Arterial Catheter Insertion Sites

Arterial catheters may be placed in the radial, ulnar, brachial, axillary, or femoral arteries. The radial site is preferred because it is readily accessible and usually has adequate collateral circulation through the ulnar artery. The radial site is easy to monitor and provides a stable location for blood sampling. The femoral artery provides pressure measurements that are less affected by peripheral vasoconstriction, but significant leakage of blood into the surrounding tissue can occur without detection. The femoral site also is more prone to contamination than the other locations.

Equipment Set-Up

Figure 15-1 shows the basic equipment used for an indwelling vascular line, in this case a brachial artery catheter. Once inserted, the catheter connects to a disposable continuous flush device by low-compliance tubing. The flush device keeps the line open by providing a continuous low flow of fluid (2 to 4 mL/hour) through the system. To maintain continuous flow, the intravenous bag supplying these systems must be pressurized, usually by a hand bulb pump. A pressure transducer, connected to the flush device, provides an electrical signal to an amplifier or monitor, which displays the corresponding pressure waveform. A sampling port (not shown) typically is included to allow blood withdrawal. CVP and pulmonary artery monitoring systems use the same basic set-up.

Procedure for Inserting an Arterial Line

Box 15-2 outlines the key procedural steps for inserting a radial artery line. Note that if the Allen test for collateral circulation is negative, the opposite wrist should be assessed. If collateral circulation cannot be confirmed on either side, the brachial site typically is used.

There are two common arterial line insertion methods: direct cannulation and the guidewire (Seldinger) technique. The direct cannulation method uses a puncture needle sheathed with the catheter. Using this approach, once a flash of blood is observed in the needle hub, the catheter sheath is advanced over the needle into the artery, and the needle is then removed.

With the Seldinger technique, a needle is used to penetrate the artery, with a soft-tipped guidewire then threaded through the needle into the vessel. Next, the needle is removed, leaving the guidewire in place. Finally, the indwelling catheter is advanced over the guidewire into position, and the guidewire is removed.

Arterial Pressure Waveforms

An arterial pressure waveform should have a clear upstroke on the left, with a dicrotic notch representing aortic valve closure on the downstroke to the right (Fig. 15-2). If the dicrotic notch is not visible, the pressure tracing is dampened and probably inaccurate, and the measured pressures likely lower than the patient’s actual values. The dicrotic notch disappears in some patients when the systolic pressure drops below 50 or 60 mm Hg.

Arterial pressure waves take on many different configurations (Fig. 15-3). The left side of the pressure wave may become straight and even pointed on the top when there is an increase in circulating catecholamines that increase cardiac contractility (a positive inotropic response). A tall, narrow pressure wave is also seen in patients with a stiff aorta due to arteriosclerotic vascular disease. In these patients, the diastolic pressure may also fall, producing an exaggerated tall and narrow complex. Increases in heart rate and vascular resistance increase diastolic pressure. On the other hand, vasodilation decreases vascular resistance and can cause a fall in diastolic pressures. Because approximately 70% of coronary artery perfusion occurs during diastole, coronary artery perfusion may be compromised if the diastolic pressure falls below 50 mm Hg.

Interpretation of Arterial Pressure Measurements

Normal arterial pressure in the adult is approximately 120/80 mm Hg and increases gradually with age. Systolic pressures greater than 140 and diastolic pressures greater than 90 are considered hypertensive (see Chapter 4 for the current classification of hypertension). A pressure below 90/60 mm Hg in adults is termed hypotension.

Although arterial pressure is one of the most frequently monitored vital signs, it reflects only the general circulatory status. Pressure is the product of flow and resistance. Because neurovascular compensatory mechanisms can maintain blood pressure by vasoconstriction while flow is decreasing, low blood pressure is a late sign of hypovolemia or impaired cardiac function. Earlier evidence of decreased blood volume or CO includes cold, clammy extremities caused by catecholamine-mediated peripheral vasoconstriction.

