Factors associated to hypoxemia in the immediate postoperative period

SCIENTIFIC ARTICLE

Factors associated to hypoxemia in the immediate postoperative period^*

Fatores associados com a ocorrência de hipoxemia no período pós-anestésico imediato

Factores asociados con la ocurrencia de hipoxemia en el período pós-anestésico inmediato

Getúlio Rodrigues de Oliveira Filho, TSA, M.D.^I; Jorge Hamilton Soares Garcia, TSA, M.D.^III; Márcia Regina Ghellar, TSA, M.D.^II; Marcos Antônio Nicolodi, TSA, M.D.^III; Asmir Luiz Boso, TSA, M.D.^II; Adilson José Dal Mago, TSA, M.D.^III

^IResponsável pelo CET/SBA

^IIInstrutor do CET/SBA

^IIIAnestesiologista do Hospital Governador Celso Ramos

^{Correspondence}

SUMMARY

BACKGROUND AND OBJECTIVES: Postanesthetic hypoxemia is a critical respiratory event which increases postoperative morbidity. This study aimed to identify hypoxemia-related factors in the immediate postoperative period.

METHODS: Participated in this study 204 patients admitted to the recovery room while breathing room air. Peripheral oxy-hemoglobin saturation (SpO₂), systolic (SBP) and diastolic (DBP) blood pressure, and heart rate (HR) were measured at 5-minute intervals since admission to the recovery room until 20 minutes later. Sedation, pain and ventilation adequacy scores were attributed to patients at the same moments. Hypoxemia was defined as a SpO₂ below 92% and recommended oxygen therapy. The following data were recorded for each patient: age, gender, weight, height, smoking, history of diabetes and chronic obstructive pulmonary disease (COPD), preanesthetic SpO₂ at arrival in the operating room, type and duration of anesthesia, surgery site, use of neuro-axial opioids and perioperative drugs. The association of those factors to postoperative hypoxemia was defined by logistic regression.

RESULTS: Forty-nine patients (24.01%) had SpO₂ below 92% during the observation period. The following factors [odds ratio (95% confidence limits)] were considered hypoxemia predictors: more than 55 years old [4.32 (1.70; 10.95)], preanesthetic SpO₂ lower than 95% [7.47 (1.50; 37.11)], general anesthesia with enflurane [14.53 (2.54; 82.93)], and clinically detectable hypoventilation [34.82 (11.46; 105.84)]. SBP and HR were significantly higher in hypoxemic patients.

CONCLUSIONS: There are factors closely related to the incidence of postoperative hypoxemia and the use of pulse oximetry allows for selective oxygen therapy during the immediate postanesthetic period.

Key Words: COMPLICATIONS: hypoxemia

RESUMO

JUSTIFICATIVA E OBJETIVOS: A hipoxemia pós-anestésica é um evento respiratório crítico que aumenta a morbidade pós-operatória. Este estudo visou identificar fatores relacionados à ocorrência de hipoxemia no período pós-operatório imediato.

MÉTODO: Foram incluídos 204 pacientes admitidos à sala de recuperação pós-anestésica respirando ar ambiente. A saturação periférica da oxi-hemoglobina (SpO₂), as pressões arteriais sistólica (PAS) e diastólica (PAD) e a freqüência cardíaca (FC) foram medidas a intervalos de 5 minutos desde a admissão do paciente até 20 minutos após. Escores de sedação, de dor e de adequação da ventilação foram atribuídos nos mesmos momentos. SpO₂ menor que 92% foi definida como hipoxemia e indicação para oxigenioterapia. Foram registrados: idade, sexo, peso, altura, história de fumo, de DPOC e de diabete melito, SpO₂ na chegada à sala de cirurgia, tipo de anestesia, região operada, duração da anestesia, uso de opióides neuro-axiais e drogas utilizadas no período peri-operatório e respectivas doses. A associação destes fatores com hipoxemia foi definida por regressão logística.

