血流動(dòng)力學(xué)監(jiān)測(cè)C初步教程杜斌.

1、PAC Q & A from Henry FesslerPAC Q & A from Henry Fessler北京協(xié)和醫(yī)院杜 斌血流動(dòng)力學(xué)監(jiān)測(cè)-初步教程PAC Q & A from Henry FesslerPAC Q & A from Henry FesslerPAC Q & A from Henry FesslerA patient is to be monitored with a PAC, and his newly hirednurse assembles the transducer systemThe transducer is clam

2、ped to an IV pole beside the bed, and is 10 cm below the level of the supine patients right atriumA bag of saline is pressurized to 300 mmHg, connected to an IV drip chamber and connected to the appropriate flush port of the transducerThe transducer is connected to the hub of the distal lumen of the

3、 PAC by 100 cm of high-pressure extension tubing, which is filled with saline and flushed free of bubbles» 1The transducer is zeroed by opening to air a stopcock which is held at the level of the right atrium. After zeroing the stopcock is closed and placed on the mattressThe catheter is then i

4、n sortedThe single-best-answer questions below (#1-4) refer to this system, which is illustrated hereFUihMflJOO rmHflAs shown, the flush bag:A. Does not cause bubble formation in the absence of fast flushingB. Has been over pressurized, causing a significant over-estimation of pulmonary artery press

5、ureC. Risks air embolization from turbulenee in the drip chamber during fast flushingShould have been connected to the transducer by high-pressure tubingPAC A 1 from Henry FesslerAnswer CThe flush system a pressurized bag of saline, often heparinized, connected to the transducer through a high resis

6、ta nee flush valveThe resistor reduces the pressure on its downstream end to only 1-2 mmHg, even when the bag is pressurized to 200- 300 mmHg. Therefore, it has trivial effects on the pressure recorded by the transducerNo need to use special tubing between the flush bag and transducer, because IV tu

7、bing can withstand a much higher pressure than 300 mmHg, and this segment of tubing not part of the path for pulse wave transmissionPAC A 1 from Henry FesslerAnswer CThe flush system maintains a slow but continuous flow of saline, which helps keep the catheter free of clotsWhen the resistor is remov

8、ed by depressing a lever or button on the flush valve, the catheter is exposed to the full force of the upstream pressure, allowing a rapid flushIf an air-filled drip chamber is attached to the saline bag, turbulenee during a rapid flush can entrain bubbles of airPAC A 1 from Henry FesslerAnswer CTh

9、ese will degrade the fidelity of the wave recording if trapped in the tubing downsiream from the resistor, and will cause air embolism if flushed into the patientThe latter is obviously more dangerous with systemic arterial than PA cathetersEven without flushi ng, the flush device con tributes to th

10、e formation of microbubbles in the downstream tubingThe high pressure In the saline bag drives gas into solution. This gas comes out of solutlon in the lower pressure system beyond the resistor, and will accumulate Into small bubbles over several hoursPAC A 1 from Henry FesslerRefere nces:Gardner RM

11、, Warner HR, Toronto AF, Gaisford WD. Catheter-flush system for continuous monitoring of central arterial pulse waveform. J Appl Physiol 1970:29;911-3.Gardner RM, Bond EL, Clark JS. Safety and efficacy of continuous flush systems for arterial and pulmonary artery catheters. Ann Thorac Surg 1977;23:5

12、34-8.Soule DT, Powner DJ. Air entrapment in pressure monitoring lines. Crit Care Med 1984; 12:520-2PAC A 1 from Henry FesslerWhich of the following is true regarding the zeroing of this tran sducerA. The zeroing was made inaccurate when the stopcock, previously used to zero, was placed on the mattre

13、ssB. The zeroing was in accurate from the start, because the transducer was not leveled to the right atriumC. The zeroing will be made inaccurate when the bed, together with the stopcock previously used to zero, is raisedD. After the PA catheter is placed, zero errors can be corrected by raising the

14、 transducer to the right atriumPAC A 2 from Henry FesslerAnswer is C"Zeroing” a transducer means defining a reference pressure as ambient, from which all vascular pressures will be measuredBy standard convention, vascular pressures are measured relative to the level of the right atrium一 In the

