During pH-stat acid-base management, the patient’s pH is maintained at a constant level by managing pH at the patient’s temperature. pH-stat pH management is temperature-corrected. Compared to alpha-stat, pH stat (which aims for a pCO2 of 40 and pH of 7.40 at the patient’s actual temperature) leads to higher pCO2 (respiratory acidosis), and increased cerebral blood flow. Often CO2 is deliberately added to maintain a pCO2 of 40 mm Hg during hypothermia.
During alpha-stat acid-base management, the ionization state of histidine is maintained by managing a standardized pH (measured at 37C). Alpha-stat pH management is not temperature-corrected – as the patient’s temperature falls, the partial pressure of CO2 decreases (and solubility increases), thus a hypothermic patient with a pH of 7.40 and a pCO2 of 40 (measured at 37C) will, in reality, have a lower pCO2 (because partial pressure of CO2 is lower), and this will manifest as a relative respiratory alkalosis coupled with decreased cerebral blood flow. During alpha-stat management you have no idea what the patient’s pCO2 is, your goal is to maintain a constant dissociation state of histidine.
Alpha Stat vs. pH Stat
A study by Kiziltan et al., in which 52 patients were randomized to alpha-stat versus pH stat management, showed that pH stat management led to increased jugular venous oxygen concentrations, implying increased CBF. A study by Sakamoto et al., comparing pH stat to alpha stat during repair of cyanotic neonatal congenital heart disease, demonstrated that pH stat management led to less pulmonary collateral circulation as well as higher oxyhemoglobin and lower deoxyhemoglobin levels on cerebral near-infrared spectroscopy, suggesting greater cerebral oxygenation through improved oxygen delivery with pH stat. A prior study by Murkin et al. comparing pH stat to alpha stat showed that during pH stat, CBF and CMRO2 become uncoupled (CBF is pressure-dependent), whereas during alpha-stat CBF is related to metabolic needs (CMRO2) and not to cerebral perfusion pressure. The major concern with pH stat is the potential for increasing the cerebral embolic load.
H Tarik Kiziltan, Mehmet Baltali, Ahmet Bilen, Gülşah Seydaoglu, Muzaffer Incesoz, Atilay Tasdelen, Sait Aslamaci Comparison of alpha-stat and pH-stat cardiopulmonary bypass in relation to jugular venous oxygen saturation and cerebral glucose-oxygen utilization. Anesth. Analg.: 2003, 96(3);644-50, table of contents [PubMed:12598237]
Takahiko Sakamoto, Hiromi Kurosawa, Toshiharu Shin’oka, Mitsuru Aoki, Yukihisa Isomatsu The influence of pH strategy on cerebral and collateral circulation during hypothermic cardiopulmonary bypass in cyanotic patients with heart disease: results of a randomized trial and real-time monitoring. J. Thorac. Cardiovasc. Surg.: 2004, 127(1);12-9 [PubMed:14752407]
J M Murkin, J K Farrar, W A Tweed, F N McKenzie, G Guiraudon Cerebral autoregulation and flow/metabolism coupling during cardiopulmonary bypass: the influence of PaCO2. Anesth. Analg.: 1987, 66(9);825-32 [PubMed:3113288]