BREATHING FREQUENCY IN SLEEP RELATED RESPIRATORY DISTURBANCES
M Á R C I A P R A D E L L A *
S U M M A R Y — I n n o r m a l a d u l t s , b r e a t h i n g f r e q u e n c y ( f ) i s h i g h l y r e p r o d u c i b l e w i t h i n a n
i n d i v i d u a l r a n g i n g f r o m 8 t o 25 m i n - 1 ; d u r i n g s l e e p , f isi k n o w n t o c h a n g e o n l y m i n i m a l l y .
T h i s v a r i a b l e i s r a r e l y r e p o r t e d i n s t u d i e s o f a d u l t s w i t h v a r i o u s s l e e p r e l a t e d r e s p i r a t o r y
d i s t u r b a n c e s ( S R R D ) . W e o c c a s i o n a l l y o b s e r v e d a s p e c t a c u l a r i n c r e a s e o f f d u r i n g s l e e p i n
p a t i e n t s w i t h S R R D a s s o c i a t e d w i t h h y p o p n e a a n d 0 2 d e s a t u r a t i o n . W e u n d e r t o o k a r e t r o
-s p e c t i v e -s t u d y o f 6 5 0 c o n -s e c u t i v e l a l l - n i g h t p o l y -s o m n o g r a p h i c r e c o r d i n g -s , i n o r d e r t o e v a l u a t e
h o w o f t e n s u c h a n i n c r e a s e i n b r e a t h i n g f r e q u e n c y o c c u r s a n d w i t h w h i c h f a c t o r s it i s
a s s o c i a t e d . W e e x c l u d e d p a t i e n t s w i t h m a j o r r e s p i r a t o r y f a i l u r e . W e f o u n d 16 p a t i e n t s (11
m a l e s ; m e a n ± S D , a g e : 45 ± 13 y r s ; b o d y m a s s i n d e x ( B M I ) : 4 4 ± 1 1 k g / m 2 ) w i t h f > 25 m i n - 1
d u r i n g s l e e p ( t a c h y p n e i c g r o u p , T ) . W e p e r f o r m e d a o n e - f o r - o n e m a t c h i n g f o r s e x , a g e a n d
B M I b e t w e e n t h e T a n d a c o n t r o l ( C ) g r o u p ( a g e : 4 5 ± 1 2 y r s ; B M I 4 4 ± 1 0 k g / m 2 ) , w i t h
s i m i l a r s l e e p d i s t u r b a n c e s b u t n o r m a l f d u r i n g s l e e p . W e c o m p a r e d t h e f i n e a c h v i g i l a n c e
s t a t e b y a v e r a g i n g f i v e m e a s u r e m e n t s o f f, e a c h o f o n e m i n u t e d u r a t i o n i n a s t a b l e p e r i o d .
W e o b s e r v e d t h a t : C p a t i e n t s s h o w e d n o s i g n i f i c a n t c h a n g e i n f i n a n y v i g i l a n c e s t a t e ; T
p a t i e n t s s h o w e d a h i g h e r f a l r e a d y d u r i n g w a k e f u l n e s s ( p < 0 . 0 5 ) a n d f i n c r e a s e d s i g n i f i c a n t l y
i n a l l s l e e p s t a g e s ( < 0 . 0 1 ) . W e c o m p a r e d t h e t w o g r o u p s f o r m a n y c l i n i c a l a n d p o l y s o m n o
-g r a p h i c v a r i a b l e s . S i -g n i f i c a n t s t a t i s t i c a l d i f f e r e n c e s w e r e o b s e r v e d f o r : f, a r t e r i a l C 0 2 t e n s i o n
( P a C 0 2 ) , v i t a l c a p a c i t y ( V C ) , f o r c e d e x p i r a t o r y v o l u m e a t o n e s e c o n d ( F B V 1 ) a n d p u l s e f r e
-q u e n c y ( P F ) .
K E Y W O R D S : b r e a t h i n g f r e q u e n c y , p o l y s o m n o g r a p h i c r e c o r d i n g , s l e e p r e l a t e d r e s p i r a t o r y
d i s t u r b a n c e s .
F r e q u ê n c i a r e s p i r a t ó r i a o b s e r v a d a e n i p a c i e n t e s c o m p r o b l e m a s r e s p i r a t ó r i o s a s s o c i a d o s a o s o n o .
