Discussion
In our experiment, we developed an expiratory gas modification bellow,
which modified the expiratory flow during the exhalation phase. This
model was developed to sample and measure tracheal oxygen concentrations
with various breathing patterns of both VT and frequency
under normal, restrictive and obstructive lung mechanics. In addition,
we provided equations for estimating inspired oxygen in the trachea for
clinical reference.
During oxygen therapy with nasal cannula, the rule of thumb is that for
patients with a normal rate and depth of breathing, each 1L/min of
oxygen supplied increases FIO2 by approximately
4%;1,2,4 therefore, the expected delivery is a
FIO2 of 0.24–0.44. However, if the patient’s f
approaches or exceeds 20breaths/min, FIO2 will likely be
well below estimates, and the delivered FIO2 will only
increase by approximately 0.025 for each 1L/min above the ambient oxygen
level.1,31 In those studies, FIO2 was
measured by an oxygen analyzer placed in the low airway, such as an
oxygen analyzer port of the TTL bellow, which is the distal airway. In
our experiment in which we designed the test lung, there was no effect
of fresh oxygen flow from the exhaled carbon dioxide because the
expiratory gas modification compartment was designed to produce a
modified expired gas. In addition, we measured FO2 insp.
and FO2 pre-insp. at the carina, which is the proximal
airway. Lower FO2 insp. and FO2pre-insp. values than estimated were found at all settings, which may be
due to differences in monitoring sites and the experimental design. By
contrast, Gibson et al. reported that in healthy subjects with a
percutaneously placed tracheal sensing catheter, the highest absolute
inspired tracheal oxygen concentration with a nasal cannula was 23.6%
at 3L/min and 25.4% at 5L/min during normal breathing
[VT:690mL, f:17breaths/min, minute ventilation
(MV):11L/min, peak inspiratory flow rate (PIFR):37L/min], 22.4% at
3L/min and 23.8% at 5L/min during quiet breathing
(VT:400mL, f:16breaths/min, MV:6.4L/min, PIFR:21L/min),
and 22.7% at 3L/min and 25.2% at 5L/min during hyperventilation
(VT:1400mL, f:14breaths/min, MV:19.5L/min, PIFR:63L/min)
.32 In another report, in patients with 97% oxygen
supply via nasal cannula, the effective FIO2 was 22.8 ±
0.1%, 27.6 ± 0.5% and 31.8 ± 0.5% with 1, 3 and 5L/min based on
trachea sampling, respectively.33 Our findings concur
reasonably well with those data, but contrast with a previously
published formula.
Oxygen concentration is also influenced by VT in our
model in the same sense as previously described by Chikata et al., who
reported that statistically significantly changes in VTaffected measured FIO2 at all flow levels: at 2L/min and
VT of 300, 500 and 700mL, FIO2 was 0.37
± 0.01%, 0.32 ± 0% and 0.29 ± 0%, respectively; at 4L/min,
FIO2 was 0.45 ± 0.01%, 0.39 ± 0.01% and 0.34 ± 0%,
respectively.34 Indeed, various authors have reported
decreasing FIO2 values during increasing MV and
f.24 Our model clearly demonstrates that changes in
VT affect measured oxygen concentration. Specifically,
as VT increases with a fixed f in the normal,
restrictive and obstructive lung models, there is a reduction in the
effective oxygen concentration due to more room air inhalation and
dilution of the oxygen concentration from the nasal cannula at all flow
levels. For example, the measured FO2 insp. in normal
lung mechanics at a flow rate of 1L/min, VT of 300mL and
f of 10breaths/min was 22.7 ± 0.10%, whereas it was 22.33 ± 0.15% and
21.60 ± 0.20% at VT of 500mL and 700mL, respectively.
Our study found that FO2 insp. and FO2pre-insp. differed in different respiratory cycles. During the
inspiratory phase, FO2 insp. measured at the carina was
significantly lower than that measured during the expiratory period. In
the previously described by O’Reilly-Nugent et al, the variability of
FIO2 in different respiratory cycles during low-flow
nasal cannula at oxygen flow rates of 2–4L/min. Their novel method of
FIO2 measurement involved sampling via a catheter placed
at the distal trachea; this has clarified the uncertainties of other
studies that were sampled near the proximal airway where inadequate gas
mixing had occurred. The researchers found that because of the bolus of
oxygen from the nasopharynx or relatively low inspiratory flow at the
beginning of each breath, the peak FIO2 is significantly
higher than the remainder of inspiration and then declines rapidly as
the inspiratory flow reaches its peak. After inspiration,
FIO2 then increases steadily, coinciding with a
reduction in inspiratory flow.35 Conversely, the
oxygen delivered from the nasal cannula is inhaled into the lungs for
gas exchange, and the oxygen concentration measured at the carina is
relatively low. However, during the expiratory period, carbon dioxide in
the lungs is exhaled, but oxygen inhalation does not occur. Therefore,
during this period, the measured oxygen concentration is higher, which
is most pronounced particularly between the end-expiratory period and
the next inspiratory period. Thus, any variation in the total expiratory
time affects the composition of the next
inspiration.4,21,27
Finally, our study provides scientific data that f, VTand oxygen flow rate significantly influence the inspired oxygen
concentration under different lung mechanics. In addition, during
spontaneous breathing, we have successfully proposed equations for
clinical practice for nasal cannula oxygen therapy. These equations are
based on different lung mechanics, VT and f to estimate
the actual patient oxygen concentration; they will provide clinicians a
reference when using nasal cannula among patients with normal,
restrictive and obstructive lung diseases.
Although we devised a spontaneously breathing lung model with three lung
mechanics conditions and different VT and f, but our
study was limited to did not consider open or closed mouth breathing
states even though oxygen concentration is influenced by the reservoir
space in the oral cavity during nasal cannula. In addition, we did not
investigate the effect of water vapours on oxygen concentration.