Hemodynamic Monitoring

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Chapter 12

Hemodynamic Monitoring

I Systemic Arterial Blood Pressure

Systemic arterial blood pressure is expressed as a systolic pressure over a diastolic pressure (Figure 12-1).

FIG. 12-1 Graphic representation of arterial pressure tracing depicting systolic, diastolic, mean, and pulse pressure (see text).

A Systolic pressure is the highest pressure attained in the artery and is determined by three major factors:

1. Stroke volume of left ventricle

a. Increased stroke volume generally causes increased systolic pressure.

b. Decreased stroke volume generally causes decreased systolic pressure.

2. Rate of blood ejection from left ventricle

a. An increased rate of left ventricular ejection generally results in increased systolic pressure.

b. A decrease in the rate of ejection generally results in decreased systolic pressure.

3. Elasticity of the arterial tree

a. Increased arterial elasticity generally results in increased systolic pressure.

b. Decreased arterial elasticity generally results in decreased systolic pressure.

B Diastolic pressure is the lowest pressure attained in the artery and is determined by three major factors:

1. Magnitude of preceding systolic pressure

a. In general, the higher the preceding systolic pressure, the higher the resulting diastolic pressure.

b. The lower the preceding systolic pressure, the lower the resulting diastolic pressure.

2. Length of ventricular diastolic interval

a. The longer the diastolic interval, the greater the time available for blood to leave the arterial system and the lower the resultant diastolic pressure.

b. The shorter the diastolic interval, the higher the diastolic pressure by the opposite mechanism.

3. State of peripheral resistance

a. The greater the peripheral resistance, the lower the rate of arterial runoff and the higher the resultant diastolic pressure.

b. The lower the peripheral resistance, the higher the rate of arterial runoff and the lower the resultant diastolic pressure.

C Measurement of arterial blood pressure generally assesses left ventricular function by systolic pressure and peripheral resistance by diastolic pressure. The most significant factor responsible for systolic pressure is stroke volume, and the factor principally responsible for diastolic pressure is the state of peripheral resistance.

D Thus it is assumed that the greater the difference between systolic and diastolic pressures, the greater the resultant flow. The difference between systolic and diastolic pressures is called the pulse pressure.

1. By the equation:

< ?xml:namespace prefix = "mml" />Q=P×1R (1)

(1)

increases in systolic pressure indicate increases in the pressure gradient (P) across the systemic circulation, and decreases in diastolic pressure indicate decreases in peripheral resistance (R).

2. Therefore widening of pulse pressure is generally thought to indicate increased blood flow

3. By the opposite mechanism, narrowing of pulse pressure is generally thought to indicate decreased flow.

4. It should be noted that widening of pulse pressure could occur without increased blood flow. Normal blood flow also can exist with a narrow pulse pressure. These phenomena occur by alterations in the other factors (i.e., diastolic interval and arterial elasticity) that determine systolic and diastolic pressure. Therefore it is imperative to assess all factors responsible for systolic and diastolic pressure before blindly stating that an increased or decreased pulse pressure represents increased or decreased blood flow in a given patient.

5. In the absence of pulmonary artery catheterization and/or serial cardiac output measurements, the trend in pulse pressure is used as a gross indicator of trends in cardiac output.

E The mean arterial pressure (MAP) represents the average pressure over one complete systolic and diastolic interval.

1. The MAP can be directly measured via a systemic arterial line or estimated by the following formula:

MAP=(2×diastlic pressure)+(systolic pressure)3˙ (2)

(2)

2. The MAP is the average pressure in the arterial tree over a given time and therefore generally is used to assess the average pressure to which the arterial system is exposed.

3. The pressure gradient across the systemic circulation is generally expressed as MAP – central venous pressure (CVP; see Section II, Central Venous Pressure).

4. The MAP is also commonly used as an indicator of left ventricular afterload, thus representing the resistance (in terms of pressure) against which the left ventricle must pump.

a. Afterload generally reflects an inverse relationship with stroke volume the more compromised the state of the left ventricle.

b. Afterload always represents a direct relationship with myocardial oxygen consumption and therefore myocardial work.

