37 .7 + 0 .62 Strain Gauge It was observed that these two types of measurements were equal at a pressure close to 100 mm Hg . Above 100 mm Hg, the tail cuff pressures were found to be progressively lower than the strain
1824
Problems in Blood Pressure Measurement
Vol . 2 2, No . 20, 1978
gauge values . This finding agrees with that of Maistrello and Matscher (4) who, in using an 18 mm tail cuff found the indirect and direct measurements made in their laboratory to be equal at 109 mm Hg . Above that pressure they found their indirect readings to be progressively lower than their direct measurements . Table 1 shows direct and indirect blood pressure measurements obtained using the tail cuff and the strain gauge procedures on rats fed the experimental diets and the control diets . Indirect blood pressure measurements of rats fed oatmeal, alone or supplemented, were significantly higher than those observed in rats fed the standard stock diet or the nutritionally defined complete diet . In the control animals there was good agreement between the systolic blood pressures as measured by direct and indirect methods . In the case of the standard stock diet the means were 131 and 130 mm Hg, respectively, while in the case of the nutri-
TA~
1
BLOOD PRESSURES OF RATS FED THE EXPERIMENTAL AND CONTROL DIETS
BLOOD PRESSURE QMM HG) DIET
TAIL CUFF
STRAIN GAUGE
SYSTOLIC
SYSTOLIC
DIASTOLIC
OATMEAL
161 ±
138 ± 11
101 ± 10
OATMEAL + 6% CASEIN
166 ± 18**
138 ±
103 ± 10
1]] + 18 **
144 + 12*
STANDAND STOCK
13O ±
131 +
NUTRITIONALLY DEFINED, COMPLETE
131 + 13
OATMEAL + 6% CASEIN
+ COMPLETE MINERAL MIX
9**
9
9
]
133 + 20
107 +
5
99 +
]
108 + 1]
** P ~ 0,01 WHEN COMPARED WITH GROUPS OF RATS FED THE STOCK OR NUTRITIONALLY DEFINED COMPLETE DIETS * 0,01 ` P ` 0,05 WHEN COMPARED TO GROUPS OF RATS FED THE STOCK DIET
Vol . 22, No . 20, 1978
Probleme in Blood Pressure Measurement
1825
tionally defined, complete diet the means were 133 and 131 mm Hq . This, taken together with the data of Fig . 1, would seem acceptable proof that the indirect procedure was operating satisfactorily and would provide useful results when applied to rats fed different experimental diets . Table 1 shows that this assumption is not necessarily true . The feeding of oatmeal produces a statistically signif_cant increase in systolic blood pressure, when this pressure is measured by an indirect method . This observation has been repeated in five independent experiments, including two previously published (7, 12), with pressures ranging from 160 to 190 mm Hg in each case . The slope of the line in Fig . 1 indicates that each 6 mm Hg increase in pressure, as measured by tail cuff methods, is associated with a 10 mm Hg increase in pressure, as measured with a strain gauge placed in direct contact with arterial pressure . Hence, one would expect the strain gauge measurements of systolic pressure shown in Table 1 to be in the range of 180-230 mm Hg for all rats fed oatmeal diets . They were not, and indeed only one strain gauge mean was significantly elevated (P < 0 .05) when compared with a selected one of the control means . Clearly, the two procedures yield divergent interpretations of the nutritional data . If one uses the tail cuff procedure, the data of Table 1 indicate that the feeding of oatmeal produces an increase in blood pressure, and that supplementation of this oatmeal with casein, or casein and minerals, has no effect in reducing this hypertension . If one uses the strain gauge method, the data of Table 1 indicate that the feeding of oatmeal has no effect on blood pressure, but that the addition of a mineral supplement might produce a modest increase . Obviously, these conclusions lead to directly conflicting dietary advice . The tail cuff method is cheap, convenient and easy to use (with young rats) . Two persons working together can obtain measurements from perhaps 30 rats per hour . This indirect blood pressure method is producing a rapidly increasing literature on hypertension . The important question to ask at this point concerns the physiological significance of this literature . The data of Table 1 suggest that one should be slow to accept any conclusion relating to blood pressure changes unless the experimental result is demonstrated by direct methods as well as by indirect pressure methods . Açknowledgements This work was supported in part by the University of Georgia College of Agriculture Experiment Stations . References l. 2.
3.
BYROM, F . B ., and C . WILSON, J . Physiol . 93 : 301-304 (1938) . FREGLY, M. J ., J . Lab . Clin . Med . 62 : 223-230 (1963) . FRANGIPANE, G ., and F . APORTI, J . Lab . Clin . Med . _73 :872-876
4.