Arterial pressure decreases in the following circumstances:

Diastolic pressure must be watched carefully during the administration of vasodilators such as sodium nitroprusside, which may reduce diastolic pressure more rapidly than systolic or mean pressure. Diastolic pressure less than 50 mm Hg and mean pressure less than 60 mm Hg in an adult may result in compromised coronary perfusion.

Arterial pressure increases with the following:

Administration of inotropic agents may or may not increase blood pressure. If a positive inotropic drug stimulates the heart under conditions of inadequate myocardial oxygenation or hypovolemia, the pressure may fall. Additionally, if the inotropic agent also causes vasodilation, the pressure may stay the same or fall as the medication is increased. In addition to systolic and diastolic blood pressure, arterial pressure monitoring allows assessment of pulse and mean arterial pressure.

The pulse pressure is the difference between the systolic and diastolic pressure. Normal pulse pressure is 30 to 40 mm Hg and is a reflection of left ventricular stroke volume (SV) and arterial system compliance. A decreasing pulse pressure is a sign of low SV. An increasing SV in a patient receiving fluid therapy is consistent with improved preload.

Mean arterial pressure (MAP) is an average of pressures in the systemic circulation and thus the pressure best associated with the adequacy of tissue perfusion. The normal reference range for MAP is 70 to 105 mm Hg. MAP is not an arithmetic average of systolic and diastolic pressures because the cardiac cycle spends about twice as long in diastole as in systole when the heart rate is normal. Most monitors compute MAP and display it digitally. MAP can be estimated mathematically by either of the following formulas:

MAP=1/3 Pulse pressure+Diastolic pressure

or

MAP=Systolic pressure+(diastolic pressure × 2)3

Circulation to the vital organs (i.e., kidneys, coronary arteries, and brain) may be compromised when MAP falls below 60 mm Hg. In such cases, the patient may need fluid therapy or medications to increase left ventricular contractility (inotropics) or to increase vascular resistance. Elevated MAP is associated with increased risk for stroke and heart failure. Pharmacologic treatment of elevated MAP may include vasodilators or negative inotropic agents. MAP is used in calculating derived hemodynamic variables such as systemic vascular resistance, left ventricular stroke work, and cardiac work (see Chapter 16).

Ischemia resulting from embolism, thrombus, or arterial spasm is the major complication of direct arterial monitoring. It is evidenced by pallor distal to the insertion site and usually is accompanied by pain and paresthesia (numbness and tingling). Ischemia can proceed to tissue necrosis if the catheter is not repositioned or removed. Thrombosis is prevented by irrigation with diluted heparinized solution. Bolus irrigation is done in very small amounts because flushing the line can result in retrograde flow and cerebral embolization.

Indications for CVP Monitoring

CVP monitoring is indicated to assess the circulating blood volume (adequacy of cardiac filling), adequacy of venous return, or right ventricular function. Patients who have had major surgery or blood loss caused by trauma and those suspected of severe dehydration may benefit from placement of a CVP catheter to guide fluid replacement therapy. Patients with either cardiogenic or noncardiogenic pulmonary edema also need CVP monitoring to guide fluid therapy. In addition, CVP measurements are useful in evaluating patients suspected of having right ventricular damage due to myocardial infarction. Once the catheter is in place, the line can be used for rapid infusion of fluids or medications and to obtain blood samples for measurement of routine laboratory studies (e.g., complete blood counts and electrolytes).

CVP Catheters and Insertion Sites

The most common central venous catheters are 7-French, triple-lumen catheters with one distal port and two ports 3 to 4 cm from the distal end of the catheter (Fig. 15-4). The multiple-lumen catheter allows infusion of blood and various medications and solutions through different ports and permits aspiration of blood samples or injections for CO measurements without interrupting the infusion of medication. Catheters with walls that are impregnated with antibiotics are less commonly associated with infection than standard catheters.

