Automated infusion of nitroglycerin to control arterial hypertension during cardiac surgery.

OBJECTIVE: To evaluate the feasibility of closed-loop blood pressure control during cardiac surgery. DESIGN: A closed-loop system regulated peroperative hypertension by controlling the infusion rate of the vasodilator nitroglycerin (NTG). The controller consisted of a regulator which was monitored by a supervisory computer program. Mean arterial pressure (MAP) was calculated every 5 s from measurements of the radial artery pressure signal. The regulator calculated an NTG infusion rate with each new MAP measurement. The supervisory computer program monitored the regulator's actions and adapted or overruled the regulator when required. SETTING: The cardiac surgery operating room. PATIENTS: 46 patients who were scheduled for cardiac surgery and who developed peroperative hypertension. INTERVENTIONS: Patients were scheduled for either bypass or valve replacement surgery. The closed-loop system was used to control hypertension before and after cardiopulmonary bypass. The use of the closed-loop system did not require deviation from the protocol normally used during cardiac surgery. All patients received standard continuous anaesthesia with opioids. MEASUREMENTS AND RESULTS: Initial automatic control was achieved in 9.4 (4.1 SD) min. The percentage of time that MAP remained in a range around the target MAP of +/- 10 and +/- 20 mmHg was 74 and 94%, respectively. The mean NTG infusion rate while MAP was within 5 mmHg of target MAP was 1.14 (0.84 SD) micrograms kg-1 min-1. Target MAP was set between 65 and 90 mmHg. There was a small group of patients (6 out of 46) who did not respond to NTG and required alternative drug therapy. CONCLUSIONS: The controller provided fast and stable control in all patients. The expert knowledge implemented through the supervisory computer program enabled the controller to respond adequately to the rapid changes in arterial pressures commonly associated with cardiac surgery. We conclude that closed-loop control of arterial pressure is feasible not only in the cardiac surgical care unit but also during cardiac surgery.

Tracing best PEEP by applying PEEP as a RAMP.

OBJECTIVE: The aim of this study was to show the feasibility of a slow, continuously increasing level of positive end-expiratory pressure (PEEP) (ramp manoeuvre) in selecting best PEEP and to evaluate whether best PEEP, as defined by maximal oxygen transport, coincides with best systemic arterial oxygenation or best compliance. DESIGN: In 11 anaesthetized piglets, PEEP was increased between 0 cmH2O (zero end-expiratory pressure; ZEEP) and 15 cmH2O (PEEP15) with a constant rate of 0.67 cmH2O x min(-1). This ramp manoeuvre was performed both under normal conditions and after induction of an experimental lung oedema. During the ramp manoeuvre, haemodynamic and pulmonary variables were monitored almost continuously. RESULTS: During the rise in PEEP, cardiac output declined in a non-linear way. In the series with normal conditions, best PEEP was always found at ZEEP. In the series with experimental lung oedema, best PEEP, as defined by maximum oxygen transport, was found at PEEP1-6, as defined by maximal compliance, at PEEP7.5 and by maximal arterial oxygen tension (PaO2) at PEEP10-14. CONCLUSIONS: Best PEEP according to oxygen transport is lower than best PEEP according to compliance and PaO2; the use of PEEP as a ramp might prevent unnecessarily high levels of PEEP.

Correction for respiration artifact in pulmonary blood pressure signals of ventilated patients.

OBJECTIVE: To develop an algorithm that corrects pulmonary artery pressure signals of ventilated patients for the respiration artifact. The algorithm should test the validity of the pulmonary pressure signal and differentiate between the cyclic respiration artifact and true measurement artifacts. METHODS: The shape of each pulmonary pressure beat is described by eight characteristic features, including mean pressure value and the systolic and diastolic timing and pressure values. The features are corrected for the respiration artifact by fitting them in a least-squares sense on the first and second harmonics of the ventilator frequency. The corrected features are used by a signal validation algorithm, which adds a validity flag to each pressure beat. The validation algorithm rejects pressure beats with sudden changes in their shape but adapts itself when the changes persist. RESULTS: The performance of the correction and validation technique was evaluated using pulmonary artery pressure signals of 30 patients who were scheduled for open heart surgery. The algorithm correctly recognized as invalid data those pressure signals disturbed by coagulation, surgical manipulations, or flushes of the pressure line. The algorithm marked on average 77 +/- 11% of the pulmonary pressure beats as valid. CONCLUSIONS: The validation algorithm marked sufficient pressure beats as valid to update a trend display every 5 sec. The correction algorithm enabled the validation algorithm to differentiate between true measurement artifacts and the respiration artifact.

