Effect of deep hypothermic circulatory arrest followed by low-flow cardiopulmonary bypass on brain metabolism in newborn piglets: comparison of pH-stat and α-stat management

Abstract

Objective: To compare the effects of pH-stat and α-stat management before deep hypothermic circulatory arrest followed by a period of low-flow (two rates) cardiopulmonary bypass on cortical oxygenation and selected regulatory proteins: Bax, Bcl-2, Caspase-3, and phospho-Akt.

Design: Piglets were placed on cardiopulmonary bypass, cooled with pH-stat or α-stat management to 18 °C over 30 mins, subjected to 30-min deep hypothermic circulatory arrest and 1-hr low flow at 20 mL/kg/min (LF-20) or 50 mL/kg/min (LF-50), rewarmed to 37 °C, separated from cardiopulmonary bypass, and recovered for 6 hrs.

Subjects: Newborn piglets, 2-5 days old, assigned randomly to experimental groups.

Interventions: None.

Measurements and main results: Cortical oxygen was measured by oxygen-dependent quenching of phosphorescence; proteins were measured by Western blots. The means from six experiments ± sem are presented as % of α-stat. Significance was determined by Student's t test. For LF-20, cortical oxygenation was similar for α-stat and pH-stat, whereas for LF-50, it was significantly better using pH-stat. For LF-20, the measured proteins were not different except for Bax in the cortex (214 ± 24%, p = .006) and hippocampus (118 ± 6%, p = .024) and Caspase 3 in striatum (126% ± 7%, p = .019). For LF-50, in pH-stat group: In cortex, Bax and Caspase-3 were lower (72 ± 8%, p = .001 and 72 ± 10%, p = .004, respectively) and pAkt was higher (138 ± 12%, p = .049). In hippocampus, Bcl-2 and Bax were not different but pAkt was higher (212 ± 37%, p = .005) and Caspase 3 was lower (84 ± 4%, p = .018). In striatum, Bax and pAkt did not differ, but Bcl-2 increased (146 ± 11%, p = .001) and Caspase-3 decreased (81 ± 11%, p = .042).

Conclusions: In this deep hypothermic circulatory arrest-LF model, when flow was 20 mL/kg/min, there was little difference between α-stat and pH-stat management. However, for LF-50, pH-stat management resulted in better cortical oxygenation during recovery and Bax, Bcl-2, pAk, and Caspase-3 changes were consistent with lesser activation of proapoptotic signaling with pH-stat than with α-stat.

Figure 1. Comparison of cortical oxygenation when using pH-stat or alpha-stat management during cooling, DHCA, low flow of 20 ml/kg/min and post-bypass recovery

Histograms of oxygen distribution in the microcirculation of the cortex were determined by the distribution of phosphorescence lifetimes, as described in Methods. The presented histograms are for control conditions, at the end of the 60 min of low flow, and after 60 and 120 min of recovery. The histograms are for 6 independent experiments presented as the mean ± SEM for each oxygen pressure.

Figure 2. Comparison of cortical oxygenation when using pH-stat or alpha-stat management during cooling, DHCA, low flow of 50 ml/kg/min and post-bypass recovery

Histograms of the oxygen distributions in the microcirculation of the cortex were determined by the distribution of phosphorescence lifetimes as described in Methods. The presented histograms are for control conditions, at the end of the 60 min of low flow, and after 60 and 120 min of recovery. The histograms are for 6 independent experiments presented as the mean ± SEM for each oxygen pressure.

Figure 3. Expression of Bax, Bcl-2, phospho-Akt, and Caspase-3 in frontal cortex of newborn piglets: comparison pH-stat and alpha-stat management

The means for DHCA with pH-stat (n=6) ± SEM expressed in % to the corresponding values obtained for the alpha-stat group of piglets (n=6). p values for significant difference from alpha-stat values as determined by two tailed t-test. p < 0.05 was considered statistically significant.

Figure 4. Expression of Bax, Bcl-2, phospho-Akt, and Caspase-3 in hippocampus of newborn piglets: comparison pH-stat and alpha-stat management

The means for DHCA with pH-stat (n=6) ± SEM expressed in % to the corresponding values obtained for the alpha-stat group of piglets (n=6). p values for significant difference from alpha-stat values as determined by two tailed t-test. p < 0.05 was considered statistically significant.

Figure 5. Expression of Bax, Bcl-2, phospho-Akt, and Caspase-3 in striatum of newborn piglets: comparison pH-stat and alpha-stat management

The means for DHCA with pH-stat (n=6) ± SEM expressed in % to the corresponding values obtained for the alpha-stat group of piglets (n=6). p values for significant difference from alpha-stat values as determined by two tailed t-test. p < 0.05 was considered statistically significant.