Hemodynamic Monitoring

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Hemodynamic Monitoring

Systemic Arterial Blood Pressure

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

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

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

1. Magnitude of preceding systolic pressure

2. Length of ventricular diastolic interval

3. State of peripheral resistance

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.

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)

image (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 image

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.

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)

image (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.

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.

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 image

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.

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

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).

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.

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:

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:

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.

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

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

The PAP and image 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).

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 image

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

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

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.

Normal values for the PWP in the adult are 2 to 12 mm Hg.

Calculation and Comparison of Systemic and Pulmonary Vascular Resistance

R=ΔPQ˙ (3)

image (3)

where R = vascular resistance expressed in mm Hg/L/min; ΔP = change in pressure across the circulation or pressure gradient, expressed in mm Hg; and image = cardiac output or flow expressed in L/min.

Systemic vascular resistance (SVR) (Figure 12-6)equals:

Pulmonary vascular resistance (PVR) (see Figure 12-6) equals: