Patient with Persistent Low Oxygen Saturation for Emergency Cesarean Section.

A 24-year-old woman with previous undiagnosed congenital methemoglobinemia presented for the emergency cesarean section in view of fetal distress. The patient had a persistent low oxygen saturation on pulse oximetry with normal heart rate and blood pressure. Arterial blood gas values were normal. The patient was asymptomatic and respiratory and cardiovascular system examination was normal. The patient underwent cesarean section under general anesthesia and was shifted to the Intensive Care Unit postoperatively. The oxygen saturation was low throughout the intraoperative and postoperative period. A complete evaluation was done postoperatively. Mass spectrophotometry was done, which confirmed methemoglobinemia in the patient.

INTRODUCTION

Methemoglobinemia is a hemoglobinopathy characterized by oxidation of ferrous iron to ferric iron within the hemoglobin molecule. The oxidized hemoglobin is unable to transport oxygen to the tissues causing hypoxia and cyanosis, impaired aerobic respiration, metabolic acidosis, and death in severe cases. It is of two types: congenital and acquired. Congenital methemoglobinemia is caused by a deficiency of enzyme cytochrome b5 reductase.[1] This enzyme is responsible for the reduction of methemoglobin to hemoglobin. Acquired methemoglobinemia could be dietary or drug induced.[1] In acquired methemoglobinemia, the rate of methemoglobin formation exceeds the rate of methemoglobin reduction. We report a patient who was operated for the emergency cesarean section and was diagnosed to be suffering from methemoglobinemia.

CASE REPORT

We report a case of a 24-year-old unbooked pregnant patient with a period of gestation 38 weeks for emergency cesarean section in view of fetal distress. Her previous pregnancy was uneventful, and she had a 3-year-old daughter. A detailed history was clinically insignificant. On examination, the patient had pallor with hemoglobin of 8 g/dl and was being transfused one unit blood. Preoperative vitals were heart rate 101/min, blood pressure 118/64 mmHg with oxygen saturation of 66% on room air. The plethysmographic waveform was good and the value of oxygen saturation did not change with the change in site of the probe or change in the probe itself. The patient denied any symptom pertaining to the cardiovascular or respiratory system, and the clinical examination was unremarkable. The patient was oxygenated with 100% oxygen after which the pulse oximeter reading rose to 70%. Meanwhile, the fetal heart rate picked up to 130/min. An arterial blood sample revealed dark brown blood color [Figure 1] with pH 7.413, PaCO228.3, PaO2163.4, SaO299.4, bicarbonate 18.8, and base excess of 2.1. In the meanwhile, fetal heart rate slowed further to 110/min. The obstetrician insisted on proceeding with cesarean section urgently. As the clinical examination ruled out any cardiorespiratory pathology and there was a classical discrepancy between pulse oximetry and arterial blood gas (ABG), the cause was suspected to be some form of dyshemoglobinemia. Since there was a category 1 indication for the cesarean section as told by obstetricians, we decided to proceed with general anesthesia after discussing our dilemma with surgeons and the patient's relatives. Induction of anesthesia was done with etomidate 12 mg and succinylcholine 75 mg intravenous and the patient was intubated. As we were suspecting dyshemoglobinemia, anesthesia was maintained with FiO2 100% and sevoflurane 2–2.5%. Injection fentanyl 80 mcg was administered intravenously after delivery of the baby. Throughout the surgery, pulse oximeter readings varied between 74% and 78% on FiO2 of 100%. The intraoperative blood loss was minimal, and intraoperative and postoperative heart rate and blood pressure were normal, and the patient was shifted to the Intensive Care Unit (ICU) for mechanical ventilation and further evaluation. The neonate had an Apgar score of 7/10 and was shifted to the neonatal ICU. The mother was transfused one unit packed red blood cells postoperatively. Postoperative pulse oximetry values varied between 74% and 78%. Serial postoperative ABG values, chest X-ray, and pulmonary function tests were normal, and respiratory physician consultation ruled out the respiratory disease. Electrocardiography and echocardiography were normal, and cardiology consultation ruled out the cardiovascular cause. The patient was extubated the next day. Mass spectrophotometry was done, which diagnosed methemoglobinemia of 5.5%. As the patient had undergone blood transfusion, a repeat test was done after 2 months and methemoglobin levels were found to be 8.2%. The pulse oximetry readings of the husband and the first child were 99% and 78% on room air, respectively. Mass spectrophotometry of blood sample of the first child showed methemoglobinemia 7.8 g% whereas the husband had a normal result. The second child had an oxygen saturation of 98% on room air. Therefore, the pediatrician refused for further workup of the second child. The genetic evaluation could not be done due to financial constraints.

