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*Department of Anesthesiology, Gent University Hospital, Gent, Belgium;
Department of Anesthesiology, Brugmann University Hospital, Brussels, Belgium;
Department of Anesthesiology, Erasme University Hospital, Brussels, Belgium;
Department of Cardiac Anesthesia and Intensive Care, Gent University Hospital, Gent, Belgium;
||Department of Thoracic and Vascular Surgery, Gent University Hospital, Gent, Belgium; and
¶Department of Anesthesiology, Geneva University Hospital, Geneva, Switzerland
Address correspondence and reprint requests to Laszlo L. Szegedi, MD, Department of Anesthesiology 2K12, Gent University Hospital, De Pintelaan, 185, 9000 Gent, Belgium. Address e-mail to laszlo.szegedi{at}ugent.be
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Abstract |
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IMPLICATIONS: This clinical study suggests that hemodilution impairs gas exchange during one-lung ventilation in patients with pulmonary hyperinflation and chronic obstructive lung disease, but not in patients with normal lungs; the reasons for this finding are not clear.
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Introduction |
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Increasing awareness of potential severe complications associated with blood transfusion has prompted the development of blood-conservation strategies. Among these strategies, acute isovolemic hemodilution (IH) is a safe procedure (2). The IH technique decreases a patient’s hemoglobin (Hb) at the start of the operation so fresh units of the patient’s blood are available when needed (2).
Few studies have investigated the effects of hemodilution on pulmonary gas exchange, particularly during OLV. Acute hemodilution may improve the gas-exchange efficiency of the normal lung (3,4), as evidenced by higher PaO2 and lower alveolar-arterial oxygen partial pressure difference, but this effect is not consistently observed. Experimentally, the relationship between anemia and PaO2 is not well defined. Deem et al. (4) analyzed studies concerning the relationship between anemia and PaO2. These studies have examined the effects of hemodilution or blood transfusion in the absence of known lung disease, suggesting an inverse relationship between PaO2 and hematocrit (Hct), a direct relationship, or an inconsistent effect. Moreover, only a few studies included concurrent controls for the effects of time, anesthesia, or both (4).
The purpose of this study was to assess the effect of IH on arterial oxygenation during OLV. Patients scheduled for thoracic surgery may have compromised preoperative lung function. Therefore, we evaluated the effect of IH on arterial oxygenation in patients with normal preoperative lung function and in patients with stable chronic obstructive pulmonary disease (COPD) with moderate degrees of pulmonary hyperinflation. A third group of patients with COPD who did not undergo IH was added to serve as control for the effects of time, anesthesia, or both.
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Methods |
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On the basis of their preoperative pulmonary function, patients were included either in a group with normal lung function (group NL; n = 17) or in a group with stable COPD and mild to moderate pulmonary hyperinflation (functional residual capacity more than 120% of the predicted value) (group COPD; n = 17). After preliminary results on 12 patients in each group, a third group of patients with stable COPD was added (group CTRL; n = 13). The assignment of the patients either to group COPD or to group CTRL was randomized. The rationale for including a concurrent control group (group CTRL) was to eliminate the effects of time, anesthesia, or both that could influence our results. Hence, these patients underwent the same sequences of OLV but no IH. Patients with cardiac or renal function impairment were not included in the study.
Anesthesia was conducted in a standardized manner. All patients were given oral alprazolam 0.5 mg approximately 60 to 90 min before arrival in the operating room.
A thoracic epidural catheter was inserted at the midthoracic level (T6 to T9) to ensure analgesia during and after surgery. A test dose of 3 mL of 2% lidocaine with epinephrine 1:200,000 was used; the initial dose (8 mL of 2% lidocaine with 100 µg of fentanyl) was given only after the end of the study, followed by a continuous infusion of bupivacaine 0.5% at 2–5 mL/h.
The induction and maintenance of general anesthesia was achieved with fentanyl (100 µg) and propofol (initial dose of 2 mg/kg followed by continuous infusion of 3–5 mg · kg–1 · h–1). Cisatracurium (initial dose of 0.15 mg/kg and top-up doses of 0.05 mg/kg) was used to allow tracheal intubation and to maintain neuromuscular blockade throughout surgery. The neuromuscular blockade was assessed by regular measurements of posttetanic count during the procedure.
