Anesth Analg 1999;88:213-219
© 1999 International Anesthesia Research Society
SPECIAL ARTICLE
The Influence of Surgical Sites on Early Postoperative Hypoxemia in Adults Undergoing Elective Surgery
Fu S. Xue, MD*,
Bai W. Li, MB
,
Guo S. Zhang, MB
,
Xu Liao, MD*,
Yan M. Zhang, MD*,
Jian H. Liu, MD*,
Gang An, MD*, and
Lai K. Luo, MD*
*Department of Anesthesiology, Plastic Surgery Hospital, Chinese Academy of Medical Sciences and Peking Union Medical College;
Department of Anesthesiology, Beijing Rail General Hospital, Beijing; and
Department of Anesthesiology, Wen-Xian People's Hospital, HeNan Province, People's Republic of China
Address correspondence and reprint requests to Fu S. Xue, MD, Department of Anesthesiology, Plastic Surgery Hospital, Chinese Academy of Medical Sciences and Peking Union Medical College, Ba-Da-Chu Rd., Beijing, People's Republic of China 100041.
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Abstract
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To determine the influence of the surgical sites on early postoperative hypoxemia, we studied postoperative hypoxemia in 994 patients, ASA physical status I or II, aged 18–68 yr, scheduled for various types of elective surgery. Patients were divided into three groups on the basis of the surgical sites: Group 1 = elective superficial plastic surgery (n = 288); Group 2 = upper abdominal surgery (n = 452); and Group 3 = thoracoabdominal surgery (n = 254). Anesthesia was maintained with 1%–2% enflurane and 67% nitrous oxide in oxygen; thiopental or fentanyl was given IV as required. SpO2 levels were recorded while patients breathed room air shortly after arrival in the recovery room (0 min) and 5, 10, 15, 20, 30, 40, 50, 60, 120, and 180 min thereafter. The results showed that during the early postoperative period, the degree of arterial desaturation and the incidences of hypoxemia (SpO2 86%–90%) and severe hypoxemia (SpO2 85%) were closely related to the operative sites and were greatest for thoracoabdominal operations, less for the upper abdominal operation, and least for the peripheral surgery. The incidence of hypoxemia and severe hypoxemia in the recovery room was 7% and 0.7%, respectively, in Group 1, 38% and 3% in Group 2, and 52% and 20% in Group 3. Mild airway obstruction and hypothermia in the postanesthesia recovery unit (PAR) were the predictive factors of early postoperative hypoxemia. We conclude that during the early postoperative period, there were significant differences in SpO2 levels and incidences of hypoxemia and severe hypoxemia among the three groups.
Implications: We found that the severity of arterial desaturation and the incidence of hypoxemia during the early postoperative period are closely related to the surgical sites and are strongest for thoracoabdominal surgery, less for upper abdominal surgery, and least for peripheral surgery.
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Introduction
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Early postoperative hypoxemia has been a matter of concern for a number of years. Because of the introduction of pulse oximeters into clinical practice, the factors that may influence the occurrence of early postoperative hypoxemia, such as age (1–3), gender (3), weight (3,4), intraoperative opioid administration (1–3), smoking (2), duration of anesthesia (2–6), and preexisting heart and lung disease (7,8), have been studied extensively. Obesity, old age, smoking, and preexisting heart and lung disease clearly predispose patients to early postoperative hypoxemia (7). Postoperative arterial desaturation and mechanical impairment of respiratory function are probably the most frequent for thoracic and upper abdominal surgery versus lower abdominal surgery and peripheral surgery (1,9,10). However, there is still controversy concerning the relationship between the operative sites and occurrence of early postoperative hypoxemia. Some studies found that the operative site was associated with postoperative hypoxemia (11), but not in the early postoperative period (12). In the study of Meiklejohn et al. (3), the incidence of early postoperative hypoxemia was similar in patients undergoing peripheral, low abdominal, and upper abdominal operations. The purposes of this study were 1) to observe the incidence, severity, and duration of early postoperative hypoxemia in otherwise healthy adults undergoing elective plastic, upper abdominal, and thoracoabdominal operations; and 2) to evaluate the influence of the surgical sites on the incidence and severity of early postoperative hypoxemia.
