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:

Long stethoscope tubing

Poor hearing in the observer

Calibration errors in the sphygmomanometer

Decreased blood flow in the extremities caused by either hypovolemia or the use of vasopressors

Severe atherosclerosis that prevents occlusion of the artery at suprasystolic pressures

Inappropriate cuff size

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:

Blood pressure changes may be rapid.

Moderate blood pressure changes may cause end-organ damage.

Frequent arterial blood gases may be needed.

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:

Duration of catheterization.

Increased catheter size.

Catheter type (Teflon catheters cause more thrombosis than catheters made of polypropylene).

Proximal emboli.

Prolonged shock.

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.

Some arterial cannulation kits have a combined needle-guidewire-cannula system, in which the guidewire is advanced into the lumen after good blood flow is obtained and the catheter is then advanced over the guidewire. After cannulation low-compliance pressure tubing is fastened to the catheter, a sterile dressing is applied, and the catheter is fastened securely in place. Care must be taken to ensure that the pressure tubing is free from bubbles before connection. After the procedure it is advisable to remove any devices that have dorsiflexed the wrist because of concerns for median nerve palsy.

6 Describe the normal blood supply to the hand

The hand is supplied by the ulnar and radial arteries. These arteries anastomose via four arches in the hand and wrist (the superficial and deep palmar arches, the anterior and posterior carpal arches) and between their metacarpal and digital branches. Because of the dual arterial blood supply, the hand usually has collateral flow, and the digits can be supplied by either artery if the other is occluded. Both ulnar and radial arteries have been removed and used successfully as coronary artery bypass grafts without ischemic sequelae to the hand.

7 Describe Allen’s test. Explain its purpose

Allen’s test is performed before radial artery cannulation to determine whether ulnar collateral circulation to the hand is adequate in case of radial artery thrombosis. The hand is exsanguinated by having the patient make a tight fist. The radial and ulnar arteries are occluded by manual compression, the patient relaxes the hand, and the pressure over the ulnar artery is released. Collateral flow is assessed by measuring the time required for return of normal coloration. Return of color in less than 5 seconds indicates adequate collateral flow, return in 5 to 10 seconds suggests an equivocal test, and return in more than 10 seconds indicates inadequate collateral circulation. In 25% of the population the collateral circulation to the hand is inadequate.

8 Is Allen’s test an adequate predictor of ischemic sequelae?

Although some clinicians advocate use of Allen’s test, others have demonstrated that Allen’s test of radial artery patency has no relationship to distal blood flow as assessed by fluorescein dye injection. There are many reports of ischemic sequelae in patients with normal Allen’s tests; conversely, patients with abnormal Allen’s tests may have no ischemic sequelae. Apparently Allen’s test alone does not reliably predict adverse outcome.

Furthermore, it appears that arterial thrombosis is not uncommon after decannulation. Rates of thrombosis of 25% and greater have been noted without evidence of ischemia. Arterial thrombosis also seems to be a self-limited event.

9 What alternative cannulation sites are available?

The ulnar, brachial, axillary, femoral, dorsalis pedis, and posterior tibial arteries are all acceptable cannulation sites. The ulnar artery may be cannulated if the radial artery provides adequate collateral flow. The brachial artery does not have the benefit of collateral flow, but many studies have demonstrated the relative safety of its cannulation. Cannulation of the axillary artery is also relatively safe, but the left side is preferred because of a lower incidence of embolization to the carotid artery. The femoral artery is an excellent site for cannulation because of the large size, the technical ease of cannulation, and the low risk of ischemic sequelae. Although some studies have indicated a slightly higher incidence of infection with a femoral catheter, other studies have not demonstrated this increase. The small size of the dorsalis pedis and posterior tibial arteries makes cannulation difficult and poses an increased risk for ischemic complications; cannulation of these arteries is relatively contraindicated for patients with peripheral vascular disease and diabetes mellitus. The ulnar site should not be attempted after the radial artery on the same side has been punctured on previous attempts.

10 How does a central waveform differ from a peripheral waveform?

As the arterial pressure is transmitted from the central aorta to the peripheral arteries, the waveform is distorted (Figure 27-1). Transmission is delayed, high-frequency components such as the dicrotic notch are lost, the systolic peak increases, and the diastolic trough is decreased. The changes in systolic and diastolic pressures result from a decrease in the arterial wall compliance and from resonance (the addition of reflected waves to the arterial waveform as it travels distally in the arterial tree). The systolic blood pressure in the radial artery may be as much as 20 to 50 mm Hg higher than the pressure in the central aorta.

