Measurement of the oxygenation status of the isolated perfused rat heart using near infrared detection

Regional cardiac tissue oxygenation as a function of blood flow and pO2: A near-infrared spectroscopic imaging study

Near-infrared spectroscopic imaging (NIRSI) is useful to assess cardiac tissue oxygenation in arrested and beating hearts, and it shows potential as an intraoperative gauge of the effectiveness of bypass grafting. The purpose of this study was to determine whether NIRSI can reliably differentiate among a range of cardiac oxygenation states, using ischemia and hypoxia models independently. An ischemia-reperfusion model was applied to isolated, beating, blood-perfused porcine hearts, in which the left anterior descending (LAD) artery was cannulated. LAD flow was decreased stepwise to approximately 50, 20, and 0% of normal flow and was completely restored between ischemic episodes. Upon completion of the ischemia-reperfusion protocol, the hearts were further subjected to periods of increasingly severe global hypoxia. Regional oxy- and deoxy-hemoglobin (myoglobin) levels were derived from spectroscopic images (650 to 1050 nm) acquired at each step. Oxygenation maps vividly highlighted the area at risk for all degrees of ischemia. Oxygenation values differed significantly for different LAD flow rates, regardless of whether intermediate reperfusion was applied, and oxygenation values during progressive hypoxia correlated well with blood oxygen saturation. These results suggest that NIRSI is well suited, not only to identify ischemic or hypoxic regions of cardiac tissue, but also to assess the severity of deoxygenation.

Microvascular and interstitial PO(2) measurements in rat skeletal muscle by phosphorescence quenching

To clarify the transport of O(2) across the microvessels in skeletal muscle, we designed an intravital laser microscope that utilizes a phosphorescence quenching technique to determine both the microvascular and tissue PO(2). After we injected the phosphorescent probe into systemic blood, phosphorescence excited by a N(2)-dye pulse laser was detected with a photomultiplier over a 10 microm in diameter area. In vitro and in vivo calibrations confirmed that the present method is accurate for PO(2) measurements in the range of 7-90 Torr (r = 0.958) and has a rapid response time. This method was then used to measure the PO(2) of microvessels with different diameters (40-130 microm) and of interstitial spaces in rat cremaster muscle. These measurements showed a significant drop in PO(2) in the arterioles after branching (from 74.6 to 46.6 Torr) and the presence of a large PO(2) gradient at the blood-tissue interface of arterioles (15-20 Torr). These findings suggest that capillaries are not the sole source of oxygen supply to surrounding tissue.

Dynamics of tissue oxygenation in isolated rabbit heart as measured with near-infrared spectroscopy

We investigated the role of myoglobin (Mb) in supplying O2 to mitochondria during transitions in cardiac workload. Isovolumic rabbit hearts (n = 7) were perfused retrogradely with hemoglobin-free Tyrode solution at 37 degrees C. Coronary venous O2 tension was measured polarographically, and tissue oxygenation was measured with two-wavelength near-infrared spectroscopy (NIRS), both at a time resolution of approximately 2 s. During transitions to anoxia, 68 +/- 2% (SE) of the NIRS signal was due to Mb and the rest to cytochrome oxidase. For heart rate steps from 120 to 190 or 220 beats/min, the NIRS signal decreased significantly by 6.9 +/- 1.3 or 11.1 +/- 2.1% of the full scale, respectively, with response times of 11.0 +/- 0.8 or 9.1 +/- 0.5 s, respectively. The response time of end-capillary O2 concentration ([O2]), estimated from the venous [O2], was 8.6 +/- 0.8 s for 190 beats/min (P < 0.05 vs. NIRS time) or 8.5 +/- 0.9 s for 220 beats/min (P > 0.05). The mean response times of mitochondrial O2 consumption (VO2) were 3.7 +/- 0.7 and 3.6 +/- 0.6 s, respectively. The deoxygenation of oxymyoglobin (MbO2) accounted for only 12-13% of the total decrease in tissue O2, with the rest being physically dissolved O2. During 11% reductions in perfusion flow at 220 beats/min, Mb was 1.5 +/- 0.4% deoxygenated (P < 0.05), despite the high venous PO2 of 377 +/- 17 mmHg, indicating metabolism-perfusion mismatch. We conclude that the contribution of MbO2 to the increase of VO2 during heart rate steps in saline-perfused hearts was small and slow compared with that of physically dissolved O2.