Intratracheal oxygen administration increases FIO2 and PaO2 compared with intranasal administration in healthy, standing horses

Dario Floriano Department of Clinical Studies–New Bolton Center, School of Veterinary Medicine, University of Pennsylvania, Kennett Square, PA

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Amanda R. Watkins Department of Clinical Studies–New Bolton Center, School of Veterinary Medicine, University of Pennsylvania, Kennett Square, PA

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Klaus Hopster Department of Clinical Studies–New Bolton Center, School of Veterinary Medicine, University of Pennsylvania, Kennett Square, PA

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Abstract

OBJECTIVE

To evaluate the efficacy of 2 different oxygen delivery strategies—intranasal and tracheal insufflation—on the inspired fraction of oxygen (FIO2) in standing horses and to determine the time needed for arterial oxygen partial pressure (PaO2) equilibration.

ANIMALS

6 healthy adult horses.

PROCEDURES

In this blinded, randomized crossover design study, horses were randomly assigned to receive oxygen via nasal cannula (group N) or transcutaneous tracheal catheter (group T). After placement of venous and arterial catheters, FIO2 was measured through a catheter placed into the distal portion of the trachea. After baseline measurements were obtained, horses received oxygen at up to 25 mL/kg/min for 1 hour via either intranasal or intratracheal catheter. The FIO2 and PaO2 were recorded at 5, 10, 15, 20, 25, 30, 45, and 60 minutes during and 5, 10, 15, 20, and 30 minutes after oxygen insufflation. Data were analyzed by use of a 2-way repeated measures ANOVA with Tukey-Kramer post hoc testing for pairwise comparisons (P < 0.05).

RESULTS

During oxygen administration, FIO2 and PaO2 increased significantly when compared with baseline, resulting in significantly higher values for group T (37.7 ± 2.4%; 214.6 ± 18 mm Hg) than for group N (34.3 ± 3.9%; 184.1 ± 11 mm Hg). The equilibration time was less than 10 minutes.

CLINICAL RELEVANCE

Intratracheal oxygen administration resulted in better oxygenation than nasal insufflation and should therefore be considered in standing horses that are experiencing severe respiratory compromise. The equilibration between FIO2 and PaO2 is rapid in adult horses.

Abstract

OBJECTIVE

To evaluate the efficacy of 2 different oxygen delivery strategies—intranasal and tracheal insufflation—on the inspired fraction of oxygen (FIO2) in standing horses and to determine the time needed for arterial oxygen partial pressure (PaO2) equilibration.

ANIMALS

6 healthy adult horses.

PROCEDURES

In this blinded, randomized crossover design study, horses were randomly assigned to receive oxygen via nasal cannula (group N) or transcutaneous tracheal catheter (group T). After placement of venous and arterial catheters, FIO2 was measured through a catheter placed into the distal portion of the trachea. After baseline measurements were obtained, horses received oxygen at up to 25 mL/kg/min for 1 hour via either intranasal or intratracheal catheter. The FIO2 and PaO2 were recorded at 5, 10, 15, 20, 25, 30, 45, and 60 minutes during and 5, 10, 15, 20, and 30 minutes after oxygen insufflation. Data were analyzed by use of a 2-way repeated measures ANOVA with Tukey-Kramer post hoc testing for pairwise comparisons (P < 0.05).

RESULTS

During oxygen administration, FIO2 and PaO2 increased significantly when compared with baseline, resulting in significantly higher values for group T (37.7 ± 2.4%; 214.6 ± 18 mm Hg) than for group N (34.3 ± 3.9%; 184.1 ± 11 mm Hg). The equilibration time was less than 10 minutes.

CLINICAL RELEVANCE

Intratracheal oxygen administration resulted in better oxygenation than nasal insufflation and should therefore be considered in standing horses that are experiencing severe respiratory compromise. The equilibration between FIO2 and PaO2 is rapid in adult horses.

Introduction

Establishing an airway, facilitating ventilation, and providing oxygen are the key features of emergency resuscitation of patients presenting with respiratory tract diseases.1 Supplemental oxygen therapy is one of the most used interventions when caring for hypoxemic patients, both in human and veterinary medicine.1,2 In equine medicine specifically, supplemental oxygen is used frequently during the perianesthetic period, when treating cases of pneumonia,3 and in smoke inhalation.4 Many of these compromised patients are oxygen dependent and therefore can benefit from treatment with an increased inspired fraction of oxygen (FIO2).5 Clinically, an increased FIO2 causes improved arterial oxygen tension as well as arterial oxygen saturation, which results in higher tissue oxygen delivery.5

The efficacy of oxygen therapy depends on the FIO2, which is dictated by the oxygen supply and delivery method.6 In human medicine, noninvasive techniques such as blow-by or wafting oxygen can be provided via oxygen tubing or a simple mask.7 In adult horses, this technique is often not feasible as untrained horses will rarely tolerate a mask. Therefore, in both foals and adult horses, placement of a nasal cannula is the most common option for oxygen delivery,1 even though the effect on the FIO2 is not well studied. Wilson et al8 demonstrated that the FIO2 measured in adult horses with a single nasal cannula and oxygen delivered at 15 L/min (approx 30 mL/kg) increased up to 50%, while bilateral nasal cannulas and oxygen flow rates of 30 L/min resulted in an increase of up to 70%.8

