No abstract

Patients with sickle cell disease usually have mild hypoxaemia and their oxyhaemoglobin dissociation curve is shifted to the right. It follows that oxygen saturation in sickle cell disease should be lower than normal. Most subjects in this clinic had normal oxygen saturation by pulse oximetry, however. To improve the understanding of this paradox, arterialised capillary oxygen tension (Po2) and oxygen saturation were compared with simultaneously measured pulse oximeter saturation in 20 children with sickle cell disease. In addition, the Po2 at 50% haemoglobin saturation (Pso) was compared with saturation measured by pulse oximetry in all 20 patients. It was found that saturation measured by pulse oximetry was, on the whole, similar to that calculated from the sampled blood. Individual deviations were not random, however, and were partly explained by (IL 237). With the water bath preheated to 37°C and kept constant throughout, a test gas was pumped through each cuvette at 400 ml/min. The test gases used were 5% carbon dioxide balance nitrogen and 5% carbon dioxide balance oxygen such that blood in one tonometer would be fully saturated while that in the other desaturated (co-oximeter So2 <5%). After 15 minutes of blood-gas equilibration aliquots of blood were withdrawn from each cuvette in varying ratios such that total volume was approximately 0 5 ml. A few stainless steel pellets were added to the syringe and, with the outlet sealed, the syringe was shaken to ensure adequate mixing of saturated and desaturated blood. We avoided prolonged periods of desaturation in the process, or re'eated deoxy-oxygenation cycles, as these have been shown to lead to irreversible biophysical changes in red cells with sickle haemoglobin (HbS).5 The blood was then analysed for pH, Po2, and carbon dioxide tension (Pco2) (Radiometer ABL30) and oxygen saturation (Radiometer OSM2 Hemoximeter). The Po2 value obtained was corrected to a standard pH of 7 40 using an IL Po2 pH nomogram (Instrumentation Laboratory). The points obtained were then plotted using Po2 on the abscissa and Sao2 on the ordinate. P50 (the Po2 at 50% saturation) was determined using three to four points in the 40-60% saturation range by linear interpolation. Using a larger amount of blood from four of these patients the entire oxyhaemoglobin dissociation curve was plotted, fitting a sigmoid curve by eye, to eight to 10 data points for each patient. To obtain better resolution for curve fitting, two oxyhaemoglobin dissociation curves were constructed by combining Po2 and S02 data from patients with similar P50 values. This was carried out to verify that there was a right shifted oxyhaemoglobin dissociation curve in our patient group. To validate our methodology, blood from two controls (HbAA) was also obtained for 735 on 9 May 2018 by guest. Protected by copyright.

Adults with sickle cell anemia (SCA) have restrictive lung impairment, increased alveolar dead space, and hypoxemia. These factors, together with increased anaerobic metabolism, are thought to cause exercise hyperventilation. To assess the role of each of these in children, 34 patients with SCA and 16 control subjects performed pulmonary function and exercise tests. Twenty-eight patients with SCA had spirometric values and lung volumes, and all but two patients with SCA had arterial saturation greater than 91% during exercise. Despite a low VO2max (30.07 +/- 6.55 ml/min/kg), the ventilatory anaerobic threshold (VAT) in the patients occurred at a similar %VO2max as in the control subjects (69 +/- 9% versus 63 +/- 12%). The slope of the delta VE/delta VCO2 relationship for sub-VAT work was steeper in the patients (29.4 +/- 6.5 versus 24.7 +/- 5.2, p = 0.01), and the ventilatory equivalent for CO2 (VE/VCO2) in steady-state exercise was greater in the patients than in the control subjects (33.2 +/- 3.5 versus 30.8 +/- 3.5, p = 0.03). End-tidal PCO2 did not differ (38.3 +/- 3.0 versus 39.2 +/- 3.1), indicating equivalent alveolar ventilation. The patients had a higher dead space:tidal volume ratio (VD/VT) than did the control subjects (0.204 +/- 0.033 versus 0.173 +/- 0.024, p = 0.0005). The PaCO2 was significantly lower in those with lower Hb, but there was no difference in pH. In conclusion, children with SCA have an increased exercise ventilatory response caused in part by increased physiologic dead space, and in part by their low Hb. The greater dead space may be the result of sickle cells impairing capillary perfusion to ventilated alveoli.

Desaturation in patients with sickle cell anemia (SCA) can lead to intravascular sickling and vascular occlusion. The increased metabolic demands of exercise tend to increase oxygen extraction, giving rise to a fall in saturation in the capillary bed that may predispose to sickling. This could be minimized with an increase in cardiac output. The aims of this study were to assess the role of increased stroke volume (SV) in augmenting cardiac output (Q) and to estimate the role of enlarged arteriovenous O2 content difference in maintaining O2 transport in children with SCA. A group of 30 children with SCA (Hb 65 to 133 g/L) and 16 healthy controls of the same racial group and of similar height and weight performed incremental and steady-state exercise at 50% Wmax. Cardiac output (Q) was measured by the indirect (CO2) Fick method during steady state. The slope of delta HR/delta VO2 during incremental exercise was higher in SCA subjects compared with controls (4.01 +/- 1.73 versus 2.80 +/- 0.61 bpm per ml/min/kg VO2, p = 0.001). Q for VO2 was abnormally high in patients, particularly older ones with lower Hb levels. HR (% predicted) was higher in patients than in controls (106 +/- 11 versus 92 +/- 8% predicted, p less than 0.0001), as was SV (113 +/- 16 versus 98 +/- 14% predicted, p = 0.002). Multiple linear regression of Q % predicted and SV % predicted on Hb and age showed a positive correlation with age and a negative correlation with Hb (r = 0.84 for Q and r = 0.76 for SV).(ABSTRACT TRUNCATED AT 250 WORDS)