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Revista Brasileira de Anestesiologia

Print version ISSN 0034-7094

Rev. Bras. Anestesiol. vol.51 no.4 Campinas  2001 



SpO2 - SaO2 gap during mechanical ventilation in anesthesia and intensive care*


Gradiente SpO2 - SaO2 durante ventilação mecânica em anestesia e terapia intensiva


Gradiente SpO2 - SaO2 durante ventilación mecánica en anestesia y terapia intensiva



Pablo Escovedo Helayel, M.D.I; Getúlio R de Oliveira Filho, TSA, M.D.II; Lúcia Marcon, M.D.III; Flávio Hülse Pederneiras, M.D.IV; Marcos Antônio Nicolodi, TSA, M.D.V; Sérgio Galluf Pederneiras, TSA, M.D.VI

IIResponsável pelo CET/SBA da SES/SC
IIIEnfermeira da Unidade de Tratamento Intensivo
VAnestesiologista do Hospital Governador Celso Ramos
VICo-Responsável do CET/SBA da SES/SC





BACKGROUND AND OBJECTIVES: Peripheral oxy-hemoglobin saturation (SpO2) is commonly used to guide ventilator settings. SpO2 values equal to, or higher than 96% are necessary to assure arterial oxy-hemoglobin saturation (SaO2) higher than 90% in intensive care patients. This study aimed to determine SpO2 - SaO2 gap and to establish the lowest SpO2 associated to SaO2 values equal to, or higher than 90%.
METHODS: Participated in this prospective study 120 adult patients of both genders submitted to general anesthesia and mechanical ventilation or under intensive care. Arterial blood samples were obtained by arterial puncture or central line aspiration using the anaerobic technique and heparin-containing syringes. Samples were analyzed immediately after collection. SpO2 was measured during collection with the pulse oximeter placed on the opposite second finger and using arterial pulse dichroic wave as the adequacy parameter.
RESULTS: Two hundred and twenty-eight samples were analyzed. The difference between SpO2 and SaO2 ranged between -7.10% and 15.2%, being -0.20% ± 2.02% the mean difference (bias). SpO2-SaO2 higher than 4.04% (two standard deviations of the difference) was observed in 4.72% of samples. Graphical analysis of the lowest SpO2 to assure an SaO2 higher than 90% has shown that only SpO2 values equal to, or higher than 99% were not associated to SaO2 values below 90%.
CONCLUSIONS: Although SpO2-SaO2 gap remaining between plus or minus 5% for 97% of the samples, only SpO2 values higher than 99% excluded patients with SaO2 values below 90%.

Key words: MONITORING: pulse oximetry; VENTILATION: mechanical ventilation


JUSTIFICATIVAS E OBJETIVOS: A saturação periférica da oxihemoglobina (SpO2) é freqüentemente utilizada para guiar alterações do regime ventilatório. Valores de SpO2 iguais ou superiores a 96% são necessários para garantir saturação arterial da oxihemoglobina (SaO2) superiores a 90%, em pacientes de terapia intensiva. Este estudo teve por objetivo determinar concordância entre valores de SpO2 e SaO2 e delimitar a menor SpO2 associada a valores de SaO2 iguais ou superiores a 90%.
MÉTODO: Foram incluídos prospectivamente 120 pacientes adultos, de ambos os sexos, submetidos à anestesia geral com ventilação mecânica ou em tratamento intensivo. Amostras de sangue arterial foram coletadas por punção arterial ou por aspiração de linha arterial, utilizando técnica anaeróbia, em seringas heparinizadas. As amostras foram analisadas imediatamente após a coleta. A SpO2 foi medida por oxímetro de pulso durante a coleta, no indicador da mão contra-lateral, utilizando a onda dicrótica de pulso arterial como parâmetro de adequação.
RESULTADOS: Foram analisadas 228 amostras. A diferença entre os valores de SpO2 e SaO2 variou entre -7,10% e 15,20%, sendo a diferença média igual a -0,20% ± 2,02%. SpO2-SaO2 maiores que 4,04% (dois desvios padrão da diferença) ocorreram em 4,72% das amostras. A determinação gráfica da SpO2, mínima para garantir a SaO2 acima de 90%, mostrou que somente valores de SpO2 iguais ou superiores a 99% não se associaram à ocorrência de nenhum valor de SaO2 inferior a 90%.
CONCLUSÕES: Embora a diferença entre os valores de SpO2 e SaO2 tenham se localizado entre mais e menos 5% em 97% dos pares analisados, somente valores acima de 99% excluíram pacientes com SaO2 menor que 90%.

