pH measurements are most commonly measured by blood gas analyzers. They are comprised of a:

• pH sensor: Contains a measuring and reference electrode.
• PCO₂ electrode: A pH sensitive glass probe that is surrounded by a bicarbonate solution and is enclosed by a CO₂ permeable membrane.
• PO₂ electrode: A platinum probe bathed in an electrolyte solution and separated from the blood sample by an O₂ permeable membrane (1) [A].

• pH is a measure of the hydrogen ion (H⁺) concentration in a liquid. pH is defined as the negative logarithm to the base 10 of H⁺. for example, if H⁺ equals 0.000001, then pH is equal to the negative log10[0.000001], or 6.0.
• Modern blood gas analyzers directly measure:
  • pH: Sensing (or measuring) electrodes produce an electrical signal or potential (voltage) that is directly related to the [H⁺]. Reference electrodes produce a stable potential by which the potential difference is measured and converted into a pH value.
  • CO₂ tension: CO₂ is relatively soluble in blood, and exists in other forms: HCO₃^-, CO₂, H₂CO₃ (CO₂ + H₂OH₂CO₃H⁺ + HCO₃^-). It is attached to hemoglobin as carbaminohemoglobin. These dissolved and bound forms exist in equilibrium with CO₂ in its gas form, but do not exert partial pressure and are not included in the PaCO₂ measurement.
  • Arterial oxygen tension: O₂ is poorly soluble in blood; the solubility coefficient is 0.003. Most of the O₂ is carried in blood attached to hemoglobin. Dissolve and bound forms exist in equilibrium with O₂ in its gas form, but do not exert partial pressure and are not included in the PaO₂ measurement.
  • Current equipment may also have the capability of measuring hemoglobin, electrolytes, glucose, and lactate.
  • Samples are warmed to 37°C prior to measurement.
• Modern blood gas analyzers calculate:
  • Arterial oxyhemoglobin saturation
  • Base excess: A reflection of the amount of strong acid necessary to bring the pH to 7.4
  • Bicarbonate concentration. This value is derived from the pH and PCO₂ measured values.
• Factors that can affect the accuracy of pH measurements:
  • Gas solubility: Changes in temperature affect the kinetic energy of O₂ and CO₂ and consequently the solubility of gases in blood (partial pressures). for example, decreased temperature will consequently decrease the kinetic energy of O₂ and CO₂; this increases their solubility and decreases their partial pressures (less in gas form). In addition, there is an increase in O₂ and CO₂ binding affinity to hemoglobin which further decreases the amount of oxygen and carbon dioxide in its gas form.
  • Water dissociation constant (pKw): Changes in temperature affect water's self-ionization (proton from one water molecule is transferred to another to form hydronium and hydroxide ions). H₂O + H₂O H₃O⁺ + OH^-. Water is the primary source of hydrogen ions. Thus, decreased temperature will decrease [H⁺] and hence increase the pH (alkalosis).
  • Electroneutrality: Maintained due to the compensatory pK adjustment of histidine's imidazole moieties (alpha residues) that parallel pKw. Imidazole retains its buffering capacity at all temperatures.
  • Storage time: A sample that is stored longer than 20 minutes before analyzing it can artificially change measured and calculated values, due to environmental cooling and cellular metabolism. Blood samples are warmer than the ambient temperature, and cooling can cause increased oxygen solubility, thereby reducing PO_2. In addition, as cellular metabolism ensues, PCO₂ increases and PO₂ and pH decrease.
  • Number of cells in the sample: Increases in the WBC, platelet, or hemoglobin levels result in increased metabolism. To reduce this effect, samples are normally placed on ice after they are obtained to maintain their stability and the addition of sodium fluoride or cyanide can reduce cellular oxygen consumption.
• Cardiopulmonary bypass (CPB) management presents a unique dilemma due to the implementation of significant hypothermia. To that extent, two techniques of management are utilized. Controversy exists regarding the most optimal methods. Blood gas samples need to be warmed to 37°C prior to measurement. Whether these values are then inserted into a table or nomogram in order to temperature "correct" for partial pressures and pH to the patient's actual temperature is what is referred to as alpha stat and pH stat measurements.
  • pH stat utilizes a nomogram or table to adjust to the patient's current hypothermic temperature. The readings, therefore, will reflect in vivo partial pressures and pH (reduced partial pressures, increased pH). Management based upon these values will result in the addition of CO₂ to the CPB pump in order to increase PaCO₂ and reduce pH.
  • Alpha stat pH management does not utilize a nomogram or table to adjust to the patient's hypothermic temperature. This is referred to as an "uncorrected" temperature system. Thus, although in vivo partial pressures may be lower and pH higher than measured values, management using this technique focuses on "total" content and does not add CO₂ to the CPB pump or affect electroneutrality.
• Transcutaneous blood gas analysis is a continuous, non-invasive method to monitor blood gas levels. The device induces hyperperfusion by local heating and PtcO₂ and PtCO₂ are measured electrochemically; these values are used to estimate PaO₂ and PaCO₂. This technique may be inaccurate in hypoperfused states and when vasoactive drugs are used.

