ChronobiologyInternational Vol. 8, No. 6, pp. 444-450
0 1991 International Society of Chronobiology
Circadian Rhythm of Blood Pressure: Internal and External Time Triggers
Chronobiol Int Downloaded from informahealthcare.com by University of North Carolina on 11/11/14 For personal use only.
Peter Baumgart Medizinische Poliklinik, University of Miinster, Miinster, F.R.G
Summary: Diurnal blood pressure (BP) fluctuations are superimposed by a 24-h rhythm with usually lower levels during the night and higher levels during the day. In contrast to other rhythmic bioparameters, the diurnal BP rhythm is largely dependent on activity and sleep rather than on clock time. This has been demonstrated by the BP characteristics after shifted sleeping and working phases, during transition from sleep to wakefulness, and by the influence of sleep and activities on the 24-h BP curve during normal daily routines. Whereas the circadian rhythm of BP is predominantly governed by external time triggers, endogenous rhythmicity can only be detected by time microscopic analysis or in conditions where effects of external time triggers are almost excluded. Key Words: Blood pressureCircadian rhythm-Shift work-Time trigger-Ambulatory blood pressure monitoring-Twenty-four-hour blood pressure.
Blood pressure (BP) fluctuates at a diurnal rhythm with usually higher levels during the day and lower levels during the night. This diurnal rhythm may be of clinical importance, since it is frequently deranged in certain diseases (1). Circadian rhythms of several biovariables such as body temperature or plasma cortisol have been shown to depend on an internal clock in humans. Reentrainment of the 24-h body temperature rhythm after flights across time zones occurs with considerable delay (2); circadian cortisol levels are not significantly altered by sleep deprivation (3). In animals active during the nighttime, there is a diurnal fall and a nocturnal rise in BP coinciding with the sleep-wakefulness cycle (4). When rats are kept in constant darkness or constant light, many physiological parameters will exhibit a so-called free-running rhythm (5,6). It has not yet been established whether BP keeps following sleep and activities in these conditions. There is an increasing number of investigations regarding circadian BP variation ~~
~~
~~
~
~~~
Received April 6 , 199 1; accepted with revisions April 30, 1991. Address correspondence and reprint requests to Priv. Doz. Dr. med. P. Baumgart at Medizinische Poliklinik, University of Miinster, Albert Schweitzer Str. 33, D-4400 Miinster, Federal Republic of Germany.
444
CIRCADIAN BLOOD PRESSURE RHYTHM 168
445
...
**......
148
138 FIG. I. Twenty-four-hour blood pressure profiles during morning shift (-) and night shift ( - .) Hourly mean systolic and diastolic BP of 17 shift workers Honzontal straight lines indicate working penods
..
Chronobiol Int Downloaded from informahealthcare.com by University of North Carolina on 11/11/14 For personal use only.
-
98
6 8 - , . . . , . .. , e 4 8
-.
...,...,...,..., 12
2e
16
24
time (h) since noninvasive 24-h assessment has become feasible by portable recorders for ambulatory blood pressure monitoring (ABPM). The impact of internal and external time triggers on blood pressure rhythm can be studied in various experimental and clinical conditions.
