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Year : 2017  |  Volume : 11  |  Issue : 2  |  Page : 463-466  

Capnographic analysis of minimum mandatory flow rate for hudson face mask: A randomized double-blind study

Department of Anesthesiology, Shri Sathya Sai Medical College and Research Institute, Kancheepuram, Tamil Nadu, India

Date of Web Publication16-May-2017

Correspondence Address:
Pranjali Kurhekar
Department of Anesthesiology, Shri Sathya Sai Medical College and Research Institute, Tiruporur-Guduvanchery Main Road, Ammapettai, Kancheepuram, Tamil Nadu
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/aer.AER_29_17

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Background: Oxygen supplementation through Hudson type face mask is frequently used in perioperative settings. Hudson mask is a variable rate performance device with the risk of rebreathing. Studies using capnography to find out an actual fraction of rebreathing in spontaneously breathing patients are not available. Aims: In this study, we analyzed the effects of different flow rates through Hudson mask with capnography on fractional inspired carbon dioxide (FiCO2). Setting and Designs: Forty patients posted for minor surgeries under monitored anesthesia care were divided into four groups. They received oxygen flow rate of 3 L/min, 4 L/min, 5 L/min, and 6 L/min as per group division, through Hudson mask. Materials and Methods: Parameters such as pulse rate, noninvasive blood pressure, oxygen saturation, respiratory rate (RR), end-tidal carbon dioxide (EtCO2), and FiCO2 were noted at baseline. After starting oxygen supplementation, these parameters were monitored every minute for ten minutes. Statistical analysis was done using analysis of variance and Kruskal–Wallis test. Pearson correlation was used to establish a relation between RR and FiCO2. Results and Conclusions: EtCO2 and FiCO2 were comparable in all four groups with no statistical significance. There was strong positive correlation seen between RR and FiCO2 at a flow rate of 3 L/min while negative correlation in other groups. We conclude that FiCO2 is similar at all flow rates while breathing through Hudson mask. Increase in RR can cause increase in FiCO2 at lower flow rate, which is within physiological tolerance limits.

Keywords: Capnography, carbon dioxide, minor surgical procedures, monitoring, oxygen mask

How to cite this article:
Kurhekar P, Prasad T K, Rajarathinam B, Raghuraman M S. Capnographic analysis of minimum mandatory flow rate for hudson face mask: A randomized double-blind study. Anesth Essays Res 2017;11:463-6

How to cite this URL:
Kurhekar P, Prasad T K, Rajarathinam B, Raghuraman M S. Capnographic analysis of minimum mandatory flow rate for hudson face mask: A randomized double-blind study. Anesth Essays Res [serial online] 2017 [cited 2021 Aug 5];11:463-6. Available from:

   Introduction Top

Oxygen is the most widely used drug perioperatively and during critical care.[1] Oxygen supplementation through Hudson face mask (simple face mask) is frequently used in perioperative settings. This method of oxygen delivery is cheap, easy to administer and acceptable to both patients and anesthesiologists in routine cases.[2] Hudson mask is a variable rate performance device which depends on the oxygen flow rate, reservoir effect size of the mask, peak inspiratory flow rate (PIFR), and respiratory rate (RR). The fraction of inspired oxygen concentration (FiO2) varies depending on these factors, and there is always risk of carbon dioxide (CO2) rebreathing. Although there are different types of oxygen delivery devices available which perform predictably better, Hudson type mask is used most commonly.[3] The literature about the performance of Hudson type masks mentions about flow rates, FiO2 and arterial partial pressure of CO2 (PaCO2).[3],[4] The data regarding the relationship between CO2 rebreathing and oxygen flow rate through Hudson mask is limited. The recommended flow rate for Hudson face mask to avoid rebreathing is 4-10 L/min.[5],[6],[7] However, in clinical practice use of 3 L/min of flow rate is frequently seen. The increase in flow rate does not improve oxygenation and performance can deteriorate with an increase in minute ventilation.[2],[8] Hence, the flow rate at which no CO2 rebreathing is seen appears to be minimum mandatory flow for Hudson mask. The previous study done on healthy human volunteers with the help of plethysmography recommends minimum flow of 5 L/min to avoid rebreathing. CO2 rebreathing can lead to increase in work of breathing which cannot be detected by clinical signs or pulse oximetry.[7] Existing literature about required flow rates and rebreathing is based on plethysmography or oxygraphy and the amount of rebreathing is calculated based on various factors.[4],[7] To the best of our knowledge, studies using capnography to find out an actual fraction of inspired CO2 (FiCO2) in spontaneously breathing patients are not available. In this study, we compared the effect of different flow rates through Hudson mask on CO2 rebreathing (FiCO2) with capnography.

