Detection of hypoxia in the early postoperative period.

The aim of this study was to determine the incidence of early postoperative hypoxia after general anesthesia and to evaluate the need of oxygen supplementation. A total of 150 patients aged between 18-60 years belonging to ASA I or II were studied. Patients were alternately allocated to two groups of 75 each. Group-I (n=75) patients received 2 L of oxygen via nasopharyngeal catheter during transfer from operation theatre to recovery room and in the recovery room for a period of 4 h. Group-II (n=75) patients did not receive oxygen either during transfer or in the recovery room. Twenty percent in Group-II developed hypoxaemia during transfer from operation theatre to recovery room out of which 24% developed mild (SaO2 86-90%), 2.66% moderate (SaO2 85-81%), and 1.33% extreme (SaO2 <76%) hypoxaemia. None of the patients in Group - I, who received oxygen supplementation, developed hypoxaemia in the early postoperative period.

RCT Entities:   Population Interventions Outcomes

INTRODUCTION

Oxygen is essential for life as a source of energy. Its lack not only stops the machine, but also wrecks the machinery (Haldane – 1847). Hypoxaemia is defined as inadequate tissue oxygenation due to either inadequate blood flow or low arterial oxygen content. Hypoxaemia is one of the most feared critical events during anesthesia and the recovery period.[1] Arterial hypoxaemia can occur anytime during anesthesia with astonishing suddenness. Severe hypoxaemia can kill a patient or leave the victim with devastating neurological handicaps.[2] Early diagnosis of hypoxaemia with the help of pulse oximeter would lead to early correction of the hypoxaemic event, which might cause postoperative complications or even death.[3]

Tyler et al.[4] showed that SaO2 decreases significantly in a large number of patients who were transported without supplemental oxygen from the operating room to the recovery room.

Therefore, we planned to conduct a study at our institution with the following aims and objectives.

To determine the incidence and degree of fall in SaO 2 during transfer from operation theatre to recovery room and in the recovery room.

To identify predictive factors; if any.

MATERIALS AND METHODS

The study was conducted at our institution between Jan 2009 and Dec 2010 after obtaining the permission from the Hospital Ethical Committee and informed consent from the enrolled patients. A total of 150 patients at age group 18–60 yrs of either sex, belonging to ASA – I or II undergoing elective noncardio-thoracic surgery under general anesthesia were included in this study.

The selected patients were alternately allocated into two groups with 75 patients in each group. Group–I patients received 2 L of O2 via nasopharyngeal catheter during transfer from the operation theatre (OT) to recovery room (RR) and also in the RR for a period 4 h. Group-II patients did not receive O2 during transfer or in the RR. Rest of all the anaesthetic procedures was same for both the groups: For example Inj Fortwin 30 mg, Phenergan 25 mg, and Glycopyrrolate 0.2 mg IM as premedication, maintenance by standard technique, and reversal by atropine/glycopyrrolate and neostigmine.

The drugs known to produce respiratory depression, for example, Morphine, Pethidine, Fentanyl etc. were avoided during anesthesia. After extubation all the patients were given 100% O2 at the rate of 6–8 L/min via open circuit (Mapleson A) for at least 5 min before transfer.

The patient's level of consciousness was scored as unresponsive (0), arousable (1), and awake (2). Before shifting the patients from OT to RR, a pulse oximeter probe was applied to a finger and SaO2 was recorded at 1-min intervals during transfer for both the groups. Group I received 2 L oxygen via nasopharyngeal catheter soon after removal of facemask and Group II patients did not receive oxygen via nasopharyngeal catheter unless SaO2 fell <90% during transfer or in the RR. SaO2, pulse rate, respiratory rate, BP, temperature and color were recorded half hourly for a period of 4 h in the RR for both the groups. It was planned, if hypoxia persists in spite of O2 via nasopharyngeal catheter, appropriate corrective measure would be taken.

Hypoxaemia will be graded as follows:

Mild hypoxaemia – 86–90%

Moderate hypoxaemia - 81–85%

Severe hypoxaemia - 76–80%

Extreme hypoxaemia - <76%

Student's ‘t’ test, Chi square or Fisher's exact test were used for statistical analyses.

