ARTERIAL HAEMOGLOBIN SATURATION REFLECTED BY CHANGES IN BLOOD OXYGEN LEVEL-DEPENDENT SIGNAL INTENSITY OF THE LEFT VENTRICULAR BLOOD POOL

Signal intensity (SI) in blood oxygen level-dependent (BOLD)-sensitive cardiovascular magnetic resonance (CMR) is dependent upon the oxygenation status of haemoglobin (Hb). Therefore, changes in arterial Hb saturation (SaO2) contribute to changes in SI. In this study, we investigated the change in SI within the blood pool of the LV cavity in comparison to the expected Hb desaturation during a one-minute period of apnea using BOLD-sensitive CMR. Arterial blood gas levels of six anaesthetized swine were manipulated through six stages consisting of paO2 levels of 100 and 80 mmHg and paCO2 levels of 30, 40 and 50 mmHg through alteration of ventilation parameters. At each stage an induced breath hold of one-minute occurred at end-expiration. Immediately prior to and after apnea, arterial blood gases (paCO2, paO2) were measured, during which BOLD-sensitive CMR images were continuously acquired using a modified SSFP cine sequence. Employing the Severinghaus method, the expected SaO2 at the beginning and end of each level was calculated with the measured arterial gases. This accounts for modifications to the Hb saturation curve by paO2, paCO2, pH and temperature. SI of the LV-blood pool was analyzed for the first and final two BOLD cines during apnea and averaged for each phase of the cardiac cycle. The mean SI of all phases was calculated and expressed as %SI change from baseline. After one-minute of apnea changes in arterial blood gases were significant (**P < 0.015) as well as in BOLD-SI (*P ≤ 0.001). The change in BOLD-SI at paO2 of 80 and 100 mmHg levels over one-minute of apnea reflect the expected ΔSaO2 at paCO2 levels of 30 and 40 mmHg, but differed at a paCO2 of 50 mmHg. Additionally the decrease in myocardial SI and SaO2 was significantly greater in the paO2 levels of 80 mmHg than in the 100 mmHg levels (#P < 0.001). The findings suggest that changes of SI in the LV blood pool of BOLD-sensitive CMR images reflect the extent of desaturation of arterial blood during one-minute apnea at low to physiologic levels of pCO2. These differences between the paO2 and paCO2 levels are expected owing to the characteristics of the oxygen-binding curve of Hb. However, there was a disparity between calculated ΔSaO2 and %SI-change at the hypercapnic levels of 50 mmHg. Consequently, the application of BOLD-CMR in the LV blood pool is promising in the non-invasive assessment of the effect of Hb saturation on BOLD-SI.Tabled 1View Large Image Figure ViewerDownload Hi-res image Download (PPT)