Low-flow oxygen delivery in patients with disturbed consciousness

Authors: Shoko Merrit Yamada

Abstract

Background:

When low-flow oxygen (O2) delivery is required, a nasal cannula is commonly used. However, no reports have described arterial blood gas analysis in actual patients receiving O2 simultaneously through a mask and a nasal cannula.

Methods:

This prospective observational study included 110 patients with disturbed consciousness due to brain damage. Arterial lines were inserted in all patients for blood pressure monitoring, and arterial blood was collected through the line. Arterial O2 saturation (SaO2) and partial pressures of O2 (PO2) and carbon dioxide (PCO2) were measured while patients received O2 at flow rates of 1, 2, and 3 L/min through either a mask or a nasal cannula.

Results:

SaO2 was significantly higher with a mask than with a nasal cannula when O2 was delivered at 2 and 3 L/min (p < 0.01). PO2 was significantly higher with a mask at 1, 2, and 3 L/min; however, PCO2 did not differ significantly among room air, nasal cannula, and mask O2 delivery. In O2 administration of 1, 2, and 3 L/min, no significant differences in PO2 or PCO2 were identified between closed-mouth and open-mouth breathing with either a mask or a nasal cannula.

Discussion:

Low-flow O2 delivery through a mask provides better oxygenation in comatose patients. For patients with decreased PO2 levels, low-flow O2 administration using a mask is more effective in increasing PO2 than using a nasal cannula, regardless of closed- or open-mouth breathing; however, one drawback is that the mask must be temporarily removed during oral care and suctioning procedures.

INTRODUCTION

It has been reported that approximately one quarter of hospitalized patients regularly receive oxygen (O2) therapy.[9,21] The indications for O2 administration include shortness of breath, chest pain, and hypoxemia.[4] In the neurosurgery field, patients with disturbed consciousness have decreased oxygenation because of irregular breathing or apnea, especially those who are comatose as a result of brain damage, and therefore frequently require low-flow O2 delivery.[3,17,20,24,26] Compared with high-flow O2 delivery, which can cause substantial damage to several organ systems, low-flow O2 is considered relatively safe.[2] However, determining whether low-flow O2 should be delivered through a mask or a nasal cannula can be challenging in clinical practice.[14] In general, it is standard practice to use a nasal cannula when delivering low-flow O2 at 4 litter per minute (L/min) or less.[28] One reason is that when delivering low-flow O2 through a mask, exhaled carbon dioxide (CO2) may remain within the mask, potentially leading to rebreathing of accumulated CO2 and an increase in partial pressure of CO2 (PCO2) in arterial blood.[7] Another reason is that when O2 is delivered through a mask at a flow rate below the patient’s inspiratory flow rate, room air around the mask may be entrained into the mask, resulting in an inconsistent fraction of inspired O2 (FiO2). In contrast, nasal cannula delivery at 2–4 L/min or high-flow mask delivery provides a more predictable range of FiO2.[1,19] However, few studies have investigated how much PCO2 increases when O2 delivery is switched to a mask from a nasal cannula. Many comatose patients with brain damage breathe with their mouths open. We therefore hypothesized that it may be preferable to deliver O2 through a mask rather than a nasal cannula to such patients. This study aimed to analyze differences in arterial blood gas parameters, including arterial blood O2 saturation (SaO2), PCO2, and partial pressure of O2 (PO2) in arterial blood, when administering low-flow O2 through either a mask or a nasal cannula to comatose patients.

MATERIALS AND METHODS

The methods and protocols applied in this study were approved by the Teikyo University Ethics Committee (Teirin 20-059). The study details were explained to the families of each patient, and written informed consent was obtained from them.

Patients

Comatose patients due to brain dysfunction were included in this study. When O2 is administered by a mask to patients who are fully conscious, they frequently remove the mask because of the discomfort of pressure on the face or unpleasant odor. On the other hand, patients with disturbed consciousness are less likely to be troubled by discomfort of the mask, enabling more reliable evaluation of oxygenation during mask wearing. Patients with impaired consciousness caused by cerebral stroke, brain tumors, or head injury were enrolled in this study. An arterial line for continuous blood pressure measurement, along with electronic cardiac monitoring and SaO2 monitoring, was placed in all patients. Low-flow O2 was delivered alternately through a mask and a nasal cannula to allow assessment of PO2 and PCO2 with each O2 delivery method. Patients whose SpO2 remained <95% while receiving O2, as well as those who had aspiration pneumonia on admission and who had tongue base collapse before the start of the study requiring oral intubation, were excluded from the study.