F Arterial blood pressure and mean arterial blood pressure can be directly measured by an intraarterial line (catheter). Alternately arterial blood pressure can be indirectly measured using a sphygmomanometer (Figure 12-2), and the MAP can be calculated from the obtained systolic and diastolic values.

FIG. 12-2 Sphygmomanometry. When the pressure within the sphygmomanometer cuff is increased above arterial blood pressure, the arteries under the cuff are occluded and no pulse can be palpated at the wrist. As the cuff pressure is gradually released, the systolic peaks of pressure finally exceed cuff pressure and blood spurts into the arteries below the cuff, producing palpable pulses at the wrist. The sudden acceleration of blood below the cuff produces vibrations that are audible through a stethoscope. The pressure in the mercury manometer at the time the pulse is heard or felt indicates systolic pressure. As cuff pressure is further diminished, the sounds increase in intensity and then rather suddenly become muffled at the level of diastolic pressure where the arteries remain open throughout the entire pulse wave. At still lower pressures, the sounds disappear completely when laminar flow is reestablished.

G Normal values for arterial blood pressure in the adult are as follows:

Systolic:	90 to 140 mm Hg
Diastolic:	60 to 90 mm Hg
Mean:	70 to 105 mm Hg

II Central Venous Pressure

The CVP usually is expressed as a single number representing the mean right atrial pressure

A The numerical pressure value of CVP is the result of the following factors:

1. The pump capabilities of the right side of the heart in part determine the CVP. If the right ventricle pumps what it receives, blood will not back up in the atrium, and the CVP should be normal. If the right side of the heart is not pumping adequately, there will be a backup of blood in the atrium that will be reflected in elevated CVP.

2. The venous tone determines CVP because venous tone is responsible for determining the venous vascular space. It thus has major implications in venous return and filling pressure of the right atrium.

3. Blood volume, which in part determines CVP, must be adequate to fill the venous vascular space, or venous return to the heart will be impeded.

4. If the pump capabilities of the right side of the heart are adequate, the CVP will directly reflect the venous vascular volume (blood volume)-to-venous vascular space relationship. Fluid therapy and diuresis are frequently gauged in terms of the CVP’s reflection of this relationship.

B The CVP is commonly used as an indicator of right ventricular preload when measured as the right ventricular end-diastolic pressure (RVEDP).

1. The RVEDP represents compliance of the right ventricle.

2. The RVEDP also represents the filling pressure necessary for the ensuing right ventricular contraction.

C The CVP is measured directly through a catheter inserted in a peripheral vein that traverses the vena cava with its tip resting in the right atrium (Figure 12-3) or from a four-lumen pulmonary artery catheter having a proximally located open channel in the right atrium (see Figure 12-8).

FIG. 12-3 Central venous pressure (CVP) catheter inserted through the jugular vein with the tip positioned in the right atrium.

FIG. 12-8 Four-channel pulmonary artery catheter.

D Normal values for CVP in the adult are 0 to 8 mm Hg.

III Pulmonary Artery Pressure (PAP)

The PAP is expressed as systolic pressure over diastolic pressure.

A Systolic pressure is the highest pressure attained in the pulmonary artery and is determined by the same three factors that determine systolic pressure in the systemic arterial system:

1. Size of right ventricular stroke volume

2. Rate of blood ejection from right ventricle

3. Elasticity of pulmonary arterial tree.

B Diastolic pressure is the lowest pressure attained in the pulmonary artery and is determined by the same three factors that determine diastolic pressure in the systemic arterial system:

1. Magnitude of preceding systolic pressure

2. Length of ventricular diastolic interval

3. State of peripheral resistance of the pulmonary arterial tree.

C Measurement of PAP generally assesses right ventricular function by systolic pressure and pulmonary arterial resistance by diastolic pressure. Thus PAP is used in precisely the same fashion as systemic arterial pressure. In this light it should be noted that all factors contributing to systolic and diastolic PAP should be fully assessed before inferences concerning blood flow are made from these values.

D The mean pulmonary artery pressure represents the average pressure over one complete systolic and diastolic interval.

1. The is the average pressure in the pulmonary artery over a given time and is used to assess the average pressure head (or front) to which the pulmonary arterial system is exposed.

2. Thus the pressure gradient across the pulmonary circulation is generally represented by the expression – pulmonary wedge pressure (PWP), or mean left atrial pressure, also referred to as the pulmonary artery opening pressure (PAOP) or pulmonary capillary wedge pressure (PCWP; see Section IV, Pulmonary Wedge Pressure).

3. The is commonly used to assess right ventricular afterload, thus representing the resistance (in terms of pressure) against which the right ventricle must pump.

a. Afterload generally reflects an inverse relationship with stroke volume the more compromised the state of the right ventricle.

b. Afterload always represents a direct relationship with myocardial oxygen consumption and therefore myocardial work.

c. Afterload of the right and left ventricles may vary dramatically and, when monitoring allows, should be independently evaluated.

E The PAP and are directly measured using a pulmonary artery catheter. The pulmonary artery catheter is inserted through a peripheral vein and traverses the vena cava, right atrium, and ventricle, with its tip resting in the pulmonary artery. As the catheter is advanced, the typical pressure curves (deflection) generated on an oscilloscope are observed to monitor its distal location (Figure 12-4).

FIG. 12-4 From left to right: Right atrial pressure; right ventricular pressure—note that diastolic pressure is zero baseline or below; pulmonary artery pressure—note that diastolic pressure is significantly above zero baseline; pulmonary wedge pressure—catheter tip senses only pulmonary capillary back pressure since balloon obstructs arterial flow. Deflation of the balloon should result in immediate return of the pulmonary artery pressure pattern (see text).

F Normal values for pulmonary arterial blood pressure in the adult are as follows:

Systolic:	15 to 28 mm Hg
Diastolic:	5 to 16 mm Hg
Mean:	10 to 22 mm Hg

IV Pulmonary Wedge Pressure (PAOP or PCWP)

PWP is expressed as a single number representing the mean left atrial pressure

A The numerical pressure value of the PWP will be the result of the following factors:

1. The pump capabilities of the left side of the heart in part determine the PWP. If the left ventricle pumps what it receives, blood will not back up into the atrium, and the PWP should be normal. If the left ventricle is not pumping adequately, there will be a backup of blood into the atrium that will be reflected as elevated PWP.

2. Blood return to the left atrium is largely the result of an adequate blood volume-to-pulmonary venous (venomotor tone) vascular space relationship.

3. If the left ventricle is pumping adequately, the PWP depends on the aforementioned vascular volume-to-vascular space relationship.

B The PWP is commonly used as an indicator of left ventricular preload when measured as the left ventricular end-diastolic pressure (LVEDP).

1. The LVEDP represents compliance of the left ventricle.

2. The LVEDP also represents the filling pressure necessary for the ensuing left ventricular contraction.

3. Preload generally varies directly with the size of the ensuing stroke volume and therefore cardiac output as may be experienced with fluid therapy.

4. However, in the failing ventricle or fluid-overloaded patient, preload varies inversely with stroke volume and cardiac output. Such a situation also represents increases in myocardial oxygen consumption and work.

5. Preload of the left and right ventricles may also vary dramatically and therefore should be separately assessed with the availability of a pulmonary artery catheter.

C The PWP is measured directly through a pulmonary artery catheter by the intermittent inflation of a balloon that occludes the local branch of the pulmonary artery (Figure 12-5). Pressure readings are taken from the tip of the catheter, which is distal to the balloon. The inflated balloon obstructs the systolic and diastolic pulmonary artery pressures (the characteristic pressure contour should be absent). Therefore the pressure measurement reflects backpressure (through a low resistance system) from the left atrium.