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Critically ill patients require continuous assessment of their cardio-vascular system to diagnose and manage their complex medical conditions. This is most commonly achieved by the use of direct pressure monitoring systems, often referred to as hemodynamicmonitoring. Central venous pressure (CVP), pulmonary arterypressure, and intra-arterial BP monitoring are common forms of hemodynamic monitoring. Patients requiring hemodynamic monitoring are cared for in specialty critical care units. Some crit-ical care step-down units also admit stable patients with CVP or intra-arterial BP monitoring. Noninvasive hemodynamic moni-toring is used in some facilities.
To perform invasive monitoring, specialized equipment is necessary and includes the following:
· A CVP, pulmonary artery, or arterial catheter, which is in-troduced into the appropriate blood vessel or heart chamber
· A flush system composed of intravenous solution (which may include heparin), tubing, stopcocks, and a flush device, which provides for continuous and manual flushing of the system
· A pressure bag placed around the flush solution that is main-tained at 300 mm Hg of pressure; the pressurized flush system delivers 3 to 5 mL of solution per hour through the catheter to prevent clotting and backflow of blood into the pressure monitoring system
· A transducer to convert the pressure coming from the artery or heart chamber into an electrical signal
· An amplifier or monitor, which increases the size of the elec-trical signal for display on an oscilloscope
The CVP, the pressure in the vena cava or right atrium, is used to assess right ventricular function and venous blood return to the right side of the heart. The CVP can be continuously measured by connecting either a catheter positioned in the vena cava or the proximal port of a pulmonary artery catheter to a pressure mon-itoring system. The pulmonary artery catheter, described in greater detail later, is used for critically ill patients. Patients in general medical-surgical units who require CVP monitoring may have a single-lumen or multilumen catheter placed into the superior vena cava. Intermittent measurement of the CVP can then be ob-tained with the use of a water manometer.
Because the pressures in the right atrium and right ventricle are equal at the end of diastole (0 to 8 mm Hg), the CVP is also an indirect method of determining right ventricular filling pres-sure (preload). This makes the CVP a useful hemodynamic pa-rameter to observe when managing an unstable patient’s fluid volume status. CVP monitoring is most valuable when pressures are monitored over time and are correlated with the patient’s clin-ical status. A rising pressure may be caused by hypervolemia or by a condition, such as HF, that results in a decrease in myocardial contractility. Pulmonary artery monitoring is preferred for the patient with HF. Decreased CVP indicates reduced right ven-tricular preload, most often caused by hypovolemia. This diag-nosis can be substantiated when a rapid intravenous infusion causes the CVP to rise. (CVP monitoring is not clinically useful in a patient with HF in whom left ventricular failure precedes right ventricular failure, because in these patients an elevated CVP is a very late sign of HF.)Before insertion of a CVP catheter, the site is prepared by shaving if necessary and by cleansing with an antiseptic solution. A local anesthetic may be used. The physician threads a single-lumen or multilumen catheter through the external jugular, ante-cubital, or femoral vein into the vena cava just above or within the right atrium.
Once the CVP catheter is inserted, it is secured and a dry, sterile dressing is applied. Catheter placement is confirmed by a chest x-ray, and the site is inspected daily for signs of infection. The dressing and pressure monitoring system or water manometer are changed according to hospital policy. In general, the dressing is to be kept dry and air occlusive. Dressing changes are performed with the use of sterile technique. CVP catheters can be used for infus-ing intravenous fluids, administering intravenous medications, and drawing blood specimens in addition to monitoring pressure.
To measure the CVP, the transducer (when a pressure mon-itoring system is used) or the zero mark on the manometer (when a water manometer is used) must be placed at a standard reference point, called the phlebostatic axis (Fig. 26-12).
After lo-cating this position, the nurse may make an ink mark on the patient’s chest to indicate the location. If the phlebostatic axis is used, CVP can be measured correctly with the patient supine at any backrest position up to 45 degrees. The range for a normal CVP is 0 to 8 mm Hg with a pressure monitoring system or 3 to 8 cm H2O with a water manometer system. The most common complications of CVP monitoring are infection and air embolism.