Unitermos: MONITORIZAÇÃO: oximetria de pulso; VENTILAÇÃO: ventilação mecânica


JUSTIFICATIVAS Y OBJETIVOS:  La saturación periférica de la oxihemoglobina (SpO2) es frecuentemente utilizada para guiar alteraciones del régimen ventilatorio. Valores de SpO2 iguales o superiores a 96% son necesarios para garantizar saturación arterial de la oxihemoglobina (SaO2) superiores a 90%, en pacientes de terapia intensiva. Este estudio tuvo por objetivo determinar concordancia entre valores de SpO2 y SaO2 y delimitar la menor SpO2 asociada con valores de SaO2 iguales o superiores a 90%.
MÉTODO: Fueron incluidos prospectivamente 120 pacientes adultos, de ambos sexos, sometidos a anestesia general con ventilación mecánica o en tratamiento intensivo. Muestras de sangre arterial fueron colectadas por punción arterial o por aspiración de línea arterial, utilizando técnica anaeróbia, en jeringas heparinizadas. Las muestras fueron analizadas inmediatamente después de la colecta. La SpO2 fue medida por oxímetro de pulso durante la colecta, en el indicador de la mano contra-lateral, utilizando la onda dicrótica de pulso arterial como parámetro de adecuación.
RESULTADOS: Fueron analizadas 228 muestras. La diferencia entre los valores de SpO2 y SaO2 varió entre -7,10% y 15,20%, siendo la diferencia media igual a -0,20% ± 2,02%. SpO2-SaO2 mayores que 4,04% (dos desvíos padrones de la diferencia) ocurrieron en 4,72% de las muestras. La determinación gráfica de la SpO2, mínima para garantizar la SaO2 arriba de 90%, mostró que solamente valores de SpO2 iguales o superiores a 99% no se asociaron con la ocurrencia de ningún valor de SaO2 inferior a 90%.
CONCLUSIONES: Aun cuando la diferencia entre los valores de SpO2 y SaO2 se hallan localizados entre más y menos 5% en 97% de los pares analizados, solamente valores arriba de 99% excluyeron pacientes con SaO2 menor que 90%.




Peripheral oxy-hemoglobin saturation is a widely used estimate of its saturation in the arterial blood for monitoring patients under anesthesia and/or intensive care. During mechanical ventilation, SpO2 has been used to determine the adequacy of changes in ventilator settings, such as oxygen inspired fraction, positive tele-expiratory pressure values and weaning. SpO2 values traditionally associated to SaO2 higher than 90%, that is, normoxemia, vary between 92% and 94% 1. Recently, however, the lack of hypoxemia has been only observed with SpO2 values equal to, or higher than 96%. Such findings have obvious clinical implications.

This study aimed to evaluated matching limits between SpO2 and SaO2 in anesthetized and critically ill patients during mechanical ventilation to establish minimum SpO2 values associated to more than 90% arterial oxy-hemoglobin saturation.



After Hospital Governador Celso Ramos Medical Ethics Committee approval, participated in this prospective study 120 patients of both genders submitted to mechanical ventilation during general anesthesia or intensive care, with inspired oxygen fraction between 40% and 100%, regardless of other ventilation parameters or of hemodynamic status. Patients in cardio-respiratory bypass, icteric, with nail polish or anemic (hemoglobin lower than 9 were excluded from the study. Patients with SaO2 equal to, or lower than 75% were also excluded due to the possibility of venous contamination of the sample during arterial puncture. Arterial blood samples were collected by radial artery puncture or arterial line aspiration using an anaerobic technique and heparin-containing syringes. Samples were analyzed immediately after collection. During collection, SpO2 values were obtained with a pulse oximeter with reusable digital sensors, using the arterial pulse dichroic wave as the adequacy parameter.

SpO2 and SaO2 values were compared by Mann-Whitney U test and their matching was evaluated by Bland-Altman test. To determine the lowest SpO2 value associated to an SaO2 equal to, or higher than 90% an SaO2, SpO2 chart was used where two lines were plotted. The vertical line defined SaO2 limit in 90% and the horizontal line crossed the ordinate axle at an SpO2 value above which no SaO2 values below 90% were observed.



Demographics data of 120 patients of whom 228 samples were analyzed are shown in table I. SpO2 values were significantly different from SaO2 values (Table II). Differences between SpO2 and SaO2 varied from -7.10% to 15.20%. Mean difference was -0.20% with a standard deviation of 2.02%. SpO2 - SaO2 gap remained between plus or minus 4.04% (two standard deviations of the difference) in 95.28% of samples (Figure 1).

