a. Review capnography data in the patient’s chart (Code: IA7e) [Difficulty: ELE: R; WRE: Ap]

Capnography is the analysis of graphic and numeric data from a patient showing the pattern and amount of exhaled carbon dioxide (CO₂). It is wise to look for previous capnography data before measuring the patient’s exhaled CO₂ level again. Look for numeric values as well as a printout of the tracing of exhaled CO₂. Be prepared to compare the previous information with the new data to help evaluate the patient’s condition. It is also useful to compare the patient’s pressure of CO₂ in arterial blood (PaCO₂) with the capnography information, even though they should not be expected to be the same.

b. Recommend capnography to obtain additional data (Code: IC10) [Difficulty: ELE: R, Ap; WRE: An]

There are three main reasons to recommend capnography. The first reason is to assess a patient’s ventilation. General anesthesia and even conscious sedation can result in a blunting of the patient’s normal drive to breathe. Several clinical or pathologic situations (discussed later in this chapter) can result in an abnormal increase or decrease in the carbon dioxide level. Second, capnography helps evaluate the effectiveness of cardiopulmonary resuscitation efforts. See the discussion in Chapter 11. Third, capnography can be used to help identify the proper placement of an endotracheal tube. If the tube is within the patient’s trachea or a main stem bronchus, the exhaled tidal volume will contain carbon dioxide. See the discussion in Chapter 12.

c. Perform the bedside procedure (Code: IB9c and IIIE3d) [Difficulty: ELE: R; WRE: Ap, An]

The capnometer is a device that measures the concentration of CO₂ in a gas sample from a patient. The principle of operation of most bedside units is based on carbon dioxide’s absorption of infrared light in a narrow wavelength band (4.3 μ). Infrared light at this wavelength is passed through the gas sample to a receiving unit. The difference between what is transmitted and what is received is directly related to how much carbon dioxide is in the gas sample. In other words, the greater the difference between the sent and the received infrared light, the greater the concentration of carbon dioxide in the gas sample.

The capnometer is calibrated by comparing a gas sample without carbon dioxide (possibly room air) with a second gas sample containing a known amount of carbon dioxide. The first gas sample without CO₂ should give a “zero” reading. Adjust the calibration control to zero if needed. The second sample usually contains 5% to 10% carbon dioxide. The capnometer should display a CO₂ level that matches the amount in the known gas sample. Adjust the calibration control as necessary. The carbon dioxide level can be documented as a percent or fraction (F_ACO₂) or as a partial pressure (P_ACO₂).

The capnograph is a strip chart recorder that provides a copy of the patient’s exhaled carbon dioxide curve. There are at least two paper speeds that are useful for different purposes. The fast speed is most useful for evaluating sudden changes in the patient’s condition. Each individual breath is easily seen (Figure 5-1). The slow speed is most useful for trend monitoring (Figure 5-2).

Figure 5-1 Normal capnograph tracing taken at a fast speed. The percentage of exhaled alveolar CO₂ is shown on the left vertical scale as F_ACO₂. The partial pressure of exhaled alveolar CO₂ is shown on the right vertical scale as P_ACO₂. The tracing of exhaled gas can be divided into these four components: A, the beginning of exhalation, which shows no carbon dioxide in the upper airway anatomic dead space; B, the addition of alveolar gas, rich in carbon dioxide, to the anatomic dead space gas causes a rapid rise in measured CO₂ level; C, pure alveolar gas with a stable amount of CO₂ causes a plateau—the end-tidal CO₂ point is shown at C just before inspiration; and D, inspiration with a rapid drop in carbon dioxide concentration to zero. Compared to a slow-speed tracing, a fast-speed tracing is more useful for determining the cause of a patient’s changing condition.

Figure 5-2 Normal capnograph tracing taken at a slow speed. The percentage of exhaled alveolar CO₂ is shown on the left vertical scale as F_ACO₂. The partial pressure of exhaled alveolar CO₂ is shown on the right vertical scale as P_ACO₂. The slow speed results in a blending of parts A, B, and C of the fast-speed tracing (Figure 5-1). Each spike is part C of the curve and marks an exhalation. A slow-speed tracing is more useful in trend monitoring of a patient than a fast-speed tracing.