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Method
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After obtaining informed consent and approval from our ethics committee, 994 patients, ASA physical status I or II, aged 18–68 yr, scheduled for elective surgery, were included in this study. Patients were excluded if they had cardiac, pulmonary, renal, hepatic, neurological, muscular, and inflammatory diseases or a long history of smoking, as were pregnant women and patients undergoing major reconstructive surgery for burns and with palatoplasty. Those with a body weight >10% above the ideal, and patients with anemia (i.e., hemoglobin concentration <8 g/L) at the end of surgery because of major blood loss during surgery were also not studied. Ideal body weight (IBW) was defined as follows: IBW (male) = 50 kg + 0.9 kg/cm above 152 cm height; IBW (female) = 45.4 kg + 0.9 kg/cm above 152 cm height (13). Patients were divided into three groups on the basis of the surgical sites: Group 1 = patients underwent elective superficial plastic surgery (face, neck, limbs, or abdominal or chest wall, n = 288); Group 2 = patients received upper abdominal surgery (partial gastrectomy and cholecystectomy, n = 452); and Group 3 = patients underwent thoracoabdominal surgery (two-stage esophagectomy with gastric replacement for carcinoma of the esophagus, patients were subjected to an upper abdominal incision and a right thoracic incision, n = 254). Patient characteristics are shown in Table 1.
Patients were premedicated with diazepam (0.2 mg/kg, maximum 10 mg) and hyoscine 5 µg/kg IM 1 hour before anesthesia. Anesthesia was induced with IV thiopental 4–6 mg/kg, fentanyl 2–5 µg/kg, and succinylcholine 1.5 mg/kg, followed by endotracheal intubation. Anesthesia was maintained with enflurane (1%–2%, expired concentration), 67% nitrous oxide in oxygen, and vecuronium or pancuronium for muscle relaxation. Thiopental 2 mg/kg or fentanyl 2 µg/kg was supplemented IV as required. During surgery, the lungs were mechanically ventilated (tidal volume [VT] 8–10 mL/kg, respiratory rate [RR] 12–15 breaths/min), and one-lung ventilation was also used for patients in Group 3, as required. During one-lung ventilation, nitrous oxide was not used, and end-tidal carbon dioxide was kept within the normal limits.
During the surgical procedure, the usual monitors were used. Inspired and end-tidal concentrations of oxygen, carbon dioxide, nitrous oxide, and enflurane were measured and displayed digitally (Anesthesia Gas Monitor Type 1304; Bruel & Kjaer, Gentofte, Denmark). VT, RR, and minute volume of ventilation were measured by using a spirometer (Dragerwerk AG, Lubeck, Germany). A cannula was placed in the radial or femoral artery for sampling arterial blood gas, hematocrit, and hemoglobin, and electrolytes were determined by using a Model-5 blood-gas-electrolyte analyzer (Nova Biomedical Company, Hoboken, NJ). During surgery, blood loss was measured by swab and drape weighing and from suction bottles. Minimal or moderate blood loss was replaced with lactated Ringer's solution as required. Hemodynamic variables remained stable throughout surgery in all the patients. During anesthesia and after surgery, SpO2 was monitored continuously by using pulse oximeters.