O2 administration and arterial blood gas analysis

Figure 1a and b shows the mask and the nasal cannula (Avion Medical SP Z.O.O, Poland, Swinoujscie) used in this study and how to use them, respectively. The mask was a simple type with perforations on both sides [Figure 1a, black arrow], and the nasal cannula had straight prongs with a tube length of 1.8 m. Arterial blood was collected from the arterial line for gas analysis (ABL 800 FLEX; Radiometer, Japan, Tokyo); thus ensuring that the arterial samples could not be contaminated with venous blood. The processes of delivering O2 and arterial blood gas measurements are summarized in a flow chart [Figure 2]. Arterial blood gases were first measured in room air. O2 was then administered at 3 L/min through either a mask or a nasal cannula for 15 min, and the blood gas analysis was performed. The patients then breathed room air again for 15 min, followed by substitution of the nasal cannula for the mask, or vice versa, again delivering O2 at 3 L/min, and blood gas was measured after another 15 min. The same procedure was repeated with O2 flow rates of 2 L/min and 1 L/min. Because all three O2 flow rates were administered to each patient through both the mask and the nasal cannula, adjustments for patient age, sex, body size, lung capacity, or respiratory condition were not required.

Patients’ safely

To ensure patients’ safety in this study, ongoing inclusion required that SpO2 be maintained at 95% or higher under all O2 delivery conditions.[8] Patients were withdrawn from the study if their SpO2 dropped below 94% while receiving O2, and the O2 flow rate was increased until the SpO2 exceeded 94%. Oral endotracheal intubation and ventilatory support were provided if SpO2 was ≤94%, even with an O2 flow rate of 10 L/min through a mask. In our system, 10 L/min was the maximum achievable O2 flow rate.

Statistical analysis

Excel 2019 software (Microsoft, Redmond, WA, USA) was used to calculate the mean and standard deviation. Student’s t-test was applied for comparisons of mean values between two groups, and the Steel–Dwass test was used for comparisons of mean values among three groups. A p < 0.05 was considered statistically significant.

RESULTS

Patients

Twenty-eight patients were unable to participate in the study because they required oral intubation before the study began. Only one woman with cerebral infarction was excluded from this study after being included; she was 104 years old, and her SpO2 dropped to 93% while receiving O2 through a mask at 3 L/min. The final study cohort comprised 110 patients (42 men and 68 women); 63 cerebral infarction, 29 intracerebral hemorrhage, 5 subarachnoid hemorrhage, 8 brain tumors, and 5 traumatic brain injury patients. The overall mean age of the patients was 74.9 ± 14.7 That of men was 74.1 ± 15.8 years, and that of women 75.4 ± 14.2 years, without showing a statistical difference.

SaO2 in room air and under receiving O2 through a mask or a nasal cannula

SaO2 was significantly higher under O2 delivery through either a nasal cannula or a mask compared with in-room air. At O2 flow rates of 2 and 3 L/min, SaO2 was significantly higher when O2 was delivered by a mask than by a nasal cannula (room air vs. nasal p < 0.01; room air vs. p < 0.01; nasal cannula vs. p < 0.01). Even at an O2 flow rate of 1 L/min, SaO2 was highest when delivered by a mask; however, there was no statistically significant difference between nasal cannula and mask (room air vs. nasal p < 0.01; room air vs. p < 0.01; nasal cannula vs. p > 0.05) [Figure 3]. These results indicate that even a flow rate as low as 1 L/min yields a significantly higher SaO2 than in room air. Moreover, it is reasonable that low-flow O2 administration at 1 L/min through nasal cannula is beneficial for patients receiving home O2 therapy.