Two different gas sampling methods exist: mainstream and sidestream. The mainstream method involves having the infrared sensing unit at the airway; usually it is attached directly to the endotracheal/tracheostomy tube. If the patient is on a ventilator, the sampling adapter must be placed between the endotracheal tube and the ventilator circuit (with or without mechanical dead space). All inspired and expired gas passes through the sensor (Figure 5-3).

Figure 5-3 Schematic drawing of a mainstream exhaled carbon dioxide analyzer sensor. The infrared sensor is attached to the patient’s airway so that all of the exhaled and inhaled gas passes through it.

Rights were not granted to include this figure in electronic media. Please refer to the printed book.

(From Szaflarski NL, Cohen NH: Heart Lung 20:363-374, 1991.)

The sidestream method employs a capillary tube placed so that a small sampling of the patient’s exhaled gas can be drawn into the capnometer for analysis. It is not necessary for the patient’s entire breath to pass through the sampling adapter; therefore it can be used in an unintubated patient by taping the sampling catheter a short distance into a nostril. If the patient is on a ventilator, the sampling adapter must be placed between the endotracheal tube and the ventilator circuit (with or without mechanical dead space). Remember that the patient’s exhaled tidal volume (V_T) and minute volume (_E) are reduced by the amount that is drawn into the capnometer (Figure 5-4).

Figure 5-4 Representation of a sidestream capnometry system. A capillary tube is placed into the ventilator circuit to sample the exhaled and inhaled gases.

d. Interpret the results from the procedure (Code: IB10c, IIIE4e) [Difficulty: ELE: R, Ap; WRE: An]

It is known that carbon dioxide diffuses from the higher concentration in the tissues to the venous blood and to the lungs to be exhaled. Figure 5-5 shows the normal physiology behind capnography. This diffusion or “flow” of CO₂ results in a measurable gradient or difference. In a healthy, upright sitting person, a close relationship exists between carbon dioxide levels in both venous blood and arterial blood and the amount of exhaled carbon dioxide gas. The carbon dioxide level at the end of exhalation is most frequently monitored during patient care. This is called the end-tidal carbon dioxide pressure (PetCO₂). When ventilation and perfusion match well, as in a healthy upright person, the gradient between the arterial carbon dioxide level (PaCO₂) and the PetCO₂ is between 2 and 3 torr, with a range of 1 to 5 torr. The gradient will show the PetCO₂ level to be less than the PaCO₂ level. This is because the PetCO₂ value is an average of exhaled carbon dioxide levels from all lung areas.

Figure 5-5 Representation of the alveolar-capillary membrane showing the diffusion of oxygen and carbon dioxide and the arterialization of venous blood. Also shown are alveolar CO₂ values measured by capnography.

Box 5-1 lists normal values of capnography. Three factors influence capnography’s use and the interpretation of the results. (1) The first factor is the patient’s metabolism. The average resting adult produces about 200 mL of CO₂ per minute, and fever and exercise increase this value. Hypothermia, sleep, and sedation decrease CO₂ production. Exhaled CO₂ is monitored during a cardiopulmonary resuscitation (CPR) attempt to determine the effectiveness of circulation and ventilation attempts and to decide if the efforts should be continued or stopped. If no carbon dioxide is being exhaled despite proper CPR procedures, the physician may conclude that the patient’s metabolism has stopped altogether and death has occurred. There would then be nothing to gain by continuing cardiopulmonary resuscitation (CPR) efforts.

(2) Although not a major factor, the patient’s cardiac output is another factor that influences the use of capnography. Sepsis, which might double a patient’s cardiac output, reduces the PCO₂ level only a few torr (millimeters of mercury [mm Hg]). Cardiogenic shock, which reduces the cardiac output, raises the partial pressure of CO₂ (PCO₂) only a few torr.

