12 CENTRAL VENOUS AND PULMONARY ARTERY PRESSURE MONITORING
Dipin Gupta M.D., Glenn J.R. Whitman M.D., Alden H. Harken M.D.

1. What does a catheter in the central venous circulation measure?

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All intrathoracic veins have nearly the same pressure. A catheter in the central venous circulation (anywhere) measures this central venous pressure (CVP) (or right atrial pressure). CVP, plus a little right atrial “kick,” pushes blood into the right ventricle. This right ventricular “filling pressure” is also termed preload.

2. What does a pulmonary artery (PA) catheter measure?

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A PA catheter (Swan-Ganz catheter) is threaded through the central venous circulation out into the PA. The catheter has three ports-one at the tip and side ports at 4 cm from the tip (the “VIP port”) and 29 cm from the tip (the “CVP port”). With inflation of the balloon at the distal catheter tip and subsequent occlusion of a pulmonary capillary vessel, the transducer at the tip of the catheter “sees” only a static column of blood between it and the left atrium. This pulmonary capillary wedge pressure approximates left atrial pressure or left ventricular filling pressure or LV preload.
When pulmonary vascular resistance is normal, PA diastolic pressure can be used as a substitute for wedge or left atrial pressure. It is not necessary in this circumstance to inflate the balloon to estimate the wedge pressure. This spares the patient the risk of PA rupture from balloon inflation (another advantage is that you do not need to get up to replace the Swan-Ganz catheter when the balloon breaks-usually at 2 a.m.).
A PA catheter can measure blood pressure at three points:

1. The level of the superior vena cava (CVP)
2. The PA (with the balloon deflated)
3. The pulmonary venous pressure/left atrial pressure (with the balloon inflated)

Other important parameters, most importantly cardiac output and mixed venous oxygen saturation, can be measured or calculated based on numbers derived from the PA catheter (see questions 9 and 10).

3. Discuss the complications of central venous catheters and PA catheters.

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Immediate complications are pneumothorax (2%); inadvertent arterial cannulation (2%); catheter malposition (7%); and, more rarely, air embolism, hemothorax, chylothorax, arrhythmia, brachial plexus injury, vocal cord paralysis, and death (each substantially less frequent than 1%).4 Additionally, “floating” a hard PA catheter across the tricuspid valve and through the right ventricular outflow tract holds the potential for ventricular tachycardia (and if you “nudge” the atrioventricular node, you can provoke complete heart block).
Delayed complications are thrombosis (33% by radiographic studies) and less commonly bacteremia, endocarditis, or clavicular osteomyelitis. Fibrin forms on the catheter within hours of insertion, and the incidence of vessel thrombosis increases with time. PA-related bloodstream infections occur in 4.8 cases per 1000 catheter-days.2 This is roughly equivalent to one bloodstream infection among 100 patients with a catheter in place for 2 days. In autopsy series (clearly not healthy patients), the incidence of infective endocarditis is usually < 2% but increases dramatically with increasing insertion duration.2

4. What are the relative contraindications to percutaneous subclavian or internal jugular venous catheterization?

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In a patient who is anticoagulated or who has a platelet count < 50,000, it is typically safer to place a central venous line by peripheral cutdown. Inadvertent arterial puncture is tolerated fairly well unless the patient is coagulopathic. A patient with hyperinflated lungs (chronic obstructive pulmonary disease) is more likely to have a pneumothorax during catheter placement.

5. How do you percutaneously place a sheath for PA catheter placement?

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1. Place the patient in mild head-down (Trendelenburg) position and turn the head toward the contralateral side.
2. Using sterile technique and after administering local anesthesia, insert an 18-G needle on a 10-mL syringe at the point where the deltopectoral groove abuts the clavicle and pointing just north of the suprasternal notch. Hugging the undersurface of the clavicle, apply gentle suction with the syringe. When you hit the vein, dark (nonpulsatile) blood easily flows back into the syringe.
3. Remove the syringe, and insert a soft, flexible wire through the 18-G needle.
4. Remove the needle, leaving the flexible wire in place.
5. Slide a plastic sheath (with the dilator inside) over the guidewire. Remove the wire and the dilator, leaving the sheath in place (if the sheath bleeds profusely, you are in the right place). Aspirate the catheter fully to evacuate all air and flush with saline (Figure 12-1).
6. A chest x-ray must be obtained to confirm proper position and exclude pneumothorax and hemothorax.

Figure 12-1 Catheter placement by percutaneous subclavian vein puncture.

6. As a PA catheter passes through the central venous circulation, what do the pressure waveforms look like?

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See Figure 12-2.