At the end of anesthesia, all the patients breathed 100% oxygen for at least 5 min after nitrous oxide was discontinued. Residual muscle relaxation as detected by thenar muscle in response to peripheral nerve stimulation (train-of-four ratio 
7) was reversed with atropine 0.03 mg/kg and neostigmine 0.07 mg/kg. When protective reflexes had recovered and adequate spontaneous breathing function was reestablished, i.e., patients were able to maintain a normal PaCO2 and a PaO2 of 
90 mm Hg with an inspired oxygen fraction of 0.3 during spontaneous breathing, or when patients were awake, tracheal extubation was performed in the operating room. The airway was then assessed, and 100% oxygen was administered via a face mask for 3–5 min before patients were transferred to the recovery room in a 5o head-down position. Respiratory status was observed and recorded in the recovery room. Adequacy of ventilation was assessed clinically by observing the expiratory air flow from the nose or mouth and the movement of the thorax with or without auscultation with a stethoscope. Airway obstruction was scored as follows: 1 = absent, 2 = mild (light snoring with adequate ventilation), 3 = moderate (necessitating a position change or a nasopharyngeal airway), and 4 = severe (obstructive dyspnea or heavy snoring with ventilation disturbances or requiring reintubation) (14). In addition, other early postoperative complications, such as inadequate hemostasis, abnormal temperature (hypothermia = temperature 36°C; fever = temperature 37.5°C), upper airway and intrathoracoabdominal infections, and cardiopulmonary arrest, were also recorded.
SpO2 levels were recorded for all the patients when breathing room air shortly after arrival in the recovery room (0 min) and 5, 10,15, 20, 30, 40, 50, 60, 120, and 180 min thereafter. Patients were in the supine position when SpO2 values were measured in the PAR. SpO2 levels 90% (86%–90%) were classified as hypoxemia, and those 85% were classified as severe hypoxemia (15). If SpO2 decreased to 85%, 100% oxygen was supplied immediately via a disposable plastic face mask with flows of at least 6 L/min. The patients receiving supplemental oxygen were allowed to breath room air for at least 3 min before the next recording of SpO2. Supplemental oxygen was readministered if hypoxemia occurred with air breathing.
All data were stored in a computer and analyzed with POMS statistical software Version 2.00 (Shanghai Scientific and Technical Publishers, Shanghai, People's Republic of China). The comparisons of SpO2 and demographic data among groups were made by using one-way analysis of variance (ANOVA), and the comparisons of SpO2 within groups were performed by using repeated-measures ANOVA. For cases in which the calculated F-value exceeded the critical value for the 0.05 probability level, the Student-Newman-Keuls multiple range test was used to determine which differences were significant. The comparisons of the incidences of hypoxemia and severe hypoxemia between and within groups, of the gender distribution, the incidence of early and late postoperative complications among groups, and the comparisons of the incidences of hypoxemia between patients with and without postoperative complications were performed by using a 
2 test. The Bonferroni correction was used when multiple comparisons were performed. The relationship between the SpO2 levels on arrival in the PAR and patients' age in each group was analyzed by using linear regression analysis. Data are expressed as mean ± SD. A P value <0.05 was considered statistically significant.
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Results
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Except for length of hospital stay, there were no significant differences among the three groups in terms of demographic data (Table 1). Surgical procedures in all the patients were uneventful, and blood loss was minimal or moderate. Mean blood loss in Groups 1, 2, and 3 were 315 ± 187, 298 ± 124, and 375 ± 164 mL, respectively, and there was no significant difference among groups (P > 0.05). The duration of one-lung ventilation in Group 3 was 1.2 ± 0.8 h (45 min to 2.2 h). On arrival in the recovery room, all the patients had clinically adequate ventilation. Sixty-five patients showed mild upper airway obstruction. The incidences of mild upper airway obstruction were 5.6%, 7.1%, and 6.7% in Groups 1, 2, and 3, respectively, without differences among the groups. No patient had moderate or severe airway obstruction or required an artificial airway. Opioid analgesics and sedative drugs were only allowed after the observation of 3 h. Patients had no clinical signs of respiratory depression from intraoperative administration of fentanyl, i.e., RR was <12 breaths/min while VT was well sustained.
While patients breathed room air, SpO2 decreased significantly after surgery in all cases, then increased gradually. During the early postoperative period, the decrease in the SpO2 and in incidences of hypoxemia and severe hypoxemia were closely related to the surgical sites and were most frequent for thoracoabdominal operations, less for upper abdominal surgery, and least for plastic surgery. There were significant differences among the three groups in the SpO2 levels and incidences of hypoxemia and severe hypoxemia in the early postoperative period. (Table 2 and Figs. 1–3).