Differences in PCO2 and PO2 between room air and O2 delivery through a nasal cannula or a mask

PCO2 values in room air and under O2 administration of 1, 2, or 3 L/min through either a nasal cannula or a mask did not differ significantly (room air vs. nasal p > 0.05; room air vs. p > 0.05; nasal cannula vs. p > 0.05). PCO2 was higher with a mask than with a nasal cannula in 49 participants (44.5%) at O2 flow rate of 3 L/min, in 66 participants (60%) at 2 L/min, and in 69 participants (62.7%) at 1 L/min [Figure 4]. The PCO2 tended to increase as the O2 flow rate decreased during mask delivery; however, the increases were small and did not reach statistical significance when compared with nasal cannula O2 delivery.

In contrast, marked differences in PO2 were observed among room air, nasal cannula O2, and mask O2 delivery. At O2 flow rates of 1, 2, and 3 L/min, both nasal cannula and mask O2 administration resulted in significant increases in PO2 compared with room air. Mask O2 delivery achieved substantially greater increases in PO2 than nasal cannula delivery at 2 and 3 L/min (room air vs. nasal p < 0.01; room air vs. p < 0.01; nasal cannula vs. p < 0.01). Even at the low flow rate of 1 L/min, a significantly higher increase in PO2 was produced by mask O2 delivery (room air vs. nasal p < 0.01; room air vs. p < 0.01; nasal cannula vs. p < 0.05). At an O2 flow rate of 3 L/min, PO2 was higher with mask O2 delivery than with nasal cannula in all 110 participants. However, five participants (4.5%) demonstrated higher PO2 with a nasal cannula at 2 L/min, and 21 participants (19.1%) at 1 L/min.

Closed- and open-mouth breathing

There is no clear definition of open-mouth breathing. In this study, open-mouth breathing was defined as breathing with the mouth open and primarily inhaling through the mouth, whereas closed-mouth breathing was defined as consistently breathing with the mouth closed. Patients who opened their mouths slightly and inhaled air through both the mouth and nose were categorized as open-mouth breathers. Closed-mouth breathing was observed in 74 patients (67.3%), and open-mouth breathing in 36 patients (32.7%). When O2 was delivered through a nasal cannula, the mean PO2 at O2 flow rates of 1, 2, and 3 L/min tended to be higher in patients with closed-mouth breathing; however, these differences were not statistically significant. Conversely, when O2 was delivered through a mask, mean PO2 values at O2 flow rates of 1, 2, and 3 L/min tended to be higher in open-mouth breathers, although these differences were also not significant [Figure 5].

DISCUSSION

The efficacy of O2 therapy depends on FiO2, which is influenced by both the O2 flow rate and delivery method.[14] Several researchers have attempted to directly measure FiO2 in the oral cavity and pharynx, and some have reported successful measurements of FiO2 with different O2 delivery systems and varying O2 flow-rates using various methods.[6,8,11,15,18,22,23,27-29] Nasal cannulas have been reported to achieve inspired O2 concentrations ranging from 25% to 39% at flow rates of 1–4 L/min.[11] However, the accuracy of the measured FiO2 in a space where inspiration and expiration are constantly changing remains contentious.

Moreover, FiO2 depends on several factors, including breathing pattern, oral cavity volume, and tidal volume, all of which vary among individuals.[19,29] Another limitation is the difficulty in precisely measuring oral cavity O2 concentration in a person with closed-mouth breathing. Therefore, we considered it more practical to evaluate changes in SaO2, PCO2, and PO2 rather than to measure FiO2 in the oral cavity or pharynx. This study is unique in that SaO2, PCO2, and PO2 were measured in the same patients with different O2 delivery devices at various O2 flow rates.

In this study, as the O2 flow rate decreased, PCO2 with mask O2 delivery tended to increase slightly, suggesting the possibility of re-inhalation of exhaled CO2 within the mask or entrainment of air around the mask; however, the degree of this increase is negligible. Most importantly, contrary to the traditionally accepted theory, O2 delivery through a mask achieved significantly better arterial oxygenation than administration through a nasal cannula without increasing PCO2, even at a low O2 delivery flow rate of 3 L/min or less. At an O2 flow rate of 3 L/min, 100% of patients demonstrated higher PO2 with a mask than with a nasal cannula, as did 95.5% of patients at 2 L/min and 80.1% at 1 L/min.