R E S U M O — E m a d u l t o s n o r m a i s , a f r e q u ê n c i a r e s p i r a t ó r i a ( f ) v a r i a d e 8 a 25 m i n - 1 c o m
g r a n d e r e p r o d u t i v i d a d e i n t r a i n d i v i d u a l : d u r a n t e o s o n o , f é q u a s e i n v a r i á v e l . E s t a v a r i á v e l
t e m s i d o r a r a m e n t e r e f e r i d a e m e s t u d o s d e p a c i e n t e s c o m p r o b l e m a s r e s p i r a t ó r i o s a s s o c i a d o s
a o s o n o ( S R R D ) . A o b s e r v a ç ã o o c a s i o n a l d e e s p e t a c u l a r a u m e n t o d a f d u r a n t e o s o n o e m
p a c i e n t e s c o m S R R D ' , p r i n c i p a l m e n t e a p n é i a s / h i p o p n é i a s a s s o c i a d a s à d e s a t u r a ç ã o d a o x i h e
m o g l o b i n a , n o s l e v o u a p r o c e d e r a e s t u d o r e t r o s p e c t i v o d e 650 e x a m e s c o n s e c u t i v o s d e p o l i s s o
-n o g i r a f i a ( P S G ) , c o m a f i -n a l i d a d e d e a v a l i a r a f r e q u ê -n c i a d a o c o r r ê -n c i a d e s t a a c e l e r a ç ã o d a f
e d o s p r o v á v e i s f a t o r e s a e l a a s s o c i a d o s . F o r a m e x c l u i d o s d e s t e e s t u d o p a c i e n t e s c o m i n s u
-f i c i ê n c i a r e s p i r a t ó r i a g r a v e . O b s e r v a m o s a o c o r r ê n c i a d e s t e -f e n ô m e n o e m 1 6 p a c i e n t e s (11
h o m e n s c o m m é d i a d e i d a d e 4 5 ± 1 3 a n o s , í n d i c e d e m a s s a c o r p o r a l ( B M I ) 4 4 ± 1 1 k g / m 2 ) p a r a
o s q u a i s a f f o i m a i o r q u e 25 m i n - 1 d u r a n t e o s o n o ( g r u p o d e t a q u i p n é i c o s , T ) . R e a l i z a m o s
c o m p a r a ç ã o d a f e n t r e T e g r u p o c o n t r o l e ( C ) d e p a c i e n t e s d e m e s m o s e x o e s e m e l h a n t e s
•
S l e e p L a b o r a t o r y , S t . L u c T r a i n i n g H o s p i t a l U n i v e r s i t é C a t h o l i q u e d e L o u v a i n , B r u s s e l s . A c e i t e : l l - s e t e m b r o - 1 9 9 2 .
i d a d e s ( 4 5 ± 1 2 a n o s ) e, B M I ( 4 4 ± 1 0 k g / n i 2 ) e S R K D , e m c a d a e s t a d o d e v i g i l â n c i a , f a z e n d o
m é d i a d e 5 c o n t a g e n s d a f, c a d a u m a f e i t a d u r a n t e 1 m i n u t o e d u r a n t e u m p e r í o d o e s t á v e l
d o d e t e r m i n a d o e s t a d o d e v i g i l â n c i a , O b s e r v a m o s q u e : o s p a c i e n t e s d o g r u p o C n ã o a p r e
-s e n t a r a m a l t e r a ç ã o -s i g n i f i c a t i v a d a f e m q u a l q u e r e -s t a d o dei v i g i l â n c i a ; o -s p a c i e n t e -s d o g r u p o
T j á t i n h a m f m a i o r q u e o s p a c i e n t e s d o g r u p o C ( p <^ 0.05) q u a n d o a c o r d a d o s e e s t a
a u m e n t o u s i g n i f i c a t i v a m e n t e e m t o d o s o s e s t a d o s d o s o n o ( p < ^ 0 . 0 1 f q u a n d o a c o r d a d o s v s f
d u r a n t e o s o n o , p < 0 . 0 0 1 T v s C d u r a n t e o s o n o ) . C o m p a r a m o s o s d o i s g r u p o s p a r a v á r i o s
p a r â m e t r o s c l í n i c o s e p o l i s s o n o g r á f i c o s . D i f e r e n ç a e s t a t i s t i c a m e n t e s i g n i f i c a n t e f o i o b s e r v a d a
p a r a o s d a d o s d e : PaCO2 ( p r e s s ã o a r t e r i a l d o C 0 2 ) , c a p a c i d a d e v i t a l ( V C ) , v o l u m e e x p i r a
-t ó r i o e m 1 s e g u n d o ( F E Í V 1 ) e d a f r e q u ê n c i a d o p u l s o ( P F ) d u r a n -t e r e p o u s o ( a c o r d a d o s ) e
d u r a n t e o s o n o .