TWENTY-FOUR-HOUR BP RHYTHM IN SHIFTED ACTIVITY AND SLEEPING PERIODS The dependence of the BP rhythm on internal or external time triggers can be evaluated by the resistance against shifts of activity and sleep. Slowly rotated shift work is an appropriate model for shifted phases of activity and sleep. There are four reports in the literature on 24-h blood pressure monitoring in shift workers (7- 10). We performed ABPM in 17 shift workers at a chemical factory on a weekly rotated three-shift schedule (7). ABPM was performed during the morning shift and the night shift on the last working days (Fridays) of the respective weeks. The BP was recorded for 24 h at intervals of 30 min using oscillometnc devices (SpaceLabs 90202, Redmon, WA, U.S.A.). Working times were 06:OO- 14:OO in the morning shift and 22:OO-06:OO h in the night shift. As indicated by the group 24-h BP
TABLE I . Systolic and diastolic BP (mean f SEM, mm Hg) in 17 shift workers during the morning and night shifis
24 h Day (0800-2000 h) Night (2000-0800 h) Sleep Work
Morning shift
Night shift
127.6 t 2.1178.4 f 1.5 133.4 f 2.2183.8 t 1.7 121.3 t 2.2172.5 t 1.5 112.5 k 1.9164.4 k 1.6 132.4 f 2.3/83.8 _+ 1.7
127.6 t 1.4177.9 t 1.2 122.2 t 1.5172.8 f 1.1 132.5 t 1.7182.6 1.5 114.1 f 1.5165.3 t 1.4 133.3 t 1.7B3.8 f 1.6 +_
Chronobiol Inl, Vol. 8, No. 6. 1991
P. BAUMGART
446
c,
8
F I G . 2 . Cross-correlationsbetween systolic 24-h BP profiles of the morning and night shift. Maximal correlation occurs at a phase difference of 8 h-corresponding with the lag between the two working periods.
x i
Chronobiol Int Downloaded from informahealthcare.com by University of North Carolina on 11/11/14 For personal use only.
0
0
u
-@.
-0
s ~ , . . . , . . I. , . . . , . . . , . . . , . . . , -12
-8
-4
8
8
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12
profiles (Fig. l), the BP was low during sleep and high during work in both shifts. The amplitudes of diurnal fluctuations were similar in the two shifts (Fig. 1). Table 1 presents the group average BP values over 24 h and the various time segments. The 24-h mean values during sleep or work were almost identical for the 2 days. In the morning shift, the mean daytime BP corresponded to the nighttime BP in the night shift. The nighttime BP in the morning shift corresponded to the daytime BP in the night shift. Cross-correlationanalysis revealed that there was a phase difference between the two 24-h BP curves over 8 h (Fig. 2). This is in accordance with the lag of 8 h between the two shifts. To study the rapidity of adaptation, ABPM was repeated in five of these volunteers during day I (Monday) of the night shift. As indicated by Fig. 3, there were no
136
FIG. 3. Twenty-four-hour blood pressure profiles from day 1 (-) and day 5 ( ) of the night shift. Hourly mean systolic and diastolic BP of five shift workers.
- -
96
t
76 66 r l 8
. . . I . . . l . r . I . . . I . . . I . . . I 4
8
12
16
time (h)
Chronobiol Int. Vol. 8, No. 6, 1991
28
24
CIRCADIAN BLOOD PRESSURE RHYTHM 96
1..
....
44 7 0.0
FIG. 4. Twenty-four-hour heart rate profiles during morning shift (-) and night shift ( -). Hourly mean values of 17 shift workers. Horizontal straight lines indicate working periods.
Chronobiol Int Downloaded from informahealthcare.com by University of North Carolina on 11/11/14 For personal use only.
--
691,. . . , . . . , e 4 0
.
. ., . .., . . . , . . . , 12 16 28 24
time (h) substantial differences between the 24-h BP curves on days 1 and 5 after shift rotation. Twenty-four-hour heart rate profiles were reversed in a similar pattern: The heart rate was low during sleep and high at work (Fig. 4). Comparison of 24-h heart rate profiles on days 1 and 5 indicated that adaptation to the new rhythm occurred almost immediately (Fig. 5). Sundberg et al. (8) also investigated the 24-h BP in shift workers. Their results are in accordance with our findings: The BP rhythm was reversed on the first day of the night shift and closely followed the sleep-wakefulness cycle. Again, the heart rate rhythm tended to change in parallel although less pronounced than the BP. Chau et
n
106
\
FIG. 5. Twenty-four-hour heart rate profiles from day 1 (-) and day 5 ( ) of the night shift. Hourly mean values of five shift workers.
--.
76
66f . . . . . . . . . . . . . . . . . . . . . . . . 8 4 0 12 16 2 0
I
24
time (h)
Chronobiol Int. Vol. 8, No. 6 , 1991
P. BAUMGART
448
A
Chronobiol Int Downloaded from informahealthcare.com by University of North Carolina on 11/11/14 For personal use only.
