title: Low-frequency changes in finger volume in patients after surgery, related to respiration and venous pressure.
abstract: OBJECTIVE
  In patients after surgery, we observed large-amplitude low-frequency changes in digital plethysmograph measurements when DC coupling of the signal was used. We set out to assess factors that might contribute to these events and in particular to test the possibility that low-frequency signals could be used to assess respiratory disturbances.
  
  
  METHODS
  We recorded values in 23 patients who had undergone gynaecological surgery. We measured nasal flow, abdominal pressure (by urinary catheter), venous pressure in the hand, and DC-coupled optical transmission plethysmography. Signals were replayed and analysed to assess the incidence of specific patterns of events.
  
  
  RESULTS
  Most patients received morphine for postoperative analgesia. Respiratory irregularity and expiratory muscle action were very frequent. Increases in abdominal pressure during expiration caused increases in venous pressure and pulsation. In 12 out of 23 patients, a characteristic response consistent with vasoconstriction was noted after increases in breath size, and, in seven patients, very-low-frequency (0.2-0.7 Hz) oscillations of finger volume were present that appeared unrelated to respiratory events. Patients who did not receive morphine had very different plethysmograph patterns, with significantly smaller pulse amplitude.
  
  
  CONCLUSIONS
  Low-frequency changes in finger volume can be simply obtained and provide considerable information about peripheral circulatory dynamics. Diverse patterns can be recognized, but the range of responses suggests that current techniques cannot be used alone to assess cardiorespiratory status. However, a combination of plethysmography with respiratory measurements shows characteristic events.
authors.affiliation: University Department of Anaesthesia, Critical Care, and Pain Medicine, Edinburgh University, Royal Infirmary of Edinburgh, Edinburgh, UK.
authors.forename: Angus
authors.initials: A
authors.lastname: Murray
authors.forename: Gordon B
authors.initials: GB
authors.lastname: Drummond
authors.forename: Stephanie
authors.initials: S
authors.lastname: Dodds
authors.forename: Lucy
authors.initials: L
authors.lastname: Marshall
mesh_terms: D005385:Fingers / Q000033:anatomy & histology*; D006801:Humans; D008875:Middle Aged; D011184:Postoperative Period; D011312:Pressure; D012119:Respiration; D001743:Urinary Bladder / Q000502:physiology; D014690:Venous Pressure / Q000502:physiology