Departments of 1 Anesthesiology, 2 Cardio-Thoracic Surgery and 3 Biostatistics, Erasmus Medical Centre, Rotterdam, The Netherlands
* Corresponding author: Department of Anesthesiology, Erasmus Medical Centre, Postbox 1738, 3000 DR Rotterdam, The Netherlands. E-mail: b.lachmann{at}erasmusmc.nl
Accepted for publication March 20, 2004.
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Abstract |
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Methods. We studied 24 stable patients after coronary artery surgery and/or valve surgery with cardiopulmonary bypass. Patients were randomly assigned to OLC or conventional mechanical ventilation (CMV). In the OLC group, recruitment manoeuvres were applied until was greater than 50 kPa (reflecting an open lung). This value was maintained by sufficient positive airway pressure. In the CMV group, volume-controlled ventilation was used with a PEEP of 5 cm H2O. Cardiac index, right ventricular preload, contractility and afterload were measured with a pulmonary artery thermodilution catheter during the 3-h observation period. Blood gases were monitored continuously.
Results. To achieve > 50 kPa, 5.3 (3) (mean, SD) recruitment attempts were performed with a peak pressure of 45.5 (2) cm H2O. To keep the lung open, PEEP of 17.0 (3) cm H2O was required. Compared with baseline, pulmonary vascular resistance and right ventricular ejection fraction did not change significantly during the observation period in either group.
Conclusion. No evidence was found that ventilation according to the OLC affects right ventricular afterload.
Keywords: heart, right ventricular afterload ; heart, right ventricular ejection fraction ; lung, PEEP ; lung, open lung concept ; lung, recruitment manoeuvre
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Introduction |
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Coronary bypass grafting (CABG) can be complicated by pulmonary dysfunction4 or by reduced right ventricular function.5 Patients with such complications could be vulnerable to increased right ventricular afterload. However, Dyhr and colleagues6 found that a lung recruitment manoeuvre followed by PEEP did not reduce cardiac output in patients after CABG who had been give a volume load. The increased right ventricular afterload could have been offset by the increased end-diastolic volume, since other effects of PEEP on cardiac output can be offset by preload augmentation.79
We set out to study the effect of ventilation according to the OLC on right ventricular afterload in patients ventilated after CABG and/or valve surgery.
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Methods |
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Preoperative risk factors were scored with the European System for Cardiac Operative Risk Evaluation (Euroscore), a scoring system used to predict mortality in cardiac surgery patients, and expressed as a percentage.10
Anaesthesia was induced with midazolam 0.1 mg kg1 i.v. and sufentanil 2 µg kg1. Muscle relaxation was with pancuronium 0.10.2 mg kg1 and was not reversed. Administration of enoximone 0.5 mg kg1 (effective for 36 h) was used routinely to reduce myocardial stunning after CPB. After induction of anaesthesia, a pulmonary artery catheter (CCO 774HF75 series; Edwards, Irvine, CA, USA) was inserted through the right internal jugular vein. Anaesthesia was maintained with midazolam 0.1 mg kg1 and sufentanil (1 µg kg1) as needed. None of the patients received corticosteroids. During operation the lungs were ventilated with the following settings: volume control mode, tidal volume 68 ml kg1; PEEP 5 cm H2O; I/E ratio 1:2; 0.30.5; and respiratory rate was adjusted to achieve a
between 4.5 and 6.5 kPa. These settings were called conventional mechanical ventilation (CMV). During CPB the lungs were not ventilated. After CPB lungs were re-expanded by manual inflation and ventilation was continued with the same settings until randomization. After surgery the pericardium was not closed. After sternum closure, the patients were given fluids until left ventricular function did not increase further. Left ventricular function was assessed by transoesophageal echocardiogram by an experienced operator, who assessed fractional area change on the transgastric midpapillary short-axis view. At optimal left ventricular function, right ventricular end-diastolic volume index (EDVI) measured with the pulmonary artery catheter was defined as optimal EDVI.
After surgery, patients were sedated with propofol 24 mg kg1 h1. An indwelling blood gas analyser probe was inserted in a radial artery for continuous blood gas analyses (ParaTrend 7+; Philips, Boblingen, Germany). Fluid management was guided by EDVI, aiming at optimal EDVI as assessed during operation. Hypovolaemia was treated with a set plan using starch colloids. If a maximum daily dose was reached, further fluid was given as pasteurized plasma. If mean arterial pressure was less than 45 mm Hg and hypovolaemia was excluded, an infusion of dobutamine or phenylephrine i.v. was given.
Cardiovascular and respiratory measurements were made every 30 min for 3 h. Measurements before randomization were considered baseline measurements. Patients were randomly assigned by envelope to the OLC group or the CMV group. Randomization was not stratified for type of operation. The study group was ventilated according to the OLC and in the CMV group ventilation was continued as described above.
