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Editorial II: Continuous cardiac output by pulse contour analysis?

2001; Elsevier BV; Volume: 86; Issue: 4 Linguagem: Inglês

10.1093/bja/86.4.467

1471-6771

Johannes J. van Lieshout, Karel H. Wesseling,

Non-Invasive Vital Sign Monitoring

Theoretically, tissue flow should be maintained at the lowest possible arterial perfusion pressure to minimize afterload and myocardial energy expenditure.1Feddersen K Aren C Nilsson NJ Radegran K Cerebral blood flow and metabolism during cardiopulmonary bypass with special reference to effects of hypotension induced by prostacyclin.Ann Thorac Surg. 1986; 41: 395-400Abstract Full Text PDF PubMed Scopus (19) Google Scholar In the monitoring and measurement of cardiovascular parameters in critically ill patients, however, cardiac output plays a subservient role to arterial blood pressure. Since blood pressure is the principal cardiovascular parameter monitored by the body via baroreceptors, this seems physiological. Furthermore, continuous cardiac output has proved extremely difficult to monitor. Intermittent methods such as indicator dilution are widely used, but for monitoring purposes cardiac output should be available continuously, like arterial pressure.2Gardner RM Continuous cardiac output: how accurately and how timely?.Crit Care Med. 1998; 26: 1302-1303Crossref PubMed Scopus (9) Google Scholar The paper by Linton and Linton in this issue3Linton NWF Linton RAF Estimation of changes in cardiac output from the arterial blood pressure waveform in the upper limb.Br J Anaesth. 2001; 86: 486-496Abstract Full Text Full Text PDF PubMed Scopus (112) Google Scholar offers a solution to the monitoring problem by deriving cardiac output from arterial pressure via a cascade of chained models. Such so-called ‘pulse contour methods' attempt to determine cardiac stroke volume from characteristics of the arterial pressure pulse. With a first publication in 1904,4Erlanger J Hooker DR An experimental study of blood pressure and of pulse-pressure in man.Johns Hopkins Hosp Rep. 1904; 12: 145-378Google Scholar pulse contour preceded Korotkoff's auscultation paper by 1 year. Both ideas were embraced almost immediately, yet clinical acceptance of the pulse contour and auscultatory methods has differed. Why? Because pulse contour methods are based on solid physical principles and less solid physiological models and they involve substantial computations, all factors that do not ease clinical acceptance. If it was possible to know the degree to which the aorta complies to a 1 mm Hg pressure increment and to measure its reponse, then the stroke volume ejected into the aorta could be computed by measuring the associated increase in pressure.4Erlanger J Hooker DR An experimental study of blood pressure and of pulse-pressure in man.Johns Hopkins Hosp Rep. 1904; 12: 145-378Google Scholar For instance, if this compliance was 2 ml per 1 mm Hg pressure increment, then a pressure rise of 40 mm Hg would correspond to a stroke volume of 40×2=80 ml. The aorta's compliance at low distending pressure is substantial but at higher pressures compliance progressively decreases, resisting overstretching of the aortic wall. Such ‘non-linear' behaviour effectively prevents any simple approach to pulse contour estimates of stroke volume. Typically, for a 50-yr-old patient, aortic compliance might be 3, 1.1 and 0.5 at distending pressures of 50, 100 and 150 mm Hg, respectively. Thus the same 40 mm Hg pulse pressure equates to stroke volumes of 120, 44 and 20 ml, depending on pressure. These values also change with the patient's age; non-linearity is more pronounced in the elderly. The significance of aortic non-linearity in relation to pulse contour measurement was appreciated as early as 1928,5Liljestrand G Zander E Vergleichende Bestimmungen des Minutenvolumens des Herzens beim Menschen mittels der Stickoxydulmethode und durch Blutdruckmessung.Z Ges Exp Med. 1928; 59: 105-122Crossref Scopus (37) Google Scholar and that of age dependency in 1954,6Starr I Clinical tests of the simple method of estimating cardiac stroke volume from blood pressure and age.Circulation. 1954; 9: 664-681Crossref PubMed Scopus (18) Google Scholar but these two factors were only measured with great precision in 1984.7Langewouters GJ Wesseling KH Goedhard WJA The static elastic properties of 45 human thoracic and 20 abdominal aortas in vitro and the parameters of a new model.J Biomech. 1984; 17: 425-435Abstract Full Text PDF PubMed Scopus (469) Google Scholar To our knowledge, the variables have been taken into account fully when measuring aortic flow by the Modelflow method only.8Wesseling KH Jansen JRC Settels JJ Schreuder JJ Computation of aortic flow from pressure in humans using a nonlinear, three-element model.J Appl Physiol. 1993; 74: 2566-2573Crossref PubMed Scopus (902) Google Scholar The pressure that determines stroke volume is proximal aortic pressure, which is not routinely available. More peripherally recorded pressure waves can be substituted if they bear a known, fixed, and wide bandwidth relationship to aortic pressure. This is almost the case for arterial pressures measured at sites along the arteries in the arm (see references cited by Linton and Linton3Linton NWF Linton RAF Estimation of changes in cardiac output from the arterial blood pressure waveform in the upper limb.Br J Anaesth. 