Diferències respiratòries entre dones i homes

Mentre treballàvem per trobar una manera de conèixer amb antelació la resistència a l’altitud dels alpinistes i trescadors, propòsit aconseguit només a mitges, vàrem trobar algunes dades interessants.
Òbviament, dones i homes augmentaven la ventilació per minut (VEM) en exercici. Però mentre que les dones augmentaven més la freqüència respiratòria (FR) els homes augmentaven més el volum corrent (VC). Com que VEM és igual a FR multiplicat per VC, resulta que tots dos respiren més, però tenen diferents estratègies front a l’exercici.
Curiosa característica. Es va publicar al congrés internacional d’Arica com ja s’explicava fa unes setmanes.
Peró hi havia més diferències. En repòs i a nivell del mar les dones tenien una saturació d’oxígen de l’hemoglobina de la sang un 0,7 – 0,8 % més elevada. Com que ja és sabut que els homes tenen l’hemoglobina més elevada i major afinitat de l’hemoglobina per l’oxígen que les dones, aquesta dada semblava una forma d’entendre que els equilibris de la funció respiratòria sòn diferents entre les unes i els altres.
Té això importància?
En el dia a dia, segurament no en té gaire i tant serveix una cosa com l’altra. Peró quan es tracta de malalties severes, de competicions a alt nivell o ascensions a gran altitud que porten l’organisme al límit, aquest coneixement pot fer més acurat el consell mèdic que podem donar.
Així es va publicar.
Interessats en trobar les diferències fisiològiques entre els homes i les dones de forma que en cada cas es pugui millorar el consell mèdic responent a cada persona segons les seves característiques.


A Ricart, T Pagés, G Viscor, C Leal, J L Ventura.

  • Servei de Medicina Intensiva, Hospital Universitari de Bellvitge, Barcelona. 
  • Departament de Fisiologia, Facultat de Biologia, Universitat de Barcelona, Barcelona.
  • Institut d’Estudis de Medicina de Muntanya, Barcelona.

The difference between genders has generated increasing interest in recent years. It is well known that women and men show differences in their respiratory system: different red blood cell counts, haemoglobin and 2,3-diphosphoglycerate plasma concentrations. Recently, further differences have been found in the ventilatory response to hypoxia and exercise and the evolution of some respiratory illnesses. In this study it was found that during rest at sea level, the haemoglobin oxygen saturation, as measured by pulse oxymetry, is slightly higher in women than in men (98.6 (SD 1.1)% versus 97.9 (SD 0.9)%; p=0.001). These findings are consistent with other studies, which found gender differences in the transcutaneous or tissue PaO2. The difference in oxygen saturation is not related to differences in ventilation. The disparity is modest and does not seem to produce great differences in the oxygen content of arterial blood, but combined with the different affinity of haemoglobin for oxygen or different metabolic rate, may play a role in the course of elite competition sports, high altitude ascents or the evaluation of critically ill patients. Further studies are needed to establish the degree, extent and clinical importance of these differences in the saturation of haemoglobin.

The social changes experienced by developed societies have contributed to the growth of medical interest in the differences between the sexes, from both a physiological and clinical perspective. It is known that there are differences between men and women in the evolution of some respiratory disorders (1,2,3). Differences have also been described in transcutaneous and tissue PaO2, in oxygen transport, in the control of respiration, and in the affinity of haemoglobin for oxygen (4, 5, 6, 7, 8). Pulse oxymetry is an accredited method that is used almost universally in critically ill patients. The widespread use of this method has led to the application of pulse oxymetry data for studying oxygen transport or making decisions on patients in a critical state. However, for the data obtained by any method to be reliable, it is necessary to have in-depth knowledge of their characteristics and all the variables that can affect the results. Many factors are known to affect pulse oxymetry results. Among these factors are: severe anaemia, interference with other electrical apparatus, patient movements, some intravenous contrasts, peripheral perfusion disturbances, right cardiac insufficiency and the dyshaemoglobinaemias.
To our knowledge, no previous work has been published describing differences between the sexes in pulse oxymetry.

In this basic physiological study, there was no conflict of interest on the part of the authors or any financial gain to bemade or any obligation regarding the results. All the subjects were volunteers and were informed of the experimental protocol.
Two experiments were carried out. Experiment 1 refers to results obtained in the course of medical check-ups of alpinists or travellers who were preparing for ascents to altitude. To ratify the obtained results, experiment 2 was carried out, increasing the sample number.
Experiment 1
159 subjects were studied: 36 women (mean (SD) age 33.7 (8.5) years (range 16–57 years), weight 59.1 (6.7) kg, lean body mass 45.5 (5.20) kg, height 163.8 (5.8) cm) and 123 men (age 36.9 (10) years (range 18–64 years), weight 74.9 (9.7) kg, lean body mass 63.7 (8.18) kg, height 174.6 (6.4) cm). We measured the oxygen saturation (SaO2) of the haemoglobin by pulse oxymetry of the index finger (Onyx model 9500, Nonin Medical, Minneapolis, MN, USA), the minute expired volume (MEV) and the tidal volume (Vt), both corrected for the weight and lean body mass of each subject, and the respiratory frequency (RR) (spirometer model 5420, Datex-Ohmeda, Stirling, UK).
Since basal oxygen consumption is linked to the lean body mass and not to the adipose tissue, to allow for the differences in the corporal make-up between men and women, we corrected the ventilatory data according to the theoretical lean body mass (77% of the total weight of the women and 85% of the total weight of the men). Since some authors relate ventilation to the levels of sex hormones, all of the calculations were re-done, excluding those subjects who were not between 16 and 40 years of age. No data were obtained on the menstrual cycle in the women. The measurements were done after 5 min of repose, between 17:00 and 19:00 hours, at sea level, seated, with the subjects connected to the spirometer and pulse oxymeter simultaneously. 
Experiment 2
209 subjects were studied: 132 women and 77 men. The subjects were young, healthy university students between 19 and 22 years of age. SaO2measurements were made during class hours (morning and afternoon) using the index finger and in a sitting position. No other measurements were taken. A different pulse oxymeter was used to discard a possible bias due to the characteristics of the apparatus (model Pulsox-5, Minolta Co., New Jersey, USA).
Statistical analysis
The samples were compared using a Student t test for unpaired values or the Mann-Whitney rank sum test when the values did not fit a normal distribution; values were considered significant when p,0.05.

Experiment 1
The SaO2 was 98.3 (SD 0.9)% in women and 97.8 (SD 1.0)% in men (Mann-Whitney rank sum test: p=0.041). Ventilatory data are shown in tables 1 and 2.

Experiment 2
The SaO2 was 98.6 (SD 1.1)% in women and 97.9 (SD 0.9)% in men (Mann-Whitney rank sum test: p=0.001).

The fact that the pulse oxymeter shows sex-linked differences does not necessarily mean that men and women have a different arterial SaO2. However, in a study carried out in 1988 to map transcutaneous PaO2 in humans, significant differences were found between men and women in transcutaneous PaO2 throughout the body (6). Higher values were recorded in women. Although the authors dispatched the findings with a terse commentary, attributing it to differences in skin or its vascularisation, the data was recorded and is at the disposition of the scientific community.
However, prior to this it had been stated that transcutaneous PaO2 does not depend on skin characteristics (7). Later, anaesthesiologists working with neurosurgical patients found differences in the PaO2 of cerebral tissue between men and women (8). Thus, while pulse oxymetry has its limitations, another method, such as the measurement of the partial pressure of transcutaneous or tissue oxygen, shows results in agreement with those obtained in our study, since the greater the PaO2, the greater the SaO2, according to the known scheme of the saturation curve of haemoglobin that correlates both parameters. Although the plasma and the tissue PaO2 are not equal, they are related; hence, unless both methods were inaccurate, the SaO2 of women must be somewhat higher than that of men. Later studies showed that the haemoglobin dissociation curve is different (at the same temperature and pH) in the two sexes and that women present less haemoglobin affinity for oxygen in comparison with men (the P50 was 2 mmHg and the 2,3-diphosphoglycerate was 2 mmol/g of haemoglobin higher in women). This difference between the sexes did not exist in preadolescents or after menopause (9). The subjects in our study did not show ventilatory differences between the sexes, even in the young subjects. Therefore, the differences in respiratory physiology between the sexes must be based on oxygen transport or the cellular respiratory phase. In the studies mentioned above it was suggested that the sex hormones influence the development of red blood cells, producing significant changes in the cellular phase of respiration, which is coherent with our results.
Given that it was not the initial aim of the study, we did not obtain data on the menstrual cycle of the women, who ventilate more in the luteal phase (10). However, we accept the fact that there may be ventilatory differences depending on the stage of the menstrual cycle. 
These small differences do not appear to be relevant in basal situations (health, repose, at sea level), but may be in extreme situations (illness, effort, altitude) when the PaO2 falls off in the haemoglobin dissociation curve. For example, it may be of relevance when important diagnostic or therapeutic decisions need to be made (levels of severity, for wearing off recommending mechanical ventilation, programmes for weaning off mechanical ventilation), in sports medicine (different training programmes, medical advice for ascents at altitude) and when information on the physiological differential between the sexes is required.
Further studies are necessary to establish the degree of clinical importance that can be attached to the small differences in oxygen saturation of haemoglobin measured by pulse oxymetry in men and women.

In this study it was found that during rest at sea level, the haemoglobin oxygen saturation, as measured by pulse oxymetry, is slightly but significantly higher in women than in men.
Competing interests: None.

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7. Rafferty TD, Morrero O. Skin-fold thickness, body mass and obesity indexes and the arterial to skin-surface PO2 gradient. Arch Surg 1983;118:1142.
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10. Muza SR, Rock PB, Fulco CS, et al. Women at altitude: ventilatory acclimatization at 4300 m. J Appl Physiol 2001;91:1791–9.

Referència: British Journal Sports Medicine 2008;42:620–621. 

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