Do the competition rules of synchronized swimming encourage undesirable levels of hypoxia?

Recent anecdotal reports that some synchronized swimmers have become dizzy or disorientated towards the end of their performance, and in the worst cases fainted underwater, have caused concern. However, the rules of synchronized swimming encourage slow performance of compulsory figures, and an analysis of the competition placings and duration of underwater sequences showed that the highest rankings were gained by slowly performed compulsory figures and free programmes containing a long underwater sequence. The combination of breath-holding and the vigorous exercise involved suggests that some of the symptoms complained of by the swimmers might be due to hypoxia. We therefore studied the alveolar gas tensions in nine members of the Great Britain National Squad immediately following the performance of set figures and the initial underwater sequence of their free routine in a swimming-bath. All were cyanosed after the underwater sequences of the free routine and reported being mildly confused. The mean(s.d.) alveolar PO2 at this stage was 5.07(1.1) KPa, while three girls had an alveolar PO2 below 4 KPa, the lowest being 3.67 KPa. These gas tensions suggest that potentially dangerous levels of hypoxia may develop during competitive synchronized swimming and that prolonged underwent sequences should not be encouraged.

Effect of pregnancy on pressure-volume relationships in circulation of rabbits.

Mean circulatory filling pressure (MCFP) has been measured after vagally induced cardiac arrest in 11 nonpregnant and 10 near-term pregnant rabbits, anesthetized with pentobarbital sodium. MCFP was 6.1 +/- 0.4 (SD) mmHg in the nonpregnant and 7.2 +/- 0.4 mmHg in the pregnant animals. The difference of 1.1 mmHg was significant (P < 0.001). Sympathetic blockade with bretylium tosylate (10 mg/kg) reduced MCFP by 0.4 mmHg in both nonpregnant and pregnant rabbits. Vascular capacitance was examined in the two groups. Unstressed vascular volume and vascular compliance were derived from measurements of MCFP after increasing blood volume by 8 and 16% or reducing it by 8%. The unstressed vascular volumes, 33.9 +/- 3.9 (SD) ml/kg in the nonpregnant and 35.1 +/- 3.2 ml/kg in the pregnant group, were not significantly different, but compliance in the pregnant group (4.0 +/- 0.6 ml.kg-1.mmHg-1) was significantly greater than in the nonpregnant rabbits (3.4 +/- 0.6 ml.kg-1.mmHg-1) (P < 0.05). We conclude that there are changes in vascular capacitance in rabbit pregnancy, probably not related to alterations in vasomotor activity, but these are insufficient to fully compensate for the increase in blood volume, thus leading to the rise in MCFP.

The effect of carotid sinus pressure changes on mean circulatory filling pressure in the anaesthetized rabbit.

The effect on mean circulatory filling pressure (MCFP) of changes in pressure within the vascularly isolated carotid sinus has been studied in anaesthetized rabbits. With the sinus exposed to arterial pressure (102 +/- 6 mm Hg; mean +/- SEM) MCFP was 6.18 +/- 0.40 mm Hg. At sinus pressures of 160 and 40 mm Hg, MCFP was 5.71 +/- 0.39 and 6.43 +/- 0.33 mm Hg respectively. The mean difference of 0.72 +/- 0.06 mm Hg between MCFP at these two sinus pressures was significant (P < 0.001). MCFP varied with blood volume when this was increased or reduced by 8%, but at each blood volume level the difference between MCFP at high and low sinus pressure remained significant (P < 0.01). The gradient for venous return, MCFP minus right atrial pressure, was altered by changes in carotid sinus pressure, being 0.60 +/- 0.12 mm Hg less at a sinus pressure of 160 mm Hg than at a sinus pressure of 40 mm Hg. This difference was significant (P < 0.01).

Contribution of the hindlimb vasculature to systemic resistance responses to carotid sinus pressure changes in the rabbit.

The effects on the vascular resistance of the skinned hindlimb of changes in pressure within the vascularly isolated carotid sinus have been compared with those on systemic vascular resistance in the same rabbits. With the hindlimb perfused at arterial pressure a reduction of sinus pressure from 200 mm Hg to 40 mm Hg increased hindlimb vascular resistance by only 50% compared with an increase of 100% in systemic vascular resistance. Although cardiac output was unaltered, blood flow to the hindlimb increased by 24%. However, flow to the contralateral hindlimb, which had been sympathectomized, rose by 400% and its resistance was halved. These responses to a reduction in sinus pressure show that in the absence of sympathetic tone the hindlimb has a compliant vascular bed and indicate that even though the changes in vascular resistance of the innervated limb were much less than the average systemic resistance change, there must, nevertheless, have been a substantial increase in sympathetic drive to the limb.

