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Face Masks and Nasal Prongs for Postoperative Hypoxemia

Info: 3431 words (14 pages) Essay
Published: 28th Sep 2017 in Health

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Early postoperative hypoxaemia is not uncommon when patients breath room air during their initial recovery period. Prolong hypoxaemia can result in delirium, arrhythmia, and cardiac arrest. We compared the efficacy of face masks and nasal prongs in the management of hypoxaemia following general anaesthesia relaxant technique.


This was a prospective open labelled randomized controlled study which compared the performance of Face Mask and Nasal Prongs in the management of early postoperative hypoxia. All procedures were performed using standard anaesthetic and surgical techniques adapted to the individual procedures. At the end of surgery; all the patients were fully awake and had SPO2 ≥97% before transfer to the recovery room. On arrival in the recovery room, 120 subjects were randomly allocated into Face Mask group and Nasal Prong, when their SpO2 decreased to ≤ 94%, thereafter 4L/min of oxygen was administered through either devices.


Early postoperative hypoxia occurred in 18.1%. The increase in oxygen saturation after commencement of oxygen therapy was significantly faster with Nasal prong (0.63 ±1.42 min) than with Face Mask (1.78 ±1.10min), p =0.001. The maximum SpO2 obtained was significantly higher with Nasal prong (98.77±1.29%) than Face mask (97.63 ±1.89%), p = 0.001. There was no significant association found between early postoperative hypoxemia and site or duration of surgery, and volume of infusion administered intra-operatively, p>0.05. Nasal Prongs (91.7%) was significantly more comfortable using modified VAS than Hudson Face mask (61.7%), p = 0.001. We have demonstrated that the use of Nasal Prongs was more efficient and comfortable than face mask in the management of early postoperative hypoxemia.


Early postoperative hypoxaemia with an incidence of 35%-60% occurs within minutes and two hours after surgery.1-3The risk factors implicated in its development includes age, gender, weight, intraoperative opioid administration, smoking, duration of anaesthesia, thoracic surgery, and pre-existing heart and lung disease. 2,4-5

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Hypoxaemia has been shown to increase the risk of surgical wound infection, reduce anastomosis integrity and thus result in poor wound healing.6-7 It may also contribute to loss of gastrointestinal mucosal integrity resulting in bacterial translocation into the circulation, leading to sepsis.6-7 As well as in the development of decreased cognitive function and delirium which can lead patients to remove nasogastric tubes, surgical drains and intra-vascular devices. Oxygen therapy has been very beneficial in the treatment of postoperative delirium secondary to hypoxia. 6,8-9 Other effects of hypoxaemia reported includes increased production of catecholamine, hypertension, tachycardia, cardiac arrhythmias and myocardial ischaemia,6,8 In addition, an increase in both time spent in the post anaesthetic care room and in the frequency of admission to intensive care units has been documented.6

The modes of delivery of oxygen to spontaneously breathing, non-intubated patients during their post-anesthesia recovery room stay include nasal cannula, nasal catheter, face mask, or face tent.1,6 Oxygen therapy must be prescribed, at an appropriate dose and through an appropriate device.

At our institution, variable performance oxygen devices such as Hudson face mask and Nasal prongs are widely used in the recovery room in patients at risk of postoperative hypoxaemia such as elderly, sickler and post thoracic or upper abdominal surgeries. The choice of therapy device usually depends on the experience of the anaesthetist, availability of the device and co-operation of the patient. We compared the efficacy of face masks and nasal prong in the management of hypoxaemia, following the administration of general anaesthesia relaxant technique, in a surgical population. Our hypothesis testing was that there is no significant difference in the performance of Hudson face mask and nasal prong in the management of postoperative hypoxia.

Patients and Methods

The institutional Human Research Ethic Committee approval and informed patient consent were obtained. The sample size calculation was based on a predetermined statistical formula used for the comparison of proportion of two independent groups.10 The reported incidence of early postoperative hypoxaemia was 35%-60% incidence,1-2,6 [with an average: incidence of 45%. (0.45 = π1)], we aimed to achieve reduction to 20% incidence (π2), using α = 95%, and β = 80%, 60 subjects per group was considered appropriate for the study. American Society of Anesthesiology (ASA) physical status I and II aged 18-80 years undergoing elective surgical procedures under general anaesthesia relaxant technique were recruited. Subjects excluded from the study included those with upper respiratory tract infection, chronic smokers, patients with deranged electrolyte/urea and sepsis. As well as those with pre-operative oxygen saturation <95% or subjects on oxygen therapy before surgery, ASA III to IV status, those undergoing regional anaesthetic techniques and subjects in whom the use of pulse oximeter may result in inappropriate results (peripheral vascular disease, severe anaemia or recent use of colored dyes).