15、supine patientf this is approximated by the mid- axillary line一 At that level, the transducer system is opened to the room using a threeway stopcockIt is not necessary that the transducer be at that same levelAnswer is CIf the transducer is below the mid-axillary line, a hydrostatic pressure equal t

16、o the height of the water column above the transducer will be applied to itThe transducer will likewise be exposed to a small negative pressure if it is above the open zero portHowever, when the system is then electronically zeroed, the appropriate pressure will be added or subtracted to set the tra

17、nsducer to zero, regardless of its heightPAC A 2 from Henry FesslerAnswer COnce the zero port is closed, the height of the port becomes un imports ntHowever, the height of the transducer must now be kept constant, relative to the mid-axillary line-If the bed is raised but the transducer is not, the

18、additional hydrostatic pressure will be recorded as an artifactual elevation of vascular pressure一 The transducer would require re-zeroing to regain its accuracyAnswer CSome situations, such as intraoperative use, may require that the transducer be kept a distance from the patientHowever, the transd

19、ucers may often be conveniently strapped to the patient's arm, and the port molded into the transducer body used for zeroing一 keeping the transducer and zero port at a fixed relationship to one another一 keeping them both near the mid-axillary line as the bed is raised or loweredAC A 2 from Henry

20、 FesslerRefere ncesPederson A, Husby J. Venous pressure movement: choice of zero level. Acta Med Scand 1951 ;141:186-94.Yang SS, Bentivoglio LG, Maranhao V, Goldberg H. From cardiac catheterization data to hemodynamic parameters. Philadelphia: Davis, 1988.Courtois Mf Fattal PG, Kovacs SJ, et al. Ana

21、tomically and physiologically based reference level for measurement of intracardiac pressures Circulation 1995;92:1994-2000.PAC A 1 from Henry FesslerThe 100 cm length of high-pressure tubing could be improved upon becauseA. The low-pressure PA does not require the narrow, high pressure tubingB. Sta

22、ndard IV tubing would decrease the natural frequency of the recording systemC. Two, 50 cm lengths with an extra stopcock between them would make it easier to remove bubblesD. Shorter tubing would improve the natural frequency of the recording systemPAC A 3 from Henry FesslerAnswer DA critically impo

23、rtant feature of a transducer/catheter system is that its natural frequency suit the pressure waveform that is being recordedIn a simple oscillating system like a pendulum, the natural frequency is the frequency with which the pendulum will swing back and forth when given a push-Ifa pendulum is push

24、ed repetitively at its natural frequency, its oscillations will get higher and higherPAC A 1 from Henry FesslerAnswer DA tansducer and catheter system oscillates, because a pressure wave introduced into the catheter will travel down and deflect the transducer membraneThe membrane recoils back, and s

25、ends a reflected wave back up the catheter. When it reaches the end, another reflected wave is generated, headed back to the transducerIf the transducer is repetitively stimulated at the same frequency with which waves reflect back and forth, the oscillations of the Iransducer membrane will become l

26、arger and the amplitude of the pulse pressure will be exaggerated. Thus, the systolic pressure will be over-estimated and diastolic pressure underesiimatedbiAnswer DA pulse wave has a complex form, but can be decomposed into a series of simple sine waves of increasing frequency, called harmonics, wh

27、ich summate to produce the original waveIn order to reproduce a wave accurately, a transducer/catheter system must be able to record accurately up to the 6-10th harmonicFor a pulse of 120 bpm (2 Hz), for example, a transducer must be able to record up to at least 12 H乙 If the natural frequency of th

28、e transducer/catheter system is less than that, the recorded wave will be distorted because harmonics near the natural frequency will be amplifiedPAC A 1 from Henry FesslerThe natural frequency of transducer/catheter systems is affected by several physical properties- The stiffer the catheter, the h

29、igher the natural frequency Thus, the importanee of high-pressure tubing is not that it will not burst, but that it is more rigid than standard IV tubing一 The length of tubing is another determinant of natural frequency, since the oscillations complete a cycle faster when the distances are short The