(3) The third and most important factor is alveolar ventilation. A doubling of alveolar ventilation, under steady-state conditions for carbon dioxide production, results in a halving of the PCO₂ levels in arterial blood and alveolar gas. However, a reduction of alveolar ventilation to half of its previous level will result in the PaCO₂ and P_ACO₂ levels being doubled (Figure 5-6).

Figure 5-6 Relationship between alveolar ventilation, PaCO₂, and exhaled percent CO₂.

(From Pilbeam SP: Mechanical ventilation: physiological and clinical applications, ed 4, St Louis, 2006, Mosby.)

Tidal volume (V_T) and respiratory rate are directly related to alveolar ventilation. Of the two, tidal volume is more important because it relates to the patient’s dead space (V_D) to V_T ratio (V_D/V_T). A decrease in the patient’s V_T level results in less alveolar ventilation and a rise in PCO₂ level. Conversely, an increase in the V_T level results in more alveolar ventilation and a drop in the PCO₂ level.

Capnography is most accurate and correlates best with the PaCO₂ level if the patient’s ventilation and perfusion match. The more ventilation and perfusion mismatching there is, or the more unstable the pulmonary perfusion, the wider or less reliable is the gradient between the patient’s arterial carbon dioxide and alveolar carbon dioxide levels. The following procedures should be performed to help understand the patient’s condition and interpret the capnography results.

Remember that mm Hg (millimeters of mercury) and torr (Torricelli) are equivalent units of pressure. The National Board of Respiratory Care (NBRC) uses these two units for different items in its questions. It uses mm Hg for blood pressure measurements and torr for blood gas values (such as PaCO₂ and PO₂). However, in questions regarding capnography, the NBRC has used both mm Hg and torr in its questions that relate to exhaled CO₂ (such as P_ACO₂ and PetCO₂).

a. Perform the bedside procedure (Code: IB9c and IIIE3d) [Difficulty: ELE: R; WRE: Ap, An]

The arterial–end-tidal carbon dioxide gradient [P(a-et)CO₂] is useful, because once it is reliably determined the patient’s ventilatory condition can be monitored by capnography alone. Drawing an arterial blood sample to measure the PaCO₂ level is less necessary if the patient is stable.

The normal gradient is 2 to 3 torr, with a range of 1 to 5 torr. However, most patients using capnography are not normal. The possible gradient ranges from 6 to 20 torr in unstable patients with cardiopulmonary abnormalities. For example, the patient who is breathing shallowly may have an end-tidal CO₂ level that is greater than the arterial CO₂ level. This is because the patient is not exhaling completely to empty alveolar gas. Therefore determining each patient’s own gradient is important.

Follow these steps:

b. Interpret the results from the procedure (Code: IB10c, IIIE4e) [Difficulty: ELE: R, Ap; WRE: An]

Review the components of a fast-speed capnography tracing in Figure 5-1 to understand a normal person’s expiratory pattern. Figure 5-7 shows eight different abnormal fast-speed capnography tracings. See the figure legend for an explanation of each problem. As the patient returns to normal, the tracing should approach that shown in Figure 5-1.

Figure 5-7 A series of abnormal fast-speed capnography tracings. A, A mechanically ventilated patient with a malfunctioning exhalation valve. Note that the baseline CO₂ level is elevated because the patient’s exhaled breath is measured during an inspiration. Correction of the exhalation valve should result in normal inhalation and exhalation. B, This rapidly rising baseline gas pressure and failure to return to baseline is usually seen when moisture or secretions block the capillary tube. Clearing the obstruction enables the patient’s gas to again reach the analyzer. C, Distortions in the tracing from incomplete exhalation. These may be caused by hiccups, chest compressions during CPR, or inconsistent tidal volume efforts during an asthma attack. D, An obstructive lung disease patient with ventilation and perfusion mismatching. There is no alveolar plateau with a stable CO₂ level. Inhaling a bronchodilator should result in the tracing returning closer to normal. E, A patient with restrictive lung disease showing no plateau of alveolar gas emptying. This is because the alveoli do not empty evenly. F, A sudden drop in carbon dioxide level in the middle of an exhalation indicates that the patient attempted inspiration. This “cleft” is usually seen when a patient who has been pharmacologically paralyzed begins to regain movement. G, Uneven carbon dioxide levels seen at the end of exhalation can be caused by the following: (1) the patient’s heartbeat pumping fresh blood and CO₂ to the emptying lungs; the cardiogenic oscillations should match the heart rate; (2) the ventilator’s exhalation valve is fluttering open and closed. H, The alveolar plateau is biphasic. This has been seen in patients with lungs that are different in compliance and ventilation/perfusion matching (e.g., single lung transplantation).

