Blood Gas Values During Hypothermia in Asphyxiated Term Neonates

Abstract

The authors have indicated they have no financial relationships relevant to this article to disclose.H YPOTHERMIA HAS BECOME an important novel neuroprotective strategy for asphyxiated term neonates. 1 Most of these neonates will receive mechanical ventilation. Hypothermia affects blood gas parameters such as pH and PCO 2 . At lower temperatures pH increases and PCO 2 decreases. This is relevant, because PaCO 2 is known to affect vascular tone and, hence, cerebral perfusion. 2 In addition, cerebral blood flow decreases during hypothermia, which increases the risk of insufficient blood flow during hypocapnia. 3 Most blood gas instruments, whether a central laboratory or point-of-care device, contain a temperaturecontrolled sample chamber specified to be 37°C. It is at that temperature that all measurements of pH and partial pressure of gases are performed. In the ␣-stat strategy, uncorrected values are used to keep the pH and PCO 2 close to the 37°C reference value. 4 However, most instruments can calculate and present temperature-corrected pH and PCO 2 values. In the so-called pH-stat method, the measured pH is corrected to the actual body temperature of the patient. Ventilator settings can be adjusted to keep the actual pH as close to 7.4 as possible. 4 At present, it is unclear whether the ␣-stat or pH-stat theory should be used in the neuroprotective strategy for asphyxiated term neonates.In the 2 large randomized, controlled trials on therapeutic hypothermia in term neonates with perinatal asphyxia, attending physicians were advised to correct blood gases and pH for rectal temperature. 5,6 Here we will summarize the relevance of blood gas values in and the effects on cellular function during hypothermia. HYPOTHERMIA, PCO 2 , AND pHPhysical laws determine that the solubility of a gas within a liquid decreases when lowering the temperature. During hypothermia, arterial PCO 2 decreases and pH increases compared with 37°C when measurements are made at the actual body temperature (Fig 1). In healthy subjects with a body temperature of 37°C, pH and PaCO 2 should approach 7.4 and 5.3 kPa (40 mm Hg), respectively. During hypothermia (33°C), pH will rise to 7.5 and PCO 2 will decrease to 4.5 kPa (34 mm Hg). PaCO 2 AND CEREBRAL BLOOD FLOWUnder normal conditions, term neonates autoregulate their cerebral blood flow in response to changes in systemic arterial pressure, but this mechanism can be impaired during neonatal distress. 7 In particular, changes in PaCO 2 have a well-recognized effect on cerebral blood flow. Hypocapnia induces cerebral vasoconstriction and may decrease cerebral tissue high-energy phosphates. 8,9 This effect has been extensively investigated in experiments in neonatal animals. [9][10][11] In preterm neonates, profound hypocapnia below values of 4 kPa (30 mm Hg) was related to periventricular leukomalacia. 12 There is little reason to assume that hypocapnia could not damage the brain in the sick term neonate. 13 Klinger et al 14 reported that in asphyxiated term neonates early hypocapnia was indepe...

Porcine neonates failing vitality score: physio-metabolic profile and latency to the first teat contact

“…The general importance of anapyrexia resides in the fact that it reduces the consumption of oxygen and increases the affinity of haemoglobin to oxygen (Bicego et al, 2002;Steiner et al, 2002). Variations in temperature tend to affect the entire organism (Odehnalová et al, 2008); the neonate piglets that obtained a low vitality score showed hypercapnia, results in agreement with previous work by Randall (1971) and Herpin et al (1996); whereas more recently, Groenendaal et al (2009) indicated that hypothermia affects PCO 2 diminishing this gas concentration at low temperatures.…”

Physiologic and pharmacologic considerations for hypothermia therapy in neonates

“…30–32 With every 1 °C drop in core temperature, pH increases by 0.016 points. PaCO 2 falls because of increased dissolved CO 2 and decreased CO 2 production.…”

While Waiting

“…At lower temperatures, pH increases and PaCO 2 decreases. 18 For example, in a healthy neonate with a body temperature of 37°C, the pH should approach 7.4 with PaCO 2 values near 40 mm Hg and PaO 2 values near 90 mm Hg. However, an arterial blood gas obtained from the same neonate undergoing hypothermia with a core temperature of 33°C would likely reveal a pH of approximately 7.5, a PaCO 2 of 34 mm Hg and a PaO 2 of 61 mm Hg.…”

“…However, an arterial blood gas obtained from the same neonate undergoing hypothermia with a core temperature of 33°C would likely reveal a pH of approximately 7.5, a PaCO 2 of 34 mm Hg and a PaO 2 of 61 mm Hg. 18 These values occur if the blood gas analyzer is unable to correct for body temperature. Most blood gas instruments contain a temperature-controlled sample chamber specified to be 37°C, referred to as the α-stat method.…”

“…Currently, it is unclear whether the α-stat or pH-stat method should be used for the ventilator management of the asphyxiated neonates undergoing hypothermia. 18 We recommend investigating the sampling method used at each institution, being cognizant of the aforementioned alterations, and considering modified ventilator strategies to obtain target blood gas values.…”