145 ,551
~
~
65
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3
-1
7
15
11
19
23
hours from waking up
:"$ ,
55
.,
,
,
,
,
,
,
,
,
,
,
,
,
FIG. 6. Systolic and diastolic 24-h BP profiles at intervals of IS min after synchronization by the time of awakening (arrow). A: Nomotensives (n = 1 1 1); B essential hypertensives (n = 109).
i"",
~
-1
3
7
11
is
19
23
hours from waking up al. (9) concluded some influence of internal regulation mechanisms from Fourier analysis of 24-h BP profiles in shift workers. Similar results were reported by Halberg et al. (10).However, in these studies also, the major findings were high BP coinciding with working periods and low BP coincidingwith sleep in all shifts. Time microscopic analysis of the BP rhythm after transmeridian flight showed rapid although incomplete adjustment ( 1 1,12). All experiments in shifted periods of activity and sleep indicated that the 24-h BP profile in healthy subjects is not resistant to phase shifts. The almost complete and immediate reversal of the 24-h BP curve strongly suggests that the circadian BP rhythm is largely dependent on external time triggers whereas an internal clock may only play a minor role for 24-h BP control.
Chronobiol In[, Vol. 8, No. 6. I991
~
CIRCADIAN BLOOD PRESSURE RHYTHM
449
166 -
mmHg :
/\- ----IJ -
136 -
-.\,
i’
115 -..
1
-
r-
\
Chronobiol Int Downloaded from informahealthcare.com by University of North Carolina on 11/11/14 For personal use only.
- - - /
9s -
/:> V - =/-.+ -
75;-.--J --= :
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-
-
,
7, \
?
1
.
.
.
I
.
.
*
1
.
.
-
I
BLOOD PRESSURE DURING TRANSITION FROM SLEEP TO WAKEFULNESS
If the morning rise in BP occurred prior to awakening, this would be an argument for an internally governed BP rhythm. We studied the morning BP increase in large series of normotensive and hypertensive subjects (1 3). Their individual 24-h BP profiles were synchronized to the time of awakening by clock alarm. As indicated by Fig. 6, there was no substantial BP increase prior to waking, but a steep increase within the first hour thereafter in both groups. The rapidity of this increase was dependent on the lag between waking and getting up (1 3). These findings were supported by other reports on synchronized 24-h BP profiles (14). TWENTY-FOUR-HOUR BP RHYTHM IN IMMOBILIZED AND IN UNCONSCIOUS PATIENTS Diurnal BP rhythms are preserved even in patients immobilized by plaster casts ( 15). However, the diurnal changes of BP are less pronounced in hospitalized patients
than in patients who are in their natural environment (16). In circulatory shock, some circadian rhythmicity of the BP could be observed, independent of treatment effects ( 1 7). However, this does not necessarily represent effects of endogenous time triggers, since the circadian rhythm of BP can be synchronized by light exposure even in an apallic state ( 1 8).
Chronohiol Inl. Vol 8, No. 6 , 1991
P. BAUMGART
450
Chronobiol Int Downloaded from informahealthcare.com by University of North Carolina on 11/11/14 For personal use only.
EFFECTS OF TIME AND ACTIVITIES ON 24-H BP PROFILES DURING NORMAL DAILY ROUTINES Clark et al. (19) assessed the effects of activity and time of day on the diurnal BP variations by covariance analysis. After allowing for the effects of activity on BP, where sleep was one of the activities, there was no significant diurnal variation of BP. In conclusion, there was no important circadian rhythm of BP independent of activity. The shape of the 24-h BP curve during a normal daily routine is critically dependent on behavioral influences. In contrast to younger subjects, the elderly usually have sleeping periods at noon. Hence, in contrast to a roughly constant average BP level in young subjects, we found a daytime BP dip coinciding with a sleeping period at noon in elderly normotensives (Fig. 7). This underlines the essential role of external rhythms for shaping of the 24-h BP curve during a normal daily routine.
