NMS. Surgery

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Chapter 1 ♦ Principles of Surgical Physiology

c. Mixed venous oxygen saturation (SvO 2 ): Can be assessed by either aspirating blood from the distal port or by using oximetry. (1) SvO 2 provides a means to determine if the amount of oxygen being pumped by the heart (oxygen delivery) is adequate for the amount of oxygen the body needs (oxygen consumption). (2) Normal SvO 2 : ~70%. (3) If the SvO 2 is persistently low (60% or less), then oxygen delivery is insufficient and organ dysfunction will occur. (4) Alternatively, SvO 2 can be obtained from the end of a central venous catheter, a less invasive but not as accurate technique.

CUT TO CASEBOOK See NMS Surgery Casebook , 3rd edition, Case 12.17: Management of Postoperative Problems in Trauma Patients.

d. Systemic vascular resistance (SVR)/pulmonary vascular resistance (PVR): By knowing the CO, the mean arterial pressure (MAP), and the central venous pressure (CVP), SVR and PVR can be calculated (MPAP, mean pulmonary artery pressure; 80, a conversion factor). (1) SVR = [(MAP − CVP)/CO] × 80; normal: 800–1,200 dynes. second/cm 5 . (2) PVR = [(MPAP − PCWP)/CO] × 80; normal: 20–120 dynes × second/cm 5 . (3) Low SVR indicates systemic inflammation or sepsis. (4) High SVR indicates other shock states with inadequate CO. e. Other: As noted, nearly all data can now be obtained noninvasively. (1) Pulmonary artery pressures: Estimated by echocardiography. (2) CO/SVR: Measured with A-line tracing. (3) FloTrac/Vigileo system uses arterial pressure waveforms analysis for stroke volume (SV) and CO. D. Vasoactive medications and antiarrhythmic drips: Increase blood pressure (vasoconstrictors) and/or cardiac output (inotropes). 1. Common IV vasoactive medications: a. Dopamine: Effect is concentration dependent. (1) Low dose (1–3 μg/kg/min): Affects dopamine receptors in the kidneys and intestine, leading to increased blood flow. (2) Intermediate dose (3–10 μg/kg/min): Primarily, a beta-receptor agonist, increasing cardiac contractility with resulting increase in CO. (3) High dose (>10 μg/kg/min): Primarily, an alpha agonist and vasoconstrictor; limiting effect is tachycardia; useful in shock.