Detection of dicrotic notch in arterial pressure signals.

OBJECTIVE: A novel algorithm to detect the dicrotic notch in arterial pressure signals is proposed. Its performance is evaluated using both aortic and radial artery pressure signals, and its robustness to variations in design parameters is investigated. METHODS: Most previously published dicrotic notch detection algorithms scan the arterial pressure waveform for the characteristic pressure change that is associated with the dicrotic notch. Aortic valves, however, are closed by the backwards motion of aortic blood volume. We developed an algorithm that uses arterial flow to detect the dicrotic notch in arterial pressure waveforms. Arterial flow is calculated from arterial pressure using simulation results with a three-element windkessel model. Aortic valve closure is detected after the systolic upstroke and at the minimum of the first negative dip in the calculated flow signal. RESULTS: In 7 dogs ejection times were derived from a calculated aortic flow signal and from simultaneously measured aortic flow probe data. A total of 86 beats was analyzed; the difference in ejection times was -0.6 +/- 5.4 ms (means +/- SD). The algorithm was further evaluated using 6 second epochs of radial artery pressure data measured in 50 patients. Model simulations were carried out using both a linear windkessel model and a pressure and age dependent nonlinear windkessel model. Visual inspection by an experienced clinician confirmed that the algorithm correctly identified the dicrotic notch in 98% (49 of 50) of the patients using the linear model, and 96% (48 of 50) of the patients using the nonlinear model. The position of the dicrotic notch appeared to be less sensitive to variations in algorithm's design parameters when a nonlinear windkessel model was used. CONCLUSIONS: The detection of the dicrotic notch in arterial pressure signals is facilitated by first calculating the arterial flow waveform from arterial pressure and a model of arterial afterload. The method is robust and reduces the problem of detecting a dubious point in a decreasing pressure signal to the detection of a well-defined minimum in a derived signal.

Computer control versus manual control of systemic hypertension during cardiac surgery.

BACKGROUND: We recently demonstrated the feasibility of computer controlled infusion of vasoactive drugs for the control of systemic hypertension during cardiac surgery. The objective of the current study was to investigate the effects of computer controlled blood pressures on hemodynamic stability when compared to conventional manual control. METHOD: Systemic artery blood pressures were managed either by computer (80 patients) or by a well-trained anesthesiologist (80 patients). The vasodilator drugs sodium nitroprusside and nitroglycerin were used. Hemodynamic stability was determined from the standard deviation of the mean arterial pressure samples and from the percentages of time that arterial pressure was hypertensive or hypotensive. RESULTS: The average standard deviation of the mean arterial pressure samples was smaller for the computer controlled than for the manually controlled group: 7.5+/-2.2 (mean+/-SD) versus 8.9+/-2.3 mmHg (P<0.0001). The systemic artery pressure was less hypertensive and less hypotensive in the computer controlled than in the manually controlled group: 9.4+/-5.7 versus 13.1+/-6.0% (P<0.0001) and 8.0+/-5.9 versus 11.8+/-7.4% (P<0.0001), respectively. CONCLUSION: We conclude that, compared with manual control, computer control of systemic hypertension significantly improved hemodynamic stability during cardiac surgery.

Automated infusion of vasoactive and inotropic drugs to control arterial and pulmonary pressures during cardiac surgery.