P A L A V R A S C H A V E : f r e q u ê n c i a r e s p i r a t ó r i a , r e g i s t r o p o l i s s o n o g r á f i c o , p r o b l e m a s r e s
-p i r a t ó r i o s r e l a c i o n a d o s a o s o n o .
RESPIRATORY PHYSIOLOGY DURING WAKEFULNESS AND SLEEP
1. Sleep organization. Wakefulness on the one hand is a state of cortical activation recognized by the occurence of alpha waves (during quiet periods) on the electroencephalogram (EEG) and increased muscle tone. The cortical activation is maintained by tonic activity of the reticular activating system, which both directly stimulates the cortex and also facilitates thalamic relay neuron passage of sensory information to the cortex. Sleep on the other hand is a state in which alpha activity is lost and muscle tone reduced. NREM sleep (stage 1-4) is characterized by inhibition of the reticular activating system, resulting in cor-tical suppression and functional deafferentation. In REM sleep, the reticular activating system is reactived, producing cortical stimulation. In contrast to wakefulness both sensory and motor functions are impaired during REM. There is both pre- and post-synaptic inhibition of afferent neurons resulting in raised arousal thresholds to external stimuli and post-synaptic inhibition of motor
neu-rons which produces the postural atonia typical of REM sleep1 1
.
2. Ventilation. Feedback regulation through chemoreceptors is one of the two major mechanisms controlling ventilation. It adjusts ventilation to changes in the environment and metabolic rate. The other one is under volitional control, and can override, under certain limits, the metabolic one. This usually happens when the respiratory system is disturbed so as to change the partial pressure
of 02 or C02 in the blood, smooth adjustments in ventilation are expected2 8
. The special attributes of the brain-stem respiratory neurons may not be inherent automaticity, but rather a consequence of their afferent inputs and their ability to convert these inputs into a rhythmic output. Thus, breathing rhythm would not be simply modulated by afferent respiratory stimuli but rather would be
critically and fundamentally dependent on such stimuli3 3
. The activation of each inspiratory burst (on-switch mechanism) is the critical event in the generation of the respiratory patterns and depends on adequate levels of afferent respira-tory stimuli. The major influence of incoming afferent stimuli is to determine the timing of the onset of each respiration and, in so doing, to fix the duration
of the preceding expiration, thereby generating the basic respiratory r h y t h m3 3
'3 9 . Under normal conditions, effective respiratory rhythmicity is critically linked to, and dependent on the need for removal of C02; other respiratory stimuli exert a more secondary modulating influence, but do not in themselves generate the respiratory rhythm.
3. The effect of sleep stages on ventilation.
3.1. Breathing patterns: (1) Drowsiness or unsteady NREM sleep is characteri-zed in many subjects by periodic breathing associated with swings in P02 and PC02, cortical EEG activity, systemic arterial pressure (AP), heart rate, 02
sa-turation (Sa02) and an overall moderate decrease in ventilation2 9
irre-gularity of breathing at sleep onset consists of a regular waxing and waning of breathing amplitude. The oscillations may be of low amplitude resulting in alternating hyper- and hypoventilation or of large amplitude with apneas at the nadir of the oscillations. During the amplitude oscillations there are few changes,
if any, in respiratory rate1 2
>4
5. Apneas occur more frequently in the elderly and
not only at the transition between wakefulness and sleep, but also tend to occur predominantely in sleep stages 1-2. They seem to be an exaggeration of
the periodicity that is typical of sleep onset 6,37,44. (3) Periodic breathing
per-sists as long as sleep oscillates between arousal and stages 1 or 2. These oscilla-tions seem to be mainly due to sleep instability at these stages and different setpoints of breathing regulation in wakefulness and sleep; the natural tendency of breathing regulation to be unstable probably aggravates this situation. It disappears when stable stage 2 or stages 3-4 are reached and systemic AP drops
20-30% below basal wakefulness values 13,43. (4) R E M sleep is characterized by
erratic shallow breathing with irregularities both in amplitude and frequency. The heart rate becomes irregular and central apneas or hypopneas occur spo-radically. Hipertensive peaks, sometimes reaching 30-40 mmHg, suddenly appear and continue for the whole length of R E M sleep. These events are synckroncus with R E M bursts and probably of central origin, related to the R E M sleep process.