REFERENCES 1. Baumgart P. 24h-Blutdruck bei primarer und sekundarer Hypertonie. Herz 1989;14:246-50.
2. Gander PH, Graeber RC, Anderson HT, Lauber JK. Adjustment of sleep and the circadian temperature rhythm after flights across nine time zones. Aviat Space Environ Med 1989;60:733-43. 3. Moldofsky H, Lue FA, Davidson JR, Girczynski R. Effects of sleep deprivation on human immune functions. FASEB J 1989;3: 1972-7. 4. Smith TL, Coleman TG, Stanek KA, Murphy WR. Hemodynamic monitoring for 24 h in unanesthetized rats. Am J Physiol 1987;253:H1335-41. 5 . Rusak B, Zucker I. Neural regulation of circadian rhythms. Physiol Rev 1979;59:449-526. 6. Turek FW. Circadian neural rhythms in mammals. Annzi Rev Physiol 1985;47:49-64. 7. Baumgart P, Walger P, Fuchs G, van Eiff M, Rahn KH. Diurnal variations of blood pressure in shift workers during day and night shifts. Int Arch Occup Environ Health 1989;61 :463-6. 8. Sundberg S, Kohvakka A, Gordin A. Rapid reversal of circadian blood pressure rhythm in shift workers. J Hypertens 1988;6:292-6. 9. Chau NP, Mallion JM, De Gaudemaris R, et al. Twenty-four-hour ambulatory blood pressure in shift workers. Circulalion 1989;80:34 1-7. 10. Halberg JU, Halberg E, Cornelissen G, et al. Chronobiologically deviant blood pressure in shift working police on metropolitan street duty. Prog Clin Biol Res 1990;341B:28 1-90. I 1. Cornelissen G. Incomplete though very rapid circadian cardivascular adjustment after a transmeridian flight. J Minn Acad Sci 1988;53:18-23. 12. Pangerl A, Marz W, Halberg F. Rapid but not abrupt transmeridian adjustment ofcircadian acrophase of systolic blood pressure. J Minn Acad Sci 1986;51: 15-6. 13. Baumgart P, Rahn KH. Morgendlicher Blutdruckanstieg: Vor oder nach dem Aufwachen? Klin Wochenschr 1990;68:320-3. 14. Vaisse B, De Gaudemaris R, Asmar R, P o g i L, Mallion JM, Safar M. Mesure ambulatoire non sanglante de la pression arterielle sur 24 heures: effets du labetalol sur la montee tensionelle du petit matin. Cardiol Angeiol 1988;37:621-6. 15. Athassaniadis D, Drayer GJ, Honour AJ, Cranston WI. Variability of automatic blood pressure measurements over 24 hour period. Clin Sci 1969;36:147-56. 16. Young MA, Rowlands DB, Stallard TH, Watson RDS, Littler WA. Effect of environment on blood pressure: home versus hospital. Br Med J 1983;286:1235-6. 17. Cugini P, Gasparetto A, Antonelli M, et al. Occurence of a circadian rhythmicity for blood pressure in patients with circulatory shock. Pros CIin Biol Res 1987;227B:173-82. 18. Kunimoto M, Ugawa Y, Sakamoto M, Inoue K, Sakuta M. Light synchronization of the circadian rhythm of blood pressure and plasma cortisol in a case of Shy-Drager syndrome in an apallic state. Nippon Naika Gakkai Zasshi 1987;76:718-24. 19. Clark LA, Denby L, Pregibon D, et al. A quantitative analysis of the effects of activity and of time of day on the diurnal variations of blood pressure. J Chron Dis 1987;40:671-81.
Chronobiol Int. Val. 8, No. 6, I991
0 1991 International Society of Chronobiology
Circadian Rhythm of Blood Pressure: Internal and External Time Triggers
Chronobiol Int Downloaded from informahealthcare.com by University of North Carolina on 11/11/14 For personal use only.
