Tuesday, December 12, 2017

Invasive Monitoring in Surgery

Invasive Monitoring in Surgical situations is quiet common today. In ICUs, invasive monitoring is employed using arterial lines (for blood pressure), pulmonary artery catheters (for cardiac output, pulmonary artery wedge pressure), and intracranial catheter (for intracranial pressure).

Arterial catheter:
How it’s done: can put it through radial, femoral, axillary, or brachial. Small amounts of heparin is infused to prevent clotting.
What it’s used for: It is useful for not only measuring output or blood pressure but also for taking blood samples. Blood pressure measurements include systolic, diastolic, and mean (BPsystolic + 2xBPdiastolic)/3.

Pulmonary artery catheter:
How it’s done: can put it through subclavian or jugular and goes through the heart into the pulmonary artery.
What its used for:
-A balloon is inflated once in the distal pulmonary artery and this allows measurement of the PCWP (pulmonary capillary wedge pressure) or PCOP (occlusion pressure). This represents a measure of left ventricular preload.
-Pulmonary catheters can also measure cardiac output by looking at temperature changes in blood (after introducing a cold IV fluid into right atrium or by using a heating probe).
-In addition, pulmonary artery catheters can assess the mixed oxygen venous saturation (SvO2) which is normally 70%. A decrease in SvO2 occurs in shock. Decreases for a sustained period of time can result in organ dysfunction. Central venous catheters (used to measure central venous pressure) can also be used in assessing SvO2.

Calculating SVR and PVR:
– Systemic Vascular Resistance = [(MAP-CVP)/CO] x 80 (normal= 800-1200 dynes*sec/cm^5). You get high SVR with inadequate cardiac output and low SVR in systemic inflammation.
– Pulmonary Vascular Resistance = [(MPAP-PCWP)/CO] x 80 (normal = 20-120 dynes*sec/cm^5)

Terminology References:
*MAP is the mean airway pressure
*MPAP is the mean pulm. artery pressure
*Central venous pressure (CVP) describes the pressure of blood in the thoracic vena cava, near the right atrium of the heart. CVP reflects the amount of blood returning to the heart and the ability of the heart to pump the blood into the arterial system.

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