3.2. Ventilation. In normal healthy adults the minute ventilation (VE) decrea-ses by a mean of 8.2 ± 2.3% from wakefulness to sleep and does not vary
sig-nificantly between sleep stages 18,38,45. The greatest decrease in ventilation has
been observed during phasic REM sleep n.i4. Mean inspiratory flow fell
signi-ficantly but timing was unchanged 33,38. Arterial oxygen saturation (Sa02) also
fell and could be accounted for entirely by the associated reduction in ventila-tion. 02 (V02) consumption and C02 (VC02) production also fell by 8.5 ± 1.6% and 12.3 ± 1.7% respectively during sleep and again did not differ between sleep stages. Ventilation correlated quite closely with V02 and VC02 during sleep with similar levels of correlation found for each sleep stage. These results
are similar to those observed in younger 14,21 or older subjects 3 7
>4 i .
3.3. Breathing frequency (f). Breathing frequency (f) varies significantly from one individual to an other but a remarkable stability is observed for the same
subject 1 4 4 , 3 1 , 3 5 - 3 8 , 4 2 . p0r healthy awake subjects the mean is 8 to 24.8 breaths/
min for the young and 8.14 to 18.06 breaths/min for the elderly (>65 years). In sleep f ranges from 9.9 to 23.6 breaths/min for the young and 5.16 to 19.64 brea-ths/min for the elderly. The lower respiratory frequency observed in the elderly was primarily due to a significantly longer inspiratory time compared with values observed in wakefulness and each sleep stages in the young 37. in children,
Guilleminault is describes the association of tachypnea with the rare occurrence
or absence of apnea during sleep with or without a decrease of oxygen satu ration which could be proposed as a marker of pathological breathing during
sleep. Aubert-Tulkens et a l2
reported a case of a woman with obesity, loud snoring, restless sleep, hypersomnia, personality changes and systemic hyper-tension, who on polysomnographic examination revealed tachypnea of 35 brea-ths/min, continuous loud snoring and an Sa02 with periodic oscillations between 95-60%.
3.4. Upper airway. Breathing implies maintenance of a normal upper airway patency: many local reflexes at the naso-oro-hypopharyngeal and laryngeal level are thought to counteract the progressive development of the negative intratho-racic pressure generated by the inspiratory muscles (scaleni, intercostals, diaph-ragm) contraction. The reflexes involved in the maintenance of upper airway patency are, of course* activated just prior to the onset of the diaphragmatic contraction.
The passage from wakefulness to sleep is normally associated with an increased upper airway muscle tone. Whether the increase in upper airway resistance alone explains the decrease in ventilation during sleep is not yet clear 1 , 1 7 , 2 0 , 2 5 , 1 0 , 4 1 .
4. Sex differences — Cater all and co-workers 5 reported that there was
no sex difference in sleep disorders in subjects with normal body weight. In
female subjects, disordered breathing was reduced during pregnancy4
stimu-lant n . 2 2 , 4 6 . Obese women (but not obese men) show an increased response to
both hypoxia and hypercapnia2
?.
5. Diseased subjects. Tobin et al.4 2
studied f in awake normal and diseased subjects and observed that respiratory rate was increased above the normal in smokers and in patients with COPD, restrictive lung disease and pulmonary hypertension but remained normal in asthmatic patients (11 to 22.2 breaths/min in normals, 12.1 to 43.7 breaths/min in diseased subjects).
PROPOSAL. The present study's main aim is to determine the frequency with which tachypnea occurs during sleep in patients referred to our laboratory (Sleep Laboratory, St Luc Training Hospital, Brussels, Belgium) for clinical assessment of sleep apnea syndrome. We were impressed by the fact that con-trary to what is normally described in current literature on the subject, instead of reducing f during sleep, several subjects showed a spectacular increase in f. With this in mind, we undertook a retrospective study of 650 all night polysom-nography recordings (excluding patients with major respiratory failures) and found 16 patients who presented a significant increase of f during sleep (p<0.01). We set up a control group (n=16) of patients with similar sleep disturbances but normal f during sleep, matched for sex, age and body mass index in an attempt to establish any distinguishing factors between them.