RESULTADOS: Quarenta e nove pacientes (24,01%) apresentaram SpO₂ menor que 92% durante o período de observação. Foram indicadores de hipoxemia [relação de chances (limites de 95% de confiança)]: idade maior que 55 anos [4,32 (1,70; 10,95)], SpO₂ pré-operatória menor que 95% [7,47 (1,50;37,11)], anestesia geral com enflurano [14,53 (2,54;82,93)], hipoventilação detectada clinicamente [34,82 (11,46;105,84)]. A PAS e a FC foram significativamente mais elevadas nos pacientes hipoxêmicos.

CONCLUSÕES: Existem fatores significativamente associados à ocorrência de hipoxemia pós-operatória, enquanto o uso do oxímetro de pulso permite a utilização seletiva de oxigenioterapia no período pós-anestésico imediato.

Unitermos: COMPLICAÇÕES: hipoxemia

RESUMEN

JUSTIFICATIVA Y OBJETIVOS: La hipoxemia pós-anestésica es un evento respiratorio crítico que aumenta la morbidad pós-operatoria. Este estudio busca identificar factores relacionados a la ocurrencia de hipoxemia en el período pós-operatorio inmediato.

MÉTODO: Fueron incluidos 204 pacientes admitidos a la sala de recuperación pós-anestésica respirando aire ambiente. La saturación periférica de la oxi-hemoglobina (SpO₂), las presiones arteriales sistólica (PAS) y diastólica (PAD) y la frecuencia cardíaca (FC) fueron medidas a intervalos de 5 minutos desde la admisión del paciente hasta 20 minutos después. Escores de sedación, de dolor y de adecuación de la ventilación fueron atribuidos en los mismos momentos. SpO₂ menor que 92% fue definida como hipoxemia e indicación para oxigenioterapia. Fueron registrados: edad, sexo, peso, altura, historia de fumo, de DPOC y de diabetes melito, SpO₂ en la llegada a la sala de cirugía, tipo de anestesia, región operada, duración da anestesia, uso de opioides neuro-axiales y drogas utilizadas en el período peri-operatorio y respectivas dosis. La asociación de estos factores con hipoxemia fue definida por regresión logística.

RESULTADOS: Cuarenta y nueve pacientes (24,01%) presentaron SpO₂ menor que 92% durante el período de observación. Fueron indicadores de hipoxemia [relación de veces (limites de 95% de confianza)]: edad mayor que 55 años [4,32 (1,70; 10,95)], SpO₂ pré-operatoria menor que 95% [7,47 (1,50;37,11)], anestesia general con enflurano [14,53 (2,54;82,93)], hipoventilación detectada clínicamente [34,82 (11,46;105,84)]. La PAS y la FC fueron significativamente mas elevadas en los pacientes hipoxémicos.

CONCLUSIONES: Existen factores significativamente asociados a la ocurrencia de hipoxemia pós-operatoria, en cuanto el uso del oxímetro de pulso permite la utilización selectiva de oxigenioterapia en el período pós-anestésico inmediato.

INTRODUCTION

The routine administration of oxygen in the immediate postoperative period is based on studies showing the high incidence of hypoxemia during this period ^1,2, or those observing a lower incidence of hypoxemia among patients receiving oxygen ^3,4. There are, however, evidences that oxygen is not needed in up to 63% of patients during the immediate postoperative period ^5,6.

There is hypoxemia even with oxygen administration ^2,7,8 and hypoxemic episodes are very often not diagnosed by the PACU staff ². Nevertheless, the use of pulse oximetry in the PACU helps identifying hypoxemic patients and selectively recommending oxygen therapy ^5,9.

Several factors, dependent on patients, anesthetic technique and surgery, are associated to more hypoxemia in the immediate postoperative period ^7,8. Such factors may help to identify patients more prone to develop hypoxemia who, should be more closely monitored by the PACU staff for being candidates to the prophylactic and selective use of oxygen therapy.

This study aimed to detect factors dependent on patients, surgeries and anesthesia, associated to the incidence of hypoxemia in the immediate postoperative period.

