COUPLED VS. UNCOUPLED PERICARDIAL CONSTRAINT: EFFECTS ON CARDIAC
CHAMBER INTERACTIONS 
Under some circumstances, the pericardium acts on the surface of the
heart to constrain it. Such constraint can either be "coupled,"
when the pericardium exerts a liquid pressure on the entire surface of
the heart, or "uncoupled," when the pericardium exerts a more
local constraining action via a regional surface pressure. In the
former, changes in the volume of one chamber of the heart influence the
volumes of other chambers, whereas in the latter this interaction among
chambers does not occur. Takata et al. (p. 1799) developed a
mathematical model of atrial and ventricular interactions in classic
pericardial diseases. The numerical solution of the model revealed that
coupled constraint accounted for pressure waveforms observed in
pericardial tamponade, whereas uncoupled constraint accounted for those
observed in constrictive pericarditis. The success of the model
indicates that the concept of coupled vs. uncoupled constraint may
offer a coherent general theory for understanding the hemodynamic
events of pericardial diseases. The paper is discussed in an Invited
Editorial by Tyberg (p. 1797).
SURFACTANT PROTEINS AND IMMATURE LUNG MECHANICS
In anesthetized immature rabbits, Kobayashi et al. (p. 1849) studied
the effects of reconstituted surfactant proteins SP-B and SP-C on
tidal volume with sufflation pressure of 25 cmH2O and on static volume
deflated to 5 cmH2O airway
pressure. For a fixed SP-C concentration (1.4%), there were
significant increases in tidal volume from 7 to 26 ml/kg and in static
volume from 6 to 32 ml/kg, as SP-B concentration increased from 0 to 0.7%. Similar results were obtained for a fixed SP-B
concentration (0.7%) when SP-C concentration (0-1.4%) was varied.
However, maximum static volumes were attained with a lower
concentration of SP-C (0.18%). Thus both SP-B and SP-C were required
for optimal lung mechanics, but less SP-C was needed.
RESPONSES TO AIRWAY OCCLUSION DURING SLEEP
The ventilatory response to airway occlusion is reduced during
rapid-eye-movement (REM) sleep. To examine the mechanism of this
reduction, Smith et al. (p. 1923) studied the responses of airway
pressure and diaphragm electromyographic activity to airway occlusions
during REM and non-REM sleep in chronically instrumented dogs. The
results suggest that the reduced mechanical response in REM sleep is
chiefly attributable to reduction or fractionation of diaphragmatic
activity. Chest wall dis-tortion may also contribute to the reduction
in mechanical output.
HYDRALAZINE IN SLEEPING RATS
Stimulation of carotid baroreceptors is known to reduce ventilation.
Carley et al. (p. 1954) reasoned that reducing arterial blood pressure
pharmacologically during sleep might stimulate ventilation and prevent
sleep-related central apnea. To test this hypothesis, they injected
rats with hydralazine or a placebo and monitored arterial pressure,
sleep state, and ventilation following the injection. Hydralazine
lowered blood pressure, increased ventilation, and reduced the
incidence of apneic periods during sleep. The findings indicate that,
at least under the conditions of the study, mild hydralazine-induced
hypotension stimulates breathing and supresses sleep-associated apneic
episodes.
ORAL CONTRACEPTIVES AND SKIN BLOOD FLOW
Changes in female reproductive hormones associated with phases of
the menstrual cycle are known to alter thermoregulatory reflex control of skin blood flow. The effects on these
phenomena of synthetic estrogen and progesterone in oral contraceptives were examined by Charkoudian and Johnson (p. 2012). Women at the end of
the third week of hormone treatment or at the end of a week
of placebo treatment were exposed to whole body heating with a
water-perfused suit. Laser Doppler flowmetry was used to monitor blood
flow in normal skin and at a skin site where noradrenergic vasoconstrictor control was blocked with bretylium. The oral
temperature at which skin vasodilation and sweating were observed in
both the normal skin and the site where noradrenergic vasoconstriction was blocked was higher in the hormone-treated women. These results indicate that the exogenous hormones had effects similar to those seen
in the normal menstrual cycle. The authors conclude that the similar
threshold shifts at control and bretylium-treated skin sites suggest
that hormones shift the function of the active vasodilator system to
higher core body temperatures through a centrally mediated action.
CONTROL OF SKELETAL MUSCLE PERFUSION DURING EXERCISE
Buckwalter et al. (p. 2037) examined the effects of 
-adrenergic- and
muscarinic-receptor blockade on the increase in blood flow to the
hindlimbs of dogs exercising on a treadmill. They found that injection
of propranolol or atropine into the iliac artery of one hindlimb had no
effect on the increase in blood flow evoked by dynamic exercise. In
contrast, these antagonists abolished the increase in blood flow evoked
by iliac arterial injections of isoproterenol and acetylcholine. The
authors concluded that -adrenergic and muscarinic receptors are not
involved in the control of blood flow to skeletal muscle during
moderate dynamic exercise.
CREATINE INTAKE AND MUSCLE PERFORMANCE
Does creatine supplementation enhance performance? Vanden-berghe et al.
(p. 2055) investigated this question in 19 young women during 10 wk of
resistance training. Creatine was supplemented at a high dose (20 g/day) for only 4 days; thereafter, the dose was reduced to 5 g/day for
the remainder of the 10-wk training period. As measured by nuclear
magnetic resonance, muscle phosphocreatine concentration increased, as
did strength and intermittent exercise capacity, compared with a
placebo group. When the supplement was removed, muscle phosphocreatine
declined. Thus, under the particular conditions of the study, it
appears that creatine supplementation enhances the development of
muscle strength during resistance training.
TRAINING AND DECONDITIONING ALTER PLATELET FUNCTION IN
WOMEN ATHLETES
The effects of chronic exercise and detraining on platelet function
were studied by Wang et al. (p. 2080) in women during the
midfollicular phase of the menstrual cycle. After training 1) resting platelet functions were
decreased, whereas plasma nitrite/nitrate levels and platelet guanosine
3
,5-cyclic monophosphate (cGMP) contents were enhanced;
and 2) potentiation of platelet
functions by acute strenuous exercise was decreased, whereas the
increases in plasma nitrite/nitrate levels and platelet cGMP contents
by acute exercise were enhanced. Deconditioning reversed these training effects. The authors concluded that training-induced platelet functional changes in women in the midfollicular phase may be mediated
by nitric oxide.
COUPLED VS. UNCOUPLED PERICARDIAL CONSTRAINT: EFFECTS ON CARDIAC
CHAMBER INTERACTIONS