The effect of pregnancy on the changes in hind-limb vascular resistance following haemorrhage in the rabbit.

The change in vascular resistance of the skinned hind limb perfused at constant flow has been measured following rapid removal of 10% of the blood volume in anaesthetized pregnant rabbits (27-29 days gestation) and compared with that of non-pregnant rabbits. 5 s after haemorrhage was completed vascular resistance had risen in all rabbits, but in the pregnant ones this increase was only one-fifth of that in the non-pregnant ones. After section of either the carotid sinus and aortic nerves or the sympathetic innervation to the hind limb the increases of vascular resistance at this time were abolished. It is concluded that the previously described diminution during pregnancy of the protective function of the baroreflexes during haemorrhage results, to some extent, from their diminished role in a vascular bed not associated with the reproductive process. The blood withdrawn was not re-infused for 10 min, throughout which, in all rabbits with sinus, aortic and sympathetic nerves intact, the increase in vascular resistance was sustained. After section of the carotid sinus and aortic nerves and the sympathetic innervation there was, during this 10 min, a slow increase in vascular resistance comparable in final size to that which occurred immediately in the respective innervated preparations, i.e. it was smaller in the pregnant than in the non-pregnant rabbits. A possible difference in action during pregnancy of a vasoconstrictor substance released during haemorrhage is discussed briefly.

Effect of pregnancy in the rabbit on the pressor response to angiotensin and noradrenaline.

The pressor responses to angiotensin II injections of 29, 117 and 468 pmol/kg and infusion at 54 pmol/kg per min were compared in near-term pregnant and matched non-pregnant rabbits. The responses to noradrenaline injections of 652, 2608 and 10 432 pmol/kg and infusion at 1185 pmol/kg per min were similarly compared. At all doses of angiotensin and noradrenaline, whether by injection or infusion, the response of the pregnant animals was significantly greater. However the control arterial pressures of the pregnant rabbits were lower and fell in a range where the baroreceptor reflex might be less effective. Thus less efficient buffering could contribute to the greater response to the vasoconstrictor agents. To test the effect of pregnancy on the vascular response to angiotensin and noradrenaline when reflex and central effects were excluded, hexamethonium was used to produce autonomic ganglion blockade. The response to both vasoconstrictors was now less in the pregnant rabbits than in the non-pregnant. Thus in assessing the effects of pregnancy on the responses of the rabbit to angiotensin and noradrenaline the effect of pregnancy on mean blood pressure must be taken into account, since this can influence the extent to which the arterial baroreceptors may modulate the response.

Arterial pressure maintenance after haemorrhage in the pregnant rabbit.

The changes in arterial blood pressure and cardiac output following rapid removal of 10% of the blood volume have been compared in non-pregnant and pregnant anaesthetized rabbits close to term (28 or 29 days gestation). Immediately after the end of haemorrhage the fall in arterial pressure was greater in the pregnant group (27.7 vs. 17.8 mmHg), but as recovery proceeded the difference diminished and by the tenth minute the reduction from pre-haemorrhage levels was similar in pregnant and non-pregnant rabbits. Cardiac output fell by about 30% in both groups and increased only very slightly during the ensuing 10 min. 1 h after section of the carotid sinus and aortic nerves the haemorrhage was repeated. The fall in arterial pressure in the non-pregnant rabbits was now more than doubled (47.2 vs. 17.8 mmHg). By contrast it was little changed in the pregnant rabbits (32.0 vs. 27.7 mmHg). It is concluded that pregnancy diminishes the role of the baroreceptor reflexes in the response of the rabbit to haemorrhage. Other mechanisms, such as an increased compliance of the resistance vessels and/or a more effective chemically mediated vasoconstriction, probably become of more importance to the maintenance of arterial pressure.

Reflex response of the rabbit hind-limb muscle vascular bed to baroreceptor stimulation and its modification by pregnancy.

