Intraoperative Monitoring

Continuous automated ST-segment analysis during thoracic surgery is especially important given the potential for cardiac ischemia, arrhythmias, and hemodynamic instability. Oxygenation during one-lung ventilation is determined by many factors including cardiac output, blood pressure, ventilation-perfusion matching, anesthetic effects on hypoxic pulmonary vasoconstriction, airway mechanics and reactivity, oxygen consumption, and preexisting pulmonary disease. Pulse oximetry with occasional intermittent arterial blood gas analysis provides warning of significant hypoxemia. The typical CO₂ vs. time waveform, displayed on most anesthesia monitors, has characteristic intervals that represent different physiologic events during ventilation. Continuous breath-by-breath spirometry (monitoring of inspiratory and expiratory volumes, pressures, and flows) enables the early detection of a malpositioned double-lumen tube and can reduce the potential for ventilatory-induced lung injury by guiding the optimization of ventilatory settings. Invasive arterial pressure monitoring is commonly used to assess beat-by-beat blood pressure, and it can also be used to derive functional hemodynamic information. However, systolic pressure variation (SPV) and pulse pressure variation (PPV) can reflect fluid responsiveness by measurement of cardiorespiratory interaction at controlled mechanical ventilation with tidal volumes >8 cc/kg, but most thoracic surgical operations are conducted with lower tidal volumes, and hence SVV and PPV do not have the ability to reveal fluid responsiveness. Minimally invasive hemodynamic monitoring (such as the esophageal Doppler, arterial pressure waveform-based devices, and/or central venous oximetry) may be coupled with certain maneuvers to identify whether “flow (cardiac output) is adequate to meet global tissue demands” during and after surgery.