M E T H O D S
P o p u l a t i o n . I n a s t u d y o f 650 c o n s e c u t i v e a l l - n i g h t p o l y s o m n o g r a p h y r e c o r d i n g s , w e f o u n d 16 p a t i e n t s w h o h a d f h i g h e r t h a n 2 5 m i n 1 d u r i n g s l e e p ( T g r o u p ) . T h e m a i n d i a g n o -s i -s of S R R D c o m p r i -s e d 9 p a t i e n t -s w i t h -s l e e p a p n e a - h y p o p n e a -s y n d r o m e , 3 w i t h a n o v e r l a p s y n d r o m e , 1 w i t h a h y p o v e n t i l a t i o n s y n d r o m e , 1 w i t h S h y - D r a g e r s y n d r o m e a s s o c i a t e d w i t h s n o r i n g , 1 w i t h c h r o n i c o b s t r u c t i v e p u l m o n a r y d i s e a s e ( C O P D ) a n d 1 w i t h m a j o r o b e s i t y a s -s o c i a t e d w i t h S a 0 2 d e -s a t u r a t i o n -s e -s -s e n t i a l y i n R E M -s l e e p . W e c o n -s t i t u t e d a c o n t r o l g r o u p o f p a t i e n t s ( n = 1 6 ) w i t h s i m i l a r S R R D ( i . e . a p n e a - h y p o p n e a , s n o r i n g , S a G 2 d e s a t u r a t i o n s ) b u t n o r m a l f d u r i n g s l e e p , m a t c h e d w i t h T f o r s e x , a g e a n d B M I ( C g r o u p ) a n d a g r o u p o f y o u n g h e a l t h y s u b j e c t s (15 m a l e s ; a g e : 24 ± 2 ; B M I : 21 ± 2 ( N g r o u p ) f o r c o m p a r i s o n w i t h t h e T g r o u p .
P o l y s o m n o g r a p h y s t u d i e s . T h e p o l y s o m n o g r a p h y r e c o r d i n g s w e r e p e r f o r m e d a c c o r d i n g t o s t a n d a r d t e c h n i q u e s 34: e l e c t r o e n c e p h a l o g r a m ( E E G ) , e l e c t r o o c u l o g r a m ( E O G ) , e l e c t r o m y o -g r a m ( E M G ) , e l e c t r o c a r d i o -g r a m ( E C G ) , o r a l a n d n a s a l a i r f l o w ( t h e r m o c o u p l e s p l a c e d i n f r o n t o f m o u t h a n d n o s t r i l s ) , r e s p i r a t o r y m o v e m e n t s ( s t r a i n g a u g e ) , b r e a t h i n g s o u n d s , i . e . , s n o r i n g ( m i c r o p h o n e a t t a c h e d t o t h e s k i n o v e r t h e l a r y n x ) . T h e s e p a r a m e t e r s w e r e r e c o r d e d o n a p o l y g r a p h a t a p a p e r s p e e d o f 15 o r 1 0 m m / s .
S i n c e 1986, t r a n s c u t a n e o u s o x y g e n s a t u r a t i o n ( S a 0 2 ) ( N e l l c o r p u l s e o x y l e t e r N - 1 0 0 , H a y w a r d , C A , U S A ) h a s a l s o b e e n r e c o r d e d . T h e m e a n of t e n m e a s u r e m e n t s p e r s e c o n d o f S a 0 2 a n d p u l s e f r e q u e n c y ( P F ) f r o m t h e o x i m e t e r , a l o n g w i t h t h e i n t e g r a t e d s i g n a l f r o m t h e m i c r o p h o n e w e r e s t o r e d i n a n i n t e r m e d i a t e d a t a b a n k e v e r y 6 s e c o n d s . T h e f i l e c o n t a i n i n g t h e s e m e a -s u r e m e n t -s a l -s o h e l d i n f o r m a t i o n o n t h e e x a c t t i m e o f t h e -s t a r t o f r e c o r d i n g . T h i -s a l l o w e d u s t o c o r r e l a t e t h e s e d a t a a t a l a t e r s t a g e a l o n g w i t h o t h e r d a t a , n a m e l y d a t a o f t h e v i g i l a n c e s t a g e s a n d a p n e a s . T h e d e s a t u r a t i o n i n d e x ( D I ) w a s t a k e n a s t h e n u m b e r o f t h e d e s a -t u r a -t i o n s e p i s o d e s ^ 4 % e x p r e s s e d p e r h o u r o f s l e e p .