RESULTS

Between the two groups of variables body weight, duration of surgery, and haemoglobin levels did not show any statistical significance [Table 1]. However, difference in the mean ages of both groups is statistically significant when the student's ‘t’ test was applied but this difference is of no clinical significance. Sex distribution between two groups were almost similar (Male 54.66% in Group I and 50.66% in Group II).The vital signs (PR, BP, RR, Temperature, and colour) were recorded every half an hour for both the groups and they were comparable. Level of consciousness after extubation was studied and it was comparable in both the groups [Table 2 and Figure 1]. The incidences of different grades of hypoxaemia between two groups are compared in Table 3 that clearly shows the importance of O2 supplementation during transfer.

Table 1

Demographic data

Table 2

Incidence of hypoxaemia in different levels of consciousness

Figure 1

Level of consciousness

Table 3

Degree of hypoxaemia during transfer

DISCUSSION

Hypoxaemia is one of the feared critical events during anesthesia and in the recovery room.[1] Postoperative hypoxaemia occurs commonly and has been documented even after minor procedure under general anesthesia.

Arterial hypoxaemia has two main causes

Diminished arterial oxygen tension.

Increased arterial oxygen tension differences.

Lung function almost always deteriorates during the course of anesthesia. This deterioration may be due to decrease functional residual capacity and increase airway closure, a V/Q imbalance or the development of atelectasis.

Further CO2 retention because of hypoventilation can in turn cause hypoxaemia by displacing oxygen from the alveoli, but this is important only when the inspired air is not oxygen enriched (Tyler et al.[4]). During the early postoperative period, desaturation may also result from diffusion hypoxia, hypo-ventilation due to central nervous system depression, or inadequate antagonism of neuromuscular blockade.

Before the advent of pulse oximeter in order to measure the adequacy of oxygenation, the clinician had to depend on cyanosis, which manifests only when 15% or 2–3 gm of hemoglobin is desaturated. Where as pulse oximeter will detect early hypoxaemia, which cannot be detected clinically. Pulse oximetry displays a beat to beat value of the oxygen saturation of blood[5] and has been accepted as part of minimum mandatory monitoring during anesthesia.[6]

In our study none of the patients in Group I who received oxygen during transport showed any hypoxaemia where as in Group II, 21 patients (28%) developed hypoxaemia during transport. Studies abroad and in India showed similar results during transfer from operation theatre to recovery room. Table 4 shows a comparative analysis of other studies with this study.

Table 4

Comparative study of incidence of hypoxaemia

In our study, among the three subgroups of age in group II, we found that the higher age group, that is, 46–60 years, had the maximum incidence of hypoxaemia (43.47%) and it was least in the younger age group (17.24%) and the difference was statistically significant (P=0.03). Similar to the studies done by Tyler et al.[4] and Blair et al.[7] we also did not find any statistically significant difference in the incidence of hypoxemia with regard to duration of surgery.

In our study, we found that the higher incidence of hypoxaemia (45.45%) was in the sedated but arousable group as compared with the awake patient (14.28%). This is statistically significant (P=0.028). None of our patients were unresponsive. Similar studies done by Tyler et al.[4] did not show any significant difference in the incidence of hypoxaemia as related to the level of consciousness.

Smith et al.,[8] in a study conducted on 350 patients to assess postoperative hypoxaemia in the recovery room, found that 1.1% of patients who receive oxygen supplementation showed a saturation below 90% where as 16.7% of patients who did not receive oxygen showed a saturation below 90%. The lower incidence of hypoxaemia in the recovery room in our study as compared to the study done by Smith et al. may be because patients who developed hypoxaemia during transfer were given supplemental oxygen before reaching recovery room.

Based on the findings of the present study, we can conclude that oxygen supplementation should be given to all the patients who have undergone general anesthesia, both during transfer from the OT to the RR and for at least two hours in the RR. Though our study did not demonstrate any incidence of hypoxaemia during transport in patients who received oxygen via nasopharyngeal catheter, it is advisable to monitor all the patients for SaO2 by pulse oximeter during transfer to pick up any case of hypoxaemia at the earliest opportunity.