(From Shapiro BA, Peruzzi WT, Templin R: Clinical application of blood gases, ed 5, St Louis, 1994, Mosby.)

a. Perform the bedside procedure (Code: IB9c and IIIE3d) [Difficulty: ELE: R; WRE: Ap, An]

Have a cooperative patient exhale maximally to residual volume (RV) through the capnography unit. The graphic results will be similar to that shown in Figure 5-8. The usual P(a-RV)CO₂ gradient in a healthy person is about 3 to 5 torr, and should be less than 7 torr. The wider the gradient, the greater is the ventilation to perfusion (/) mismatching. A gradient of more than 13 torr is considered to be markedly abnormal. This may be the case in patients with chronic obstructive pulmonary disease (COPD), pulmonary emboli, left heart failure (LHF), or hypotension.

Figure 5-8 Comparison of several fast-speed capnograph tracings showing how the exhaled carbon dioxide level changes as the patient exhales to residual volume. A normal tracing is matched against abnormal tracings of pulmonary emboli, left heart failure (LHF), and chronic obstructive pulmonary disease (COPD). The normal patient has the same end-tidal CO₂ level as residual volume CO₂ level, showing good matching of ventilation and perfusion. LHF and COPD patients have a narrowing of the gradient as residual volume is approached. The patient with a pulmonary embolism keeps the same gradient at the residual volume as that found at the end of the tidal volume.

(From Pilbeam SP: Mechanical ventilation: physiological and clinical applications, ed 4, St Louis, 2006, Mosby.)

b. Interpret the results from the procedure (Code: IB10c, IIIE4e) [Difficulty: ELE: R, Ap; WRE: An]

When comparing the normal (solid line) tracing in Figure 5-8 with the / mismatching (dashed line) tracing, note the increased gradient at end-tidal CO₂. With continued exhalation to residual volume, the left heart failure and COPD patients have a narrowing of the gradient. This can be used clinically to follow these patients’ progress and response to treatment. The patient with a large pulmonary embolism will not have such a narrowing of the gradient as he or she exhales to residual volume. This patient’s gradient will narrow to normal as the embolism is resolved and the physiologic dead space returns to normal.

Past exams have often had one question that requires the interpretation of capnography results, especially the end-tidal CO₂ value (PetCO₂). Examples include a change in alveolar ventilation, shallow breathing, and CPR attempt.

a. Determine the decimal fraction or percentage of dead space

Place both carbon dioxide values into this formula, which is derived from the original Bohr formula:

In which:

V_D/V_T or V_D the patient’s physiologic dead space

PaCO₂ = the patient’s arterial carbon dioxide pressure

PĒCO₂ = the patient’s average exhaled carbon dioxide pressure

The following example is based on an abnormal adult:

The patient’s V_D fraction can be recorded as 0.6 or 60%. (Note that this equation must be used when the patient’s tidal volume is not known.)

b. Determine the dead space volume

Place both carbon dioxide values into this formula, which is derived from the original Bohr formula:

In which:

V_D V_T or V_D the patient’s physiologic dead space

V_T = the average exhaled tidal volume

PaCO₂ = the patient’s arterial carbon dioxide pressure

PĒCO₂ = the patient’s average exhaled carbon dioxide pressure

The following example is based on a normal adult:

The patient’s physiologic V_D volume = 150 mL. (Note that this equation must be used when the patient’s tidal volume is known.)

In addition, the preceding equation allows calculation of the patient’s V_D/V_T ratio. In this example, it is 150/500, 0.3, or 30%, depending on how it is written.

