27: Arterial Catheterization and Pressure Monitoring

Published on 06/02/2015 by admin

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Last modified 06/02/2015

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CHAPTER 27 Arterial Catheterization and Pressure Monitoring

James Duke, MD, MBA

1 Why is arterial blood pressure monitored?

Blood pressure monitoring is fundamental in determining the effects of anesthesia on the cardiovascular system. Because decisions about patient care may be based on blood pressure data, it is important to understand how the data are obtained. Arterial pressure is monitored either noninvasively with a blood pressure cuff or invasively with arterial cannulation and a pressure transducer.

2 How do noninvasive blood pressure devices work?

Blood pressure is usually measured either manually (ausculatory method) or with an automated device (oscillometric method). With the ausculatory method, a pneumatic cuff is inflated to occlude arterial blood flow. As the cuff is deflated, audible frequencies called Korotkoff sounds are created by turbulent blood flow in the artery. The pressure at which the sounds are first audible is taken as the systolic pressure, and the pressure at which the sounds become muffled or disappear is taken as the diastolic pressure. Errors in measurement may be caused by:

image Long stethoscope tubing
image Poor hearing in the observer
image Calibration errors in the sphygmomanometer
image Decreased blood flow in the extremities caused by either hypovolemia or the use of vasopressors
image Severe atherosclerosis that prevents occlusion of the artery at suprasystolic pressures
image Inappropriate cuff size
image Too rapid of a deflation rate

With the oscillometric method, a pneumatic cuff is also inflated to occlude the arterial blood flow. As the cuff is deflated, the arterial pulsations cause pressure changes in the cuff that are analyzed by a computer. The systolic pressure is taken as the point of rapidly increasing oscillations, the mean arterial pressure as the point of maximal oscillation, and the diastolic pressure as the point of rapidly decreasing oscillations. Errors in measurement may occur from inappropriate cuff size or factors that prevent detection of cuff pressure variations, such as patient shivering. Prolonged use of the stat mode, in which the cuff reinflates immediately after each measurement is obtained, may lead to complications such as ulnar nerve paresthesia, thrombophlebitis, or compartment syndrome.

3 What are the indications for intra-arterial blood pressure monitoring?

Intra-arterial blood pressure monitoring is indicated when:

image Blood pressure changes may be rapid.
image Moderate blood pressure changes may cause end-organ damage.
image Frequent arterial blood gases may be needed.
image Noninvasive blood pressure monitoring is inaccurate.

Clinical examples include anticipated cardiovascular instability (e.g., massive fluid shifts, intracranial surgery, significant cardiovascular disease, valvular heart disease, diabetes), direct manipulation of the cardiovascular system (e.g., cardiac surgery, major vascular surgery, deliberate hypotension), frequent sampling of blood gases for pulmonary disease acid-base status or single-lung ventilation, or morbid obesity, which prevents accurate noninvasive measurements.

4 What are the complications of invasive arterial monitoring?

Complications include distal ischemia, arterial thrombosis, hematoma formation, catheter site infection, systemic infection, necrosis of the overlying skin, and potential blood loss caused by disconnection. The incidence of infection increases with duration of catheterization. The incidence of arterial thrombosis increases with:

image Duration of catheterization.
image Increased catheter size.
image Catheter type (Teflon catheters cause more thrombosis than catheters made of polypropylene).
image Proximal emboli.
image Prolonged shock.
image Preexisting peripheral vascular disease.

5 How is radial artery catheterization performed?

The wrist is dorsiflexed and immobilized, the skin is cleaned with an antiseptic solution, the course of the radial artery is determined by palpation, and local anesthetic is infiltrated into the skin overlying the artery (if the patient is awake). A 20-G over-the-needle catheter apparatus is inserted at a 30- to 45-degree angle to the skin along the course of the radial artery. After arterial blood return, the angle is decreased, and the catheter is advanced slightly to ensure that both the catheter tip and the needle have advanced into the arterial lumen. The catheter is then threaded into the artery. Alternatively the radial artery may be transfixed. After arterial blood return, the apparatus is advanced until both the catheter and the needle pass completely through the front and back walls of the artery. The needle is withdrawn into the catheter, and the catheter is pulled back slowly. When pulsatile blood flow is seen in the catheter, it is advanced into the lumen. If the catheter will not advance into the arterial lumen and blood return is good, a sterile guidewire may be placed into the lumen through the catheter, and the catheter advanced over the wire.

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