S l e e p s t a g i n g w a s p e r f o r m e d a c c o r d i n g t o s t a n d a r d m e t h o d s i n 4 0 o r 6 0 s e c o n d s e p o c h s . T o t a l s l e e p t i m e ( T S T ) w a s m e a s u r e d f r o m t h e m o m e n t w h e n t h e p a t i e n t f e l l a s l e e p u n t i l t h e f i n a l a w a k e n i n g , e x c l u d i n g w a k e t i m e d u r i n g s l e e p . T h e t i m e o f t h e v a r i o u s s l e e p s t a g e s w a s c a l c u l a t e d a s p e r c e n t a g e o f T S T . M o v e m e n t a r o u s a l i n d e x ( M A ) i s t h e n u m b e r o f e p i -s o d e -s o f i n c r e a -s e i n E M G a c c o m p a n i e d b y a l p h a r h y t h m i n t h e E E G f o r a t l e a -s t 2 -s e c o n d -s , d u r i n g a s l e e p e p o c h , e x p r e s s e d p e r h o u r o f s l e e p .
S t u d y . W e c o m p a r e d f ( f r o m a i r f l o w ) i n e a c h v i g i l a n c e s t a t e b y a v e r a g i n g f i v e m e a s u r e m e n t s o f f, e a c h o f o n e m i n u t e d u r a t i o n i n a s t a b l e p e r i o d f o r : ( a ) t h e T g r o u p ( w a k e -f u l n e s s v s s t a g e s 1, 2, 3, 4 a n d R E M s l e e p ) ; ( b ) t h © C g r o u p ( i d e m t o T g r o u p ) ; ( c ) t h e N g r o u p ( i d e m t o T g r o u p ) a n d T v s C g r o u p ( i n e a c h v i g i l a n c e s t a t e ) . W e c o m p a r e d t h e T a n d C g r o u p s f o r o t h e r p o l y s o m n o g r a p h y v a r i a b l e s : % s t a g e 1, 2, 3, 4 a n d R E M s l e e p ; M A ; S a 0 2 ; D I a n d P F a n d f o r s o m e c l i n i c a l v a r i a b l e s : b l o o d p r e s s u r e ( B P ) ; b l o o d g a s ( P a 0 2 a n d P a C 0 2 ) ; v i t a l c a p a c i t y ( V C ) a n d f o r c e d e x p i r a t o r y v o l u m e a t o n e s e c o n d ( F E V 1 ) .
R E S U L T S
A n a l y s i s o f t h e r e s u l t s h i g h l i g h t e d t h e f o l l o w i n g e l e m e n t s :
1 . b y d e f i n i t i o n , T a n d C g r o u p s w e r e w e l l m a t c h e d f o r a g e a n d B M I ( m e a n ± S D ; a g e : 4 5 ± 1 3 y r s v s 4 5 ± 1 2 y r s ; B M I : 4 4 ± 1 1 k g / m 2 v s 4 4 ± 1 0 k g / m 2 ) ;
2 . T p a t i e n t s s h o w e d a h i g h e r f d u r i n g w a k e f u l n e s s t h a n C p a t i e n t s ( 2 3 . 4 ± 6 . 5 m i n - 1 v s 1 7 . 9 ± 3 . 0 m i n - 1 ; p < 0 . 0 5 ) ;
3 . C p a t i e n t s s h o w e d n o s i g n i f i c a n t m o d i f i c a t i o n o f f i n a n y s l e e p s t a g e ;