The recent advent of volumetric capnography technology allows the patient’s dead space to be rapidly determined. These units can simultaneously measure the patient’s exhaled tidal volume and the variable percentage of carbon dioxide found during the exhalation (Figure 5-10). The computer that is integrated with the volumetric capnography unit then calculates the volume of dead space as a fraction of the exhaled tidal volume. Figure 5-11 shows how volumetric capnography can be set up to measure the dead space of a patient requiring mechanical ventilation. This technology permits the rapid assessment of a patient as treatment is being performed. For example, if a patient had a large pulmonary embolism resulting in significant dead space, a clot-dissolving medication such as streptokinase (Kabikinase or Streptase) or alteplase (Activase) would be given. These “clot buster” medications will rapidly dissolve the patient’s pulmonary embolism. With volumetric capnography, the dead space volume can be monitored as it normalizes when blood flow through the lungs is restored.

Figure 5-10 A graphic representation of the factors involved in the calculation of a patient’s dead space volume, or dead space to tidal volume (V_D/V_T) ratio, through volumetric capnography. These units can measure exhaled carbon dioxide (by pressure or percentage) at time intervals as the patient’s tidal volume is exhaled. Review Figure 5-1, as needed, for a normal capnograph tracing.

Figure 5-11 Measurement of volumetric capnography on a patient receiving mechanical ventilation. All measurements are taken between the patient’s endotracheal tube and mechanical ventilator circuit. The mainstream capnograph sensor sends data on exhaled carbon dioxide concentration to the computer within the capnograph monitor. The pressure and flow sensors send data to the computer to determine the exhaled volume. The computer integrates the information to determine anatomic dead space (white area) and alveolar dead space (black area). With areas q and p being calculated as equal, the shaded area is alveolar volume where gas exchange occurs.

a. Decreased V_D/V_T ratio

b. Increased V_D/V_T ratio

c. Increased dead space effect with ventilation greater than perfusion ( > )

Many clinicians believe that a dead space/tidal volume ratio of 0.6 (60%) or more is an indication for mechanically ventilating the patient. A person who is wasting 60% or more of his or her ventilation will soon tire from the work of breathing and go into ventilatory failure.

a. Review blood pressure data in the patient’s chart (Code: IA8b) [Difficulty: ELE: R; WRE: Ap]

Systemic arterial blood pressure (BP) is the force exerted against the walls of the arteries when blood is pumped through them. As one of the vital signs, the patient’s BP should be found in the chart. Review it to know the patient’s normal blood pressure reading, and if it has been stable or variable with the patient’s changing condition.

b. Recommend blood pressure measurement to obtain additional data (Code: IC11) [Difficulty: ELE: R, Ap; WRE: An]

The patient’s blood pressure, and other vital signs, should be monitored as often as clinically prudent. If the patient’s cardiovascular or pulmonary condition is changing frequently, the blood pressure should be measured frequently, even continuously.

c. Perform blood pressure measurement (Code: IB9m, IIIE3e) [Difficulty: ELE: R, Ap; WRE: An]

The general steps in measuring blood pressure were described in Chapter 1. The BP is usually measured on either of the patient’s arms. Necessary equipment includes the proper size of blood pressure cuff, a sphygmomanometer to measure the pressure, and a stethoscope to hear the sounds of bloodflow returning through the brachial artery. Figure 5-12 shows the basic elements of the procedure. The cuff is inflated to a pressure that is greater than the patient’s systolic pressure. As the pressure in the cuff is gradually decreased, the first sound heard (Korotkoff sounds) is the systolic pressure. The pressure reading at which this sound ceases is the diastolic pressure. Blood pressure measurement can usually be performed manually by a respiratory therapist or other trained health care professional. If BP measurement is needed on a frequent basis, an automated blood pressure measurement system can be set up on the patient’s arm. This unit can be programmed to measure the BP on a schedule and can also have high and low blood pressure alarm limits established as a safety feature.