Pulmonary artery pressure monitoring is an important tool used in critical care for assessing left ventricular function, diagnosing the etiology of shock, and evaluating the patient’s response to medical interventions (eg, fluid administration, vasoactive med-ications). Pulmonary artery pressure monitoring is achieved by using a pulmonary artery catheter and pressure monitoring sys-tem. Catheters vary in their number of lumens and their types of measurement (eg, cardiac output, oxygen saturation) or pacing capabilities. All types require that a balloon-tipped, flow-directed catheter be inserted into a large vein (usually the subclavian, jugu-lar, or femoral vein); the catheter is then passed into the vena cava and right atrium. In the right atrium, the balloon tip is inflated, and the catheter is carried rapidly by the flow of blood through the tricuspid valve, into the right ventricle, through the pulmonic valve, and into a branch of the pulmonary artery. When the cathe-ter reaches a small pulmonary artery, the balloon is deflated and the catheter is secured with sutures. Fluoroscopy may be used during insertion to visualize the progression of the catheter through the heart chambers to the pulmonary artery. This procedure can be performed in the operating room or cardiac catheterization laboratory or at the bedside in the critical care unit. During in-sertion of the pulmonary artery catheter, the bedside monitor is observed for waveform and ECG changes as the catheter is moved through the heart chambers on the right side and into the pul-monary artery.
After the catheter is correctly positioned, the following pres-sures can be measured: CVP or right atrial pressure, pulmonary artery systolic and diastolic pressures, mean pulmonary artery pressure, and pulmonary artery wedge pressure (Fig. 26-13). If a thermodilution catheter is used, the cardiac output can be mea-sured and systemic vascular resistance and pulmonary vascular re-sistance can be calculated.
Normal pulmonary artery pressure is 25/9 mm Hg, with a mean pressure of 15 mm Hg (see Fig. 26-5 for normal ranges). When the balloon tip is inflated, usually with 1 mL of air, the catheter floats farther out into the pulmonary artery until it becomes wedged. This is an occlusive maneuver that impedes blood flow through that segment of the pulmonary artery. A pressure measurement, called pulmonary artery wedge pressure, is taken within seconds after wedging of the pulmonary artery catheter; then the balloon is immediately deflated and blood flow is restored. The nurse who obtains the wedge reading ensures that the catheter has returned to its normal position in the pulmonary artery by evaluating the pul-monary artery pressure waveform. The pulmonary artery diastolic reading and the wedge pressure reflect the pressure in the ventricle at end-diastole and are particularly important to monitor in criti-cally ill patients, because they are used to evaluate left ventricular filling pressures (preload). At end-diastole, when the mitral valve is open, the wedge pressure is the same as the pressure in the left atrium and the left ventricle, unless the patient has mitral valve dis-ease or pulmonary hypertension. Pulmonary capillary wedge pres-sure is a mean pressure and is normally 4.5 to 13 mm Hg. Critically ill patients usually require higher left ventricular filling pressures to optimize cardiac output. These patients may need to have their wedge pressure maintained as high as 18 mm Hg.
Catheter site care is essentially the same as for a CVP catheter. As in measuring CVP, the transducer must be positioned at the phlebostatic axis to ensure accurate readings (see Fig. 26-12). Complications of pulmonary artery pressure monitoring include infection, pulmonary artery rupture, pulmonary thromboem-bolism, pulmonary infarction, catheter kinking, dysrhythmias, and air embolism.
Intra-arterial BP monitoring is used to obtain direct and contin-uous BP measurements in critically ill patients who have severe hypertension or hypotension (Fig. 26-14). Arterial catheters are also useful when arterial blood gas measurements and blood sam-ples need to be obtained frequently.
Once an arterial site is selected (radial, brachial, femoral, or dorsalis pedis), collateral circulation to the area must be con-firmed before the catheter is placed. This is a safety precaution to prevent compromised arterial perfusion to the area distal to the arterial catheter insertion site. If no collateral circulation exists and the cannulated artery became occluded, ischemia and infarc-tion of the area distal to that artery could occur. Collateral circu-lation to the hand can be checked by the Allen test to evaluate the radial and ulnar arteries or by an ultrasonic Doppler test for any of the arteries. With the Allen test, the nurse compresses the ra-dial and ulnar arteries simultaneously and asks the patient to make a fist, causing the hand to blanch. After the patient opens the fist, the nurse releases the pressure on the ulnar artery while maintaining pressure on the radial artery. The patient’s hand will turn pink if the ulnar artery is patent.
Site preparation and care are the same as for CVP catheters. The catheter flush solution is the same as for pulmonary artery catheters. A transducer is attached, and pressures are measured in millimeters of mercury (mm Hg). Complications include local obstruction with distal ischemia, external hemorrhage, massive ecchymosis, dis-section, air embolism, blood loss, pain, arteriospasm, and infection.
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