Figure 12-2 Pressure waveforms after insertion of a Swan-Ganz catheter.

7. What is the value of the CVP and PA pressure?

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Starling’s law states that (up to a point) increasing end-diastolic volume (preload) increases stroke volume (volume of blood ejected during systole, which is multiplied by heart rate to yield cardiac output). Clinically, we cannot measure end-diastolic volume, so filling pressures are used as a surrogate.
CVP is an estimate of the pressure with which blood flows into the right side of the heart. This number does not reflect left-sided filling pressures. As stated earlier, PA diastolic or wedge pressures allow a better estimate of left-sided filling pressure.

8. Name other parameters that can be measured or calculated with use of a PA catheter.

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Cardiac output, venous oxygen saturation, pulmonary and systemic vascular resistance.

9. How is cardiac output measured?

There are two ways to use a PA catheter to calculate cardiac output:

1. The technique of thermodilution, in which a volume (10 mL) of saline with known temperature (108°C) is injected into the proximal port of a PA catheter. A temperature probe at the distal catheter tip measures the change in temperature of blood from the time when the cold saline was injected and the time that it passes by the probe. The precise volume and temperature of the injectate allow calculation of the amount of blood passing by the probe, which is a measure of cardiac output. Because cardiac output changes by 15% during the respiratory cycle, injection should be synchronized with end-expiration. A left-to-right intracardiac shunt adds warm blood to the cold saline bolus, giving a falsely elevated measurement of cardiac output.
2. The Fick principle, which relates cardiac output to venous oxygen saturation (see question 11).

10. How is the oxygen content of blood calculated?

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An oximetric PA catheter has a fiberoptic monitor at its distal tip that continuously measures hemoglobin saturation [So2 (%)]. The catheter tip in the PA measures mixed venous blood (Svo2) oxygenation. After 24 hours of placement, the catheter becomes covered with fibrin, and measurements become less reliable.
The amount of oxygen in blood (Cao2) comprises that portion dissolved in blood (almost nothing) and that portion attached to hemoglobin (lots).
The amount dissolved is calculated by:

O2 dissolved = 0.003 x PaO2

The amount attached to hemoglobin is calculated by:

O2 attached = 1.38 x [Hb] x SaO2

For example, if hemoglobin = 12 g/dL, Pao2 = 60 mmHg, and Sao2 = 90%, then Cao2 = (0.003 × 60) + (1.38 × 12 × 0.90) = 15.08 mL oxygen/100 mL blood. Dissolved oxygen usually comprises only a small percentage of Cao2 (< 1% in this example). Clinically, it is excluded from calculations (see Chapter 6).

11. How is the oxygen content of the blood used?

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Assuming normal parameters of hemoglobin = 15 g/dL, Sao2 = 96%, and Svo2 = 75%, the difference in oxygen content between the arterial circulation and the venous circulation (A-Vo2) is 4.35 vol%. For every 100 mL of blood that travels around the body, the tissues extract 4.35 mL of oxygen. The normal range for the A-Vo2 is 3-5 vol%.
The Fick principle uses this A-Vo2 to determine cardiac output. Nonstressed patients typically consume oxygen at the rate of 125 mL/min/m2. This is really a “wild guess” because we do not usually determine A-VO2 unless a patient is stressed. By measuring the A-Vo2, we can determine the oxygen contribution for each 100 mL of blood that travels around the body. If the measured A-Vo2 difference = 4.35 mL of oxygen, every 100 mL of cardiac output contributes 4.35 mL to the Vo2 of 250 mL (for a person who is 2 m2, or 2 × 125 mL/min/m2). A total of 5.75 L of blood must travel around the patient’s body each minute to meet the oxygen requirement. By “assuming” Vo2 (typically a big assumption) and by calculating the A-VO2, one can approximate cardiac output.

12. Explain the significance of the Svo2.

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This is a “poor man’s” cardiac output measure. In a patient with a fixed metabolic rate (or stable oxygen consumption), as cardiac output increases (delivering more blood/min and more oxygen/min), the patient extracts less oxygen per 100 mL of blood peripherally, and more oxygen per 100 mL returns to the right side of the heart (as your patient gets healthier, Svo2 rises). Conversely, as cardiac output decreases (delivering less oxygen/min peripherally to meet fixed demand), the patient extracts more oxygen per 100 mL of blood. Returning venous blood contains less oxygen, and Svo2 decreases. Knowing the the differential diagnosis of a falling Svo2 is important: (1) progressive anemia, (2) cardiac failure, (3) decreasing arterial saturation, and (4) increased basal metabolic rate. The differential diagnosis of a rising Svo2 is (1) sepsis, (2) left-to-right intracardiac shunt, (3) left-to-right peripheral shunt (dialysis access), and (4) inadvertent wedging of the pulmonary artery catheter. The other more gratifying possibility is that your patient is improving in response to your therapy!

