Cardiovascular Dynamics after

Transcription

1 Effects of PEEP on Cardiovascular Dynamics after Open-Heart Surgery: A New Postoperative Monitoring Technique Thomas A. Angerpointner, M.D., Alan E. Farworth, F.R.C.S., and Bryn T. Williams, F.R.C.S. ABSTRACT The circulatory effects of incremental increases in positive end-expiratory pressure ventilation (PEEP) were assessed on 11 occasio in postoperative cardiac surgical patients by phasic aortic blood flow measurements and high-fidelity pressure recordings from which flow and pressure-flow derivatives of cardiac performance were calculated. At 15 cm H,O PEEP, mean arterial blood pressure was decreased by lo%, stroke volume and cardiac output by 15%, and peak left ventricular power by 21%. Although these indices promptly returned to control levels when PEEP was discontinued, levels of PEEP above 10 cm H20 should be used with caution for this group of patients. Ventilation with positive end-expiratory pressure (PEEP) is important in the management of some respiratory iufficiency states [l, 91. Such factors as massive blood trafusion, anesthetic gases, extracorporeal circulation, pulmonary venous congestion, low cardiac output, and the thoracic operative site can impair respiratory function during operation. Although PEEP ope peripheral airways that diminish the physiological shunt and improve arterial oxygen teion [l], oxygen delivery may actually diminish due to il lowered cardiac output [B, 101. These adverse hemodynamic effects merit special examination in patients after cardiac operation. Most hemodynamic studies with PEEP, which average cardiac output over a period of time and which do not provide a continuous beat-by-beat record, have been performed with From the Department of Cardiothoracic Surgery, St Thomas Hospital, Lambeth Palace Rd, London, England. Accepted for publication Dec 1, Address reprint requests to Mr. Williams, Department of Cardiothoracic Surgery, St Thomas Hospital, London, SEl 7EH, England. indicator-dilution techniques that have recognized drawbacks [5, 151. In this study the effect on cardiovascular hemodynamics of progressive increments of PEEP has been evaluated by high-fidelity pressure recordings and phasic aortic blood flow measurements using an extractable flow probe. This method permitted continuous monitoring of circulatory events and the calculation of pressure-flow derivatives of cardiac performance. Materials and Methods Seven patients, 6 men and 1 woman, were investigated on 11 separate occasio. Their ages ranged from 18 to 63 years and body surface area from 1.56 to 2.02 mz. Four patients had undergone coronary artery bypass grafting, 1 had had mitral valve replacement, 1 aortic valve replacement, and 1 had had closure of an atrial septa1 defect. Phasic aortic blood flow was measured by an extractable electromagnetic flow probe* positioned around the ascending aorta before closure of the chest. The cable of the probe was brought through an extrapleural tunnel to the exterior at the second right intercostal space and connected to a fl0wmeter.t The method of calibration and the details of iertion and removal of this probe have been reported previously [171. High-fidelity arterial pressure recordings were obtained with a catheter-tip traducer$ from a major branch of the aorta. A polyethylene catheter was ierted into the left atrium through the right superior pulmo- *CME Series 900 flow probe, Carolina Medical Electronics Inc, King, NC. tcliniflow Model 601D, Carolina Medical Electronics Inc. SMillar Mikrotip pressure traducer PC-530, Millar Itruments, Houston, TX. 555

2 556 The Annals of Thoracic Surgery Vol 23 No 6 June 1977 nary vein to measure mean left atrial pressure (LAP). A bedside computer developed at St Thomas' Hospital was used to analyze the signals and to calculate pressure-flow derivatives of cardiac performance [2]. The layout of this monitoring system is shown in Figure 1. The following indices were itantaneously and continuously recorded: heart rate (min-'); arterial blood pressure: systolic, diastolic, mean (mm Hg); mean LAP (mm Hg); phasic aortic flow (Llmin); cardiac output (Llmin); stroke volume (ml); stroke work (joules); ventricular power (watts); and vascular afterload (stroke index units X lo6). The pressure and flow signals, LAP, and electrocardiogram were stored on magnetic tape to be analyzed off-line by a digital computer.* The patients were ventilated with either an Elema-Schonander 900 Servot or an Engstrom 300 Respirator$ on which PEEP was variable to 50 cm H,O. The patients were hemodynamically Varian digital computer 6201L100, Varian Associates, Palo Alto, CA. telema-sieme, Brow., Stockholm, Sweden. SLKB Medical, AB, Brom, Stockholm, Sweden. Fig 1. Monitor combining pressure and flow measurements. n COMPUTER I t ' stable and had been sedated with papaveretum or diazepam 30 minutes prior to study. There was no significant blood loss or replacement, and routine inteive c.are procedures, which could disturb the patient, were avoided during the study period. The respirator rate was set between 14 and 18 breaths per minute, and ventilation was adjusted to maintain Pace, between 35 and 40 mm Hg. The respirator settings were not altered during this study. PEEP was applied in 5 cm HzO increments to 15 cm HzO, and each level of PEEP was maintained for 5 minutes. PEEP was discontinued after the last period and a further 5 minute period at zero PEEP was recorded. The study was terminated if the cardiac output was depressed below 20% of control (this occurred only once, at 15 cm H20 PEEP). An average value for each hemodynamic variable measured was obtained from at least 20 beats in the final minute of each 5 minute study period. This value was coidered representative for that particular level of PEEP and was compared statistically to the control period using the paired Student t-test. Results Although the number of patients studied was small, the results were remarkably uniform, and a coistent trend of depression of cardiac per- AORTIC FLOW I MEAWREO VALUES DERIVED VALUES WORK POWER VASCULAR AFTERLOAD ' --(PRESS +FLOW) :

