Detecting a Volume Deficit During Spontaneous Breathing

Thirteen healthy volunteers (4 women) at an age of 25 years (range 18-36) were recruited through www.forsøgsperson.dk.

The volunteers were placed on a tilt table and provided with a facemask with an opening of 30-33 mm. A Hook ring was fitted with an inspiratory resistor, an expiratory resistor, a combination of the two resistors, or with no resistance and each resistor provides a 7.5 cmH2O threshold resistance. Each resistor was applied for two minutes with variables obtained in the last minute. The table was then tilted 45° head-up to accumulating blood in the legs and therefore a reduction in the central blood volume. Conversely, 20° head-down tilt was used to expand the central blood volume. For each volunteer measurements with the four respiratory interventions was randomized, whereas the tilt table position was in fixed order: supine, head-up tilt, and head-down tilt.

Three-lead ECG recorded heart rate. A 20 G arterial catheter was placed in the brachial artery of the non-dominant arm and connected to a transducer for reading of blood pressure and stroke volume variation (Vigileo-Flotrac™, version 1.07, Edwards Lifesciences, Nyon, Switzerland). Also SV and CO variation and the arterial pressure curve were recorded for subsequent determination of arterial pressure variations. Additionally, a catheter was placed via a brachial vein to the superior caval vein for recording of central venous pressure and SvO2. While the subjects were breathing spontaneously, the respiratory rate was measured by capnography and peripheral oxygen saturation by fingerprobe.

Variation in the arterial pulse pressure (PPV) was 100 x ((PPmax - PPmin) / ((PPmax + PPmin)/2)), where PPmax and PPmin is the maximal and minimal difference between systolic and diastolic pressure during the respiratory cycle, respectively. By the same formula systolic pressure variation (SPV) was calculated.