A three-lead electrocardiogram, invasive radial arterial and central venous blood pressures, and arterial oxygen saturation were continuously monitored. Expired end-tidal carbon dioxide tension and flow-volume and pressure-volume loops were also continuously displayed (S/5 Anesthesia Monitor AM; Datex-Ohmeda Division, Instrumentarium Corp., Datex-Ohmeda, Finland).
In all patients, the bronchus of the dependent left lung was intubated with a double-lumen endotracheal tube (DLT) (Broncho-cathTM; Mallinckrodt Laboratories, Athlone, Ireland) of an appropriate size determined by the height of the patient (5). The correct position of the DLT was ascertained with fiberoptic bronchoscopy.
A constant tidal volume of 10 mL/kg was delivered with an S/5 ADU ventilator throughout the study. The ventilatory pattern consisted of a volume-controlled square-wave flow pattern at a rate of 10 breaths/min, with a fraction of inspired oxygen in air of 0.5. Inspiratory time/expiratory time was 1:2, and the end-inspiratory pause was 10% of the total respiratory cycle. End-expiratory pressure was set to 0. Ventilatory variables were kept constant during the study during both TLV and OLV.
A transesophageal echocardiograph probe was inserted after tracheal intubation to measure the CO with the method of the effective aortic valve area (6). Blood gas samples were analyzed immediately after they were drawn and were temperature-corrected. Arterial and central venous oxygen saturation were measured with a cooximeter.
This investigation was performed with closed chest before the surgical procedure, with the patients in the supine position. After intubation in the supine position and fiberoptic bronchoscopic control of the correct DLT position, the tracheal lumen of the DLT was clamped, and the nonventilated right lung was allowed to deflate to atmospheric pressure. After 15 min of OLV with the above-described ventilatory settings, end-inspiratory and end-expiratory occlusions were performed to determine the mechanical characteristics of the respiratory system (peak inspiratory airway pressure, end-inspiratory plateau pressure, and intrinsic positive end-expiratory pressure [PEEPi]), and arterial and venous blood gas samples were drawn and analyzed. The CO was measured.
After these baseline measurements (before hemodilution), the lung was sighed manually (insufflation pressure up to 40 cm H2O) (7), and TLV was restored with unaltered ventilatory settings. An IH was performed during the period of TLV by simultaneous withdrawal of blood and infusion of 6% hydroxyethyl starch 130/0.4 at equal volumes. The exchange was standardized to 500 mL for each patient.
Thereafter, OLV was restored for 15 min. At the end of this period, ventilatory data were again recorded, arterial and venous gas samples were drawn and analyzed, and CO was measured (after IH).
In group CTRL, the sequences of OLV were the same but no IH was performed. The temperature of the patients was kept constant during the entire study with an air convection system.
Statistical analysis was performed with the GraphPad InStat software package, Version 3.05, for Windows 95/NT. First, the assumption that data were sampled from populations with identical standard deviations was tested by using the method of Bartlett. The assumption that the differences were sampled from populations that follow Gaussian distribution was verified by using the method of Kolmogorov and Smirnov. Thereafter, differences between groups and inside each group were analyzed with one-way analysis of variance (ANOVA) with the Tukey-Kramer multiple comparisons test. Values of P < 0.05 were accepted as statistically significant. Data are presented as mean ± SD.
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Discussion |
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HPV is the major protective mechanism that diverts blood flow away from the atelectatic lung (1). Although HPV is an intrinsic mechanism of the pulmonary vasculature (8), several studies indicate that it may be influenced by red blood cells. McMurtry et al. (9) reported that isolated rat lungs perfused with plasma had rapidly decaying HPV when compared with lungs perfused with blood. Another study reported similar results in isolated rat, cat, or rabbit lungs (10).
Deem et al. (11) studied the effect of anemia on intrapulmonary shunt during left lung atelectasis in rabbits and concluded that isovolemic anemia has a deleterious effect on pulmonary gas exchange, possibly through attenuation of HPV. Another study (12) suggested that in rabbits with previous lung injury induced by gas embolism, IH resulted in improved oxygen exchange, but it did not find a clear mechanism for this improvement. The same authors reported an improvement in gas exchange in the normal rabbit lung as a result of an improvement in overall ventilation/perfusion matching (13). Kleen et al. (14) suggested that severe acute normovolemic hemodilution in dogs causes different alterations in the heterogeneity of regional pulmonary blood flow in hyperoxic conditions. Other authors (15) found no influence of changes in Hb concentration on HPV efficiency.
