An extensive literature review was performed to compare the data from several studies focusing on responses and outcomes between the use of pH stat or alpha stat blood gas strategy during pediatric hypothermic cardiopulmonary bypass.
To achieve pH stat blood gas management during pediatric hypothermic cardiopulmonary bypass, the addition of carbon dioxide to the oxygenator may be necessary.1 Modification of the air/oxygen mixer with the addition of a carbon dioxide flow meter is a solution to adding carbon dioxide to the cardiopulmonary bypass circuit. An in-line blood gas analyzer, capable of operating in pH stat or alpha stat modes, provides immediate detection for modification of air/oxygen/carbon dioxide parameters. Therefore, pH stat can be clinically achieved with the addition of carbon dioxide and the use of an in-line blood gas analyzer.
The use of alpha stat versus pH stat blood gas management during hypothermic cardiopulmonary bypass has been greatly debated because of theoretical advantages and disadvantages of each technique. In the 1960s and 1970s, pH stat was widely accepted for cardiopulmonary bypass; the shift to alpha stat occurred in the 1980s after results of cold-blooded vertebrate research studies were published. The alpha stat method was also widely accepted because of ease of clinical use without the need for additional carbon dioxide.
The theory for alpha stat management follows results of physiologic studies of cold-blooded vertebrates. Blood gases are managed at 37°C, with a pH of 7.40 and a pCO2 of 40 mm Hg, despite the patient temperature. This method follows the normal shift of the oxyhemoglobin dissociation curve. Alpha stat management preserves autoregulation of cerebral vasculature and intracellular pH. Conversely, pH stat philosophy follows the behavior of hibernating mammals. Blood gases are managed at the patient temperature, with a pH of 7.40 and a pCO2 of 40 mm Hg. To maintain these results, carbon dioxide must be added to the oxygenator. The use of pH stat causes a rightward shift of the oxyhemoglobin dissociation curve.2
Several investigators have focused on the differences between alpha stat and pH stat during hypothermic cardiopulmonary bypass. Duebener et al.3 studied the effects of alpha stat and pH stat management on the diameter of the cerebrocortical microvessels in piglets. During cooling, the diameter of the alpha group decreased (89% ± 11%), while the pH group significantly increased (132% ± 13%) compared with baseline diameters. During the first 10 minutes of rewarming, the vessels in the pH group were also significantly larger. The pH group also had significantly greater tissue oxygenation at the end of cooling (p = 0.008). This study concluded that pH stat was the preferred method for patients requiring deep hypothermia, because of increased cerebral tissue oxygenation.
Nagy et al.4 studied the effects of blood gas management on serum troponin-T levels in 101 newborns and children to mark myocardial damage. Cardiac troponin-T and cTnT levels were measured before bypass and then 30 minutes and 4 and 24 hours after bypass. The cTnT levels were significantly higher in the alpha stat group at 30 minutes and 4 and 24 hours after bypass (p = 0.01, 0.02, and 0.01, respectively). The ventilation time (55.9 ± 50.5 hours vs. 35.6 ± 22.1 hours) and length of intensive care unit stay (4.03 ± 3.09 days vs. 2.75 ± 1.52 days) were significantly longer in the alpha stat group compared with the pH stat group. These results suggest that pH stat may provide better myocardial protection to an overloaded or hypoxic pediatric heart, or in techniques involving long ischemic times or circulatory arrest.
Perioperative outcomes between alpha stat and pH stat blood gas managements were compared by du Plessis et al.5 for infants requiring deep hypothermic cardiopulmonary bypass for corrective surgery. The pH stat strategy was associated with shorter recovery time to electroencephalographic activity (p = 0.03), lower postoperative morbidity (p = 0.058), shorter intubation times in D-transposition patients (p = 0.01), and length of intensive care unit stays (p = 0.01).
Bellinger et al.6 studied the effects of alpha stat and pH stat on developmental and neurologic outcomes after deep hypothermic cardiopulmonary bypass in infants. Psychomotor Development Index scores of 110 patients did not differ significantly between the groups (p = 0.97). The results of the Mental Development Index scores were dependent on diagnosis. In all but the ventricular septal defect subgroup, the pH stat group had nonstatistically higher Mental Development Index scores. Abnormalities on electroencephalogram (p = 0.77) and neurologic examination (p = 0.70) were also not significantly different between the methods of blood gas management. Bellinger et al.6 concluded that the use of alpha stat or pH stat strategy is not associated with improved or impaired early neurodevelopmental outcomes in infants undergoing deep hypothermic cardiopulmonary bypass.
The use of pH stat during pediatric hypothermic bypass has been favored because of the results from several animal and patient studies.7,8 Both techniques are manageable with modification to the air/oxygen blender and use of an in-line blood gas analyzer. The pH stat strategy is associated with increased tissue oxygenation and cerebral blood flow while cooling on cardiopulmonary bypass. Data also suggests that pH stat management results in better patient outcomes with shorter ventilation times and intensive care unit stays following pediatric cardiac surgery. However, there does not seem to be a significant difference in long-term outcomes of infants or children.
1. Calvert KS, Gustafson RA, Rosen DA: A simple technique of pH-stat strategy for infants undergoing deep hypothermic circulatory arrest. Ejournal of Perfusion Technology. Available: perfline.com.ejournal/2002/Kca0102.html
. Accessed May 10, 2005.
2. Varjavand N, Kaye JM, Wang S, Primiano FP: The interactive oxyhemoglobin dissociation curve. Available at: Ventworld.com/resources/oxydisso
. Accessed May 10, 2005.
3. Duebener LF, Hagino I, Sakamoto T, et al: Effects of pH management during deep hypothermic bypass on cerebral microcirculation: Alpha stat versus pH-stat. Circulation 106: I103, 2002.
4. Nagy ZL, Collins M, Sharpe T, et al: Effect of two different bypass techniques on the serum troponin-T levels in newborns and children. Circulation 108: 577, 2003.
5. du Plessis AJ, Jonas RA, Wypij D, et al: Perioperative effects of alpha-stat versus pH-stat strategies for deep hypothermic cardiopulmonary bypass in infants. J Thorac Cardiovasc Surg 114: 991–1000, 1997.
6. Bellinger DC, du Plessis AJ, Rappaport LA, et al: Developmental and neurologic effects of alpha-stat versus pH-stat strategies for deep hypothermic cardiopulmonary bypass in infants. J Thorac Cardiovasc Surg 121: 374–383, 2001.
7. Sakamoto T, Zurakowski D, Duebener LF, et al: Interaction of temperature with hematocrit level and pH determines safe duration of hypothermic circulatory arrest. J Thorac Cardiovasc Surg 128: 220–232, 2004.
8. Priestley MA, Golden JA, O’Hara IB, et al: Comparison of neurologic outcome after deep hypothermic circulatory arrest with alpha-stat and pH-stat cardiopulmonary bypass in newborn piglets. J Thorac Cardiovasc Surg 121: 336–343, 2001.