30、 transducers should be positioned as close as possible to the site of pressure measurement By the time a transducer is attached to the usual assemblage of tubing, catheters and connectors, natural frequency is reduced to only about 12-15 Hzf barely adequate for the taskPAC A 3 from Henry FesslerAnsw

31、er DIt is important to clear all bubbles from the tubing, because they decrease the natural frequencyHowever, it is also important to minimize the number of connections and stopcocksAny irregularity in the path of the waves creates additional reflections, and creates blind pockets for bubbles to col

32、lectThe figure on the left shows an arterial pressure tracing in which the natural frequency of the catheter system is too low, together with a fast flushThe figure on the right is the same patient moments later, after bubbles have been cleared from the system. The systolic pressure differs by 20-30

33、 mmHgPAC A 3 from Henry FesslerRefere nces:Milnor WR. Pulsatile blood flow. N Engl J Med 1972:287:27-34.O'Rourke MF, Yaginuma T. Wave reflections and the arterial pulse. Arch Intern Med 1984;144:366-71 Kleinman B Understanding natural trequency and damping and how they relate to the measurement

34、of blood pressure. J Clin Monit 1989;5:137-47.Gardner RM. Direct blood pressure measurement-dynamic response requirements. Anesthesiology 1981;54:227-36Heimann PA, Murray WB. Construction and use of cathetermanometer systems. J Clin Monit 1993;9:45-5.PAC Q 4 from Henry FesslerQuestions #4 and 5 refe

35、r to the simulated waveform below which shows the pulmonary artery pressure and the response to a brief fast flush in a patient in the intensive care unitPAC Q 4 from Henry FesslerPAC Q 4 from Henry FesslerPAC Q 4 from Henry FesslerWhich one of the following statements is true:A. The pulmonary arter

36、y mean pressure will be un derestimatedB. The pulmonary artery diastolic pressure will be underestimatedC. The pulmonary capillary wedge pressure will be underestimatedD. The pulmonary artery pulse-pressure will be un derestimatedWhich one of the following conditions is NOT likely to cause a tracing

37、 like that aboveA. A bubble of air in the tubingB. A partially wedged catheterC. A partially closed stopcockD. A fibrin clot on the catheter tipPAC A 4 & 5 from Henry FesslerQuestion 4Which one of the following statements is true:A The pulmonary artery mean pressure will be underestimatedB. The

38、pulmonary artery diastolic pressure will be underestimatedC The pulmonary capillary wedge pressure will be underestimatedD. The pulmonary artery pulse-pressure will be underestimatedWhich one of the following conditions is NOT likely to cause a tracing like that aboveA. A bubble of air in the tubing

39、B. A partially wedged catheterC. A partially closed stopcockD. A fibrin clot on the catheter tipQuestion 5Another characteristic of transducer/catheter systems essential for high fidelity recordings is damping- Returning to the example of a pendulum, damping is the characteristic that slows the pend

40、ulum to a stopIn a transducer with a wave oscillating back and forth, it is the quality that makes the amplitude of the wave decay with successive reflections- A certain amount of damping is useful to prevent excessive oscillations when important harmonics of a wave approach the natural frequency of

41、 a recording system- Too much damping begins to filter out important features of the wave, making it appear more like a sine wave or eventually, like a mean pressurePAC A 4 & 5 from Henry FesslerDamping is provided by electronic filters, and by physical characteristics of the recording systemFac

42、tors leading to in crease in the dampi ng一 high resistance, such as provided by a catheter partially occluded by clot or partially closed stopcock一 increased compliance of the conducting system, due either to the inadvertent use of standard IV tubing or the presence of air bubblesPAC A 4 & 5 fro

43、m Henry FesslerThe degree of damping can be qualitatively estimated by inspection of the response to a brief fast flush, as the flush is released and the wave comes back within scale一 In a properly clamped system, one should observe 2-3 quick oscillations and a return to the underlying wave-In an ov

44、er damped system, the pressure will gradually return to the waveform, without oscillations. Alternatively, it may go off-scale in the negative direction, and gradually returnWhile the waveform itself may be con fused with a "partial wedge；* the response to a fast flush will be similar in a prop