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Figure 1. The preoperative SpO2 and the SpO2 in the early postoperative period while patients were breathing room air. Group 1 = superficial plastic surgery; Group 2 = upper abdominal surgery; and Group 3 = thoracoabdominal surgery. Points express mean ± SD. *P < 0.01, Group 1 versus Groups 2 and 3. #P < 0.05, Group 2 versus Group 3.
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Figure 2. The incidence of hypoxemia in the first 180 min after surgery while patients were breathing room air. Group 1 = superficial plastic surgery; Group 2 = upper abdominal surgery; and Group 3 = thoracoabdominal surgery. Points are incidence of hypoxemia (SpO2 of 86%–90%). *P < 0.01 compared with Group 1. #P < 0.05 compared with Group 2.
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Figure 3. The incidence of severe hypoxemia in the first 180 min after surgery while patients were breathing room air. Group 1 = superficial plastic surgery; Group 2 = upper abdominal surgery; and Group 3 = thoracoabdominal surgery. Points are incidence of severe hypoxemia (SpO2 = 85%). *P < 0.01 compared with Group 1. #P < 0.05 compared with Group 2.
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The postoperative recovery of SpO2 was also significantly different among the three groups. (Table 3). Recovery rates for SpO2 were fastest 0–60 min after surgery. SpO2 returned to >90% in all cases of Group 1 at 1 h after the operation. However, even 3 h after the operation, SpO2 levels had not yet reached >90% in 6.2% of patients in Group 2 and 20.1% of patients in Group 3. In addition, the mean SpO2 levels returned to baseline in Group 1 by 180 min after surgery, but not in Groups 2 and 3.
The early and late postoperative complications of all the patients are shown in Table 4. Only mild airway obstruction and hypothermia in the PAR were related to the occurrence of early postoperative hypoxemia. The incidences of hypoxemia in patients with and without mild airway obstruction were 69% and 3.3%, respectively, in Group 1 (P < 0.01), 78% and 35% in Group 2 (P < 0.01), and 88.3% and 70.5% in Group 3 (P > 0.05). The incidences of hypoxemia in patients with a temperature 36°C and a temperature 36.1–37.5°C were 33.3% and 6.1%, respectively, in Group 1 (P < 0.01), 61.9% and 37.2% in Group 2 (P < 0.05), and 100% and 70% in Group 3 (P < 0.05). No significant correlation was found between the decrease in SpO2 on arrival in the PAR and patients' age in any group by linear regression analysis.
In patients with an SpO2 85%, SpO2 can be rapidly increased to the desired levels (95%) after O2 therapy via a face mask with flows of at least 6 L/min. No patient had postoperative complications relating to hypoxemia. One patient in Group 3 died 16 days after surgery because of fistula of esophagogastric anastomostic site and severe intrathoracic-abdominal infections. There were no death from anesthetic reasons during the perioperative period.
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Discussion
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Our data show that the degree of arterial desaturation and the incidence of hypoxemia (SpO2 <90%) were closely related to the operative sites in the immediate postoperative period. Compared with the patients who underwent elective peripheral surgery, patients who underwent upper abdominal and thoracoabdominal surgery had much more severe desaturation and much higher incidences of hypoxemia and severe hypoxemia, particularly those patients who underwent thoracoabdominal operation. The findings are in keeping with the results of some previous studies (1,16). Contrary to our results, however, Meiklejohn et al. (3) did not find that the incidences of hypoxemia and severe hypoxemia were significantly different among patients undergoing peripheral, lower abdominal, and upper abdominal operations. This may be related to the small sample in each group. No statistical significance was achieved in the numbers studied, but there was a trend toward a higher incidence of severe hypoxemia in patients after upper abdominal surgery (34.4%) compared with that in patients who underwent peripheral (20.8%) and lower abdominal operations (14.3%) (3).