4 . T p a t i e n t s i n c r e a s e d f i n a l l s l e e p s t a g e s ( s t a g e 1 : 3 1 . 5 ± 7 . 7 m i n - 1 ; s t a g e 2 : 3 1 . 9 ± 7 . 2 m i n - l ; s t a g e 3 : 3 1 . 0 ± 6 . 0 m i n - 1 ; s t a g e 4 : 3 0 . 3 ± 6 . 5 m i n - 1 ; s t a g e R E M : 2 9 . 3 ± 4 . 0 m i n - l ; a l l p < 0 . 0 1 v s w a k e f u l n e s s ) ;
5 . N s u b j e c t s s h o w e d n o s i g n i f i c a n t m o d i f i c a t i o n o f f i n a n y v i g i l a n c e s t a g e ;
6 . T a n d C p a t i e n t s s h o w e d n o s i g n i f i c a n t d i f f e r e n c e f o r t h e d i s t r i b u t i o n o r t h e s l e e p s t a g e s ;
7 . N s u b j e c t s s h o w e d a n o r m a l d i s t r i b u t i o n of s l e e p s t a g e s : s t a g e 1 : 1 0 . 0 ± 4 . 7 ; s t a g e 2 : 4 8 . 5 ± 9 . 1 ; s t a g e 3 : 7 . 7 ± 4 . 2 ; s t a g e 4 : 1 5 . 2 ± 6 . 8 ; s t a g e R E M : 1 8 . 6 ± . 6 . 4 % T S T ;
8 . T a n d C p a t i e n t s s h o w e d n o s i g n i f i c a n t d i f f e r e n c e f o r : M A , S a 0 2 , D I , B P a n d P a 0 2 ;
9 . N s u b j e c t s s h o w e d a m e a n S a 0 2 d u r i n g T S T o f 9 7 . 2 ± 1 . 0 % a n d 'a, n u m b e r of M A / h s of 1 2 . 0 ± 6 . 1 ;
1 0 . a s i g n i f i c a n t s t a t i s t i c a l l e v e l o f d i f f e r e n c e ) w a s o b s e r v e d b e t w e e n T v s C g r o u p f o r : P a C 0 2 ( 4 4 . 2 ± 6 . 0 v s 3 9 . 1 ± 3 . 1 m m H g p < 0 . 0 5 ) ; V C ( 7 6 . 2 ± 1 1 . 3 v s 8 3 . 2 ± 7 . 8 p e r c e n t p r e d i c t e d n o r m a l p < 0 . 0 5 ) ; F E V 1 ( 7 7 . 0 ± 1 2 . 9 v s 8 6 . 3 ± 1 0 . 0 p e r c e n t p r e d i c t e d n o r m a l p < ^ 0 . 0 5 ) ; P F ( a w a k e : 8 4 . 2 ± 1 3 . 3 v s 7 4 . 3 ± 8 . 9 m i n - 1 ; T S T : 8 0 . 0 ± 1 3 . 6 v s 6 9 . 8 < ; 9 . 6 m i n - 1 p < ^ 0 . 0 5 f o r w a k e f u l n e s s a n d s l e e p ) a n d f i n a l l v i g i l a n c e s t a t e s ( w a k e f u l n e s s p < [ 0 . 0 5 ; s t a g e 1, 2, 3 a n d 4 s l e e p p < 0 . 0 0 1 ; R E M s l e ^ p p < 0 . 0 5 ) ( s e e F i g . 1 a n d T a b l e 1 ) .
f
C O M M E N T S
Discussion. An acceleration of f during sleep is rarely confirmed in a review of current literature. In females and children, tachypnea may be considered a marker of pathologic breathing during sleep; which could be associated with loud snoring, and oxygen desaturation with or without the occurence of apneas
or hypopneas 2,16. in this study, we found 16 patients from a population of 650
subjects (2.5%) who presented a significant acceleration of f during sleep. All
but two of these patients (one female, BMI of 19.78 kg/m2
, who suffered from
Shy-Brager syndrome and one male, BMI of 27.68 kg/m2
, who suffered from DPOC) suffered from morbid obesity and were referred to us for investigation of sleep apnea syndrome.