MAISTRELLO, I ., and R. MATSCHER, J . Appl . Physiol . _26 :188-193
(1969) . (1969) .
1826 5. 6, 7. 8. 9. 10 . 11 . 12 .
Problems in Blood Pressure Measurement
Vol . 22, No . 20, 1978
BUNAG, R . D ., J . M. MCCUBBIN and I . H . PAGE, Cardiovascular Res . 5 : 24-31 (1971) . BUNAG, R . D ., J . Appl . Physiol . 34 : 279-282 (1973) . CASTER, W . O . and M, D . PARTHEMOS, Amer . J, Clin . Nutr . _29 : 529-534 (1976) . MENEELY, G . R . and H . D . BATTARBEE, Nutr . Rev . 34 : 225-235 (1976) . COHEN, N . L ., P, REYES, J . T . TYPO and G . M . BRIGGS, J . Nutr . 91 : 482-488 (1967) . CIITHBERTSON, W. F . J ., Proc . Nutr . Soc . 16 : 70-76 (1957) . CASTER, W . 0 . and S. BLEECKER, J . Nutr . ~5 : 308-310 (1975), RESURRECCION, A . V . A ., C . J . ROESEL andW . O . CASTER, Fed . Proc . 36 : 618A (1977) .
Pergamon Press
PROBLEMS IN THE INTERPRETATION OF BLOOD PRESSURE CHANGES SEEN IN RATS FED A BREAKFAST CEREAL Anna V . A . Resurreccion and W . O . Caster Home Economics Nutrition The University of Georgia Athens, GA 30602 (Received in final form April 10, 1978)
Summary Systolic blood pressures were measured on the same animals by direct and indirect methods . A tail cuff was used for the indirect measurements, and the direct measurements were obtained with a strain gauge after a needle was inserted into a major artery . Good agreement was found between normal values and elevated pressures produced by norepinephrine, but gross differences were observed in a nutritional experiment in which oatmeal was fed . Credibility requires that blood pressure changes be demonstrated by direct as well as by indirect measurements . In 1938, Byrom and Wilson (1) demonstrated the possibility of estimating the systolic blood pressure of the rat by an indirect method making use of a blood pressure cuff around the tail . These indirect pressure measurements have been shown (2-6) to be in good agreement with direct measurements obtained by using a cannula placed into a major artery . The indirect measurements are, however, susceptible to consistent biases related to cuff width (4), position of the cuff on the rat's tail (2), use of anesthetized or unanesthetized rats (5), the temperature of the rat (2), the correct time lag necessary for stabilization of blood pressure (2) and instrumentation (6) . It has therefore been suggested (2,4) that each laboratory should standardize it's technique of indirect measurement by comparison with a direct blood pressure method . In a recent study (7), using indirect blood pressure measurements, it was found that the feeding of certain common breakfast cereals was associated with an increase in blood pressure, and that this effect was inversely correlated with the sodium content of those cereals . The sodium concentrations ranged from 60 to 1800 ppm, and the lower the sodium content, the higher the blood pressure . The correlation between sodium and blood pressure, after 30 days of feeding, was r = -0 .68 (P < 0 .01) and at 45 days was r = -0 .79 . The highest systolic blood pressures (180 and 185 mm Hg) and the lowest sodium concentrations were observed with a diet of oatmeal . 0300-9653/78/0522-1821$02 .00/0 Copyright Q 1978 Pergamon Press
1822
Problems in Blood Pressura Measurement
Vol . 22, No . 2 0, 1978
This negative correlation between dietary sodium and systolic blood pressure, is in direct disagreement with a substantial literature (8) and is of considerable nutritional interest . Before proceeding with nutritional studies, therefore, it would seem prudent to cross-check the indirect blood pressure procedure rather carefully against a direct blood pressure method . The purpose of this study is to determine the effect of feeding oatmeal on the systolic blood pressure of the rat . Materials and Methode Weanling, male, white Sprague Dawley rats were housed individually in wire cages with raised floors, and maintained on either one of the experimental or control diets . Water and feed The three experimental diets used in were supplied ad libitum. this study were as follows : Oatmeal (Quick Quaker Oats, The Quaker Oats Co ., Chicago, IL .), Oatmeal supplemented with 6$ casein, and Oatmeal supplemented with 6$ casein and a complete mineral mixture (9) . The two control diets were a standard stock diet (Purina Laboratory Chow, Ralston Purina Co ., St . Louis, MO .) and a nutritionally defined complete diet consisting