Before initiating this study, the authors postulated that, when O2 is delivered at low flow rates, a mask would be preferable for patients with open-mouth breathing, whereas a nasal cannula would be more suitable for those with closed-mouth breathing.

However, the findings indicated that this assumption was incorrect. One advantage of open-mouth breathing is that the larger oral and pharyngeal spaces function as an O2 reservoir.[28,29] People breathe through an open mouth after exercise because this facilitates inhalation and exhalation of larger volumes of air with each breath. Whether FiO2 differs between open- and closed-mouth breathers during O2 delivery through a nasal cannula has not been definitely elucidated. Some of the researchers have mentioned that there is no significant difference in FiO2 between open- and closed-mouth breathers,[11,15] whereas others have reported higher inspired O2 fractions in open-mouth breathers.[6,23,28] Yamamoto et al. reported that open-mouth breathing results in a significantly higher FiO2 than closed-mouth breathing when O2 is administered through a pharyngeal cannula.[29] In the present study, mean PO2 was slightly higher during closed-mouth breathing when O2 was administered through a nasal cannula at flow rates from 1 to 3 L/min; however, there were no significant differences in PO2 between closed- and open-mouth breathers. In contrast, mean PO2 was slightly higher in open-mouth breathers when O2 was delivered through a mask at flow rates from 1 to 3 L/min, although no significant differences were detected in PO2 between closed- and mouth-open breathing patterns. In conclusion, under low-flow O2 rates at <4 L/min, the patient’s breathing pattern is not a critical factor in the choice between a mask and a nasal cannula.

Based on reports showing no significant differences in mortality rates or disability at 3 or 12 months when O2 therapy was administered to normoxic patients with either acute cerebral stroke or severe traumatic brain injury, routine O2 administration is not necessarily recommended, even during the prehospital phase of care.[12,25] Low-flow O2 therapy is recommended to prevent cerebral hypoxia that worsens outcomes when SpO2 falls below 94% in stroke patients and below 90% in traumatic brain injury patients.[10,13] However, a recent recommendation suggests that all patients with brain injury should receive supplemental O2 in the prehospital setting, regardless of their baseline SpO2, to reduce secondary hypoxic insults.[13] Moreover, our study addresses unmet needs in neurotrauma centers within the framework of the Dasic matrix.[5] Specifically, it provides clinical evidence to guide the choice between a face mask and a nasal cannula when administering low-flow O2 to comatose patients. Our findings indicate that, when low-flow O2 is administered to improve SaO2 and PO2, mask delivery is more effective than nasal cannula delivery, although low-flow O2 administration through a nasal cannula can also contribute to significantly better SaO2 and PO2 compared with no O2 supplementation. In patients with impaired consciousness, the commonly accepted theory that nasal cannula O2 delivery is superior to mask delivery during low-flow O2 administration because of inhalation of accumulated CO2 within the mask is not supported by our results.

Patients with impaired consciousness are less likely to remove a mask voluntarily; however, masks are frequently removed temporarily for oral suctioning and oral care. Therefore, mask O2 delivery is recommended when maximizing oxygenation is the highest priority for a patient, whereas nasal cannula O2 delivery is preferable for patients who require only modest improvement in oxygenation and frequent suctioning without interrupting O2 administration.

In this study, different O2 administration methods were applied to the same individual. However, this study has limitations in drawing definitive conclusions from the data because of the small sample size (110 cases) and the lack of analysis of factors affecting respiratory function, such as respiratory rate, smoking history, anemia, and congestive heart failure.[16]

CONCLUSION

When low-flow O2 delivery is necessitated for patients with disturbed consciousness, administration through a mask provides better oxygenation acquiring higher PO2 level than delivery through a nasal cannula. However, a disadvantage of mask use is that it must be removed temporarily during oral care and suctioning procedures. Therefore, the choice between a mask and a nasal cannula should be based on the patient’s clinical condition.