Cerebrospinal fluid and arterial acid-base equilibria in spontaneously breathing third-trimester pregnant women

Francesco Zadek, Giorgio Giudici, Chiara Ferraris Fusarini, Maria T Ambrosini, Adriana di Modugno, Vittorio Scaravilli, Alberto Zanella, Roberto Fumagalli, Nino Stocchetti, Edoardo Calderini, Thomas Langer, Francesco Zadek, Giorgio Giudici, Chiara Ferraris Fusarini, Maria T Ambrosini, Adriana di Modugno, Vittorio Scaravilli, Alberto Zanella, Roberto Fumagalli, Nino Stocchetti, Edoardo Calderini, Thomas Langer

Abstract

Background: Acid-base status in full-term pregnant women is characterised by hypocapnic alkalosis. Whether this respiratory alkalosis is primary or consequent to changes in CSF electrolytes is not clear.

Methods: We enrolled third-trimester pregnant women (pregnant group) and healthy, non-pregnant women of childbearing age (controls) undergoing spinal anaesthesia for Caesarean delivery and elective surgery, respectively. Electrolytes, strong ion difference (SID), partial pressure of carbon dioxide ( [Formula: see text] ), and pH were measured in simultaneously collected CSF and arterial blood samples.

Results: All pregnant women (20) were hypocapnic, whilst only four (30%) of the controls (13) had an arterial [Formula: see text] <4.7 kPa (P<0.001). The incidence of hypocapnic alkalosis was higher in the pregnant group (65% vs 8%; P=0.001). The CSF-to-plasma Pco2 difference was significantly higher in pregnant women (1.5 [0.3] vs 1.0 [0.4] kPa; P<0.001), mainly because of a decrease in arterial Pco2 (3.9 [0.3] vs 4.9 [0.5] kPa; P<0.001). Similarly, the CSF-to-plasma difference in SID was less negative in pregnant women (-7.8 [1.4] vs -11.4 [2.3] mM; P<0.001), mainly because of a decreased arterial SID (31.5 [1.2] vs 36.1 [1.9] mM; P<0.001). The major determinant of the reduced plasma SID of pregnant women was a relative increase in plasma chloride compared with sodium.

Conclusions: Primary hypocapnic alkalosis characterises third-trimester pregnant women leading to chronic acid-base adaptations of CSF and plasma. The compensatory SID reduction, mainly sustained by an increase in chloride concentration, is more pronounced in plasma than in CSF, as the decrease in Pco2 is more marked in this compartment.

Clinical trial registration: NCT03496311.

Keywords: acid–base imbalance; cerebrospinal fluid; hyperventilation; hypocapnic alkalosis; metabolic acidosis; pregnancy; respiratory alkalosis; respiratory physiology; water–electrolyte imbalance.

Copyright © 2022 The Authors. Published by Elsevier Ltd.. All rights reserved.

Figures

Fig 1
Fig 1
Relationships of CSF Pco2 and pH. Data are represented by scatter plot, linear regression (dashed line), and 95% confidence interval (continuous line) of CSF Pco2 and CSF pH in the two populations. White dots represent the pregnant group (r=–0.52; P=0.02) and black dots the control group (r=–0.60; P=0.03). Pco2, partial pressure of carbon dioxide.
Fig 2
Fig 2
Relationships of CSF Pco2 and SID. Data are represented as scatter plot, linear regression (dashed line), and 95% confidence interval (continuous line) of CSF Pco2 and CSF SID in the two populations. White dots represent the pregnant group (r=0.64; P=0.002) and black dots the control group (P=0.48). Pco2, partial pressure of carbon dioxide; SID, strong ion difference.
Fig 3
Fig 3
Relationships of arterial Pco2 and arterial SID. Data represented as scatter plot, linear regression (dashed line), and 95% confidence interval (continuous line) of arterial Pco2 and arterial SID in the two populations. White dots represent the pregnant group (r=0.61; P=0.004) and black dots the control group (P=0.96). Pco2, partial pressure of carbon dioxide; SID, strong ion difference.

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