Autonomic Challenges From Mild Hypovolemia and Mechanical Ventilation

Effects of Low Central Volume and Intermittent Positive Pressure Ventilation on the Heart Rate Variability

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.

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.

Inclusion Criteria:

• healthy volunteers

Exclusion Criteria:

• history of cardiac, metabolic, respiratory, renal, neurological or hematologic disease of any kind
• chronically assuming drugs of any kind
• non sinus cardiac rhythm
• ectopic beats >5% of all cardiac beats
• claustrophobia or unable to tolerate noninvasive ventilation via facial mask