Figure 27-1 Configuration of the arterial waveform at various sites in the arterial tree.

(From Blitt CD, Hines RL: Monitoring in anesthesia and critical care medicine, ed 3, New York, 1995, Churchill Livingstone, with permission.)

11 What information can be obtained from an arterial waveform?

The arterial waveform provides valuable information about the patient’s hemodynamic status:

The waveform determines the heart rate during electrocardiographic electrocautery interference and whether the electrical spikes from a pacemaker result in ventricular contractions.

The slope of the upstroke may be used to evaluate myocardial contractility.

Large respiratory variations suggest hypovolemia.

The waveform provides a visual estimate of the hemodynamic consequences of various arrhythmias.

12 How is the arterial waveform reproduced?

Reproduction of the arterial waveform requires the following equipment:

Intravascular catheter

Fluid-filled pressure tubing and a stopcock

Electromechanical transducer

Electronic analyzer and display system

The mechanical energy at the catheter tip is transmitted to the transducer by the fluid-filled tubing and then converted to an electrical signal. The electrical signal is then converted to a waveform by the analyzer and displayed.

13 Define damping coefficient and natural frequency

Natural frequency, a property of the catheter-stopcock–transducer apparatus, is the frequency at which the monitoring system resonates and amplifies the signals it receives. The natural frequency is directly proportional to the diameter of the catheter lumen and an inverse function of the length of the tubing, the system compliance, and the density of the fluid contained in the system. Because the natural frequency of most monitoring systems is in the same range as the frequencies used to recreate the arterial waveform, significant amplification and distortion of the waveform may occur.

The damping coefficient reflects the rate of dissipation of the energy of a pressure wave. This property may be adjusted to counterbalance the erroneous amplification that results when the natural frequency of the monitoring system overlaps with the frequencies used to recreate the waveform.

14 What are the characteristics of overdamped and underdamped monitoring systems?

The damping coefficient is estimated by evaluating the time for the system to settle to zero after a high-pressure flush (Figure 27-2). An underdamped system continues to oscillate for three or four cycles; it overestimates the systolic and underestimates the diastolic blood pressure. An overdamped system settles to baseline slowly without oscillating; it underestimates the systolic and overestimates the diastolic blood pressure. However, in both cases the mean blood pressure is relatively accurate.

Figure 27-2 Underdamped, adequately damped, and overdamped arterial pressure tracings after a high-pressure flush.

15 How can the incidence of artifacts in arterial monitoring systems be reduced?

The incidence of artifacts can be reduced by meticulous care of the monitoring system:

The connecting tubing should be rigid with an internal diameter of 1.5 to 3 mm and a maximal length of 120 cm.

The lines should be kept free of kinks, clots, and bubbles, which cause overdamping of the system.

Only one stopcock per line should be used to minimize possible introduction of air.

The mechanical coupling system should be flushed with saline to maintain the patency of the arterial line and minimize the risk of distal embolization and nosocomial infection.

The transducer should be placed at the level of the right atrium, the midaxillary line in the supine position.

The transducer should be electrically balanced or rezeroed periodically because the zero point may drift if the room temperature changes.

16 What is distal pulse amplification?

The shape of the arterial pressure wave changes as it travels from the low-resistance ascending aorta to the high-resistance arteriolar bed. This dramatic change in resistance causes the pressure wave to be reflected back toward the ascending aorta, influencing the shape of the pressure wave. Although the changes in shape do vary with age, sympathetic tone, and vascular impedance, generally peripherally measured arterial waveforms seem amplified and demonstrate higher systolic pressures, wider pulse pressures, and somewhat lower diastolic pressures. The mean arterial pressure is only slightly reduced.

17 What are the risks and benefits of having heparin in the fluid of a transduction system?

Heparinized infusions have been used for over 25 years for the purposes of reducing the incidence of arterial catheter thrombosis. However, a recent comparison of use of heparinized vs. nonheparinized solutions for arterial transducer systems found no increase in catheter thrombosis when a nonheparinized solution was used. The benefit of not using heparinized solutions would be decreased exposure to heparin and the possibility of leading to heparin-induced thrombocytopenia.

18 Are any risks associated with flushing the catheter system?

On occasion arterial catheter systems are flushed to improve the qualilty of the arterial waveform. Retrograde embolization of air or thrombus into the cerebral vasculature is a risk. Flush rates of greater than 1 ml/min have been observed to result in retrograde flow. It should be noted that adverse neurologic events associated with arterial cannulation are extremely rare events and remain anecdotal at this time.