of 168 casein, 48 lactalbumin, 48 cellulose, 48 of the complete mineral mixture of Williams and Briggs (9), 4$ of a complete (10) vitamin mixture prepared in sucrose (11), 488 wheat starch, 16$ sucrose and 4$ corn oil . Indirect (tail cuff) measurement of systolic blood pressure was obtained by passing the rat's tail through a tail cuff 25 mm wide . A rubber bulb connected to a pneumatic pulse transducer was taped on the tail, distal to the cuff . Cuff pressure, cycling interval, and inflation and deflation rates were maintained constant by using a programmed electro-sphygmomanometer PE 300 . Maximum cuff pressure was 200 mm Hg unless a higher pressure was needed . Cycling interval was 30 sec and inflation and deflation Immediately after the rates were maintained at 17 .5 mm Hg/sec . last of three successive inflation and deflation cycles was completed, direct blood pressure (strain gauge) measurements were made by cannulation of the lower abdominal aorta with a 22 gauge hypodermic needle attached to a pressure transducer RP 1500 and a strain gauge coupler type 7172 . All indirect and direct measurements of blood pressure were recorded on a phyaiograph, and made use of matching transducers and related commercially available units (Narco Bio-Systems Inc ., Houston, TX) . After 36 days of feeding, the rats were anesthetized with ether, and blood pressures of the rats were determined by both the indirect and direct methods described above . The value for tail cuff systolic pressure of each rat was the average of three successive readings obtained during inflation of the tail cuff . The direct values for systolic and diastolic blood pressures were obtained by recording the strain gauge output and averaging the highest and the lowest reading, respectively, for each heart beat seen over a 10 - 20 second recording period .
Vol . 22, No . 20, 1978
Problems in Blood Pressure Measurement
1823
Results and Discussion Figure 1 shows a comparison of the systolic blood pressure measurements obtained in this laboratory by indirect (tail cuff) and direct (strain gauge) methods . The elevated blood pressures were obtained by injecting norepinephrine (Levo-arterenol, Winthrop Laboratories, New York, N .Y .) intraperitoneally, and making pressure measurements 8-15 minutes after injection . The tail cuff measurement was obtained 8-10 minutes after injection and the strain gauge measurement was obtained immediately thereafter . The highest blood pressures were obtained with norepinephrine at doses of 1-3 mg/Rg body weight . 2001 v~ E `r
TC = 37.7 " 0.62 SG
15 0
U
Q
H
100'
100
150
STRAIN
GAUGE
200 (mm Hq)
FIG~ 1 COMPARISON OF SYSTOLIC BLOOD PRESSURES ESTIMATED FROM DIRECT iSTRAIN GAUGE) AND INDIRECT NAIL CUFF) MEASUREMENTS Good agreement was observed between the two methods over a range of tail cuff values from 98 to 175 mm Hg and strain gauge measurements ranging from 110 to 200 mm Hg . A correlation coef ficient of r = 0 .83 (P G 0 .01) over this entire range of pressures was found . This correlation coefficient is at least as good as that obtained by Bunag et al . (5), who reported a 0 .76 correlation between tail cuff and âortic systolic pressure measurement in anesthetized rats . The quantitative relationship between tail cuff and strain gauge measurements was found to be : Tail Cuff = 37 .7 + 0 .62 Strain Gauge It was observed that these two types of measurements were equal at a pressure close to 100 mm Hg . Above 100 mm Hg, the tail cuff pressures were found to be progressively lower than the strain
1824
Problems in Blood Pressure Measurement
Vol . 2 2, No . 20, 1978
gauge values . This finding agrees with that of Maistrello and Matscher (4) who, in using an 18 mm tail cuff found the indirect and direct measurements made in their laboratory to be equal at 109 mm Hg . Above that pressure they found their indirect readings to be progressively lower than their direct measurements . Table 1 shows direct and indirect blood pressure measurements obtained using the tail cuff and the strain gauge procedures on rats fed the experimental diets and the control diets . Indirect blood pressure measurements of rats fed oatmeal, alone or supplemented, were significantly higher than those observed in rats fed the standard stock diet or the nutritionally defined complete diet . In the control animals there was good agreement between the systolic blood pressures as measured by direct and indirect methods . In the case of the standard stock diet the means were 131 and 130 mm Hg, respectively, while in the case of the nutri-