1. The pressure--flow relationship of the blood-perfused skinned hind limb has been compared in primigravid pregnant and virgin non-pregnant rabbits while pressure within the vascularly isolated carotid sinus was held at a series of non-pulsatile values. 2. At each level of sinus pressure the slope of the pressure--flow relationship was steeper in the pregnant animals. Thus, at all but the very lowest flows, the same flow was achieved at a lower perfusion pressure in the pregnant rabbits. 3. When hind-limb vascular resistance at constant flow was plotted against carotid sinus pressure it could be seen that at each level of sinus pressure vascular resistance was less in the pregnant rabbits. 4. The reflex responses of the hind-limb bed to changes in sinus pressure were compared in pregnant and non-pregnant animals; there was little difference in the threshold and saturation values of carotid sinus pressure. There seemed to be a small reduction in the range and gain of the reflex in the pregnant group but this was not statistically significant. 5. We have previously shown total peripheral resistance in the pregnant rabbit to be less than that in the non-pregnant rabbit at the same level of sinus pressure. The lower hind-limb vascular resistance of the pregnant rabbit at any given sinus pressure must contribute to this reduced total peripheral resistance.

The response of the hind-limb vascular bed of the rabbit to sympathetic stimulation and its modification by pregnancy.

1. Pressure-flow relationships in the hind limb, perfused with blood at a series of constant flows, have been compared in primigravid and virgin rabbits following section of the lumbar sympathetic chain. 2. In the absence of sympathetic stimulation perfusion pressure was lower (P less than 0.05) in the pregnant rabbits at all levels of flow to the leg. 3. It is concluded that pregnancy leads to a reduction in the vascular resistance of the acutely sympathectomized hind limb. 4. The pressure-flow curves of hind limbs from pregnant and non-pregnant rabbits in which the cutaneous circulation had been excluded by skinning the leg and ligating the ankle were also compared. Again, perfusion pressure at each flow level was lower in the limbs from the pregnant animals (P less than 0.05). 5. The sympathetic supply was stimulated at frequencies of 0.3, 1, 3, 7, 15 and 30 Hz. Irrespective of whether the cutaneous circulation was excluded, the pressure-flow curves for the hind limbs of pregnant and non-pregnant rabbits were different at the lower frequencies. As stimulus frequency rose this difference became smaller, and at a stimulus frequency of 7-15 Hz or more there was little difference in the position of the curves in pregnant and non-pregnant rabbits. 6. Examination of the pressure-flow curves suggests that when, as in these experiments, flow is held constant, the ability of sympathetic stimulation to increase the resistance to blood flow is reduced in pregnancy. However, if these pressure-flow curves are used to predict the change of flow that would occur at a constant perfusion pressure, the response to sympathetic stimulation appears to be greater in pregnancy. 7. Further analysis of the findings, based on certain assumptions, suggests that differences between the pressure-flow curves of pregnant and non-pregnant animals during sympathetic stimulation are due in the main to either a differing size of the unstimulated vascular beds or a differing compliance of their vessel walls, rather than to any intrinsic alteration in their response to sympathetic stimulation.

Changes in cardiac output and total peripheral resistance during the carotid sinus baroreceptor reflex in the pregnant rabbit.

1. The reflex cardiovascular effects of changes in pressure within the isolated carotid sinus have been examined in sixteen anaesthetized pregnant rabbits. 2. Comparison of the mean results at sinus pressures of 40 and 200 mmHg showed that heart rate fell by 32.1 beats min-1 and arterial pressure was reduced by 48.2 mmHg. Cardiac output, measured by thermal dilution, fell by 25.7 ml. min-1.kg-1 and total peripheral resistance was reduced by 0-292 mmHg. ml.-1 min. kg. 3. The corresponding changes previously reported in a group of seventeen non-pregnant female rabbits were a similar fall in heart rate of 34.5 beats min-1 but significantly greater reductions in arterial pressure of 69-8 mmHg and in total peripheral resistance of 0-432 mmHg. ml.-1 min.kg. However, the fall in cardiac output of 12.6 ml.min-1.kg-1 was significantly smaller. 4. These observations show that the smaller blood pressure response of the pregnant rabbit to alterations in sinus pressure is due to a reduced change in total peripheral resistance.

The contribution of alterations in cardiac output to changes in arterial pressure reflexly evoked from the carotid sinus in the rabbit.

1. The reflex cardiovascular effects of changes in pressure within the vascularly isolated carotid sinus were examined in seventeen anaesthetized rabbits. The opposite sinus was denervated and both aortic nerves were divided, 2. Comparison of the mean values at sinus pressures of 40 and 200 mmHg showed a large reduction in systemic arterial pressure from 126 to 58 mmHg and a moderate reduction in heart rate, from 287 to 253 beats min-1. Cardiac output, measured by thermal dilution, showed only a small change, a fall from 160 to 148 ml. min-1 kg-1. 3. By contrast with this reduction in cardiac output of just over 7%, total peripheral resistance, derived by dividing mean arterial pressure by cardiac output, was halved, falling from 0-48 to 0-41 mmHg ml.-1 min kg. 4. Thus in the anaesthetized rabbit changes in cardiac output make only a small contribution to the changes in systemic pressure evoked by alterations in carotid sinus pressure. Changes in total peripheral resistance are principally responsible for the effect on systemic pressure. 5. Though the changes in output of the heart were small, there were considerable changes in the work done by the left ventricle which was approximately halved when carotid sinus pressure was raised from 40 to 200 mmHg.

The carotid sinus baroreceptor reflex in the pregnant rabbit.

1. The reflex responses to baroreceptor stimulation have been compared in eight pregnant and eight non-pregnant anaesthetized female rabbits.2. The vascularly isolated, innervated carotid sinus was exposed for 30 sec to a series of non-pulsatile pressures ranging from 30 to 230 mmHg. The contralateral sinus nerve and both aortic nerves were cut. Systemic arterial pressure and heart rate were measured at each sinus pressure.3. The range of arterial pressure change which could be evoked from the isolated innervated sinus was less in the pregnant than in the non-pregnant rabbits. Mean changes were 98 and 61 mmHg respectively. On the other hand changes in heart rate were similar in the two groups (45 and 43 beats/min respectively).4. The smaller blood pressure response in the pregnant animals resulted from a lesser rise in systemic arterial pressure at low levels of sinus pressure. At high sinus pressures the blood pressure fell to a similar level in both groups of animals.5. Pressure on the great vessels by the gravid uterus was not a factor since there was no consistent difference between the responses obtained with the rabbit lying on its back or on its side.6. Mechanisms which might be responsible for the difference found are discussed.

The vasomotor component of the carotid sinus baroreceptor reflex in the cat during stimulation of the hypothalamic defence area.

1. The effect of moderate intensities of stimulation of the hypothalamic defence area on the baroreceptor reflex has been investigated in the cat by comparing the responses of arterial blood pressure and perfusion pressure of the isolated hind-limb muscle bed perfused at constant volume inflow, when the isolated carotid sinus was subjected to a series of non-pulsatile pressures with and without simultaneous hypothalamic stimulation.2. In the absence of hypothalamic stimulation the characteristic sigmoid curves relating sinus pressure to blood pressure or muscle perfusion pressure were obtained.3. With simultaneous stimulation of the hypothalamus a similar sigmoid relationship was found. There was no evidence of any reduction in the over-all power or maximum sensitivity of the baroreceptor reflex.4. However, in those cats which had been atropinized to abolish the cholinergically mediated muscle vasodilatation, the curves obtained during hypothalamic stimulation were displaced in such a manner as to suggest that, while baroreceptor modulation of vasoconstrictor tone continued during defence area stimulation, the blood pressure regulating mechanism had been ;reset' so that, within the physiological range of sinus pressures, any given level of sinus pressure was associated with a greater vasoconstrictor tone.5. In non-atropinized cats there was little displacement of the curves relating sinus pressure to blood pressure, while the curves relating sinus pressure to muscle perfusion pressure were displaced in the opposite direction so that over-all muscle vascular resistance was less than normal at each level of sinus pressure.

Modification of the reflex response to stimulation of carotid sinus baroreceptors during and following stimulation of the hypothalamic defence area in the cat.

1. The effects of stimulation of the hypothalamic defence area on carotid sinus baroreceptor reflexes have been investigated by examining the cardiovascular responses to a 15 sec period of increased pressure within the vascularly isolated carotid sinus before, during, and immediately following a 25 sec period of hypothalamic stimulation.2. Identification of the hypothalamic defence area was based on the occurence of atropine-sensitive muscle vasodilatation. Electrode positions were confirmed by histological examination.3. During hypothalamic stimulation the reflex fall in blood pressure resulting from a rise in sinus pressure was found to be undiminished whether sinus pressure was raised at the onset or at the 10th sec of hypothalamic stimulation.4. By contrast, in at least half the cats in which a reflex bradycardia could be evoked from the sinus, this bradycardia was largely if not completely suppressed during hypothalamic stimulation. This suppression of reflex bradycardia occurred when sinus pressure was raised at the onset as well as at the 10th sec of stimulation.5. During the first 5 sec after hypothalamic stimulation the hypotensive response to an increase in carotid sinus pressure was much reduced; on the other hand the reduction in heart rate was exaggerated, sometimes to a very marked degree.6. The results suggest that stimulation of the hypothalamic defence area can modify baroreceptor reflexes and that this modification can include selective alterations in the various components of the reflex response.