In relation to the main diagnosis of SRRD presented by our patients: The alveolar hypo-ventilation syndrome is defined as a combination of hypercapnia and alveolar hypoxemia. Nonspecific alveolar hypoventilation can occur secondary to damaged respiratory centers in the brain and can also be seen with drug intoxication (barbiturates, for example) and with abnormalities of breathing apparatus, as in muscular dystrophy, kyphoscoliosis, Pierre Robin syndrome, oostructice lung, disease, etc. The Pickwickian syndrome, in its classic form in-cludes obesity, hypersomnolence, periodic breathing with hypoventilation and cor pulmonale is. in obesity, the total lung volume and the functional residual capa-city are diminished. The decrease in functional residual capacapa-city is secondary to a marked diminution in expiratory volume. Because of the diminished volu-mes, the obese patient breaths closer to residual volume. This, coupled with a
decreased tidal volume allows for earlier airway closure in areas of decreased ventilation 46. sleep studies indicate that the incidence of obstructive sleep-apnea (OSA), obesity-hypo ventilation syndrome and desaturation in unselected,
morbi-dly obese male patients is extremely high1
^. Obese children and adolescents are
undoubtedly at risk as a group for OSA in adulthood so. Kunitomo' et a l2 7
showed that there is no sex-related difference in ventilatory or occlusion pressure res-ponses to hypoxia or hypercapnia in either obese subjects or normal control subjects. However, there is respiratory compensation with mass loading i.e. accumulation of fat about the chest wall that produce a decrease in chest wall
and lung compliance 4
apnea is often seen after the menopause. Progesterone has been considered as
a protective hormone in females 4
#. Among our tachypneic group, 4 women
pre-sented a morbid obesity and 2 were in a post-menopausal age. Harman et al1
^ suggest that for obese men anatomic factors would appear to be important. Increased fatty tissue in the hypopharynx may predispose to obstructive apnea.
Kopelman et a l2 3
believe that mechanical impedance of breathing due to abdo-minal adipose tissue combined with abnormal central respiratory control i.e.
relatively depressed chemosensitivities 2 7
are the major factors responsible for the respiratory disturbances observed during sleep. All of our obese patients presented obstructive sleep apnea-hypopnea, periodic breathing or snoring asso-ciated with Sa02 desaturation.
In patients with Shy-Drager syndrome, there is evidence of multiple system atrophy due to widespread degeneration throughout the central nervous system. Respiratory abnormalities in these patients may be due to laryngeal obstruction caused by bilateral abductor paralysis of the vocal cords, or to disorganization of central respiratory control, i.e., functional instabilities inherent in the
orga-nization of respiratory rate and rhythm8
. These may produce episodes of sleep apnea with concomitant hypoxia which mav presage death due to cardiorespi-ratory a r r e s t2 1
. Our patient had a tachypnea associated with snoring observed in 100% of the TST. These are compatible with the patophysiologic findings described above.
In chronic obstructive pulmonary disease (COPD), Tobin et a l4 2
observed an elevated respiratory rate during wakefulness, similar in patients with or without hypercapnia, when compared with normal subjects and symptomatic asthmatic
patients. They postulated that, since the severity of resistive loading as reflected by the airway resistance was comparable among the two COPD groups and symptomatic asthmatic patients, the faster respiratory rate in patients with COPD implied a proportionally greater fall in compliance which shortened the time constant of the respiratory system. Compliance of the respiratory system was equivalent to the sum of the reciprocals of the lung and thoracic wall compliances. Dynamic lung compliance probably decreased to approximately the same extent in COPD as in symptomatic asthma and would not account for the more rapid rate in COPD. The chronically hyperinflated thorax in COPD appeared to be less compliant than normal. In their patients, the most specific variability in the components of the breathing pattern appeared to be expiratory timing. This concurs with the hypothesis of Sullivan et al30 that the mechanism initiating inspiratory burst, rather than the one responsible for terminating the burst, is of fundamental importance in setting up breathing frequency.
A commentary may be made in relation to smokers. Tobin et a l4 2
observed an increased respiratory rate with heightened respiratory center drive as reflected by elevated mean inspiratory flow (VT/TI) in smokers who had little or no subjective complains. In our series of patients, 6 tachypneic patients and 4 control patients were smokers. This enabled us to exclude the effect of a higher respis ratory center set point induced by chronic intermittent stimulation with nicotine with a factor allowing us to differentiate T and C patients.
Conclusion. Our results confirm the data observed in literature for normal subjects in relation to the stability of f within a subject, independent of vigilance states and the inter-individual variability that exists 3 5 , 3 7 , 3 8 . "We believe that for the majority of our tachypneic patients, the most likely hypothesis that may explain the observed acceleration of breathing frequency during sleep is that this was due to the morbid obesity associated with the diminution of pulmonary volumes (VC and FBV1) and hypercapnia. The reduction in pulmonary volumes may potencialise ventilatory decreases normally observed during sleep, enhancing the development or underscoring and increasing the hypercapnia that already exists. This in turn induces tachypnea and tachycardia (detected as PP) as a compensatory mechanism.
R E F E R E N C E S
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1980, 3 : 2 3 5 - 2 4 6 .
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