Peter Baumgart Medizinische Poliklinik, University of Miinster, Miinster, F.R.G
Summary: Diurnal blood pressure (BP) fluctuations are superimposed by a 24-h rhythm with usually lower levels during the night and higher levels during the day. In contrast to other rhythmic bioparameters, the diurnal BP rhythm is largely dependent on activity and sleep rather than on clock time. This has been demonstrated by the BP characteristics after shifted sleeping and working phases, during transition from sleep to wakefulness, and by the influence of sleep and activities on the 24-h BP curve during normal daily routines. Whereas the circadian rhythm of BP is predominantly governed by external time triggers, endogenous rhythmicity can only be detected by time microscopic analysis or in conditions where effects of external time triggers are almost excluded. Key Words: Blood pressureCircadian rhythm-Shift work-Time trigger-Ambulatory blood pressure monitoring-Twenty-four-hour blood pressure.
Blood pressure (BP) fluctuates at a diurnal rhythm with usually higher levels during the day and lower levels during the night. This diurnal rhythm may be of clinical importance, since it is frequently deranged in certain diseases (1). Circadian rhythms of several biovariables such as body temperature or plasma cortisol have been shown to depend on an internal clock in humans. Reentrainment of the 24-h body temperature rhythm after flights across time zones occurs with considerable delay (2); circadian cortisol levels are not significantly altered by sleep deprivation (3). In animals active during the nighttime, there is a diurnal fall and a nocturnal rise in BP coinciding with the sleep-wakefulness cycle (4). When rats are kept in constant darkness or constant light, many physiological parameters will exhibit a so-called free-running rhythm (5,6). It has not yet been established whether BP keeps following sleep and activities in these conditions. There is an increasing number of investigations regarding circadian BP variation ~~
~~
~~
~
~~~
Received April 6 , 199 1; accepted with revisions April 30, 1991. Address correspondence and reprint requests to Priv. Doz. Dr. med. P. Baumgart at Medizinische Poliklinik, University of Miinster, Albert Schweitzer Str. 33, D-4400 Miinster, Federal Republic of Germany.
444
CIRCADIAN BLOOD PRESSURE RHYTHM 168
445
...
**......
148
138 FIG. I. Twenty-four-hour blood pressure profiles during morning shift (-) and night shift ( - .) Hourly mean systolic and diastolic BP of 17 shift workers Honzontal straight lines indicate working penods
..
Chronobiol Int Downloaded from informahealthcare.com by University of North Carolina on 11/11/14 For personal use only.
-
98
6 8 - , . . . , . .. , e 4 8
-.
...,...,...,..., 12
2e
16
24
time (h) since noninvasive 24-h assessment has become feasible by portable recorders for ambulatory blood pressure monitoring (ABPM). The impact of internal and external time triggers on blood pressure rhythm can be studied in various experimental and clinical conditions.
TWENTY-FOUR-HOUR BP RHYTHM IN SHIFTED ACTIVITY AND SLEEPING PERIODS The dependence of the BP rhythm on internal or external time triggers can be evaluated by the resistance against shifts of activity and sleep. Slowly rotated shift work is an appropriate model for shifted phases of activity and sleep. There are four reports in the literature on 24-h blood pressure monitoring in shift workers (7- 10). We performed ABPM in 17 shift workers at a chemical factory on a weekly rotated three-shift schedule (7). ABPM was performed during the morning shift and the night shift on the last working days (Fridays) of the respective weeks. The BP was recorded for 24 h at intervals of 30 min using oscillometnc devices (SpaceLabs 90202, Redmon, WA, U.S.A.). Working times were 06:OO- 14:OO in the morning shift and 22:OO-06:OO h in the night shift. As indicated by the group 24-h BP
TABLE I . Systolic and diastolic BP (mean f SEM, mm Hg) in 17 shift workers during the morning and night shifis
24 h Day (0800-2000 h) Night (2000-0800 h) Sleep Work
Morning shift
Night shift
127.6 t 2.1178.4 f 1.5 133.4 f 2.2183.8 t 1.7 121.3 t 2.2172.5 t 1.5 112.5 k 1.9164.4 k 1.6 132.4 f 2.3/83.8 _+ 1.7
127.6 t 1.4177.9 t 1.2 122.2 t 1.5172.8 f 1.1 132.5 t 1.7182.6 1.5 114.1 f 1.5165.3 t 1.4 133.3 t 1.7B3.8 f 1.6 +_
Chronobiol Inl, Vol. 8, No. 6. 1991
P. BAUMGART
446
c,
8
F I G . 2 . Cross-correlationsbetween systolic 24-h BP profiles of the morning and night shift. Maximal correlation occurs at a phase difference of 8 h-corresponding with the lag between the two working periods.