The primary aim of our study was to find out the minimum flow rate at which no CO2 rebreathing is seen at any time during the study. The secondary aim of the study was to compare hemodynamic parameters, RR, end-tidal CO2 (EtCO2), oxygen saturation (SpO2), and integrated pulmonary index (IPI) between groups.

   Materials and Methods Top

This was a prospective randomized double-blind study conducted over a period of 3 months from August 2016 to October 2016, after obtaining approval from the Institutional Ethics Committee (IEC No. 2016/261). The trial was registered in clinical trial registry – India, before starting enrollment of patients (CTRI/2017/02/007807). Patients between 18 and 40 years of age of physical status American Society of Anesthesiologist (ASA) class I, posted for elective surgeries under monitored anesthesia care were included in the study. Patients with physical status ASA class II and above, posted for emergency surgeries, pregnant patients and patients not willing to participate in the study were excluded from the study. The study protocol was explained to all patients and informed valid consent was obtained. No premedication was given to any patient. Patients were randomly divided into four groups by block randomization method with a block size of four depending on the flow rate they would receive. All the patients were monitored with Planet 60/Star 60 monitor, SANKRAY Technologies Pvt. Ltd., India. Before the start of the study, CO2 calibration was done with manufacturer approved kit containing calibration gas from Scott medicals. The calibration gas contained 5% of CO2, 21% of oxygen and the balance amount of nitrogen gas. Autocalibration was done by the machine and its status was confirmed before connecting to each subject. After shifting the patient to the operation theater, the monitor was switched on. Monitor alarms for all the parameters were disabled. Once the autocalibration was over and CO2 ready message was displayed, it was connected to the patient through nasal prongs of the disposable Adult CapnoLine (Microstream Technology). SpO2 probe and noninvasive blood pressure (NIBP) were connected. Pulse rate (PR)/min, NIBP, systolic/diastolic as mmHg, RR/min, EtCO2 as mmHg, FiCO2 as mmHg, IPI as number were noted at baseline before connecting to Hudson mask. Then the disposable adult Hudson mask (HUDSON RCI, Romsons) was applied with as much as possible seal. The mask was connected to the oxygen source behind workstation, which was out of the vision of monitoring anesthesiologist, through two-meter long tubing. The oxygen was given through flow meter with rotating type bobbin by operation theater assistant, who did not have visual access to the monitor facing opposite side. Oxygen flow rate was started depending on the group. Group I received flow of 3 L/min, Group II received flow of 4 L/min, Group III received 5 L/min of flow, and Group IV received 6 L/min of flow. After starting oxygen through Hudson mask, PR, NIBP, EtCO2, RR, FiCO2, and IPI were monitored every minute for 10 min. At the end of 10 min, oxygen mask was removed, and that point was taken as a study endpoint. The study protocol was handed back to the investigator in sealed envelope. After that scheduled surgical procedure was done as planned for each patient. All the patients were monitored with the same planet 60/star 60 machine by the consultant anesthesiologist assigned to the particular case.

Statistical analysis

Sample size calculation was done with the primary aim of finding flow with zero inspired CO2, extrapolating the results from the previous study.[7] Applying the effect size (es-1.98) to the analysis of variance (ANOVA) between four groups the sample size of 8 per group was needed with 1% of significance and power of 95%. Ten patients per group were included to allow possible dropouts. Statistical analysis was done with IBM Statistical Package for Social Sciences version 23 (SPSS, IBM Corporation, USA) for Windows (Microsoft Corporation, USA). Demographic characteristics were assessed with ANOVA and expressed as mean ± standard deviation and frequency. Vital parameters and capnographic parameters were assessed by Kruskal–Wallis test to compare between the groups and expressed as median (range). All the parameters were compared as values noted each minute in between the groups. P< 0.01 was considered statistically significant. The correlation between FiCO2 and RR was established using Pearson correlation coefficient and was expressed as graphical presentation. The graphs of median values of FiCO2 and EtCO2 against each minute of time interval were obtained.