Lowest SpO2 to assure an SaO2 higher than 90% has shown that only SpO2 values equal to, or higher than 99% were not associated to SaO2 values below 90% (Figure 2).



Hypoxemia is confirmed by the presence of cyanosis. However, it is only seen when SaO2 is lower than 75% and depends on reduced hemoglobin saturation, which must be higher than 5 g.dl-1. So, its absence does not rule out the possibility of hypoxemia 1. Peripheral oxy-hemoglobin saturation is strongly correlated to oxy-hemoglobin arterial saturation and, as such, it is routinely used in perioperative monitoring and in intensive care units for an early hypoxemia diagnosis 3,5.

Pulse oximetry checks the difference in absorption of two light wave lengths crossing perfused tissues, by analyzing the absorbance range of continuous and pulsatile components of tissue blood flow. Absorbance rates are related to SaO2 measurements obtained in healthy volunteers by co-oximetry, resulting in a calibration algorithm which is stored in the pulse oximeter’s digital microprocessor and generates an SaO2 estimate 6.

Arterial oxy-hemoglobin oxygen saturation may be classified as fractional and functional. The first is spectrophotometrically measured by co-oximetry and is calculated by the [HbO2/(HbO2 + reduced Hb + methemoglobin + carboxy-hemoglobin) x 100] ratio. Functional oxy-hemoglobin oxygen saturation is estimated by pulse oximetry and is calculated by the [HbO2/(HbO2 + reduced Hb) x 100] ratio 7. This way, pulse oximetry does not distinguish reduced hemoglobin from other forms of hemoglobin and may overestimate SaO2 7.

SpO2 measurements artifacts were related to a decrease in the ratio between signal quality and capturing interference.

So, the use of vasopressants, the low tissue perfusion and the inadequate placement of the sensor are responsible for signal decrease, while movement and room light increase the interference 8. The marked decrease in systemic vascular resistance may lead SpO2 to underestimate SaO2. In septic patients, pulse oximeter is sensitized by the pulsatile venous flow caused by the opening of skin arterial-venous fistulas 9.

Although acute anemia in non hypoxemic patients does not significantly affect pulse oximetry precision 6, sickle cell disease patients during crisis show SaO2 values underestimated by SpO2, however without hiding the diagnosis of hypoxemia 10,11. Dyshemoglobulinemias may affect pulse oximetry measurements. From those, the most common is carboxy-hemoglobulinemia associated to chronic smoking and CO2 poisoning, where SaO2 is overestimated by SpO2. During severe methemoglobinemia, SpO2 is constant around 85%, regardless of SaO2. Methemoglobulinemia may result from the use of sodium nitroprusside, nitroglycerin, metoclopramide, sulfas, prilocaine and benzocaine 12.

Demographics, such as age, gender or race do not influence SpO2 - SaO2 ratio 13. In blacks, SpO2 below 95% reflects SaO2 below 90%, while in Caucasians this same SpO2 value is 92% 6.

Normal SpO2 - SaO2 gap values vary between plus or minus 3% to 5% 6.

In anesthesia, and especially in intensive care, SpO2 is used to identify changes in ventilator settings. Assuming normal SpO2 - SaO2 gap values, lower SpO2 values between 92% and 94% are used to prevent SaO2 going below 90%, which corresponds to mild hypoxemia 3-6. However, in critically ill patients under mechanical ventilation, lower SpO2 limit should be 96% to avoid hypoxemia during changes in ventilator settings 2.

In our study, 95.28% of patients showed SpO2 - SaO2 gap values within mismatching limits, that is, two standard deviations above and below the mean difference, but 4.72% showed excessively high values. In such cases, if the lowest acceptable SpO2 to rule out hypoxemia during ventilator settings adjustment was established between 92% and 94%, SaO2 values would be well below 90% and patients would be hypoxemic after the intervention.

We concluded that, although SpO2 estimating SaO2 with a minor mismatch in most cases, only values equal to, or higher than 99% rule out the possibility of SaO2 below 90% in surgical or critically ill patients. It is recommended that adjustments in ventilator settings should only be done after confirmation of PaO2 and SaO2.



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Mail to:
Dr. Getúlio Rodrigues. de Oliveira Filho
Address: Rua José Cândido Silva 179/402
ZIP: 88075-250 City: Florianópolis, Brazil

Submitted for publication Deciembre 1, 2000
Accepted for publication January 9, 2001



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

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