Figure 5-12 Arterial blood pressure is most commonly measured with a sphygmomanometer cuff that is inflated to a pressure greater than the patient’s BP. When that happens, no palpable pulse can be felt. As the cuff pressure is gradually decreased, a respiratory therapist can listen, with a stethoscope placed over the brachial artery, for the return of blood flow. The first sound heard is systolic pressure (in this case 110 mm Hg) and the last sound heard is diastolic pressure (in this case 80 mm Hg).

(Modified from Rushmer RF: Cardiovascular dynamics, ed 3, Philadelphia, 1970, Saunders. In Wilkins RL, Dexter JR, Heuer AJ: Clinical assessment in respiratory care, ed 6, St Louis, 2010, Mosby.)

d. Recommend the insertion of an arterial line (WRE code: IC12) [Difficulty: WRE: R, Ap, An]

Recommend that an arterial line (catheter) be inserted in the following situations: (1) the patient needs continuous monitoring of blood pressure, (2) frequent arterial blood samples are needed for blood gas analysis. The arterial line is usually placed into the radial artery of an adult; a newborn will have the line placed into the umbilical artery.

e. Interpret the results of the blood pressure measurement (Code: IB10m, IIIE4c) [Difficulty: ELE: R, Ap; WRE: An]

Review the general discussion on blood pressure in Chapter 1. The following are normal blood pressure values:

Patients who have values that are higher or lower than these should be further evaluated. Know the following values since they represent a serious patient problem:

a. Review central venous pressure measurement data in the patient’s chart (Code: IA8b) [Difficulty: ELE: R; WRE: Ap]

The central venous pressure (CVP) is the pressure measured in a patient’s superior vena cava, just above the right atrium. See Figure 5-13. Review previous patient data to understand whether there is an abnormality. Compare the current data with the earlier information to determine if there has been a change in the patient’s condition. See Box 5-2 for normal values.

Figure 5-13 Proper position of a central venous pressure (CVP) catheter with its distal tip located in the superior vena cava, just above the right atrium. The catheter can be inserted through the internal jugular vein (as shown) or the subclavian vein. This traditional, single-lumen CVP catheter can be used to measure the pressure on the right side of the heart and to administer fluids or medications.

b. Recommend the insertion of a central venous pressure catheter to obtain additional data (WRE code: IC12) [WRE Difficulty: R, Ap, An]

A single-lumen central venous pressure catheter (also called a CVP line) is inserted into many patients for one or more reasons: (1) to monitor the patient’s right-sided (right atrium) heart pressure, (2) to rapidly administer a large volume of intravenous fluids, and (3) to administer cardiac medications during a CPR attempt (preferred route). Recently, the development of a triple-lumen CVP catheter with integrated fiberoptic channel (Edwards Lifesciences, LLC) has enabled the continuous monitoring of central venous oxygen saturation (ScvO₂), in addition to the previously mentioned uses. See Figure 5-14.

Figure 5-14 New triple-lumen CVP catheter that can be used to continuously monitor the patient’s oxygen saturation of central venous blood (SCVo₂) in addition to measuring the right heart pressure and administering fluids and medications.

(From Edwards Lifesciences LLC, Irvine, Calif.)

c. Recommend central venous pressure measurement to obtain additional data (Code: IC11) [Difficulty: ELE: R, Ap; WRE: An]

The CVP is measured in order to know the pressure in the right atrium of the patient’s heart. This information is used to help evaluate a patient’s fluid status and to guide the administration or restriction of more fluids.

d. Perform central venous pressure measurement (Code: IB9e) [Difficulty: R, Ap, An]

The catheter is usually inserted into the right jugular vein or right subclavian vein and advanced to just above the superior vena cava. When set up for monitoring pressure, it measures the right atrial pressure (see Figure 5-15 for how to perform the procedure). Note that the stopcock must be kept at the midchest (midheart) level. Usually a mark is placed at this location (on the patient’s chest) for consistency. Raising the stopcock above the mark results in an incorrectly low reading. Lowering the stopcock below the mark results in an incorrectly high reading. See Figure 5-16. Clinical practice is very important in learning how to perform this procedure. It is important that the patient breathes spontaneously if at all possible. Peak pressures during inspiration on a mechanical ventilator may artificially raise the CVP reading. Positive end-expiratory pressure (PEEP) may also raise the CVP reading. If the patient cannot be removed from the ventilator, take the reading during exhalation. Make a note of the settings and that the reading was taken with the patient on the ventilator. Record the data in the patient’s chart or flow sheet.