   Results Top

The demographic characteristics [Table 1] in all the groups were comparable. All the four groups showed rebreathing of CO2 as FiCO2 value was more than zero during the study interval. The baseline FiCO2 value was zero in all the groups. The amount of FiCO2 seen in all the groups is comparable [Table 2]. Amount of FiCO2 during each minute in all four groups is compared in graphical manner [Figure 1]. The graph appears to show high amount of FiCO2 in Group I as compared to other groups and is above 1 mmHg throughout study interval, however, P value is not significant. P values of all capnographic parameters at each minute were not significant [Table 2]. The EtCO2 values [Figure 2] for all groups were comparable with no statistical significance.
Table 1: Demographic characteristics

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Table 2: Respiratory parameters at each minute (P value)

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Figure 1: Amount of fractional inspired carbon dioxide in each minute. Fractional inspired carbon dioxide is high at all times at flow rate of 3 L/min

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Figure 2: Amount of end tidal carbon dioxide during each minute at different flow rates

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The correlation between RR and FiCO2 varies among the groups [Figure 3]. In Group I (3 L/min of flow rate), strong positive correlation was seen between FiCO2 and RR, showing an increase in amount of FiCO2 with an increase in RR. In Group II, significant negative correlation was seen throughout the study. In Group III except for 3rd and 4th min of study, there was a significant negative correlation. In Group IV, initial 2 min of the study showed significant positive correlation, 3rd min onwards significant negative correlation between FiCO2 and RR was noted. All hemodynamic parameters and IPI values [Table 3] were comparable with a nonsignificant P value.
Figure 3: Correlation between respiratory rate and fractional inspired carbon dioxide, positive at 3 L/min and negative at other flow rates

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Table 3: Hemodynamic parameters (P value)

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   Discussion Top

In this study, we have found that rebreathing is seen at all the flow rates while breathing through Hudson mask and the actual fraction of inhaled CO2 shown by capnography, at different flow rates is not significant. To the best of our knowledge, this is the first study to compare the actual fraction of inspired CO2 with the help of capnography, while breathing through Hudson mask. Hudson mask is a variable performance device; hence, FIO2 varies based on RR and tidal volume (TV). Fresh gas flow (FGF) set as flow rate is delivered lesser than PIFR and so the atmospheric air drawn through the vents is must for adequate TV. This results in breath-to-breath variation in performance of Hudson mask. Our results show that at different flow rates given through Hudson mask, the hemodynamic parameters, SpO2, and EtCO2 are comparable.

Jensen et al. studied the effect of different oxygen flow rates on ventilation and concluded that at flow rates <5 L/min, there was an increase in TV which they attributed to CO2 rebreathing. However, they did not find a significant increase in RR at flow rates <5L/min.[7] We have studied the actual amount of CO2 rebreathing and found FiCO2 and RR both were same at all flow rates. They also noted that rebreathing does not cause any change in hemodynamic parameters. In our study, we did not find any difference in hemodynamic variables as well as IPI. After studying the performance of Hudson type mask at two different flow rates, Leigh concluded that the total expiratory time which represents the O2 wash in and CO2 washout time would decide the composition of next breath.[9] In our study, we found that at lower FGF of 3 L/min, increase in RR resulted into less time for exhaled CO2 washout leading to increase in FiCO2. However, FGF of more than 4 L/min was sufficient to wash out CO2 even with decrease in expiratory time. Campkin et al. studied the effects of rebreathing through Hudson mask with the help of capnometry. The load of inspired CO2 was calculated based on the end tidal CO2 and the dead space.[10] Their observations showed that some amount of CO2 was inspired even at FGF of 8 L/min, but it was significantly higher at FGF <4 L/min. In our study also FiCO2 is seen at all flow rates and though it was higher at FGF <4 L/min it was not significant. However, Campkin et al. have calculated inspired CO2 load based on EtCO2 and dead space, while we have analyzed the actual amount of inspired CO2 during each minute.

