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C H A P T E R 19 HEMODYNAMIC MONITORING DAVE NEILIPOVITZ, JON HOOPER, AND PETER BRINDLEY A 55-year-old manpresents to thehospital withsevere septic shock. You have given 5 litres of crystalloid fluids but wereforced to intubate the patient. Despite the fluid, the patient remained hypotensive and thus a vasopressor was initiated. Presently, the blood pressure is hard to detect by non-invasive techniques. You question whether the patient has been given enough fluid or whether he requires a higher vasopressor dose—or even if another medication such as an inotrope should be started. You wonder if hemodynamic monitors would help you answer these questions. Would they? One of the most important principles when treating patients is a thorough and accurate physical exam. Unfortunately critical illness may make the interpretation of physical findings—and even a seemingly simple procedure such as blood pressure measurement—difficult. As such, technology is often applied to determine not only blood pressure but also filling pressures and cardiacoutput. Of course, these devices are not perfect. There are limitations to their accuracy as well as to the interpretation and application of the values obtained. The purpose of this chapter isto introduce the various hemodynamic measuring devices along with a discussion of potential pitfalls. • Blood Pressure Blood pressure enables perfusion oforgans and tissue beds. Since hypotension is often a major problem in shock, it needs to be accurately measured. Blood pressure (BP)can be obtained indirectly or directly. Each method has advantages and disadvantages. Auscultation is the simplest indirect method for measuring BP. Aproperly sized cuff is placed around the arm and inflated. The cuff is slowly deflated and auscultation of the Kortokoff sounds is used 187 to determine systolic and diastolic pressures. The mean arterialpressure (MAP)can then be calculated as follows: MAP=DBP+ 1/3(SBP - DBP).Although reasonably accurate in stable patients, auscultation may underestimate BPby as much as 20 mm Hg in low flow states. Therefore, direct measurements are preferred in critical care patients.1 The oscillometric method is another indirect method that is generally thought tobe moreaccurate than auscultation. Thevarious automatic oscillometric devices (e.g., Dinamap®) use plethysmography to detect pulsatile changes in an artery in order to measure mean and systolic pressures (the diastolic is then calculated).2 '3 Unfortunately, the oscillometric method can also be inaccurate in low flow states, at extremes of BP and in shivering patients and those with dysrhythmias.2 '4 Prolonged use or frequent cuff inflations can also cause tissue or nerve damage and direct methods are preferred in these settings.5 The present gold standard for assessing BP is direct measurement following insertion of an arterial catheter. Patients who may require arterial lines include those who are hemodynamically unstable (allows "beat to beat" BP observation), patients on vasoactive drugs and those requiring frequent blood sampling. Although arterial lines are considered to be more reliable than indirect measurements, they may cause complications such as needle trauma, hemorrhage, hematoma, potential infection and arterial thrombosis with distal ischemia.^10 Inaccurate values can also be recorded by arterial lines. As the pressure waves move peripherally in the arterial system, the systolic BP rises as a result of reflected waves from the periphery (systolic amplification).11 "13 Likewise, the diastolic BPdecreases with increased distance from the aorta. For example, the radial systolicpressure maybe higher than the aorticsystolic pressure. Themean BP however is more robust and is therefore a more accurate reflection of centralaortic BP (Figures19.1).The transducer can also causeerrors in the measurement of any parameter, be it the BPor central venous pressure (CVP). The transducer must be calibrated (often called zeroing) to the atmospheric pressure. Improper calibration can introduce errors in measurement. The transducer must be placed at the proper level (traditionally at the level of the right atrium). A transducer that is below this level will falsely elevate the measured value and if placed above will falsely lower it. The sensing device of the transducer, which converts the fluid wave into an electrical signal, can be wrongly amplified (hyperFigure 19.1 a is a drawing illustrating the changes in blood pressure during progression from the aorta to the periphery. Note that the systolic increases, the diastolic decreases, and the mean arterial pressure remains relatively constant. Figure 19.1b is an actual example of simultaneous femoral arterial tracing (middle trace) and radial arterial tracing (bottom trace). Figures 19.1 - Blood Pressure Tracings, Changes from Aorta to Periphery resonant) or attenuated (dampened).Amplification of the signal can...

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