Page 484 - Cardiac Nursing

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CHAPTER
H H H H Hemodynamic Monitoring
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Elizabeth J. Bridges
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Cardiovascular support off criitiic llly illl patientss requires noninvasivee re remmaiins thhe saame up to a backrest elevation of 60 degrees. Use of f
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and invasive monitoring of physiological indicators of cardiovascu- th this reference, which is alsoo the same reference recommended for
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lar ffunction, including factors that affect ca drdiac performance (pre- th thee evaluation of jugularr venous distentionn, giives a CVP measure-
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lo load, afterloadd, c nontractility, andd heartt rate [HR]) and the balance me nt t ha is m m g l ow e
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between O 2 supply and demand. This chapter reviews technolo- referenced to the phlebbostatic a ixis. 7,8 Th er is n ge l
Theree is noo general cconsen-
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gies for hemodynamiic moniitoriing ((arteriall bloodd pressure (BP) sus on which reference is most accurate; however, these studies
monitoring, central venous pressure (CVP)/ pulmonary artery highlight the importance of using a standardized reference and
(PA) catheterization, and cardiac output (CO) and monitor- also interpreting the absolute pressure measurements relative to
ing) and discusses the current recommendations for the effective the different reference levels.
use of hemodynamic monitoring in optimizing patient outcomes. Previous research on the effect of position on hemodynamic pres-
Newer techniques such as central venous oxygenation saturation sure measurements has been limited by the use of incorrect reference
( ) and functional hemodynamic monitoring and new tech- points. In many of these studies, attainment of accurate PA pressures
nologies such as transpulmonary indicator dilution (TPID) CO, was not possible because the use of a reference point above or below
pulse contour analysis, transesophageal Doppler, partial CO 2 re- the LA resulted in the inclusion of hydrostatic pressure component;
breathing, and microcirculation and tissue oxygenation monitor- thus, the measured pressures were underestimated or overestimated. 9
ing techniques are introduced. For every 1 cm the reference point is above the LA; the measured
pressure decreases by 0.73 mm Hg. Conversely, for every 1 cm the
reference point is below the LA, the measured pressure increases by
TECHNICAL ASPECTS OF 0.73 mm Hg. The position-specific reference points are summarized
INVASIVE PRESSURE in Display 21-1. In the lateral position, reference points have been
MONITORING validated for the 30- and 90-degree lateral positions with a 0-degree
backrest elevation. Further study of the lateral position with varying
Referencing degrees of backrest elevation is needed. In studies performed to eval-
uate the effects of prone position on hemodynamic indices, the
Pressure in blood vessels has three components: dynamic BP (i.e., MAL or the midanteroposterior diameter of the chest have been
the BP generated by the heart), hydrostatic pressure (related to used as the reference point. 15–21
fluid density, gravitational acceleration, and height of the column
of blood between the heart and the vessels), and static pressure (re- Zeroing Versus Referencing
lated to the volume of blood in the vascular system at zero flow). 1
The BP is the same at all points along a horizontal level. However, Zeroing is performed by opening the system to air to establish at-
pressure at different vertical levels reflects not only the dynamic mospheric pressure as zero, although changes in barometric pressure
pressure but also the hydrostatic pressure. have minimal effect on measured pressure. In addition, zeroing is
22
Referencing, which is performed to correct for the change in hy- performed to compensate for offset caused by hydrostatic pressure or
drostatic pressure in vessels above and below the heart, is accom- offset in the pressure transducer, amplifier, oscilloscope, recorder, or
plished by placing the air–fluid interface (stopcock) of the catheter digital delays. The act of simultaneously zeroing and referencing en-
system at the level of the heart to negate the weight effect of the sures that intracardiac pressures are being measured (Display 21-2).
catheter tubing. All invasive cardiovascular pressure-monitoring sys-
tems (PA, CVP, and arterial) are referenced to the heart, not to the Infection Control
catheter tip or the site of insertion. 1–3
The phlebostatic axis and phlebostatic level are the most com- Catheter-related infection remains the leading cause of nosoco-
monly used reference points for the mid-right atrium (RA) and mial infections, particularly in critical care and are associated
left atrium (LA) (Fig. 21-1). 4,5 As the patient moves from the flat with increased length of hospital stay and resource use. 24 In a
to the backrest elevated position, the phlebostatic level rotates on study of 1,140 central venous and 1,038 arterial catheters, both
the axis and remains horizontal (Fig. 21-2). In patients with nor- in situ for an average of 9.5 days, the catheter-related blood
mal chest wall configuration, the midaxillary line (MAL) is a valid stream infection (CR-BSI) incidence was 4.6% and 3.7%, re-
reference level for the RA and the LA; however, use of the MAL spectively. 25 A systematic review of 200 studies found a CR-BSI
in patients with varied chest configuration may result in a pressure incidence for nonmedicated central catheters of 2.9/1,000
difference of up to 6 mm Hg. 6 catheter days (95% cardiac index [CI] 2.6 to 3.2) and
Although the phlebostatic axis is the most commonly cited ref- 1.4/1,000 catheter days (95% CI 0.8 to 2.0) for peripheral ar-
26
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erence level, other reference levels have been suggested. For CVP terial lines. 26
measurements Magder suggests using a reference point 5 cm be- Evidence-based guidelines exist for the prevention of CR-BSI
low the angle of the sternum, as this point reflects mid-RA, which (Table 21-1). 42,43 In 2006, the results of the effect of an
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