x i
Chronobiol Int Downloaded from informahealthcare.com by University of North Carolina on 11/11/14 For personal use only.
0
0
u
-@.
-0
s ~ , . . . , . . I. , . . . , . . . , . . . , . . . , -12
-8
-4
8
8
4
12
profiles (Fig. l), the BP was low during sleep and high during work in both shifts. The amplitudes of diurnal fluctuations were similar in the two shifts (Fig. 1). Table 1 presents the group average BP values over 24 h and the various time segments. The 24-h mean values during sleep or work were almost identical for the 2 days. In the morning shift, the mean daytime BP corresponded to the nighttime BP in the night shift. The nighttime BP in the morning shift corresponded to the daytime BP in the night shift. Cross-correlationanalysis revealed that there was a phase difference between the two 24-h BP curves over 8 h (Fig. 2). This is in accordance with the lag of 8 h between the two shifts. To study the rapidity of adaptation, ABPM was repeated in five of these volunteers during day I (Monday) of the night shift. As indicated by Fig. 3, there were no
136
FIG. 3. Twenty-four-hour blood pressure profiles from day 1 (-) and day 5 ( ) of the night shift. Hourly mean systolic and diastolic BP of five shift workers.
- -
96
t
76 66 r l 8
. . . I . . . l . r . I . . . I . . . I . . . I 4
8
12
16
time (h)
Chronobiol Int. Vol. 8, No. 6, 1991
28
24
CIRCADIAN BLOOD PRESSURE RHYTHM 96
1..
....
44 7 0.0
FIG. 4. Twenty-four-hour heart rate profiles during morning shift (-) and night shift ( -). Hourly mean values of 17 shift workers. Horizontal straight lines indicate working periods.
Chronobiol Int Downloaded from informahealthcare.com by University of North Carolina on 11/11/14 For personal use only.
--
691,. . . , . . . , e 4 0
.
. ., . .., . . . , . . . , 12 16 28 24
time (h) substantial differences between the 24-h BP curves on days 1 and 5 after shift rotation. Twenty-four-hour heart rate profiles were reversed in a similar pattern: The heart rate was low during sleep and high at work (Fig. 4). Comparison of 24-h heart rate profiles on days 1 and 5 indicated that adaptation to the new rhythm occurred almost immediately (Fig. 5). Sundberg et al. (8) also investigated the 24-h BP in shift workers. Their results are in accordance with our findings: The BP rhythm was reversed on the first day of the night shift and closely followed the sleep-wakefulness cycle. Again, the heart rate rhythm tended to change in parallel although less pronounced than the BP. Chau et
n
106
\
FIG. 5. Twenty-four-hour heart rate profiles from day 1 (-) and day 5 ( ) of the night shift. Hourly mean values of five shift workers.
--.
76
66f . . . . . . . . . . . . . . . . . . . . . . . . 8 4 0 12 16 2 0
I
24
time (h)
Chronobiol Int. Vol. 8, No. 6 , 1991
P. BAUMGART
448
A
Chronobiol Int Downloaded from informahealthcare.com by University of North Carolina on 11/11/14 For personal use only.
145 ,551
~
~
65
5s
3
-1
7
15
11
19
23
hours from waking up
:"$ ,
55
.,
,
,
,
,
,
,
,
,
,
,
,
,
FIG. 6. Systolic and diastolic 24-h BP profiles at intervals of IS min after synchronization by the time of awakening (arrow). A: Nomotensives (n = 1 1 1); B essential hypertensives (n = 109).
i"",
~
-1
3
7
11
is
19
23
hours from waking up al. (9) concluded some influence of internal regulation mechanisms from Fourier analysis of 24-h BP profiles in shift workers. Similar results were reported by Halberg et al. (10).However, in these studies also, the major findings were high BP coinciding with working periods and low BP coincidingwith sleep in all shifts. Time microscopic analysis of the BP rhythm after transmeridian flight showed rapid although incomplete adjustment ( 1 1,12). All experiments in shifted periods of activity and sleep indicated that the 24-h BP profile in healthy subjects is not resistant to phase shifts. The almost complete and immediate reversal of the 24-h BP curve strongly suggests that the circadian BP rhythm is largely dependent on external time triggers whereas an internal clock may only play a minor role for 24-h BP control.
Chronobiol In[, Vol. 8, No. 6. I991
~
CIRCADIAN BLOOD PRESSURE RHYTHM
449
166 -
mmHg :
/\- ----IJ -
136 -
-.\,
i’
115 -..
1
-
r-
\
Chronobiol Int Downloaded from informahealthcare.com by University of North Carolina on 11/11/14 For personal use only.
- - - /
9s -
/:> V - =/-.+ -
75;-.--J --= :
[ , f L
-
-
-
,
7, \
?
1
.
.
.
I
.
.
*
1
.
.
-
I
BLOOD PRESSURE DURING TRANSITION FROM SLEEP TO WAKEFULNESS
If the morning rise in BP occurred prior to awakening, this would be an argument for an internally governed BP rhythm. We studied the morning BP increase in large series of normotensive and hypertensive subjects (1 3). Their individual 24-h BP profiles were synchronized to the time of awakening by clock alarm. As indicated by Fig. 6, there was no substantial BP increase prior to waking, but a steep increase within the first hour thereafter in both groups. The rapidity of this increase was dependent on the lag between waking and getting up (1 3). These findings were supported by other reports on synchronized 24-h BP profiles (14). TWENTY-FOUR-HOUR BP RHYTHM IN IMMOBILIZED AND IN UNCONSCIOUS PATIENTS Diurnal BP rhythms are preserved even in patients immobilized by plaster casts ( 15). However, the diurnal changes of BP are less pronounced in hospitalized patients
than in patients who are in their natural environment (16). In circulatory shock, some circadian rhythmicity of the BP could be observed, independent of treatment effects ( 1 7). However, this does not necessarily represent effects of endogenous time triggers, since the circadian rhythm of BP can be synchronized by light exposure even in an apallic state ( 1 8).
Chronohiol Inl. Vol 8, No. 6 , 1991
P. BAUMGART
450
Chronobiol Int Downloaded from informahealthcare.com by University of North Carolina on 11/11/14 For personal use only.
EFFECTS OF TIME AND ACTIVITIES ON 24-H BP PROFILES DURING NORMAL DAILY ROUTINES Clark et al. (19) assessed the effects of activity and time of day on the diurnal BP variations by covariance analysis. After allowing for the effects of activity on BP, where sleep was one of the activities, there was no significant diurnal variation of BP. In conclusion, there was no important circadian rhythm of BP independent of activity. The shape of the 24-h BP curve during a normal daily routine is critically dependent on behavioral influences. In contrast to younger subjects, the elderly usually have sleeping periods at noon. Hence, in contrast to a roughly constant average BP level in young subjects, we found a daytime BP dip coinciding with a sleeping period at noon in elderly normotensives (Fig. 7). This underlines the essential role of external rhythms for shaping of the 24-h BP curve during a normal daily routine.
REFERENCES 1. Baumgart P. 24h-Blutdruck bei primarer und sekundarer Hypertonie. Herz 1989;14:246-50.
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Chronobiol Int. Val. 8, No. 6, I991