Anaesthetic Technique

In the preoperative period, all patients were educated on the use of visual analogue scale to assess their level of comfort during the use of the oxygen therapy devices. Oral diazepam (5mg) was administered nocte and on call to theater. On arrival at the theatre, continuous monitoring of the blood pressure, heart rate, and oxygen saturation was taken every 5 minutes with a Multiparameter Cardiocap 7110 monitor till the end of surgery.

General anaesthesia was induced with intravenous sodium thiopentone (5mg/kg), while endotracheal intubation was facilitated with pancuronium (0.1mg/kg) and appropriate sized endotracheal tube was secured. Anaesthesia was maintained with Isoflurane 1.5-2% in 100% oxygen. Analgesia was via a multimodal approach and included IV diclofenac 1mg/kg, tramadol 1mg/kg and paracetamol 15mg/kg. Intravenous fluid was given based on maintenance of 40ml/kg/day and, the on-going loses (from nasogastric tube and surgical drains). At the end of surgery, residual neuromuscular blockade was reversed with intravenous atropine 0.02mg/kg and neostigmine 0.04mg/kg. After extubation, all patients were given 100% oxygen using a well fitted face mask via the anaesthetic machine for 3 minutes. Oxygen saturation was recorded at the end of the 3minutes (Time 0). All the patients were ensured to have maintained SPO2 ≥97% before being transferred to the recovery room by the researcher. During transfer, the patients were continuously monitored for peripheral oxygen saturation (SPO2) by pulse oximeter (Nonin). The alarm on the pulse oximeter was pre-set at SPO2 ≤95% and thus when the SPO2 fell below 95%, it was recorded. The frequency of hypoxaemic events during transportation of patients was noted from the monitor and recorded. Supplemental oxygen was not administered during transport.

On arrival in the recovery room, continuous monitoring of SpO2, blood pressure, heart rate, respiratory rate was commenced. The level of consciousness on arrival using post-anaesthetic recovery score was also monitored.22.Patients with oxygen saturation ≥94%, had continuous monitoring as per standard protocols in the recovery room. However, patients with SpO2 ≤94% had humidified supplemental oxygen therapy commenced via variable performance devices: nasal prong or face mask at an oxygen flow rate of 4litre/minutes. The patients were randomly assigned into two groups, by the researcher, from a sealed envelope which contained the groups written on a folded piece of papers. Group A: received oxygen therapy via face mask, and Group B: received oxygen therapy by nasal prong. Continuous monitoring of vital signs and the sensorium was continued every 5 minutes in the recovery room for 30 minutes if the Spo2 is >94%, subsequently every 15mins till the patients were discharged from the recovery room.

The efficacy of the devices was determined by comparing the proportion of the patients who de-saturated while on oxygen therapy as well as the time required to achieve a saturation ≥95%. In addition, the ability of the patient to maintain SpO2 >94% without CO2 retention was also used. The retention of CO2 was documented when there was an increase in respiratory rate, heart rate, and blood pressure and/or sweating in the presence of adequate analgesia and cardiovascular stability. The ability to provide good comfort /convenience for the patient during the procedure was assessed by the use of visual analog scale in the recovery room before their discharge to the ward. 11

The data collated included patient’s demographic data such as age, weight, height, haemoglobin concentration and BMI. Others included the type, duration and site of surgery and incision, the type and volume of intra-operative fluids transfused, presence of shivering, the choice of oxygen therapy device and the presence of carbon dioxide retention. .

Statistical Analysis

Numerical data were expressed as means ±SD, while categorical data was expressed as frequency. Student t-test was used for comparison of means while chi-square was used to compare frequencies. The risk of development of hypoxia such as site of surgery, duration of surgery, volume of fluid used intra-operatively and the type of incision made were analysed univariate analysis. A p value of <0.05 was considered significant for all tests. All analysis was performed using the Statistical Package for Social Sciences for Windows version 17 (SPSS, Chicago, IL).

