Autonomic Challenges From Mild Hypovolemia and Mechanical Ventilation
Condition: Autonomic Nervous System Imbalance; Hypovolemia; Positive-Pressure Respiration
MeSH terms: Hypovolemia
Study type: Interventional

Summary: 
      Heart Rate Variability (HRV) analysis has been studied in the critically ill patients
      although it is affected by several uncontrolled variables in the clinical conditions. The aim
      of this trial is to measure the effects of mildly reduced central volume and cyclic variation
      of intrathoracic pressure on the variables frequently used to describe the HRV.
    

Detailed description: 
      Twelve healthy volunteers will be studied. Mild hypovolemia will be induced by 12 hours
      fasting from food and drinks. At 8.30 AM the study protocol will start. The studied subjects
      will lie calm supine in a ICU bed able to provide passive head up and head down tilt. They
      will be connected to a Siemens SC9000 monitor showing their ECG waves and to a Nexfin (BMEYE)
      monitor for continuous noninvasive blood pressure (NBP) assessment. Both waves (ECG and NBP)
      will be recorded on a laptop PC through analogic/digital input/output converter (PowerLab
      8/35, ADinstruments). The studied subjects will undergo to a sequence of (1) spontaneous
      breathing at 10 degrees head up, (2) spontaneous breathing at 7 degrees head down, (3)
      noninvasive ventilation at 10 degrees head up, and (4) noninvasive ventilation at 7 degrees
      head down. The sequence 1-2-3-4 will be randomized. After this four phases, a fluid challenge
      of ringer acetate 15ml*kg will be intravenously administered and the a sequence 1-2-3-4 will
      be repeated after a new randomization. Noninvasive ventilation will be provided with a facial
      mask with ventilatory setting: Psupp 8 cmH2O, PEEP 5 cmH2O, FiO2 0.28. Psupp will be
      decreased by 2 cmH2O steps if the inspiratory tidal volume will be >10ml*kg. During both
      spontaneous breathing and noninvasive ventilation the subjects will breathe following a
      metronome at 18bpm.

      HRV analysis will be conducted following the recommendation of the Task Force of the European
      Society of Cardiology and the North American Society of Pacing and Electrophysiology (see
      reference).

      Furthermore, the healthy volunteers will be studied by trans-thoracic ultrasound assessment
      with a Philips EPIQ7 sonographer, during each study phase.
    

Intervention name(s): Increase of central volume; Ventilation mode
Intervention type(s): Other; Other
Intervention description(s): Mild hypovolemia will be induced in healthy volunteers by 12 hours fasting. Three conditions will be considered for the analysis, each of them both during spontaneous breathing and positive pressure ventilation:
baseline
after fluid shift induced by passive head down position at 15 degrees
after fluid challenge with Ringer acetate 15ml*kg in head down position
A total of six steps will be considered for the analysis; Each previous step will be done in two respiratory conditions:
spontaneous breathing
noninvasive ventilation via facial mask in pressure support mode at 8 cmH2O above positive end expiratory pressure of 5 cmH2O, inspiratory fraction of oxygen of 0.25 In both conditions respiratory rate will be set at 18 breaths per min following a metronome
Intervention model: Sequential Assignment
Masking: None (Open Label)
Primary purpose: Basic Science