METHODS

After the Medical Ethics Committee, Hospital Governador Celso Ramos approval, participated in this study 216 patients of both genders, aged 15 to 87 years, submitted to elective surgeries under general or regional anesthesia, sequentially admitted to the PACU. Patients under oxygen therapy or mechanical ventilation were excluded. Immediately after admission, patients were monitored with pulse oximetry and non-invasive blood pressure. Peripheral oxy-hemoglobin saturation (SpO₂), heart rate (HR), systolic (SBP) and diastolic (DBP) blood pressure were recorded at admission (M0), and at 5-minute intervals (M5, M10, M15, M20) for 20 minutes. At the same moments patients received a sedation score (0 = agitated, 1 = calm, alert, 2 = sleepy and awaking after verbal command, 3 = sleepy, awaking after tactile stimulation and 4 = commatous), a ventilation score (0 = apnea, 1 = limited breathing, respiratory obstruction, bronchospasm or HR below 8 bpm, and 2 = free breathing), a muscular strength score (1 = keeps grip for > 15 seconds or 0 = dos not keep grip or keeps it for less than 15 seconds) and a verbal pain score (0 = no pain or no answer, 1 = mild pain, 2 = moderate pain and 3 = severe pain). During the study period, patients only received oxygen via nasal catheter (2 L.min^-1) if their SpO₂ went below 92%. The following data were collected: age, gender, weight, height, smoking, history of chronic obstructive pulmonary disease (COPD) or diabetes, SpO₂ at arrival in the operating room, type and duration of anesthesia, surgery site, neuroaxial use of opioids and perioperative drugs and doses. Clinically detected hypoventilation was defined as a ventilation score equals 1. Patients were divided in hypoxemic, if at any time during the observation period they presented SpO₂ below 92% for more than 30 seconds, and normoxemic, if no SpO₂ below 92% without oxygen therapy was seen during the study period.

Demographics, patients distribution according to the anesthetic technique, surgery site, diabetes, COPD and smoking history, ventilation, sedation, pain and muscular strength scores, the incidence of hypoventilation, the frequency of SpO₂ below 95% at arrival in the operating room, age above 55 years and the frequency of utilization of each perioperative drug were compared among patients with or without hypoxemia by Student's t test for independent samples (continuous variables) or chi-square test (categorical variables) for odds ratio and their 95% confidence limits calculation. Variables with significant differences between subgroups in the Student's t test or presenting significant association with hypoxemia in the chi-square test were used as independent variables in retrograde logistic regression analysis where the presence or not of hypoxemia (SpO₂ below 92%) was the dependent variable.

Hemodynamic data were compared between hypoxemic and normoxemic patients using the bi-factorial co-variance analysis for repetitive measures being SpO₂ the co-variable in every moment, followed by post hoc Spjotvoll-Stolline test.

SpO₂ was compared by Friedmans, two-tail Wilcoxon (intragroups comparison) and Mann-Whitney (comparison moment by moment between groups) tests.

The acceptable level of significance was 5%.

RESULTS

Twelve patients receiving oxygen therapy in the PACU for reasons other than those established in our protocol, such as anemia (2 patients), shivering (2 patients), bronchspasm (1 patient), hypotension (4 patients) and unknown reason (2 patients) were excluded from the study.

Demographics of the remaining 204 patients are shown in table I. From those, 49 (23.14%) had SpO₂ lower than 92% for more than 30 seconds and received oxygen therapy. Hypoxemic patients were significantly older than normoxemic patients (Table II).

Brazilian Journal of Anesthesiology, 2001; 51: 3: 185 - 195

Factors associated to hypoxemia in the immediate postoperative period

Getúlio Rodrigues de Oliveira Filho; Jorge Hamilton Soares Garcia; Márcia Regina Ghellar;

Marcos Antônio Nicolodi; Asmir Luiz Boso; Adilson José Dal Mago

Brazilian Journal of Anesthesiology, 2001; 51: 3: 185 - 195

Factors associated to hypoxemia in the immediate postoperative period

Getúlio Rodrigues de Oliveira Filho; Jorge Hamilton Soares Garcia; Márcia Regina Ghellar;

Marcos Antônio Nicolodi; Asmir Luiz Boso; Adilson José Dal Mago

The following variables were statistically significant in the individual analysis by the chi-square test: physical status ASA III, more than 55 years, preoperative SpO₂ below 95%, history of COPD and diabetes, deep sedation (patient awakes only when touched or does not awake), inadequate muscular strength (patient does not keep grip for more than 15 seconds), clinically detected hypoventilation, the use of diazepam or the association of fentanyl and diazepam or midazolam for sedation during regional anesthesia, the association of fentanyl and morphine, the use of vecuronium, enflurane, isoflurane and N₂O, general and regional anesthesia (Table III and Table IV). The best logistic regression for our study included general anesthesia with enflurane, clinically detected hypoventilation, age above 55 years and SpO₂ at operating room arrival below 95% (Table V). With this model, 96.12% of normoxemic patients and 65.30% of hypoxemic patients were correctly classified using the 0.5 logit limit.

Brazilian Journal of Anesthesiology, 2001; 51: 3: 185 - 195

Factors associated to hypoxemia in the immediate postoperative period

Getúlio Rodrigues de Oliveira Filho; Jorge Hamilton Soares Garcia; Márcia Regina Ghellar;

Marcos Antônio Nicolodi; Asmir Luiz Boso; Adilson José Dal Mago

Brazilian Journal of Anesthesiology, 2001; 51: 3: 185 - 195

Factors associated to hypoxemia in the immediate postoperative period

Getúlio Rodrigues de Oliveira Filho; Jorge Hamilton Soares Garcia; Márcia Regina Ghellar;

Marcos Antônio Nicolodi; Asmir Luiz Boso; Adilson José Dal Mago

Brazilian Journal of Anesthesiology, 2001; 51: 3: 185 - 195

Factors associated to hypoxemia in the immediate postoperative period

Getúlio Rodrigues de Oliveira Filho; Jorge Hamilton Soares Garcia; Márcia Regina Ghellar;

Marcos Antônio Nicolodi; Asmir Luiz Boso; Adilson José Dal Mago

There were no statistically significant differences in: weight, gender, body mass index, physical status, smoking history, volume of crystalloids administered during surgery, surgery duration and site, pain scores, the use of thiopental propofol, midazolam, fentanyl, alfentanil, morphine, atracurium, sevoflurane or neuroaxial opioids (Table VI).

Brazilian Journal of Anesthesiology, 2001; 51: 3: 185 - 195

Factors associated to hypoxemia in the immediate postoperative period

Getúlio Rodrigues de Oliveira Filho; Jorge Hamilton Soares Garcia; Márcia Regina Ghellar;

Marcos Antônio Nicolodi; Asmir Luiz Boso; Adilson José Dal Mago

Systolic blood pressure, heart rate and hemoglobin peripheral saturation are shown in figure 1, figure 2 and figure 3.

Brazilian Journal of Anesthesiology, 2001; 51: 3: 185 - 195

Factors associated to hypoxemia in the immediate postoperative period

Getúlio Rodrigues de Oliveira Filho; Jorge Hamilton Soares Garcia; Márcia Regina Ghellar;

Marcos Antônio Nicolodi; Asmir Luiz Boso; Adilson José Dal Mago

Brazilian Journal of Anesthesiology, 2001; 51: 3: 185 - 195

Factors associated to hypoxemia in the immediate postoperative period

Getúlio Rodrigues de Oliveira Filho; Jorge Hamilton Soares Garcia; Márcia Regina Ghellar;

Marcos Antônio Nicolodi; Asmir Luiz Boso; Adilson José Dal Mago

Brazilian Journal of Anesthesiology, 2001; 51: 3: 185 - 195

Factors associated to hypoxemia in the immediate postoperative period

Getúlio Rodrigues de Oliveira Filho; Jorge Hamilton Soares Garcia; Márcia Regina Ghellar;

Marcos Antônio Nicolodi; Asmir Luiz Boso; Adilson José Dal Mago

DISCUSSION

Postoperative hypoxemia is caused by inadequate ventilation control or by airway patency as a consequence of residual effects of anesthetics and/or neuromuscular blockers and inadequate ventilation/perfusion ratio caused mainly by atelectasis zones in lung-dependent regions^10-12. Other causes include low cardiac output, anemia, thermogenic shivering, pain and agitation¹³. Most hypoxemic events occur during the 15 minutes following patients' admission to PACU.