For the purpose of this study the underlisted definitions were used:

Hypoxaemia was graded into four values of SPO2;9 Mild (86-90%), Moderate (81-85%), Severe (76-80%), and Extreme (<80).12

The trigger value of oxygen saturation for commencement of oxygen therapy was SPO2<94%.13

Tachycardia described as heart rate (HR) >100 beats/minute or 25% increase in heart rate from the baseline value. 14

Bradycardia descried as HR< 60 beat/minutes or 25% decrease in heart rate.14 Hypertension described as SBP >140 mmHg, or 25% increase in SBP or DBP from the baseline.14

The level of consciousness (LOC/Sensorium) was graded into : 1-fully awake; 2 – Asleep but easily aroused; 3- Asleep and difficult to arouse. 15

The Visual analogue scale was used to assess patient comfort in the use of the oxygen therapy devices: (VAS score, 100m = worst possible discomfort, 0mm = most comfortable).


There was no statistically significant difference between the two groups in regards to age, weight, BMI, and preoperative Hb concentration. Both groups showed a female preponderance (24:36 in Group A; 14:46 in Group B), Table I. The most common surgery was pelvic surgery (myomectomy) in both groups and the least was thyroidectomy, Figure 2.

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The oxygen saturation trends during theatre/recovery room transit period is depicted in Figure 3. The patients’ oxygen saturation was observed to be highest (99-100%) at the immediate post-extubation period following 3mins on supplemental oxygen (Time 0). Thereafter, saturation declined steadily during transportation, Figure 3.

Figure 4 shows the comparative performance of the two oxygen therapy devices during oxygen therapy in the recovery room. Though, the mean Oxygen saturation decreased prior to the administration of oxygen (Pre Oxygen therapy) in both groups, the difference was insignificant; face mask group (93.2 ±7.0%) versus nasal prong group (93.3 ±2.6%), p = 0.461). However, the subjects in the Nasal Prong group responded faster (0.63 ±1.42 minutes) to oxygen therapy, than the face mask group (1.78 ±1.10 minutes), p = 0.001. A significantly higher SpO2 values (97.4 ±2. 5%) were achieved in the group B compared to group A (96.0 ± 3.5%) at 5minutes duration of oxygen therapy, p= 0.002. There was no incidence of hypoxaemia or de-saturation while patients were on oxygen therapy with either of the devices, throughout the study periods.

The mean HR was significantly higher in the face mask group than the nasal prong group; at 5, 10 and 45 minutes in the recovery room. There was a gradual drop in MAP during the study period. Though patients in face mask group A exhibited higher mean values, this was not statistically significant till 30mins into the study when MAP was 70.4 ±16.5mmHg in group A compared to 57.9 ± 16.2mmHg in group B, p = 0.01 Table IV. The mean RR was significantly higher in the face mask group than the nasal prong group at 5, 25, 30, 45 and 60 minutes, p≤0.05, Table V.

Using Univariate analysis to determine the risk factors for the development of postoperative hypoxaemia, none of the confounding variables studied were implicated, Table VI.

The mean level of convenience using modified VAS pre-oxygen therapy was similar in face mask Group (0) versus nasal prong Group (0). However, a higher proportion of subjects in the nasal prong group 55 patients (91.7%), were comfortable with the device compared with the face mask group 37 patients (61.7%) p=0. 001.


We have demonstrated that early postoperative hypoxia occurred in 18.07% of subjects, which is within the stated range of 7.8%-60%. 2-4 The wide variation in incidence could be attributed to differences in subject cohort, type of oxygen delivery device, anaesthetic and surgical techniques. The lower incidence in our study was attributed to the maintenance of anaesthesia with Isoflurane in 100% oxygen compared to the former that used nitrous oxide for maintenance. The use of nitrous oxide has been associated with diffusional hypoxia if oxygen is not administered for 3-5 minutes after its discontinuation. The residual effect of incomplete reversal of neuromuscular agent may also influence the incidence of hypoxaemia. It is suggested that in patients who received neuromuscular blocker, monitoring of reversal by a peripheral nerve stimulator should be encouraged.