The incidence of early postoperative hypoxemia in different studies varies considerably because of the differences in patient selection, anesthetic techniques, observation method, and type of surgery. Previous studies found that early postoperative hypoxemia (SpO2 90%) occurred in 35%–70.8% of patients undergoing elective surgery under general anesthesia, of which 12%–34.4% experienced severe hypoxemia (SpO2 85%) (1,7,14,15). When all the patients were considered together, the incidences of hypoxemia and severe hypoxemia in our study were 39% and 6.6%, respectively. In our patients, many factors may be involved in early postoperative hypoxemia. It may have been related to the anesthetic itself, as it is an extension of the disorder of gas exchange that occurs during anesthesia. Alveolar hypoventilation, ventilation/perfusion mismatch, right to left shunting of blood, depressed cardiac output, and increased oxygen consumption due to hypertonic muscles and shivering are also potential causative factors (17). During the early postoperative period, airway tissue edema, secretion accumulation in the pharynx, and the tongue falling into the pharynx due to residual anesthetic effect are common. These factors can aggravate alveolar hypoventilation and increase respiratory efficacy. The normal hypoxic drive to ventilation may be abolished by a concentration of enflurane as low as 0.1 MAC (18). This concentration may be present in the immediate postoperative period and may cause some early-phase respiratory depression. Diffusion hypoxia after nitrous oxide use is unlikely to be a contributing factor because all the patients breathed 100% oxygen for at least 5 min at the end of the operation. Residual effects of neuromuscular blockade could aggravate early postoperative hypoxemia, but routine reversal of residual neuromuscular blockade at the end of surgery makes this unlikely.
Except for the influences of anesthesia on respiratory function described above, patients undergoing upper abdominal and thoracic operations are more at risk of hypoventilation, imbalance of ventilation and perfusion, and atelectasis compared with those undergoing other operations, probably because of more severe mechanical impairment of respiratory function (20–23). The characteristic postoperative mechanical respiratory abnormality after abdominal or thoracic surgery is a restrictive pattern with severely reduced inspiratory capacity and vital capacity (VC) plus smaller, but more important, reduction in functional residual capacity (FRC) (21–23). Patients breathe rapidly with a small VC and are unwilling or unable to inspire deeply. The reduced inspiratory capacity also limits the patients' ability to cough effectively to clear accumulated secretions, which may exaggerate impairment of ventilation. Small airway closure and impairment of the cough mechanism lead to retention of sections and atelectasis, which further reduce FRC. Movements of the thorax, abdomen, and diaphragm can be restricted by the mechanical inefficiency caused by incision of muscles and by the restrictive effects of bandages, gut distention, pneumoperitoneum, and pneumothorax (20). In addition, pain from the surgical incision is associated with a decrease in VC and a reduction in the efficiency of coughing and the ability to make maximal inspiratory efforts. Effective pain relief can ameliorate these effects, particularly in patients with an upper abdominal incision (24).
The main mechanism of early postoperative hypoxemia is probably airway closure. When closing capacity (CC) exceeds FRC, lung regions develop a low ventilation/perfusion ratio, which leads to impaired gas exchange and possible gas trapping and atelectasis (1). It is well demonstrated that the degree of postoperative hypoxemia correlates closely with reduction in FRC and with the ratio of CC to FRC (23). Twenty-four hours after upper abdominal surgery, FRC and VC decreases to approximately 70% and 40% of preoperative levels, respectively, and remains depressed for several days. However, FRC and VC show less depression and more rapid recovery after lower abdominal surgery compared with upper abdominal surgery, and they are unchanged after superficial or extremity surgery (21–23). The decreases in FRC and VC after thoracic surgery are similar to the results after upper abdominal surgery (25). The mechanism for the reduction in FRC after abdominal or thoracic surgery is the combined effect of incisional pain and reflex dysfunction of the diaphragm. Additional effects of thoracic surgery include pleural effusion, cooling of the phrenic nerve, and mediastinal widening (26).
