title: Subcutaneous bioimpedance recording: assessment of a method for hemodynamic monitoring by implanted devices.
abstract: BACKGROUND
  Diagnostic evaluation of patients with suspected symptomatic arrhythmias is limited by inability to assess the hemodynamic impact of a detected rhythm.
  
  
  OBJECTIVE
  To address this limitation, we utilized closely spaced subcutaneous electrodes, small enough to incorporate within an implantable monitor, to detect blood flow-induced pectoral muscle bioimpedance (Z) changes in a swine model of hemorrhage-induced hypotension.
  
  
  METHODS
  In seven anesthetized and ventilated adult pigs, small ring electrodes (current electrodes 5 cm apart; voltage electrodes 3.5 cm apart) were positioned on the left pectoral muscle. Z signals (Biopac system) and invasive arterial blood pressures were recorded. Hypotension was induced by hemorrhage (50% blood volume reduction). Mean arterial pressure (MAP) and pulse pressure (PP) with corresponding pulse Z (DeltaZ) and base Z (Z(o)) were measured. A longitudinal mixed model with a first-order autoregressive error structure was used to test for associations (change in DeltaZ vs change in MAP and change in DeltaZ vs change in PP) taking into account within pig correlation.
  
  
  RESULTS
  During bleeding-induced hypotension, Z(o) increased. Changes of DeltaZ correlated with both a change in MAP (coefficient = 1.17, P < 0.0001) and change in PP (coefficient = 0.98, P < 0.0001). A change in DeltaZ of 1-2 orders of magnitude corresponded to an approximate 40-70% drop in MAP and PP in a porcine model in which the baseline MAP was 69-70 mmHg.
  
  
  CONCLUSIONS
  Our findings suggest that closely spaced subcutaneous electrodes identify changes in local tissue/vascular bioimpedance that correlate well with direct invasive measures of induced hypotension in a porcine model.
authors.affiliation: Departments of Medicine (Cardiac Arrhythmia Center), University of Minnesota Medical School, Minneapolis, Minnesota 55455, USA.
authors.forename: Deviprasad
authors.initials: D
authors.lastname: Venugopal
authors.forename: Robert
authors.initials: R
authors.lastname: Patterson
authors.forename: Rajat
authors.initials: R
authors.lastname: Jhanjee
authors.forename: Scott
authors.initials: S
authors.lastname: McKnite
authors.forename: Keith G
authors.initials: KG
authors.lastname: Lurie
authors.forename: Andres
authors.initials: A
authors.lastname: Belalcazar
authors.forename: David G
authors.initials: DG
authors.lastname: Benditt
mesh_terms: D000465:Algorithms; D000818:Animals; D001783:Blood Flow Velocity / Q000502:physiology*; D003936:Diagnosis, Computer-Assisted / Q000379:methods*; D017097:Electric Impedance; D004867:Equipment Design; D019544:Equipment Failure Analysis; D005260:Female; D018670:Monitoring, Ambulatory / Q000295:instrumentation* / Q000379:methods; D018482:Muscle, Skeletal / Q000098:blood supply* / Q000502:physiology*; D010992:Plethysmography, Impedance; D019736:Prostheses and Implants; D015203:Reproducibility of Results; D012212:Rheology / Q000295:instrumentation*; D012680:Sensitivity and Specificity; D013552:Swine