Figure 1

Patient blood is dark brown due to methemoglobinemia (image on the left) when compared to the bright color of blood sample taken from a normal patient (image on the right)

DISCUSSION

In the present case, the patient showed a persistent low saturation on pulse oximetry and blood was dark brown. The causes of low oxygen saturation on pulse oximetry can be airway-related, cardiovascular, respiratory diseases or hematological causes. After ruling out airway-related, cardiovascular, and respiratory diseases, we suspected a hematological cause with abnormal hemoglobin as there was a mismatch between oxygen saturation on ABG and pulse oximetry, and oxygen supplementation did not improve saturation on pulse oximetry.

Pulse oximetry measures light absorbance at only two wavelengths: 660 and 940 nm. The pulse oximetry readings are confounded in methemoglobinemia due to its absorption spectrum.[1] ABG in patients with methemoglobinemia shows a normal PO2 and oxygen saturation value.[2] The oxygen saturation in ABG is a derived value based on the partial pressure of dissolved oxygen and assumes no abnormal hemoglobin is present. This explains the persistent saturation gap between pulse oximetry and ABG value as seen in our case.

Co-oximetry measures light at four different wavelengths that correspond to specific absorbance characteristics of deoxyhemoglobin, oxyhemoglobin, carboxyhemoglobin, and methemoglobin. A peak absorbance at 630 nm is used to measure methemoglobin. A simple bedside test to differentiate between methemoglobin and deoxyhemoglobin is to insufflate oxygen through patients’ blood. Blood containing deoxyhemoglobin appears dark initially but will turn bright on exposure to atmospheric oxygen. However, blood containing methemoglobin does not change its color.[2]

Our suspicion of methemoglobinemia in the present case was confirmed by mass spectrophotometry postoperatively. There was no history of exposure to drugs or toxins which precipitate methemoglobinemia. We suspected congenital methemoglobinemia in this patient as her first child also had raised methemoglobin levels in the blood. However, we could not proceed with the further genetic workup of the patient due to financial constraints.

Although the incidence of methemoglobinemia is rare, knowledge of the condition is very important for anesthesiologists. Methemoglobin has no oxygen-carrying capacity but high oxygen affinity. This causes decreased tissue oxygenation and shifting of oxygen dissociation curve to the left.[3]

Although spinal anesthesia should be the preferred technique for lower segment cesarean section in these patients, the urgent indication for immediate surgery strained us to proceed with general anesthesia. Previously also, general anesthesia has been administered in this patient population without any detrimental outcome.[1456] While contemplating general anesthesia, consideration should be given to avoidance of drugs precipitating methemoglobinemia such as prilocaine, lignocaine, benzocaine, metoclopramide, sodium nitroprusside, isosorbide dinitrate, and nitroglycerin. Meperidine, thiopental, propofol, succinylcholine, and inhalational anesthetics are considered safe and preferred during general anesthesia in these patients.[7] Factors influencing oxygen transport to the tissues should be optimized. The maintenance of normal cardiac output, minimal blood loss, and avoidance of hypoxia, hypothermia is important. Furthermore, the factors which increase oxygen consumption such as fever and shivering should be promptly treated. Preoperative blood transfusion should be considered in patients diagnosed preoperatively. As there is interference with normal monitoring devices, reliance should be on the values obtained by co-oximetry.

Coming to the conclusion, methemoglobinemia should be strongly suspected in patients presenting with a persistent low oxygen saturation and a classical saturation gap between pulse oximetry and ABG. Depending on the urgency and nature of surgery, both regional anesthesia and general anesthesia can be successfully employed for anesthesia. While choosing general anesthesia, one should carefully control and manage factors influencing oxygen transport to tissue level and avoid the drugs known to precipitate methemoglobinemia. Our clinical suspicion is much more important than advanced laboratory and diagnostic tools.

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Conflicts of interest

There are no conflicts of interest.