KEY POINTS: Svo2 TRENDS

1. “Poor man’s” estimation of cardic output
2. Decreased Svo2: progressive anemia, cardiac failure, decreasing arterial saturation, increased basal metabolic rate
3. Increased Svo2: sepsis, cyanide toxicity, left-to-right intracardiac shunt, left-to-right peripheral shunt, inadvertent wedging of PA catheter

13. How do you determine the systemic (peripheral) vascular resistance (SVR)?

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SVR = [(MAP - CVP)/CO] x 80
where SVR = systemic vascular resistance (dyne • sec/cm-5), MAP = mean arterial blood pressure (mmHg), CVP = central venous pressure (mmHg), and CO = cardiac output (L/min).
Normal SVR is 800-1200 dyne • sec/cm-5. Multiplying by 80 corrects SVR values from Wood units (mmHg/L/min) to standard metric units (dyne • sec/cm-5).

14. How is a PA catheter used to evaluate shock?

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Management of the patient in shock requires knowledge of intracardiac “filling” pressures (CVP, PA pressure), cardiac ouput, SVR, and Svo2. Prompt PA catheter placement guides therapy (see Chapter 4 and Table 12-1).
Hypovolemic shock. Right and left filling pressures (CVP and wedge/PA pressures) are low, as are cardiac output and Svo2. SVR is high. The diagnosis is confirmed when volume repletion with rising filling pressure is associated with increased cardiac output, normalization of system pressure, and decreased SVR.
Table 12-1. PA CATHETER EVALUATION OF SHOCK

Cardiogenic shock. Shock despite adequate filling pressures means that the pump is failing. Cardiac output and SvO2 are low. If SVR is high, infuse dobutamine, 5 μg/kg/min, to stimulate the heart and reduce SVR. If SVR is low, infuse epinephrine, 0.05 μg/kg/min, to stimulate the heart and increase SVR.
Septic shock. The hallmarks of septic shock are normal or low-normal filling pressure, supranormal cardiac output, high Svo2, and low SVR (< 600 dyne • sec/cm-5). Treatment requires fluid resuscitation and systemic vasoconstriction while the underlying cause (e.g., abdominal abscess) is treated.

15. What is the evidence supporting the use of a PA catheter?

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There is no definitive evidence in support of PA catheterization. A prospective trial of > 5700 patients with various disease processes (mostly medical patients) revealed that patients who underwent PA catheterization had higher 30-day mortality, higher hospital costs, and longer intensive care unit length of stay.1
Regardless, we recommend PA catheterization3 for patients with cardiogenic shock, unexplained shock, or unexplained acidosis; all patients undergoing peripheral vascular surgery; and high-risk patients undergoing aortic surgery. Traumatically injured patients, patients with respiratory failure, and critically ill pediatric patients may benefit as well. If you cannot determine what the patient’s volume status is, insert a PA catheter.

16. Do central venous catheters or PA catheters need to be changed on a regular basis?

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In accordance with Centers for Disease Control guidelines, central venous catheters do not need to be replaced routinely if the exit wounds are dressed properly and sterilized routinely. PA catheters should be changed every 5 days to minimize risks of thrombus and infection.
When catheter-related infection is documented, a new catheter must be placed at a different location. Removed catheters in the setting of bacteremia are always sent for culture.

References
WEB SITES

1. http://www.acssurgery.com/abstracts/acs/acs0606.htm
2. http://www.acpmedicine.com/abstracts/sam/med1401.htm

BIBLIOGRAPHY
1. Connors AF, Speroff T, Dawson NV, et al: The effectiveness of right heart catheterization in the initial care of critically ill patients. JAMA 276:889-897, 1996. Medline Similar articles Full article
2. Mermel LA, Maki DG: Infectious complications of Swan-Ganz pulmonary artery catheters. Am J Respir Crit Care Med 149:1020-1036, 1994. Medline Similar articles
3. Pulmonary Artery Consensus Conference Participants: Pulmonary artery consensus conference: Consensus statement. Crit Care Med 25:910-925, 1997.
4. Ruesch S, Walder B, Tramer MR: Complications of central venous catheters: Internal jugular versus subclavian access: A systematic review. Crit Care Med 30:454-460, 2002. Medline Similar articles Full article