The purpose of this study is to assess the suitability of the ear and forehead as a better site for measure of fluid responsiveness.

The aim of this study is to compare Pleth Variability Index (PVI) measurements at different sensor locations in respect to signal quality, sensor fit to site location and fluid responsiveness. PVI will be measured at 3 different sensor sites: the fingers, ear lobes and forehead. The ability to predict fluid responsiveness will be analyzed using cardiac output as the reference value.

Inclusion Criteria:

• 18 years of age or older
• Undergo any type of surgeries under general anesthesia in UCI Medical Center
• Mechanically ventilated during surgery
• Arterial line placement is preferred but not required
• Agree to sign the consent and HIPAA forms

Exclusion Criteria:

• Less than 18 years of age
• Pregnant women
• Refused to sign the consent and HIPAA forms
• Receive vasoactive medication prior to surgery
• Has these conditions: hypotension, hypothyroidism, moderate or poorly controlled hypertension, chronic poorly-controlled diabetes, heart failure patients, and patients with active airway flow obstruction, left ventricular dysfunction, intra-cardiac shunts, right ventricular dysfunction, atrial fibrillation, arrhythmias
• patients with surgeries at or around site of sensor placement
• Subject has skin abnormalities affecting the digits such as psoriasis, eczema, angioma, scar tissue, burn, fungal infection, substantial skin breakdown, nail polish or acrylic nails that would prevent monitoring of SpO2 levels during the study

The study population will be patients who undergo any form of surgery, where he/she is mechanically ventilated, preferably cases that require arterial line placement, use of cardiac output measurement, and where fluid administration will occur perioperatively.