In none of our subjects was the oxygen saturation ≤ 90% during theatre–recovery room transit period, neither did any of the patients de-saturate to ≤ 94%, our oxygen therapy trigger. Thus, oxygen therapy may not be required during transit to the recovery room post operatively. This is in agreement with the findings in previous studies.13,16 The use of manoeuvres such as recovery positioning or propping-up the patients during transport, which prevents airway obstruction and splinting of the diaphragm may just be enough to prevent hypoxaemia or de-saturation which may occur during transport. Nevertheless, it has been reported that when the duration of transport without supplemental oxygen is prolonged, the risk of developing postoperative hypoxaemia increased. 4

Other factors implicated in the presence of de-saturation or hypoxaemia during transport include premorbid state of the subject, airway obstruction and hypoventilation, resulting from the residual effects of inhalation agents, the use of opioids and neuromuscular blockers. To reduce the influence of patient premorbid state, only ASA I and II subjects were recruited. Subjects with sickle cell disease, anaemia following haemorrhagic shock, as seen in ectopic pregnancy, post thoracotomy, neurosurgery, cigarette smokers, patients with COPD and preoperative hypoxemia were excluded from our cohort.

We observed that desaturation in the recovery room occurred within 30 minutes of arrival, this is similar to a previous observation that early postoperative hypoxaemia occurred most commonly <1hour after anaesthesia in older children and adults.3 During oxygen therapy with either of the devices, none of our subjects developed hypoxaemia. This may suggest that either the Nasal prong or Face Mask were effective in delivering oxygen in the early postoperative period in patients with oxygen saturation ≤94%. Our findings agree with previous observations. 3,15-16, In contrast, a high incidence of hypoxaemia (25%) was observed with the use of the aerosol face tent during oxygen therapy by Graybeal and Russel.8 Its design allows significant air entrainment, reducing the FiO2. 5,17

The nasal prong was observed to be more efficient than the face mask for oxygen therapy in our study: This could be due to a larger dead space volume reservoir of the face mask, which takes longer to be filled, compared to the smaller nasopharyngeal reservoir of the Nasal Prong. It is also possible that the claustrophobic effect of the face mask affected patient compliance and hence oxygen delivery ( FiO2 ) to the patients.

The arterial blood gas analyzer is the gold standard for assessing oxygen therapy. Several scholars have reported that the results obtained from SPO2 correlated with PaO2.18-19

This study observed no association between site of surgery, duration of surgery, gender and volume of intra operative infusion used and early postoperative hypoxaemia. This may not be unrelated to the similarity in the cohort studied. As a high proportion (>50%) of our cohort had myomectomy and exploratory laparotomy performed, in regions which does not influence respiratory excursion. A similar observation was reported by other scholars in relation to the duration and site of surgery.11, 4-5 on the contrary, the site of surgery has been implicated in the development of late postoperative periods.7,20 While early postoperative hypoxaemia is mainly due to anaesthetic factors, late postoperative hypoxaemia is mainly related to reduced functional residual capacity of the lung, especially with upper abdominal surgeries. 7,20

The ability of the devices to cause carbon dioxide retention was evaluated with the haemodynamic changes that occurred in the patients during oxygen therapy. In our study, a significant difference was observed in the changes in HR, MAP and RR within the first five minutes of commencement of Oxygen therapy with the two devices. This initial cardiovascular changes during oxygen therapy could be attributed to the period in which the reservoir (dead space) of the device was equilibrating with oxygen. This was more pronounced in the Face Mask group. The observed changes cardiopulmonary changes, could also be due to central respiratory depressive effects of inhalational anaesthetic agent and the residual effect of tramadol used intraoperatively. Arterial blood gas (ABG) analysis would have been more ideal to measure PaCO2 to confirm carbon dioxide retention. However, this is invasive technique, and the use is unethical in the present situation.

Our result showed that the use of Nasal Prong was associated with better satisfaction for our patients than the Face Mask. This result is similar to the findings by other researchers.3,15 Nasal Prongs have also been shown to be cost effective, when compared with the face mask. This is very important in this period of global economic recession. This study is limited by the fact that the assessment of adequacy of oxygenation was determined with the pulse oximeter, we were unable to use the arterial blood analyser due to the cost implication. Likewise the clinical signs of carbon dioxide retention were used instead of a capnography as such was unavailable at the time of our study. Inspite of this we have demonstrated that he nasal prong was a more effective device of delivering oxygen in subjects with early postoperative hypoxia, it is also comfortable and cost effective.


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