3 557 Angerpointner, Farworth, and Williams: Effects of PEEP after Open-Heart Surgery formance with each increment of PEEP was shown. The continuous record of 1 patient (Fig 2) demotrates this depression as well as the prompt hemodynamic respoe to each change of PEEP level. Within a few cardiac cycles after cessation of PEEP, these indicators all returned to control levels. The overall respoe to PEEP in these studies is shown in the Table. Even at 5 cm H,O PEEP a depression in stroke volume, cardiac output, peak aortic blood flow, arterial blood pressure, and peak left ventricular power was obtained, and with higher PEEP levels these effects were more pronounced. At 5 cm H20 PEEP the depression in mean arterial blood pressure ( p < 0.01) and peak left ventricular power ( p < 0.05) was statistically significant. At 10 cm H20 PEEP aortic blood flow, stroke volume, cardiac output, and stroke work were also significantly depressed. A further depression in these variables was seen at 15 cm H20 PEEP. With cessation of PEEP these factors promptly reverted to control levels, and no lasting depression was observed. Fig 2. Hemodynamic respoe to progressive increments of PEEP in a 58-year-old man after coronary artery bypass gra f ting. ImrnHbl I I I 1 I LEFT ATRIAL PRESSURE ' 20 AORTIC PEAK FLOW STROKE VOLUME 0 'OI 0 lml I IW 1 VENTR POWER 51 d "9 STROKE WORK OJ cmh 0 PEEP 2 Throughout the study there was no significant change in heart rate, LAP, and vascular afterload. Comment Acute pulmonary failure is characterized by a low functional residual capacity (FRC) [4, 131, reduced compliance [ll, 131 and physiological shunting of venous blood [ll] through nonaerated lung segments. In this situation PEEP has been found useful because peripheral airways are kept open recruiting alveoli for gas exchange. FRC and compliance are raised [21, the physiological shunt is lowered, and arterial oxygenation is improved [l]. Since Cournand and associates [3] demotrated that intermittent positive-pressure ventilation lowers cardiac output, PEEP has been generally accepted as a cause of further reduction [8, 91. This effect has been attributed to the raised intrathoracic pressure obstructing venous return [15]. The concept of an optimal PEEP level at which the oxygen delivery to the tissues is maximal has been developed, this point being reached when the improvement in arterial oxygen teion is offset by the lowered cardiac output [8, 161. However, some investigators have reported a widely variable respoe to PEEP. Powers and colleagues [12] found that in half of their 70 studies the cardiac output did not fall when PEEP rose. They coidered pulmonary vascular resistance to be critical in determining the effect on cardiac output. As regional alveolar hypoxia results in a decrease in regional alveolar blood flow caused by a local increase in vascular resistance [71, they reasoned that alveolar recruitment by PEEP could reverse this state. This could result in a net reduction in pulmonary vascular resistance and an improvement in cardiac output. Kirby and associates 161 reported no depression in cardiac performance in 28 patients treated with high PEEP levels up to 44 cm H20. However, in both studies there were few cardiac surgical patients, a group for which susceptibility to the adverse hemodynamic effects of PEEP could reasonably be expected. In our group of patients who had undergone cardiac procedures a marked depression of most indicators of cardiac performance in respoe to