There are no available data regarding the influence of thoracoabdominal surgery on the early postoperative FRC, but we believe that the decrease in FRC after thoracoabdominal surgery was greater than that after upper abdominal or thoracic surgery. This is probably because the thoracic wall was involved and because the thoracoabdominal surgical procedures were much more extensive than the others, thus restraining the diaphragmatic movement more severely. Bishop and McKeown (20) reported that the forced vital capacity in patients undergoing esophagectomy with gastric replacement decreased to 33% of the preoperative value in the early postoperative period. Our data and that of a previous study by Knudsen (16) show that arterial desaturation and hypoxemia in the immediate postoperative period are more pronounced and longer lasting after thoracoabdominal than abdominal surgery. Because we also controlled the other factors known to interfere with the occurrence of early postoperative hypoxemia and no patients had clinical signs of respiratory depression from the intraoperative administration of anesthetics, the differences in postoperative SpO2 levels and hypoxemia observed among groups are likely to be mainly due to the surgical sites.
The present results showed no correlation of patients' ages with degree of arterial desaturation in the patients undergoing elective surgery of different sites during the early postoperative period. This is in agreement with the results of some previous studies in adult patients (3,15,27,28); however, other studies found that patients' ages were closely related to the occurrence of early postoperative hypoxemia (1,2,6,8,19,29). The exact reasons for the different results are unclear, but they likely include differences among these studies in patient selections, anesthetic techniques, observation method, and type of surgery. For example, the association noted in some of these may be explained by differences in the age groups examined. Closer analysis of the data of Moller et al. (29) reveals that although the 18–39 yr group had a smaller incidence of hypoxemia than those 40 yr, there was no difference between the 40–60 yr and >60 yr groups. Similarly, Morris et al. (6) reported an association with age but only examined the 40 yr and >40 yr groups. This suggests that although adults aged <40 yr have a lower risk of developing hypoxemia than those aged >40 yr, age beyond 40 yr is not an important predictive factor.
Our data also demonstrated that recovery rate of SpO2 was faster during the first 60 min. This may be due to the rapid disappearance of the depressant effects of residual anesthetics (1,8). SpO2 returned to baseline in our patients undergoing elective plastic surgery 3 h after surgery. Because of the absence of continuing mechanical respiratory abnormality in this category of patients, the effects of residual anesthetics on respiratory function may be a primary cause of early postoperative hypoxemia (7). However, SpO2 levels in Groups 2 and 3 recovered only to 95.1% ± 2.5% and 92.4% ± 2.6% (corresponds to a PaO2 of approximately 64–75 mm Hg), respectively, as late as 3 h after surgery, which were significantly lower than the preoperative levels. In addition, 6.2%–20.1% of the patients in Groups 2 and 3 still had an SpO2 of 90% (PaO2 60 mm Hg) at 3 h. This may be related to the persistence of ventilation abnormalities in these patients. Previous studies showed that ventilatory disturbance could remain for several days after upper abdominal or intrathoracic surgery (10,23,25).
The present study shows consistently, as have other studies, that the early postoperative period is a time at which patients are most apt to develop severe arterial desaturation (1,14,15,30,31). During this time, patients should be closely monitored and properly treated, particularly after thoracoabdominal and upper abdominal surgery. In the absence of adequate monitoring of arterial oxygen, oxygen administration should be routinely considered for all the patients after general anesthesia in the immediate postoperative period (32). Because pulse oximetry is now a standard of care during anesthesia and in the immediate postoperative period, the routine use of oxygen therapy may be an unwarranted expense in healthy patients at low risk of hypoxemia who have adequate oxygen saturation in the recovery room (33). In this and other studies (1,15), oxygen therapy was administered only when SpO2 was 85% and was sufficient to successfully increase SpO2 to the desired levels without any hypoxemic consequences.
In conclusion, we have confirmed that the severity of arterial desaturation and the incidences and duration of hypoxemia during the early postoperative period are closely related to the surgical sites and that they are the most pronounced for thoracoabdominal surgery, less for upper abdominal surgery, and least for peripheral surgery. These results suggest the need for close respiratory monitoring and oxygen therapy during the early postoperative period, even in healthy adult patients undergoing uncomplicated, elective plastic surgery and particularly in those undergoing elective upper abdominal or thoracoabdominal surgery.
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Footnotes
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This study is a major project supported by the Young Scientists Fund of Chinese Academy of Medical Sciences and Peking Union Medical College (grant no. 950012).
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Accepted for publication October 1, 1998.
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Abstract
Introduction