Postanesthetic hypoxemia is related to a higher incidence of cardiovascular events during the immediate postoperative period, longer PACU stay and increase in unplanned ICU admissions ⁷.

The introduction of pulse oximetry for immediate postanesthetic period monitoring has contributed for the improved identification of hypoxemia and cardiac events such as bradycardia, for decreasing myocardial ischemia, for the use of higher oxygen flows and for a more frequent use of oxygen therapy after PACU discharge, however without changing postoperative mortality or complications rate ⁹.

Hypoxemia is defined as oxy-hemoglobin arterial saturation (SaO₂) below 90%, corresponding to a PaO₂ of approximately 60 mmHg. Such saturation is found in the inclined portion of the hemoglobin dissociation curve, so that minor SaO₂ variations result in marked PaO₂ decreases ¹³. However, SpO₂ overestimates SaO₂ in 2 to 5% ^14,15 justifying lower SpO₂ limits between 92% and 96% used in several studies ^5,8,16,17.

In studies using 90% as SpO₂ limit, hypoxemia was seen in 0.9% ⁷, 4% ⁶, 8.24% ¹⁸, 20% ⁴, 55% ² and 80% ¹ of patients. SpO₂ limit of 94% was related to hypoxemia in 15% ¹⁷ and 37% ⁵ of patients. With the same SpO₂ limit, patients under general anesthesia and breathing room air in the PACU had more hypoxemia (44.7%) than patients under regional anesthesia (24%) or those submitted to general anesthesia and receiving postoperative oxygen therapy (26.1%) ³. In studies with 92% as SpO₂ limit, the incidence of hypoxemia has been 7% ¹⁶ and 15% ⁸, while in our study, hypoxemia was seen in 23.14% of patients.

In previous studies, variables associated to higher probabilities of immediate postanesthetic hypoxemia were: males, diabetes mellitus, urgency surgeries, postanesthetic medication with opioids and sedatives, fentanyl in doses above 2 µg.kg^-1.h^-1, association of fentanyl and morphine, thiopental as compared to propofol ⁷, obesity ^1,7,8,18, more than 60 years of age ^2,7,8,17,18, anesthesia for more than 4 hours ^2,7, more than 1500 ml crystalloid infusion ⁸, physical status (ASA) ⁸ and general anesthesia ⁸.

In our study, the isolated analysis of variables has shown a significant association with immediate postoperative hypoxemia of: physical status ASA III, more than 55 years of age, history of COPD or diabetes mellitus, preoperative SpO₂ below 95%, general anesthesia, association of perioperative fentanyl with postoperative morphine, use of vecuronium, enflurane, isoflurane and N₂O, deep postanesthetic sedation, inability of keeping grip for 15 seconds and clinical signs of inadequate ventilation.

On the other hand, regional anesthesia, followed or not by perioperative sedation with diazepam and associations of fentanyl and diazepam or midazolam, was related to significantly lower chances of immediate postanesthetic hypoxemia.

The isolated analysis of variables has the disadvantage of not taking into consideration the effect of other factors which may influence possible associations between each variable and a certain event. On the other hand, logistic regression has the advantage of controlling all factors involved in the model by calculating the influence of each variable on the dicotome dependent variable. That is why, after identifying variables potentially related to postoperative hypoxemia by isolated analysis, logistic regression technique was applied and has identified as independent indicators of postoperative hypoxemia only: more than 55 years of age, preoperative SpO₂ below 95%, general anesthesia with enflurane and clinically detectable hypoventilation. The remaining variables, significant in isolated analysis, have not contributed to the logistic model and were rejected.

Elderly patients have a decrease in respiratory reserve due to factors such as decrease in nervous breathing control, especially manifested by apnea during sleep, mechanical changes in the chest wall (increase in dorsal syphosis, costal cartilage calcifications and decrease in respiratory muscular mass) and changes in pulmonary volumes and capacities (decrease in maximum respiratory, total and vital pulmonary capacities and increase in airway closing volume)¹⁹. In addition, elderly patients are more sensitive to depressing effects of inhalational anesthetics, sedatives and opioids. For those reasons, they are more exposed to postoperative hypoxemia as shown by this and other studies ^2,7,8,16,18.