4 558 The Annals of Thoracic Surgery Vol 23 No 6 June 1977 Hemodynamic Resp0n:;e to Progressive Increments of PEEP" PEEP (cm H'O) Control Values Heart Rate (min-') 105 k 11 Arterial blood pressure (mm Hg) Systol1c 131 k 23-7 k Diastolic 68 f 11-2 k Mean 89 f 12-3 f Pulse 61 k 20-4 f LAP (mm Hg) 13 f 4 Of2 Aortic peak flow 11.5 f k 0.4 (Liminlm') Stroke volume (ml/m2) 27 f 8-1 f 2 Cardiac output (Llminlm') f 0.2 Peak LV power (watt:slm') 3.4 k f Stroke work (jouleslm') 0.32 k f 0.03 Vascular afterload 95 k 25-2 k 6 (SI units x lo6 [m2]) No change during study f 4-4 f f f k f f Of4-0.6 f f f 3-4 f f f f f k f k 6 +1 k 6-3 f 5-1 f f o.o f k f "Flow and pressure-flow derivatives are recorded as indices per square meter. The absolute values for each indicator in the control period are show:?; the effects of each increment of PEEP are represented as differences from this control value. Values are expressed f standard deviation; p = probability; = not significant; LAP = left atrial pressure; LV = left ventricle; SI = stroke index. PEEP was observed. Even at 5 cm HzO PEEP, the arterial blood pressure was significantly lowered. Because vascular afterload and cardiac output were not significantly altered at this PEEP level, stimulation of thoracic baroreceptors [141 may have accounted for this effect. Although stroke volume, cardiac output, peak aortic blood flow, and peak left ventricular power were decreased at 5 cm H,O PEEP, this reduction became statistically significant for all the indicators at 10 cim H,O PEEP. At 15 cm H20 PEEP mean arterial blood pressure was decreased by lo%, cardiac output by 15%, and peak left ventricular power by 21%. Surprisingly, LAP did not rise during the study, presumably due to impaired forward blood flow through the lungs. Despite the prompt restoration of control values for all hemodynamic indices after cessation of PEEP, levels greater than 10 cm H,O :should be used with caution after cardiac oper. A t' io. References 1. Asbaugh DG, Petty TL: Positive end expiratory pressure: physiology, indicatio, and contraindicatio. J Thorac Cardiovasc Surg 65:165, 1973

5 559 Angerpointner, Farworth, and Williams: Effects of PEEP after Open-Heart Surgery 2. Bourne PR, Williams BT: A cardiac monitor combining flow and pressure measurements. Biomed Eng 10:453, Cournand A, Motley HL, Werko L, et a:: Physiological studies of the effects of intermittent positive pressure breathing on cardiac output in man. J Physiol , Falke KJ, Pontoppidan H, Kumar A, et al: Ventilation with end-expiratory pressure in acute lung disease. J Clin Invest 51:2315, Jacobs RR, Williams BT, Schenk WG Jr: Cardiac output in hypovolaemia: an evaluation of the dye dilution method using an electromagnetic flowmeter as a standard. Arch Surg 102:199, Kirby RR, Dow JB, Civetta JM, et al: High level positive end expiratory pressure (PEEP) in acute respiratory iufficiency. Chest 67:156, Lopez-Majano V, Wagner HN, Twining AB, et al: Effect of regional hypoxia on the distribution of pulmonary blood flow in man. Circ Res 28:550, Lutch JS, Murray JF: Continuous positivepressure ventilation effects on systemic oxygen traport and tissue oxygenation. Ann Intern Med 76:193, McIntyre RW, Laws AK, Ramachandran PR: Positive expiratory pressure plateau: improved gas exchange durihg mechanical ventilation. Can Anaesth SOC J 16:477, Philbin DM, Patterson RW, Baratz RA: Continuous pressure ventilation and oxygen delivery. Br J Anaesth 44:667, Pontoppidan H, Geffen 8, Lowetein E: Acute respiratory failure in the adult. N Engl J Med 287:690, Powers SR, Manna1 R, Neclerio M, et al: Physiologic coequences of positive endexpiratory pressure (PEEP) ventilation. Ann Surg 178:265, Ramachandran PR, Fairley HB: Changes in functional residual capacity during respiratory failure. Can Anaesth SOC J 17:359, Rushmer RF: Cardiovascular Dynamics. Philadelphia, Saunders, 1970, p Smith HJ, Oriol A, Morch J, et al: Hemodynamic studies in cardiogenic shock: treatment with isoproterenol and metaraminol. Circulation 35:1084, Suter PM, Fairley HB, Isenberg MD: Optimum end-expiratory airway pressure in patients with acute pulmonary failure. N Engl J Med 292:284, Williams BT, Sancho-Fornos S, Clarke D, et al: The Williams-Barefoot extractable blood flow probe. J Thorac Cardiovasc Surg 63:917, 1972