• pH measurements can be drawn commonly from the radial artery located on the distal lateral wrist or from the femoral artery which is lateral to the femoral vein and medial to the femoral nerve in the crease of the groin.
• Venous pH can be measured from any of the veins, commonly from the antecubital vein.
• "Arterialized" samples may be considered from an extremity vein that has minimal tissue extraction (cold extremity).

CPB:

• pH stat management usually requires adding CO₂ to the oxygenated gas inflow. This increases total blood CO₂ content and increases cerebral blood flow (results in cerebral autoregulation uncoupling). This "luxuriant" flow may result in an increased embolic load (including air), thus potentially increasing the risk of cerebral injury (2) [B]. In ischemic stroke, pH stat management has been shown to imply better cerebral blood flow to injured brain, but it may be dangerous by elevating intracranial pressure in subacute stages (3) [B].
• Alpha stat management preserves cerebral autoregulation. There is no CO₂ added to the oxygenator; thus total CO₂ content stays constant. Normally, cerebral blood vessels are able to adjust the flow of blood by vasoconstriction when arterial BP is raised and vasodilation when arterial BP is lowered; thus if the PaCO₂ doubles, the CBF doubles. Since alpha stat management maintains CO₂ at normal levels, cerebral autoregulation in response to BP remains intact. As a result, it may have less adverse effects on enzyme function and cerebral autoregulation, and is more commonly utilized during CPB.

• Arterial blood gas measurements are useful perioperatively and on the floor when acid–base or oxygenation abnormalities are suspected.
  • Preoperatively, baseline values can be attained to assess a patient's normal respiratory status. Abnormalities can aid with risk stratification, the need for preoperative optimization, as well as ventilatory settings, invasive monitors, and bed acuity.
  • Intraoperatively, arterial blood gas measurements are used to identify respiratory and metabolic derangements. Hypoxia and alveolar–arterial oxygen gradients can aid with discerning between ventilation/perfusion mismatching, shunting or diffusion barriers. Extubation criteria are often based upon clinical signs and examination; however, in some patients, arterial blood gas measures are useful in identifying patients who may fail extubation.
  • Intensive care unit: Intubated patients are carefully assessed for the return of adequate mechanical functions as well as PO₂ and PCO₂ levels during spontaneous ventilation.
• pH stat versus alpha stat approaches during CPB: Patients managed by the pH stat approach are considered hypercarbic and have a lower pH (respiratory acidosis). Patients managed by the alpha stat approach are considered hypocarbic and have an increased pH (respiratory alkalosis). The two techniques may have different cerebral and cardiac outcomes, and controversy exists as to which approach yields a better patient outcome.
  • Adults: Several studies have shown that during moderate hypothermia, alpha stat management produces better neurologic outcomes, and is thus more widely used.
  • Pediatrics: Several studies have shown that pH stat management produces better neurologic outcomes secondary to decreased oxygen consumption, more homogenous brain cooling, and better cerebral metabolic recovery. Data also shows that pH stat management results in better outcomes with shorter ventilation times and shorter intensive care unit stays after pediatric cardiac surgery (4) [B]. Thus, pH stat is more commonly used; it may also be combined with alpha stat (pH stat used during cooling and alpha stat used during rewarming) (5) [B].

pH = 6.1 + log [HCO₃^-]/0.03 × PaCO₂

• Extubation and Intubation Criteria
• Metabolic Acidosis
• Anaerobic Metabolism
• Respiratory Alkalosis
• Respiratory Acidosis
• Cardiopulmonary Bypass

Sara Miller , MD

Priti G. Dalal , MD, FRCA