title: Pregnancy-induced analgesia: a combined psychophysical and neurophysiological study.
abstract: BACKGROUND
  To investigate changes in heat pain threshold and modifications in heat pain processing during pregnancy and labour, seventy-six nulliparous pregnant women were enrolled in two studies.
  
  
  METHODS
  In the first study (psychophysical), 60 pregnant women underwent a quantitative sensory testing (QST) investigating heat perception in two body areas (right forearm and T10 dermatome) according to these groups: 32-33 gestational weeks (GW), 39-40 GW, early stage of active labour and 24 h after the delivery. In the other study (neurophysiological), contact heat-evoked potentials (CHEPs) were recorded in other 16 women at the 32nd GW and in 11 of these also at the 40th GW.
  
  
  RESULTS
  The psychophysical study showed that heat pain threshold was significantly increased at the forearm at 32-33 GW (median ± IQR: 39.6 ± 0.7 °C), at 39-40 GW (40.6 ± 1.1 °C) and at early stage of active labour (40.8 ± 1.5 °C) as compared to 20 non-pregnant controls (p < 0.001). Heat pain threshold tested at T10 level was significantly increased at 32-33 GW (41.0 ± 1.6 °C), at 39-40 GW (42.1 ± 1.8 °C), and at early stage of active labour (42.3 ± 1.3 °C) as compared to the non-pregnant women (p < 0.001). The N2-P2 CHEP amplitude (main negative N2 and positive P2 components of the vertex biphasic potential) recorded from the pregnant women was significantly lower at the 40th than at the 32nd GW, after stimulation of both the forearm (p < 0.001) and the abdomen (p < 0.001).
  
  
  CONCLUSIONS
  In pregnant women, there is a progressive increase of heat pain threshold and a reduction of the CHEP amplitude, suggesting that a general inhibitory mechanism may be involved.
authors.affiliation: Institute of Anesthesiology and Intensive Care, Università Cattolica del Sacro Cuore, Rome, Italy. gdraisci@inwind.it
authors.forename: G
authors.initials: G
authors.lastname: Draisci
authors.forename: S
authors.initials: S
authors.lastname: Catarci
authors.forename: C
authors.initials: C
authors.lastname: Vollono
authors.forename: B A
authors.initials: BA
authors.lastname: Zanfini
authors.forename: C
authors.initials: C
authors.lastname: Pazzaglia
authors.forename: C
authors.initials: C
authors.lastname: Cadeddu
authors.forename: D
authors.initials: D
authors.lastname: Virdis
authors.forename: M
authors.initials: M
authors.lastname: Valeriani
mesh_terms: D000328:Adult; D016022:Case-Control Studies; D005073:Evoked Potentials, Somatosensory / Q000502:physiology*; D005260:Female; D006358:Hot Temperature; D006801:Humans; D007747:Labor Stage, First / Q000502:physiology; D058748:Pain Perception / Q000502:physiology*; D017288:Pain Threshold / Q000502:physiology*; D049590:Postpartum Period / Q000502:physiology; D011247:Pregnancy / Q000502:physiology*; D011263:Pregnancy Trimester, Third / Q000502:physiology