In this study, preexisting lung disease, as demonstrated by altered preoperative pulmonary function tests, was associated with a higher baseline PEEPi and a slightly lower PaO2 (group COPD and group CTRL). The most important finding of this study was a significant decrease in PaO2 after IH during OLV of the patients with compromised lung function. COPD patients are chronically hypoxic; thus, in such patients HPV may be already maximal and no more protective during OLV, because these patients already have an increased pulmonary vascular resistance and decreased pulmonary vascular bed (16). Other mechanisms that tend to decrease the degree of venous admixture during OLV include gravitational effect and surgical manipulation (1). In this study, performed with a closed chest and with patients in the supine position, these potential factors were absent.
The maintenance of tissue oxygen delivery during an acute reduction in red blood cell concentration depends on both an increase in CO and an increase in blood oxygen extraction (17). However, in our study, CO remained constant with the mild IH (Table 2). Anesthesia significantly reduces the CO response associated with acute IH. This could be related to the effects of the anesthetics on the autonomic and the cardiovascular systems (18).
The higher PEEPi values (Table 2) during OLV observed in group COPD as compared with group NL could have induced enough hyperinflation to disrupt the ventilation/perfusion matching. However, this observation is unlikely, given that in group CTRL (COPD patients who did not undergo IH) the PEEPi values were comparable to those measured in group COPD and that the PaO2 changes in group CTRL were not significant.
If the PaO2 was lower in group COPD after IH than in group CTRL (without IH) and if there were no differences in the oxygen saturation of blood collected in the superior cava vein or in the cardiac index, then the shunt must have increased in group COPD; even though difficult to explain, this may be the only reason for our findings.
Some consideration should be given to the limitations of our study. First, it was a purely clinical study; according to institutional structures, monitoring and measurements were limited. For example, a pulmonary artery catheter could not be inserted, so we were not able to perform shunt measurements. Second, given the absence of data on the subjects and the patient population studied (stable COPD with mild to moderate pulmonary hyperinflation), only a mild degree of IH was studied. Further investigations are required to evaluate the effects of more profound IH in patients with normal preoperative lung function.
It is surprising that this mild hemodilution/anemia was associated with this degree of gas-exchange impairment in the patients with COPD, given that changes in intrapulmonary shunt in the experimental model reported by Deem et al. (11) were not realized until the Hct was in the mid-20s. The different results in the experimental model and this clinical study could be for any number of reasons, including species differences and the presence of COPD, but it raises the question as to whether the observed decrease in PaO2 was in fact due to hemodilution. Although this is somewhat unlikely (18), the study protocol allows the possibility that the observed change in arterial oxygenation occurred after IH during TLV.
Third, our results could have been influenced by the repeated endotracheal tube clamping maneuvers. Benumof (19) showed that intermittent hypoxic challenges may potentiate the HPV response in an animal model. In our study design, there were, indeed, two sequences of deflating/inflating of the lung; however, the oxygenation was not better in either group in the second period of OLV. This is in accordance with the studies of Carlsson et al. (20), who found maximal HPV within 15 minutes of hypoxia, and Domino et al. (21), who observed a maximal response after the first hypoxic challenge during OLV in closed-chest dogs. To eliminate the changes over time in oxygenation during OLV, a third group of patients (group CTRL) with altered preoperative pulmonary function was studied. Sequences of OLV without IH did not result in significant difference in arterial oxygenation (Table 3).
Fourth, the study was performed with the patients in the supine position and with a closed chest instead of the usual clinical situation of a lateral position with an open chest. These differences may affect extrapolation of the results because of the absence of the gravitational effect in COPD patients and the absence of surgical manipulation—factors that may contribute to blood-flow redistribution during OLV (1).
In conclusion, mild IH significantly altered arterial oxygenation during OLV in patients with preoperative compromised lung function. Although anemia may be less tolerated by COPD patients in these conditions, this does not indicate that a more aggressive transfusion approach is required in these patients. Indeed, PaO2 could be easily increased by increasing the fraction of inspired oxygen, which is common practice during OLV. Acute IH did not appear to be a safe alternative technique to blood transfusion in patients with altered pulmonary function undergoing thoracic surgery. Further studies are required to better define the adequate transfusion trigger in this population.
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This article has been cited by other articles:
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  A. I. Levin and J. F. Coetzee Arterial Oxygenation During One-Lung Anesthesia Anesth. Analg., January 1, 2005; 100(1): 12 - 14. [Full Text] [PDF]
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