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

SCIENTIFIC ARTICLE

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

^IME₂ do CET/SBA da SES/SC

^IIResponsável pelo CET/SBA da SES/SC

^IIIEnfermeira da Unidade de Tratamento Intensivo

^IVME₁ do CET/SBA da SES/SC

^VAnestesiologista do Hospital Governador Celso Ramos

^VICo-Responsável do CET/SBA da SES/SC

Correspondence

SUMMARY

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

RESUMO

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

RESUMEN

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%.

INTRODUCTION

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.

METHODS

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 g.ml^-1) 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.

RESULTS

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).

Brazilian Journal of Anesthesiology, 2001; 51: 4: 305 - 310

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

Pablo Escovedo Helayel; Getúlio R de Oliveira Filho; Lúcia Marcon; Flávio Hülse Pederneiras;

Marcos Antônio Nicolodi; Sérgio Galluf Pederneiras

Brazilian Journal of Anesthesiology, 2001; 51: 4: 305 - 310

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

Pablo Escovedo Helayel; Getúlio R de Oliveira Filho; Lúcia Marcon; Flávio Hülse Pederneiras;

Marcos Antônio Nicolodi; Sérgio Galluf Pederneiras

Brazilian Journal of Anesthesiology, 2001; 51: 4: 305 - 310

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

Pablo Escovedo Helayel; Getúlio R de Oliveira Filho; Lúcia Marcon; Flávio Hülse Pederneiras;

Marcos Antônio Nicolodi; Sérgio Galluf Pederneiras

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).

Brazilian Journal of Anesthesiology, 2001; 51: 4: 305 - 310

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

Pablo Escovedo Helayel; Getúlio R de Oliveira Filho; Lúcia Marcon; Flávio Hülse Pederneiras;

Marcos Antônio Nicolodi; Sérgio Galluf Pederneiras

DISCUSSION

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 SaO₂⁷.

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.

REFERENCES

• 01. Powell, JF, Menon DK, Jones JG - The effects of hypoxemia and recommendations for postoperative oxygen therapy. Anaesthesia, 1996;51:769-772.
• 02. Seguin P, Le Rouzo A, Tanguy M et al - Evidence for the need of bedside accuracy of pulse oximetry in an intensive care unit. Crit Care Med, 2000;28:703-706.
• 03. Moller JT, Johannessen NW, Espersen K et al - Randomized evaluation of pulse oximetry in 20,802 patients: I. Perioperative events and postoperative complications. Anesthesiology, 1993; 78:445-453.
• 04. Jensen L.A, Onyskiw JE, Prasad NG - Meta-analysis of arterial oxygen saturation monitoring by pulse oximetry in adults. Heart Lung, 1998;27:387-408.
• 05. Smith DC - Pulse oximetry in the recovery room. Anaesthesia, 1989;44:345-348.
• 06. Jubran A - Advances in respiratory monitoring during mechanical ventilation. Chest, 1999;116:1416-1425.
• 07. Hampson N - Pulse oximetry in severe carbon monoxide poisoning. Chest, 1998;114:1036-1041.
• 08. Severinghaus JW, Spellman MJ - Pulse oximeter failure thresholds in hypotension and vasoconstriction. Anesthesiology, 1990;73:532-537.
• 09. Secker C, Spiers P - Accuracy of pulse oximetry in patients with low systemic vascular resistance. Anaesthesia, 1997;52: 127-130.
• 10. Kress JP, Pohlman AS, Hall JB - Determination of hemoglobin saturation in patients with acute sickle chest syndrome: a comparison of arterial blood gases and pulse oximetry. Chest, 1999;115:1316-1320.
• 11. Ortiz FO, Aldrich TK, Nagel RI et al - Accuracy of pulse oximetry in sickle cell disease. Am J Respir Crit Care Med, 1999;159: 447-451.
• 12. Barker SJ, Tremper KK, Hyatt J et al - Effects of methemoglobin on pulse oximetry and mixed venous oximetry. Anesthesiology, 1989;70:112-117.
• 13. Saber W, McCarthy K, Schilz R - Limitations of pulse oximetry. Chest, 2000;118:184S.

*

Recebido do Hospital Governador Celso Ramos, CET/SBA Integrado de Anestesiologia da SES/SC, Florianópolis, SC

Publication Dates

• Publication in this collection
  21 Mar 2011
• Date of issue
  2001

History

• Accepted
  09 Jan 2001
• Received
  01 Dec 2000