Figure 5-15 Procedure for determining the central venous pressure (CVP) with a manometer. A, Water-column manometer marked in centimeters, IV tubing, and patient in place. The patient must be placed in a horizontal position with the stopcock located at the midchest (midheart) point, as shown by the dashed line. B, Turn the stopcock so that the manometer fills with fluid above the expected pressure. C, Turn the stopcock off to the IV so that the fluid flows from the manometer into the patient. Look at the stable fluid level for the CVP reading. D, Turn the stopcock so that the fluid flows from the IV into the patient.

(From Daily EK, Schroeder JS: Techniques in bedside hemodynamic monitoring, ed 5, St Louis, 1994, Mosby.)

Figure 5-16 The effects of patient position compared to position of the pressure transducer. A, When the patient’s midchest (midheart) is above the pressure transducer, the measured pressure will be higher than actual. B, When the patient’s midchest (midheart) is below the pressure transducer, the measured pressure will be lower than actual. C, True pressures will be measured when the patient’s midchest is the same height as the pressure manometer. All hemodynamic pressures (CVP, arterial pressure, pulmonary artery pressure, and pulmonary capillary wedge pressure) will be adversely affected if the patient and manometer heights do not match, as shown in A and B.

(From Oblouk Darovic G: Hemodynamic monitoring—invasive and noninvasive clinical application, ed 3, Philadelphia, 2002, Saunders.)

Past exams have asked about the proper placement of the CVP stopcock or arterial pressure transducer at the midchest (midheart) location to ensure accurate pressure measurements. Usually a mark is placed on the patient’s chest for a consistent measurement point. Review Figure 5-16 for the effects of misplacement.

e. Interpret the results of the central venous pressure measurement (Code: IB10m, IIIE4c) [Difficulty: ELE: R, Ap; WRE: An]

As noted earlier, the CVP is a measure of the pressure in the right atrium. The two main factors that influence the right atrial pressure are the blood volume returning to it and the functioning of the right ventricle (see Box 5-2 for normal CVP readings).

A decreased CVP reading usually indicates that the patient is hypovolemic. Hypotension confirms this. An increased CVP reading may suggest one of the following possibilities:

Memorize the values listed in Box 5-2. Expect to see several questions in which these values are used as patient data or as options to answer a question. The normal values must be understood to identify abnormal values and know why they are abnormal.

a. Review pulmonary artery pressure data in the patient’s chart (Code: IA8b) [Difficulty: ELE: R; WRE: Ap]

The pulmonary artery pressure (PAP) is the systolic and diastolic pressure found in either pulmonary artery. It can be measured only through a pulmonary artery catheter. (The NBRC often refers to this as a flow-directed pulmonary artery catheter.) Review the patient’s previous values before taking another pressure reading to make a comparison. Box 5-2 shows normal cardiopulmonary values.

b. Recommend the insertion of a pulmonary artery catheter for additional data (WRE code: IC12) [WRE Difficulty: R, Ap, An]

To read the PAP, a pulmonary artery catheter (PAC) must be inserted through a vein and passed through the right atrium and right ventricle into the pulmonary artery. The common insertion sites, in descending order of preference, are the basilic vein in either the right or the left arm, the right internal jugular or subclavian vein, or the right or left femoral vein. The pulmonary artery catheter is also commonly called a Swan-Ganz catheter after the inventors who gave their names to a particular brand. The catheters are available in different lengths and diameters for pediatric and adult patients. See Figure 5-17 for a typical adult catheter.