OBJECTIVE: To evaluate the feasibility of a closed-loop system for simultaneous control of systemic arterial and pulmonary artery blood pressures during cardiac surgery. DESIGN: Feasibility study. SETTING: The cardiac surgery operating room. PATIENTS: The performance of the multiple-drug closed-loop system was evaluated during cardiac surgery in 30 patients who required treatment with more than one vasoactive or inotropic drug. INTERVENTIONS: A multiple-drug closed-loop system integrated five single-drug blood pressure controllers. Arterial hypertension was controlled using sodium nitroprusside or nitroglycerin, arterial hypotension was controlled using noradrenaline or dobutamine, and pulmonary hypertension was controlled using nitroglycerin. The anesthesiologist selected target pressures and single-drug blood pressure controllers. The multiple-drug closed-loop system had a set of priority rules that automatically activated from the selected single-drug controllers the optimum single-drug controller for each hemodynamic state. Drug infusion rates of the nonactive controllers were kept constant. The initial knowledge that was used to construct the priority rules was obtained from standard anesthetic protocols on perioperative management of cardiac surgical patients. A supervisory computer program defined the actions to be taken in cases of infusion pump problems, invalid pressure measurements, and during unexpected increases and decreases in systemic arterial pressure. MEASUREMENTS AND MAIN RESULTS: The activation of single-drug controllers by the priority rules was accurate and fast. On average, a different single-drug controller was activated once every 7.2 mins. As a measure of variability, the average deviation of mean arterial pressure and mean pulmonary artery pressure from their target values was evaluated and was 8.6+/-4.0 and 4.4+/-4.0 mm Hg, respectively, before cardiopulmonary bypass and 8.0+/-3.6 and 2.4+/-0.9 mm Hg, respectively, after cardiopulmonary bypass. None of the single-drug controllers showed any signs of unstable response. CONCLUSION: Closed-loop control of both arterial and pulmonary pressures using multiple drugs is feasible during cardiac surgery.

Left ventricular pressure-volume relationships before and after cardiomyoplasty in patients with heart failure.

BACKGROUND: The aim of this study was to elucidate whether beneficial effects of cardiomyoplasty (CMP) in patients with dilated cardiomyopathy are the result of a decrease in existing ventricular dilatation or a prevention of further dilatation. METHODS AND RESULTS: Combined micromanometer-conductance catheters were used to evaluate left ventricular pressure-volume relationships in six patients with dilated cardiomyopathy before and at 6 and 12 months after CMP. Acute changes in preload and afterload were induced by a standardized leg-tilting intervention and a bolus infusion of nitroglycerin. After CMP, end-diastolic volume (EDV) decreased from 138+/-10 to 103+/-18 mL/m2 (P<.01) at 6 months and to 83+/-17 mL/m2 (P<.01) at 12 months. End-diastolic pressure (EDP) decreased from 20.2+/-6.4 to 13.9+/-7.7 mm Hg (P<.01) at 6 months after CMP. Peak ejection rate and ejection fraction increased at 6 months after CMP from 594+/-214 to 799+/-214 mL/s (P<.05) and from 26.6+/-4.7% to 40.1+/-8.3% (P<.05), respectively. Peak dP/dt decreased at 12 months after CMP from -842+/-142 to -712+/-168 mm Hg/s (P<.05). Leg-tilting before CMP increased EDP from 20.2+/-6.4 to 25.6+/-5.2 mm Hg (P<.01), end-systolic pressure (ESP) from 118+/-17 to 122+/-17 mm Hg (P<.05), and tau from 50.8+/-2.8 to 53.8+/-2.3 ms (P<.05). Six months after CMP, leg-tilting also increased EDV from 103+/-18 to 110+/-22 mL/m2 (P<.05) and ESV from 62+/-14 to 66+/-14 mL/m2 (P<.05). Before CMP, nitroglycerin decreased EDP from 20.2+/-6.4 to 10.4+/-3.8 mm Hg (P<.01), ESP from 118+/-17 to 96+/-11 mm Hg (P<.05), ESV from 100+/-11 to 89+/-7 mL/m2 (P<.05), and tau from 50.8+/-2.8 to 44.5+/-3.7 ms (P<.05). Six months after CMP, nitroglycerin decreased EDP, ESP, and tau to similar values. CONCLUSIONS: Our findings show that up to 1 year after CMP, marked decreases in left ventricular volume are present. Our measurements suggest that CMP actively reduced the dilated ventricle but did not prevent a higher EDV on an increased venous return. The latissimus dorsi muscle wrap contraction results in better synchronization of contraction and more rapid emptying of the left ventricle.