Common sites for introduction of central venous catheters include the subclavian and internal jugular veins (Fig. 15-5). An advantage of the subclavian vein approach is that it results in a much more stable catheter after placement. Disadvantages of the subclavian vein approach are that it is technically more difficult because the vein is harder to find and the catheter guidewire does not follow the subclavian vein as easily as it turns to form the superior vena cava. The subclavian vein is close to the subclavian artery, which is easily punctured, and the mediastinum can hold a fair amount of blood without external evidence of blood loss. The pleural surface is not far below the vein, so pneumothorax is a potential complication of the procedure.

The internal jugular vein approach is easier because there is nearly a straight shot for the guidewire to reach the superior vena cava and less risk for pneumothorax, and hematomas are easier to see and control. Disadvantages of the internal jugular vein approach are that the catheter is much less stable after placement and is subject to kinking, breakage, and accidental removal.

A chest radiograph should be performed after central venous line insertion to ensure proper placement and to rule out pneumothorax.

Procedure for Inserting a CVP Catheter

Central venous line kits commonly include a needle for venous penetration, a stiff plastic dilator, and a guidewire coiled in a plastic sheath with a “J” tip to prevent venous wall penetration. The J tip is held straight by a small separate sheath to accommodate entry into the hub of the insertion needle.

The technique is nearly identical for subclavian and internal jugular line insertions. Normally, the head of the patient’s bed is lowered, which increases venous pressure and causes the vein to swell, making it easier to penetrate and thread the guidewire. This also decreases the risk for inadvertent air embolism. The subclavian vein is entered from an insertion site at the edge of the distal third of the clavicle. The internal jugular vein can be entered from the head of the clavicle or a site behind the brachial artery.

The catheter lumens are flushed with heparinized saline, and the cap is removed from the lumen with the distal port. The guidewire is inserted and threaded, and the needle withdrawn. A dilator is inserted over the guidewire and removed, and the distal port of the CVP catheter is threaded onto the guidewire. It is advanced to a depth that should leave the tip in the superior vena cava. The guidewire is then removed, the hub replaced, the port flushed with heparinized saline, and the catheter secured in position. A chest radiograph typically is taken after insertion to verify the tip of the catheter is in the superior vena cava just above the right atrium.

Once inserted and secured, the CVP catheter is attached to a flushed and calibrated monitoring system like that used for pressure measurement through an arterial line (see Fig. 15-1). However, because central venous pressures typically are much lower than arterial pressures, two key differences in the procedure are required. First, the monitor scale for CVP measurement should be set to the low range, typically 0 to 30 mm Hg. Second, to assure accuracy in measurement and interpretation, the pressure transducer must be placed level with the patient’s right atrium, identified externally at the phlebostatic axis. As indicated in Figure 15-6, the phlebostatic axis is located at the intersection of the fourth intercostal space and midaxillary line. Positioning of the transducer below the phlebostatic axis will result in erroneously high CVP readings, whereas positioning the transducer above this level will cause the reading to be lower than the patient’s actual value.

Central Venous and Atrial Pressure Waveforms

The CVP waveform reflects pressure changes in the right atrium and normally consists of three waves for each cardiac cycle: a, c, and v (Fig. 15-7).

The a wave results from atrial contraction and occurs during ventricular diastole. When there is no atrial contraction (atrial fibrillation), there is no a wave. Conversely, when the atrium contracts against a closed valve, as occurs during atrioventricular (AV) dissociation or with some junctional or ventricular pacemaker rhythms, large a waves called cannon waves occur. The downslope of the a wave (X descent) results from the decrease in atrial pressure as the blood moves into the ventricle and ends with the closure of the tricuspid valve (tricuspid on right, mitral on left).

The c wave occurs at the completion of AV valve closure. It represents the movement or bulging of the AV valve back toward the atrium during ventricular contraction.