Ventilation according to the OLC was initiated by switching the ventilator to pressure control mode, PEEP 10 cm H2O, 0.30.4, I/E ratio 1:1 and a pressure to obtain a tidal volume of 46 ml kg1, aiming at a
of 4.56.5 kPa. A respiratory frequency of 40 b.p.m. was chosen to achieve good carbon dioxide elimination with a low tidal volume. A lung recruitment manoeuvre was applied by increasing peak pressure to 40 cm H2O for 40 s to increase the
ratio to a value greater than 50 kPa. If not, a recruitment manoeuvre was repeated by increasing peak pressure 5 cm H2O greater than before, up to a maximum peak pressure of 60 cm H2O until the
ratio became greater than 50 kPa. If the
ratio decreased slowly below 50 kPa after recruitment, PEEP was increased by 2 cm H2O and a recruitment manoeuvre (beginning at 40 cm H2O) was repeated.
Cardiovascular measurements consisted of right atrial pressure (RAP), mean pulmonary arterial pressure (PAmean) and pulmonary capillary wedge pressure (PCWP). A cardiac output computer (Vigilence; Edwards) that was connected to the pulmonary artery catheter and the monitor recorded heart rate and calculated cardiac index (CI), EDVI and right ventricular ejection fraction (REF). From these values, pulmonary vascular resistance (PVR) was calculated.
After the 3-h study period, if temperature and cardiovascular measurements were satisfactory, sedation was stopped and the patients were weaned from ventilation. Data on outcome were not obtained in this study.
Statistics
To adjust for differences between patients, the changes from baseline measurements were calculated and used to compare the two groups.
Blood gas and cardiovascular measurements, as changes from baseline, were compared using analysis of variance (ANOVA) for repeated measurements (PROC MIXED procedures from SAS).11
Results are presented as mean (SD). The P-values given are two-sided, and P<0.05 was considered significant.
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Results |
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In the OLC group, the Euroscore was greater and more patients had surgery for CABG than in the CMV group. The groups were similar for other baseline characteristics (Table 1). In the CMV group, two patients required a greater to maintain
greater than 10 kPa. In the OLC group, 5.3 (3) recruitment attempts were made with a mean peak pressure of 45.5 (2) cm H2O to open the lung (Fig. 1). To keep the lung open, a total PEEP of 17.0 (2.7) cm H2O had to be applied in the OLC group.
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Discussion |
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To study the effects of ventilation on right ventricular performance, cardiac function is best considered in processes that affect right ventricular preload, contractility and afterload.12
Efforts were made to maintain right ventricular preload constant. Fluid management was not based on wedge pressure, because this varied with intrathoracic pressure, but on the right ventricular EDVI. Compared with baseline, EDVI was comparable between the groups, suggesting comparable right ventricular preload. However, cardiac output depends on right as well as left ventricular preload. Since the ventricles share a common interventricular septum and are housed in a common pericardial sac, which limits their volume, right ventricular EDVI will affect left ventricle EDVI.12 In particular, increased PEEP can restrict left ventricular filling by leftward displacement of the interventricular septum.13 However, ventricular interdependence can be ruled out in our patients because the pericardial sac was not closed after surgery, and therefore could not limit ventricular volume.
Right ventricular afterload is difficult to assess, since the measures that could be used are not easy to obtain and do not only depend on afterload. Commonly used measures of afterload are PVR and REF.
PVR is often criticized because the calculation of resistance assumes that the vessels have rigid walls. Because the pulmonary vessels can collapse, their pressureflow relationship is not linear, and a linear relationship is only likely if left atrial pressure is equal to or greater than pleural pressure (West zone 3). To assess pulmonary vascular resistance, Naeije14 suggests a pressureflow diagram (Fig. 3). The pressure decrease across the pulmonary circulation is displayed on the vertical axis and CI on the horizontal axis. If changes in this plot are compared with baseline values, then pulmonary vasoconstriction or dilatation may be inferred.
REF is inversely related to right ventricular afterload. In the present study, REF did not differ between groups, suggesting that the OLC treatment did not affect afterload.
Our findings contrast with previous clinical studies, in which afterload was greater when PEEP was greater.1519 In these studies, however, greater PEEP was used without a recruitment manoeuvre, so that atelectasis would persist.20 Experimentally, atelectasis can increase right ventricular afterload, causing right ventricular failure in the longer term.21 Thus, a greater PEEP without recruitment may increase right ventricular afterload by affecting atelectatic lung regions or by overdistending healthy lung parts.22 Recruitment manoeuvres could re-expand atelectatic lung, and, if combined with low tidal volumes, this would reduce the increment in the right ventricular afterload during ventilation with greater PEEP. In healthy, non-intubated volunteers without atelectasis, 12.5 cm H2O PEEP did not increase right ventricular afterload,23 supporting our results. The open lung concept advocates recruitment manoeuvres followed by elevated PEEP levels and low driving pressures, resulting in low tidal volumes.3
We found no evidence of increased right ventricular afterload during ventilation according to the OLC in patients after cardiac surgery. The question remains whether these results can be generalized to patients with an intact pericardium, and this question warrants further investigation.
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Acknowledgments |
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References |
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