The purpose with the planned study is to study Pleth Variability Index (PVI) as a non invasive indicator for mild hypovolemia during spontaneous breathing in healthy subjects. Lower body negative pressure (LBNP), a well established technique is used to create defined levels of hypovolemia.

Primary hypothesis: By applying a breathing resistance (positive end expiratory pressure PEEP) to spontaneous breathing, the respiratory synchronous variation in the pulse oximeter signal (PVI) is enhanced and enables PVI to detect and grade the level of mild hypovolemia.

Secondary hypothesis: 1) When tidal volume is increased, the respiratory synchronous variation in the pulse oximeter signal (PVI) will also increase. 2) The earlobe is superior to the finger in detecting PVI 3) Hemoglobin (Hb), detected non invasively by pulse oximetry, is not affected by hypovolemia created by LBNP

The subjects are placed supine on a bed. Non invasive equipment for recording pulse and blood pressure is placed on a finger on the right hand. From this equipment total peripheral resistance and cardiac output are recorded. A pulse oximeter for saturation, perfusion index and PVI is placed on a finger and on the ear lobe. Data are recorded in parallel from the finger and the ear lobe. In addition non invasive haemoglobin (Hb) is recorded from the pulse oximeter on the finger. The subjects breathe through a mask connected to a simple Y-connector with an inspiratory valve on one side and an expiratory valve on the other. A positive pressure valve is applied on the expiratory side. Tidal volume is measured on-line by a connector connected to a spirometer and the tidal volume is displayed to the subjects. Stroke volume, total peripheral resistance and cardiac output is measured using ultra sound via the jugular fossa.

Baseline data are recorded with the subjects in the supine position breathing with normal tidal volume without PEEP and without LBNP applied. Data are then recorded with LBNP applied in two steps (40 and 15 mmHg). Recordings are continued 5 minutes after releasing the negative pressure. Data are collected in total in four different series; normal tidal volume without PEEP; normal tidal volume with PEEP 5cm H2O; three doubled tidal volume without PEEP; three doubled tidal volume with PEEP 5 cm H20. Respiratory rate is not controlled. The order of the four series is randomized. Between the series a 20 minute period of rest is allowed to the subjects. This period is also necessary to restore steady state between provocations.

Inclusion Criteria:

• Subjects without any heart or lung disease

Exclusion Criteria:

• Smoking
• Drugs affecting circulatory or respiratory system

Healthy subjects between 18-30 years old without any medications affecting circulatory or respiratory system