45、erly damped system whether or not the catheter is wedgedPAC A 4 & 5 from Henry FesslerThe figures above show an overdamped (left) and properly damped (right) arterial pressure and fast flush. The tracings were recorded from the same patient within minutes of each other. In the tracing on the rig

46、ht, the time between successive peaks at the end of the fast flush is 1/natural frequency, and the decreased amplitude of successive peaks is a function of damping. The waveform recorded by an over-damped system will approach the mean pressure Mean pulmonary artery pressures will remain accurate (as

47、 will mean wedged pressures). Systolic pressures will be underestimated, and diastolic pressures overestimated. When an over damped pressure is recognized, reversible causes may often be found.ReferencesKleinman B. Understanding natural frequency and damping and how they relate to the measurement of

48、 blood pressure. J Clin Monit 1989:5:137-47.Morris AH, Chapman RH, Gardner RM.Frequency of technical problems encountered in the measurement of pulmonary artery wedge pressure. Crit Care Med 1984;12:164-PAC Q 6 from Henry FesslerThe simulated waveform above was ,frecordedH from the pulmonary artery

49、of a patient in septic shockPAC A 4 & 5 from Henry FesslerPAC A 4 & 5 from Henry FesslerPAC Q 6 from Henry FesslerWhich one of the following statements about this waveform is correct?A. The artifactual oscNations may sometimes be reduced by connecting the Iransducer directly to the hub of th

50、e pulmonary artery cathe怕r.B. The digital display of PA pressure filters out the oscillations electronically, prior to calculating systolic and diastolic pressuresC. The oscillations should prompt a search for a bubble in the tubingD. The "whip artifact” will also be seen as excessive oscillati

51、ons upon release of a fast flush of the catheter.PAC A 6 from Henry FesslerAnswer A"Whip artifact1* may be due to the long, flexible pulmonary artery catheter fluttering in a high velocity bloodstream These artifacts are unavoidable, given the nature of the catheter and the patient.The human ob

52、server can estimate where the true waveform will lie, in the middle of the oscillations.The waveform displayed on the bedside monitor has already been filtered.The algorithms for the digital display assumes the highest and lowest points are systole and diastole. These will therefore be over- and tin

53、der-estimated, respectivelyAnswer AArtifact looking identical to catheter whip can also occur when the natural frequency of the transducer/catheter system is too low. This is more likely to occur in the PA than in a systemic arterial tracing.The PAC is Ion ger, giving it a lov/er natural frequency f

54、or the same length of external connections. The PA waveform also has greater amplitude in Its higher harmonics, meaning the high freque ncy components con tribute more to the shape of the wave than is the case in a systemic artery.PAC A 6 from Henry FesslerAnswer AOne correctable cause of a low natu

55、ral frequency is excessive tubing between the catheter and transducer. This can be tested by connecting the transducer directly to the PAC.Air bubbles in the tubing also reduce the natural frequency, but have an even greater effect on damping. Therefore, a catheter with a bubble will appear like the

56、 waveform in Q4.If the cause of -whip artifact” is catheter movement in a properly functioning transducer/catheter system, the fast flush will appear normal. It the cause is a system with too low a natural frequency, the oscillations after a fast flush will be more widely spaced, but of the same amp

57、litudeReferencesMiller GS, Zbilut JP. Practical evaluation of cathetertransducer coupling systems for artifact. Heart Lung 1983;12:156-61 Kleinman B. Understanding natural frequency and damping and how they relate to the measurement of blood pressure. J Clin Monit 1989;5:137-47.Gardner RM. Direct bl

58、ood pressure measurement-dynamic response requirements. Anesthesiology 1981 ;54:227-36.Hunziker P. Accuracy and dynamic response of disposable pressure transducer-tubing systems. Can J Anaesth 1987；34:40914.PAC Q 7 from Henry FesslerPAWP is being recorded continuously in a paralyzed patient on positive pressure ventilation with an l:E ratio of 1:2. The pressure is seen to rise and fall in time with