TA~
1
BLOOD PRESSURES OF RATS FED THE EXPERIMENTAL AND CONTROL DIETS
BLOOD PRESSURE QMM HG) DIET
TAIL CUFF
STRAIN GAUGE
SYSTOLIC
SYSTOLIC
DIASTOLIC
OATMEAL
161 ±
138 ± 11
101 ± 10
OATMEAL + 6% CASEIN
166 ± 18**
138 ±
103 ± 10
1]] + 18 **
144 + 12*
STANDAND STOCK
13O ±
131 +
NUTRITIONALLY DEFINED, COMPLETE
131 + 13
OATMEAL + 6% CASEIN
+ COMPLETE MINERAL MIX
9**
9
9
]
133 + 20
107 +
5
99 +
]
108 + 1]
** P ~ 0,01 WHEN COMPARED WITH GROUPS OF RATS FED THE STOCK OR NUTRITIONALLY DEFINED COMPLETE DIETS * 0,01 ` P ` 0,05 WHEN COMPARED TO GROUPS OF RATS FED THE STOCK DIET
Vol . 22, No . 20, 1978
Probleme in Blood Pressure Measurement
1825
tionally defined, complete diet the means were 133 and 131 mm Hq . This, taken together with the data of Fig . 1, would seem acceptable proof that the indirect procedure was operating satisfactorily and would provide useful results when applied to rats fed different experimental diets . Table 1 shows that this assumption is not necessarily true . The feeding of oatmeal produces a statistically signif_cant increase in systolic blood pressure, when this pressure is measured by an indirect method . This observation has been repeated in five independent experiments, including two previously published (7, 12), with pressures ranging from 160 to 190 mm Hg in each case . The slope of the line in Fig . 1 indicates that each 6 mm Hg increase in pressure, as measured by tail cuff methods, is associated with a 10 mm Hg increase in pressure, as measured with a strain gauge placed in direct contact with arterial pressure . Hence, one would expect the strain gauge measurements of systolic pressure shown in Table 1 to be in the range of 180-230 mm Hg for all rats fed oatmeal diets . They were not, and indeed only one strain gauge mean was significantly elevated (P < 0 .05) when compared with a selected one of the control means . Clearly, the two procedures yield divergent interpretations of the nutritional data . If one uses the tail cuff procedure, the data of Table 1 indicate that the feeding of oatmeal produces an increase in blood pressure, and that supplementation of this oatmeal with casein, or casein and minerals, has no effect in reducing this hypertension . If one uses the strain gauge method, the data of Table 1 indicate that the feeding of oatmeal has no effect on blood pressure, but that the addition of a mineral supplement might produce a modest increase . Obviously, these conclusions lead to directly conflicting dietary advice . The tail cuff method is cheap, convenient and easy to use (with young rats) . Two persons working together can obtain measurements from perhaps 30 rats per hour . This indirect blood pressure method is producing a rapidly increasing literature on hypertension . The important question to ask at this point concerns the physiological significance of this literature . The data of Table 1 suggest that one should be slow to accept any conclusion relating to blood pressure changes unless the experimental result is demonstrated by direct methods as well as by indirect pressure methods . Açknowledgements This work was supported in part by the University of Georgia College of Agriculture Experiment Stations . References l. 2.
3.
BYROM, F . B ., and C . WILSON, J . Physiol . 93 : 301-304 (1938) . FREGLY, M. J ., J . Lab . Clin . Med . 62 : 223-230 (1963) . FRANGIPANE, G ., and F . APORTI, J . Lab . Clin . Med . _73 :872-876
4.
MAISTRELLO, I ., and R. MATSCHER, J . Appl . Physiol . _26 :188-193
(1969) . (1969) .
1826 5. 6, 7. 8. 9. 10 . 11 . 12 .
Problems in Blood Pressure Measurement
Vol . 22, No . 20, 1978
BUNAG, R . D ., J . M. MCCUBBIN and I . H . PAGE, Cardiovascular Res . 5 : 24-31 (1971) . BUNAG, R . D ., J . Appl . Physiol . 34 : 279-282 (1973) . CASTER, W . O . and M, D . PARTHEMOS, Amer . J, Clin . Nutr . _29 : 529-534 (1976) . MENEELY, G . R . and H . D . BATTARBEE, Nutr . Rev . 34 : 225-235 (1976) . COHEN, N . L ., P, REYES, J . T . TYPO and G . M . BRIGGS, J . Nutr . 91 : 482-488 (1967) . CIITHBERTSON, W. F . J ., Proc . Nutr . Soc . 16 : 70-76 (1957) . CASTER, W . 0 . and S. BLEECKER, J . Nutr . ~5 : 308-310 (1975), RESURRECCION, A . V . A ., C . J . ROESEL andW . O . CASTER, Fed . Proc . 36 : 618A (1977) .