At FGF of 3 L/min, FiCO2 was always above 1 mmHg and at FGF of more than 3 L/min it was always below 1 mmHg. CO2 rebreathing can stimulate cardiovascular responses but the previous study done on humans for the duration of 10 min has not shown any cardiovascular changes.[7] In our study also FiCO2 as high as 1 mmHg have not caused any change in hemodynamic parameters and IPI, which is composite index of different vital parameters (EtCO2, RR, SpO2, PR). CO2 tolerance studies have shown that no disabling physiological or clinical symptoms are associated with breathing of 5% CO2, which is equivalent to a concentration of CO2 in expired gases.[11]

Campkin et al. found no relation between FiCO2 and duration of respiratory cycle and concluded that expiration time affecting FiCO2 is a theoretical possibility.[10] In this study, we found a strong correlation between these parameters. Study by Campkin et al. was done on mechanical lung model, and our study was on spontaneously breathing patients. The physiological variations seen in patients could be responsible for the difference in observations in our study. Previous studies on Hudson mask have mentioned about comparable PaCO2 at different flow rates.[3],[4] In our study, we did not analyze blood gases, instead measured the EtCO2 which is a surrogate marker of PaCO2 under normal physiological conditions and a noninvasive, simple method as well. There was no change in EtCO2 at different flow rates in our study.

Wagstaff and Soni have concluded that increase in RR causes decrease in effectively inspired oxygen concentration (EIOC) while breathing through Hudson mask.[12] Patient factors such as RR and TV directly affect the EIOC and FiCO2 irrespective of FGF. All the previous studies regarding performance and FGF in Hudson mask were done either on simulating models or on human volunteers and methods to alleviate patient factors which can affect performance have not been tried.[7],[12]

Our study has some limitations. First, that it was done on patients without anxiolysis. This could have affected the RR and FiCO2. Another limitation of our study is that only capnographic parameters were analyzed and breathing pattern was not evaluated. Further studies are needed on patients or human volunteers after adequate anxiolysis and sedation to minimize the interference of physiological variation.

   Conclusions Top

FiCO2, the amount of rebreathing is similar at all flow rates while breathing through Hudson mask.

The flow of 3 L/min can be given through Hudson type face mask. Although the amount of inhaled CO2 while using Hudson type mask appears to be safe, further studies are needed to justify continuing use of Hudson type mask against better-performing oxygen delivery devices.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.

   References Top

O'Driscoll BR, Howard LS, Bucknall C, Welham SA, Davison AG; British Thoracic Society. British Thoracic Society emergency oxygen audits. Thorax 2011;66:734-5.  Back to cited text no. 1
Hett DA, Geraghty IF, Radford R, House JR. Routine pre-oxygenation using a Hudson mask. A comparison with a conventional pre-oxygenation technique. Anaesthesia 1994;49:157-9.  Back to cited text no. 2
Wagstaff AJ. Oxygen therapy. In: Bersten AD, Soni N, editors. Oh's Intensive Care manual. 7th ed. UK: Butterworth Heinemann Elsevier; 2014. p. 327-40.  Back to cited text no. 3
Waldau T, Larsen VH, Bonde J. Evaluation of five oxygen delivery devices in spontaneously breathing subjects by oxygraphy. Anaesthesia 1998;53:256-63.  Back to cited text no. 4
Kory RC, Bergmann JC, Sweet RD, Smith JR. Comparative evaluation of oxygen therapy techniques. JAMA 1962;179:767-72.  Back to cited text no. 5
Bateman NT, Leach RM. ABC of oxygen. Acute oxygen therapy. BMJ 1998;317:798-801.  Back to cited text no. 6
Jensen AG, Johnson A, Sandstedt S. Rebreathing during oxygen treatment with face mask. The effect of oxygen flow rates on ventilation. Acta Anaesthesiol Scand 1991;35:289-92.  Back to cited text no. 7
Williams AB, Jones PL, Mapleson WW. A comparison of oxygen therapy devices used in the postoperative recovery period. Anaesthesia 1988;43:131-5.  Back to cited text no. 8
Leigh JM. Variation in performance of oxygen therapy devices. Anaesthesia 1970;25:210-22.  Back to cited text no. 9
Campkin NT, Ooi RG, Soni NC. The rebreathing characteristics of the Hudson oxygen mask. Anaesthesia 1993;48:239-42.  Back to cited text no. 10
Vercruyssen M, Kamon E, Hancock PA. Effects of carbon dioxide inhalation on psychomotor and mental performance during exercise and recovery. Int J Occup Saf Ergon 2007;13:15-27.  Back to cited text no. 11
Wagstaff TA, Soni N. Performance of six types of oxygen delivery devices at varying respiratory rates. Anaesthesia 2007;62:492-503.  Back to cited text no. 12


  [Figure 1], [Figure 2], [Figure 3]

  [Table 1], [Table 2], [Table 3]

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