Patients with obstructive sleep apnea (OSA) are more likely to experience postoperative complications and impact significantly on healthcare resources. In the surgical population, the type and volume of IV fluid administration may contribute to the worsening of postoperative sleep apnea but the pathophysiological mechanisms have not been elucidated. Recent research from general population suggests that intravenous (IV) fluid administration worsens the severity of OSA by shifting of fluid from the legs to the neck thus increasing neck size and causing airway collapse. In this novel study, our objective is to examine the contribution of IV fluid administration and rostral fluid shift from the legs to the neck thereby leading to worsening of sleep apnea following surgery. In this prospective observational cohort study, 25 consecutive adult patients, requiring at least 2 days hospital stay, will be recruited. Following informed consent, patients will undergo portable home sleep study to determine the preoperative severity of sleep-disordered breathing. On the day of surgery, leg, neck and total body fluid volumes will be recorded, and patients will undergo general anesthetic. The results of this study will be used to design future clinical trials evaluating methods to decrease postoperative OSA worsening, as well as decreasing OSA-related postoperative complications.

Obstructive sleep apnea (OSA) is a common sleep-related breathing disorder, associated with increased morbidity and mortality. It is an independent risk-factor for post-operative cardio-respiratory complications and increased perioperative utilization of health care resources. In non-surgical patients, rostral fluid shift from the lower limbs to the neck has been identified as a pathophysiologic mechanism to explain OSA severity based on gender, increasing age, and in fluid-overload states. In this novel study, our objective is to examine the contribution of fluid shift from the lower limbs to the neck thereby leading to worsening of sleep apnea following surgery.

Objectives of the study

• Measure the change in the leg, neck and total fluid volume from preoperative baseline to the second postoperative day.
• Assess the impact of the change in neck fluid volume on neck circumference and postoperative AHI.

Similar to the patients with heart failure, renal failure and older men, we hypothesize the rostral shift of fluid from the lower limbs to the neck occurs in the postoperative period contributing to the postoperative worsening of OSA. In this study, we will perform serial measurements of segmental and total body water shifts occurring after surgery, at various time-points over two postoperative days. We will monitor indices such as the Apnea Hypopnea index (AHI) and oxygen desaturation index (ODI) to examine changes in the severity of OSA. Internal validity of our study will be made more robust by use of well-validated methods such as BIA for fluid measurements and portable PSG for OSA severity.

Inclusion Criteria:

• American Society of Anesthesiologists (ASA) physical status I to IV
• Undergoing elective non-cardiac surgery, requiring general anesthesia, lasting for 1-2 hours or more and requiring same day admission for a minimum of 2 days postoperatively.

Exclusion Criteria:

• Patients undergoing cardiac, intracranial surgery, procedures involving the neck such as cervical spine surgery, or carotid endarterectomy.
• Patients requiring epidural or spinal anesthesia for their surgery, as neuraxial blockade may impact fluid shifts from legs to neck and may confound our results.
• Prior history of vascular surgery in the lower limbs such as varicose vein surgery, vascular by-pass surgery or evidence of venous efficiency.
• Fluid overload states such as history of congestive heart failure, or renal failure will be excluded.
• Patients with metal implants in lower limbs or spine, due to possible interference with the leg impedance signals.
• Patients requiring urgent sleep physician referral (within four weeks) due to serious medical conditions or safety critical-occupations according to the 2011 Canadian Thoracic Society (CTS) guidelines e.g. unstable ischemic heart disease, recent cerebrovascular disease, congestive heart failure, obstructive/restrictive lung disease, pulmonary hypertension, hypercapnia respiratory failure.
• Pregnant or lactating patients, as the physiological mechanisms governing postoperative fluid shifts are different in this patient population.

Surgical patients attending the preoperative assessment clinic in Toronto Western Hospital (UHN) who will be undergoing elective non-cardiac surgery, requiring general anesthesia, lasting for 1-2 hours or more and requiring same day admission for a minimum of 2 days postoperatively.