2001; 86: 486-496Abstract Full Text Full Text PDF PubMed Scopus (112) Google Scholar). The usable physiological bandwidth is a'more than adequate'15 Hz, even to the finger, but a strong resonance is present which must be carefully compensated for by an antiresonance.9Karamanoglu M O'Rourke MF Avolio AP Kelly RP An analysis of the relationship between central aortic and peripheral upper limb pressure waves in man.Eur Heart J. 1993; 14: 160-167Crossref PubMed Scopus (746) Google Scholar Intra-arterial pressure is measured predominantly with catheter-transducer systems. With continuous flush devices these systems have become very reliable. Nevertheless, in view of the exposure to blood which this technique produces, a non-invasive method that monitors the quality of the recorded pressure pulse10Prentza A Wesseling KH Catheter-manometer system damped blood pressures detected by neural nets.Med Biol Eng Comput. 1995; 33: 589-595Crossref PubMed Scopus (8) Google Scholar is much needed; it should also signal at an early stage the presence of inadequate recordings.3Linton NWF Linton RAF Estimation of changes in cardiac output from the arterial blood pressure waveform in the upper limb.Br J Anaesth. 2001; 86: 486-496Abstract Full Text Full Text PDF PubMed Scopus (112) Google Scholar Non-invasive measurement of finger arterial pressure has no significant instrumental damping. Its innovative use with the Modelflow method in an ICU environment has been investigated recently.11Hirschl MM Binder M Gwechenberger M et al.Noninvasive assessment of cardiac output in critically ill patients by analysis of the finger blood pressure waveform.Crit Care Med. 1997; 25: 1909-1914Crossref PubMed Scopus (40) Google Scholar 12Popovich MJ Hoffman WD Noninvasive cardiac output monitoring.Crit Care Med. 1997; 25: 1783-1784Crossref PubMed Scopus (11) Google Scholar To measure stroke volume, each individual heart beat must be identified in the continuous pressure tracing. Digital computer programs can be designed that hardly ever miss or create a beat, even in the presence of severe arrhythmias or damped pressure curves or when recorded peripherally, such as in the radial artery. Yet, such algorithms are not always implemented.13Rödig G Prasser C Keyl C Liebald A Hobbhahn J Continuous cardiac output measurement: pulse contour analysis vs thermodilution technique in cardiac surgical patients.Br J Anaesth. 1999; 82: 525-530Crossref PubMed Scopus (161) Google Scholar The dependence of the properties of the aorta on age, gender, distending pressure and arteriosclerosis are now understood in great detail, but still we are left with aortic diameter at maximal pressure as the single parameter that may vary up to ±40% from the population average.7Langewouters GJ Wesseling KH Goedhard WJA The static elastic properties of 45 human thoracic and 20 abdominal aortas in vitro and the parameters of a new model.J Biomech. 1984; 17: 425-435Abstract Full Text PDF PubMed Scopus (469) Google Scholar 11Hirschl MM Binder M Gwechenberger M et al.Noninvasive assessment of cardiac output in critically ill patients by analysis of the finger blood pressure waveform.Crit Care Med. 1997; 25: 1909-1914Crossref PubMed Scopus (40) Google Scholar The effect is that absolute levels of cardiac output cannot be determined with certainty, although changes in cardiac output can be tracked with precision. These problems have been recognized since 1904. Bo¨ger and Wezler14Böger A Wezler K Zur physikalischen Bestimmung absoluter Werte des Schlag- und Minutenvolumens des Herzens beim Menschen.Arch Exp Path Pharmacol. 1937; 184: 253-275Crossref Scopus (1) Google Scholar were the first to propose a once-only calibration of pulse contour cardiac output by another method such as indicator dilution. It was recently shown15Jellema WT Wesseling KH Groeneveld ABJ Stouteribeek CP Thijs LG van Lieshout JJ Continuous cardiac output in septic shock by simulating a model of the aortic input impedance. A comparison with bolus injection thermodilution.Anesthesiology. 1999; 90: 1317-1328Crossref PubMed Scopus (181) Google Scholar that this calibration method may be applicable even after 48 h of monitoring in ICU patients. A patent aortic valve is required to prevent any back flow that is not modelled. In addition, aortic aneurysms modify the properties of the aorta in unpredictable ways. Changes with sympathetic outflow in smooth muscle tone in the aortic wall may also modify these properties. The amount of aortic smooth muscle, however, is small and major effects have not been reported. The properties of the aorta change with distending pressure and so depend on the patient's posture. Yet, between the supine and erect positions, offsets in young adult subjects appear small.16Harms MPM Wesseling KH Pott F et al.Continuous stroke volume monitoring by modelling flow from non-invasive measurement of arterial pressure in humans under orthostatic stress.Clin Sci. 1999; 97: 291-301Crossref PubMed Scopus (283) Google Scholar Obese patients may show differences in pulse contour-estimated cardiac output due to abdominal compression when moved from the supine to the prone position, however. Since the major concerns about continuous measurement of cardiac output now have solutions, methods that include these solutions will probably prove reliable and sufficiently precise in tracking changes in cardiac output in critically ill patients. Linton and Linton3Linton NWF Linton RAF Estimation of changes in cardiac output from the arterial blood pressure waveform in the upper limb.Br J Anaesth. 2001; 86: 486-496Abstract Full Text Full Text PDF PubMed Scopus (112) Google Scholar go a long way in this direction, but early reports cannot be fully accepted before well controlled17Siegel LC Pearl RG Noninvasive cardiac output measurement: troubled technologies and troubled studies.Anesth Analg. 1992; 74: 790-792Crossref PubMed Google Scholar clinical studies have established the facts in context. Fully non-invasive cardiac output monitoring from arterial pressure could at last be in sight.