Figure 5-17 A 7-French quadruple-lumen thermodilution pulmonary artery catheter. A, Distal port goes to the tip of the catheter. It is used for measuring PAP and PCWP and for sampling blood for PO₂ determination. B, Balloon inflation port is used to inflate the balloon for inserting the catheter and for obtaining a PCWP reading. C, Proximal port is used for measuring central venous pressure and for injecting iced saline for a thermodilution cardiac output study. The iced saline exits from the right atrial opening. D, Thermistor connector attaches to the cardiac output computer. A bimetallic wire runs through the catheter to the thermistor opening, where it is exposed to temperature changes of the blood and cold injectate. (NOTE: Not all catheters are capable of measuring cardiac output. Some catheters have other special features such as continuously measuring venous saturation or cardiac pacemaker leads.)

(From Oblouk Darovic G: Hemodynamic monitoring—invasive and noninvasive clinical applications, ed 3, Philadelphia, 2002, Saunders.)

c. Recommend pulmonary artery pressure measurement to obtain additional data (Code: IC11) [Difficulty: ELE: R, Ap; WRE: An]

The PAP is important to measure in patients with severe pulmonary or cardiovascular problems, pulmonary hypertension, myocardial infarction, congestive heart failure, hypertension, and hypotension.

d. Perform pulmonary artery pressure measurement (Code: IB9m, IIIE3e) [Difficulty: ELE: R, Ap; WRE: An]

Figure 5-17 is an illustration of a 7-French quadruple-lumen thermodilution pulmonary artery catheter. Besides measurement of PAP, it is capable of being used to measure cardiac output. Figure 5-18 illustrates how a pulmonary artery catheter could be arranged with a pressure transducer and pressure monitor. Figure 5-19 shows a representation of the series of pressure waveforms seen as the catheter is advanced through the heart and into the wedged position in a branch of the pulmonary artery. Figure 5-20 shows a larger cutaway view of the heart with a PAC and normal heart chambers and related pressures.

Figure 5-18 The various components of the pulmonary artery monitoring system. It includes a fluid source such as normal saline that is pressurized to 300 mm Hg to maintain a flow of fluid through the tubing system. The continuous flush device (made by Sorenson) is designed to allow three drips of fluid per minute through the tubing system; pulling on the fast-flush valve gives a continuous flow of fluid to clear air or blood from the system. The transducer converts a pressure signal to an electrical signal that is sent to the monitor for display as numeric data and a pressure waveform. The high-pressure tubing transfers the patient’s pressure accurately to the transducer. The double stopcocks are used to close off the tubing system or sample blood from the patient. This then connects to the distal port of the pulmonary artery catheter. This same monitoring system can be used with an arterial catheter for continuous pressure monitoring and blood sampling.

(Adapted from Smith RN: Invasive pressure monitoring, Am J Nurse 9:1514, 1978. Copyright 1978 The American Journal of Nursing. Used with permission in Oblouk Darovic G: Hemodynamic monitoring, Philadelphia, 1987, WB Saunders.)

Figure 5-19 Sequence of pressures and pressure waveforms seen as the pulmonary artery catheter advances through the right atrium, right ventricle, and pulmonary artery until it wedges. Be observant of premature ventricular contractions (PVCs) as the catheter is advanced through the right ventricle.

(From Wilkins RL, Dexter JR, Heuer AJ: Clinical assessment in respiratory care, ed 6, St Louis, 2010, Mosby.)

Figure 5-20 Pulmonary artery catheter (PAC) through a heart with normal chamber pressures. SVC, Superior vena cava; IVC, inferior vena cava; RA, right atrium; RV, right ventricle; PA, pulmonary artery; PCWP, pulmonary capillary wedge pressure; LA, left atrium; LV, left ventricle; and Ao, aorta.

To obtain an accurate pulmonary artery pressure measurement, the equipment must be set up and calibrated properly, the distal lumen of the catheter must be patent with a continuous fluid-filled channel from the patient’s bloodstream back to the transducer, and the catheter’s balloon must be deflated. Clinical practice is important in understanding how to perform this procedure. Record the PAP data in the patient’s chart or flow sheet.

e. Interpret the results of pulmonary artery pressure measurement (Code: IB10m, IIIE4c) [Difficulty: ELE: R, Ap; WRE: An]

Again, the PAP is the systolic and diastolic pressure found in the pulmonary artery (see Box 5-2 for normal values). Elevated PAP values are usually seen with the following conditions: