ABSTRACT
The delivery of oxygen by arterial blood to the tissues of the body has a number of critical determinants including blood oxygen concentration (content), saturation (S O2 ) and partial pressure, haemoglobin concentration and cardiac output, including its distribution. The haemoglobin-oxygen dissociation curve, a graphical representation of the relationship between oxygen satur-ation and oxygen partial pressure helps us to understand some of the principles underpinning this process. Historically this curve was derived from very limited data based on blood samples from small numbers of healthy subjects which were manipulated in vitro and ultimately determined by equations such as those described by Severinghaus in 1979. In a study of 3524 clinical specimens, we found that this equation estimated the S O2 in blood from patients with normal pH and S O2 >70% with remarkable accuracy and, to our knowledge, this is the first large-scale validation of this equation using clinical samples. Oxygen saturation by pulse oximetry (S pO2 ) is nowadays the standard clinical method for assessing arterial oxygen saturation, providing a convenient, pain-free means of continuously assessing oxygenation, provided the interpreting clinician is aware of important limitations. The use of pulse oximetry reduces the need for arterial blood gas analysis (S aO2 ) as many patients who are not at risk of hypercapnic respiratory failure or metabolic acidosis and have acceptable S pO2 do not necessarily require blood gas analysis. While arterial sampling remains the gold-standard method of assessing ventilation and oxygenation, in those patients in whom blood gas analysis is indicated, arterialised capillary samples also have a valuable role in patient care. The clinical role of venous blood gases however remains less well defined.
ABSTRACT
INTRODUCTION: Insulin resistance is implicated in the pathogenesis of polycystic ovarian syndrome (PCOS). Insulin-sensitizing agents are increasingly used in the treatment of infertility and hirsutism in PCOS. However, not all women with PCOS are insulin-resistant. OBJECTIVE: To assess the degree of insulin resistance within a clinic population of women referred for treatment of oligomenorrhoea or infertility. DESIGN: We evaluated 25 consecutive PCOS outpatients referred for treatment of menstrual dysfunction/infertility and a matched control group. All underwent a standard oral glucose tolerance test (OGTT) with serial insulin measurements. Insulin sensitivity was calculated using homeostasis model assessment (HOMA). RESULTS: Five of the 25 clinic patients had abnormal glucose handling (two had previously unknown type 2 diabetes and three had impaired glucose tolerance). Fasting and 2-h insulin levels were significantly higher in the PCOS women. Mean HOMA-S (insulin sensitivity) was even lower for PCOS women with normal GTT status (mean (95% confidence interval): 0.53 (0.34-0.72)) than for controls (0.94 (0.84-1.04)) (F = 4.2, p < 0.001). HOMA-B (pancreatic beta-cell function) was nearly tripled for normal GTT status PCOS women at 273 (205-342) versus 105 (70-139) for controls (F = 6.8, p < 0.001). CONCLUSIONS: The results suggest a role for routine measurement of HOMA-S in identifying women with PCOS with insulin resistance with a view to targeting them with insulin-sensitizing agents.