In healthy volunteers, SpO₂ below 94% was seen in just 13.3% ³. It is possible that patients with preoperative SpO₂ below 95% have some ventilation abnormality, such as hypoventilation caused, for example, by preanesthetic medication, or abnormality in lungs ventilation/perfusion ratio.

Patients submitted to general anesthesia have higher chances of developing postoperative hypoxemia ^3,8. In our study, however, only general anesthesia with enflurane was significantly related to hypoxemia in the logistic model, although general anesthesia, isoflurane and N₂O being significant in isolated analyses. Enflurane is a potent ventilation depressant and causes more hypoxemia in the immediate and late postoperative period ²⁰. Although not measuring its expired gas concentration, it is possible that the residual effect of enflurane could contribute for immediate postanesthetic hypoxemia.

Clinically detected hypoventilation was the factor more strongly related to hypoxemia. Parameters to define hypoventilation were respiratory rate, diaphragmatic movements inspection, and detection of ventilation abnormalities, such as high respiratory obstruction and bronchospasm. This way, this study confirms the value of clinical evaluation in detecting risk factors for early postoperative hypoxemia.

Similarly to other studies ^3,8, regional anesthesia was related to lower chances of postoperative hypoxemia, even with sedation with diazepam or midazolam, associated or not to fentanyl.

Among the variables studied, weight, gender, body mass index, smoking history, volume of crystalloids administered during surgery, surgery duration and site, the use of thiopental, propofol, midazolam, fentanyl, alfentanil, morphine, atracurium, sevoflurane or neuroaxial opioids, were not significant both in isolated analysis and logistic regression. Hypoxemic patients did not differ from normoxemic patients, except in anesthesia duration. In a previous study ^2,7, anesthesia duration was only a hypoxemia indicator when longer than 4 hours. It is possible that this study failed to identify procedure duration as a factor significantly related to postoperative hypoxemia because mean procedure duration was lower than 4 hours. Smoking was a significant indicator in one study ², while in others ^6,17, as well as in our study, there was no significance. The use of morphine for postoperative analgesia was not significantly related to hypoxemia, in line with other authors' findings ⁷. So, the risk for hypoxemia does not justify limiting the use of morphine for postoperative analgesia. Cavitary surgeries were no more related to hypoxemia than superficial surgeries, in line with other authors' findings ^2,7. In one study, however, cavitary surgeries were related to higher percentages of postoperative hypoxemia ²².

In our study, patients with SpO₂ below 92% had higher blood pressure and heart rate values. Hypoxemia increases circulating catecholamine levels and predisposes to myocardial ischemia ¹³, while oxygen administration decreases postoperative heart rate ²³.

The use of pulse oximetry for postanesthetic monitoring allows for a selective use of oxygen therapy, that is, oxygen is administered only for patients with oxy-hemoglobin desaturation. For example, in the PACU of a general hospital, oxygen was administered only in patients with SpO₂ below 94%, corresponding to 37% of patients and resulting in substantial cost reductions ⁵. In patients submitted to gynecological laparoscopies, using the same criteria, oxygen was administered in 15% of cases ¹⁷. In our study, oxygen therapy was administered in only 23.14% of patients. Considering that in patients monitored with pulse oximetry the incidence of hypoxemia depends on the level of motivation of PACU staff to detect it ²¹, it is possible that the identification of factors associated to hypoxemia could be used to encourage PACU staff aiming at improving surveillance and patients monitoring, thus detecting hypoxemic episodes and early treating them.

In conclusion, our study has identified factors related to higher chances of immediate postoperative hypoxemia, which may identify patients candidate to a more intensive monitoring in the PACU. Another conclusion is that SpO₂ monitoring in the immediate postanesthetic period allows for the selective use of oxygen therapy.

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Submitted for publication July 12, 2000

Accepted for publication October 17, 2000

*

Received from Hospital Governador Celso Ramos, CET/SBA Integrado de Anestesiologia da SESSC, Florianópolis, SC

Publication Dates

• Publication in this collection
  10 Feb 2011
• Date of issue
  June 2001

History

• Received
  12 July 2000
• Accepted
  17 Oct 2000