Transtracheal catheter systems are more invasive methods for oxygen delivery that have been described.9 These systems deliver oxygen at the level of the carina, reducing the oxygen flow rates necessary to achieve adequate oxygenation.9 In hypoxemic foals, transtracheal catheter oxygenation resulted in more improvement of the arterial oxygen partial pressure (PaO2) when compared with nasal insufflation.10 The authors discussed that the distal location of the catheter tip in the trachea may have resulted in a reduction of dead space and therefore increased FIO2; however, this parameter was not measured in the study.10

To our knowledge, no literature exists that measures and compares the effects of oxygen insufflation through nasal cannulae or transtracheal catheters on the actual FIO2 in standing adult horses. This information may prove useful in evaluating a patient’s oxygenation status while being treated with supplemental oxygen and in grading the severity of the pulmonary disease.11,12 Additionally, an understanding of the time required for PaO2 to reach equilibrium after changes in the FIO2 is helpful when a patient’s clinical response to oxygen therapy is evaluated.13

The goal of this study was to evaluate the effects of 2 different oxygen delivery strategies—intranasal and intratracheal insufflation—on the FIO2 in standing horses. The second goal was to measure the time needed for the PaO2 to equilibrate following changes in the FIO2.

We hypothesized that FIO2, measured at the level of the thoracic inlet, would be affected by the oxygen delivery system and that intratracheal administration would result in higher FIO2 values than the intranasal oxygen administration. We further hypothesized that the equilibration time for the PaO2, after changing the FIO2, would be less than 10 minutes.

Materials and Methods

Animals

Six horses including 3 Thoroughbreds, 1 Percheron, 1 crossbred horse, and 1 Quarter Horse (4 mares and 2 geldings) with a mean age of 12 years (range, 3 to 22 years) and body weight of 545 kg (range, 410 to 680 kg) were included in the study. The horses were considered healthy on the basis of history and physical examination and routine hematological and biochemical blood examination. The study was approved by the Institutional Animal Care and Use Committee (approval No. aaebbic 806698).

Instrumentation

The hair was clipped, and after an aseptic preparation, 2% lidocaine was infiltrated into the subcutaneous tissues over the left jugular vein and a 14-gauge catheter was placed. Horses were sedated by use of xylazine (0.5 mg/kg, IV), restrained by an experienced handler using an appropriate halter, and equipped with all catheters in the same way for both experiments: the area over the left or right transverse facial artery was clipped and cleaned. After local desensitization with 2% lidocaine (0.3 mL), a 20-gauge catheter was placed into the transverse facial artery and used for collection of arterial blood samples.

Aseptic preparation of the ventral neck and local infiltration with lidocaine was performed to place a transtracheal tube for oxygen administration and a transtracheal catheter for FIO2 measurement. A vertical stab incision was made approximately 10 cm caudal to the larynx, and the soft tissues were bluntly dissected away from the trachea. A No. 10 blade was used to create a stab incision through the tracheal annular ligament between 2 cartilaginous rings. A 12 Fr polyvinyl Levin Tube was inserted between the tracheal rings and directed distally in the tracheal lumen.

A 14-g Teflon over-needle catheter was placed approximately 20 cm caudal to the oxygen delivery tube and advanced caudally into the trachea. An 8.5 Fr (external diameter) transtracheal oxygen catheter was connected to a multiparameter monitor for inspiratory and expiratory oxygen concentration measurements.

A nasal cannula (internal diameter, 7.1 mm) was inserted via the nostrils until the tip was located approximately at the level of the medial canthus of the eye, as determined by prior measurement.

Experimental design

A blinded, randomized crossover design study was used. Horses were assigned using a computer-generated randomization list (www.randomization.com) to receive oxygen via either nasal cannula (group N) or intratracheal catheter (group T). After a minimum washout period of 1 week, the experiment was repeated and horses received oxygen via the other delivery system.

Following baseline measurements, horses received oxygen via the respective delivery system over a period of 1 hour starting with a fresh gas flow of 5 L/min for the first 10 minutes, 10 L/min for the following 10 minutes, and 15 L/min for the remaining 40 minutes.

Arterial samples were obtained and immediately analyzed prior to oxygen supplementation as a baseline. During insufflation, samples were obtained at 5, 10, 15, 20, 25, 30, 45, and 60 minutes, as well as 5, 10, 15, 20, and 30 minutes after insufflation.

Heart rate, respiratory rate, FIO2, inspired fraction of CO2, PaO2 and arterial carbon dioxide partial pressure were recorded at the aforementioned time points.

Statistical analysis

Data were assessed for normality by use of visual assessment of the q-q plots of residuals and a Shapiro-Wilk test. When normally distributed, data were reported as mean ± SD.

Influence of location of administration of oxygen (nasal vs tracheal) was analyzed by use of a 2-way repeated measures ANOVA with Tukey-Kramer post hoc testing for pairwise comparisons. Significance was set at P < 0.05.

Results

All horses tolerated placement of the nasal cannula, transtracheal catheter, and instrumentation well and stood quietly throughout the study period.

The analyzed results for FIO2 and PaO2 are summarized (Table 1).

Table 1

Mean and SD of inspired fraction of oxygen (FIO2) and arterial oxygen tension (PaO2) achieved with 2 oxygen delivery systems (intratracheal vs intranasal insufflation) and 3 different oxygen flow rates (5, 10, and 15 L/min) in 6 healthy horses at different time points; the level of significance (sig) is at P < 0.05.

FIO2 (%) PaO2 (mm Hg)
Tracheal Nasal Tracheal Nasal
Variable Mean SD Sig Mean SD Sig Mean SD Sig Mean SD Sig
T0 20.2 0.4 20.3 0.5 96.9 2.7 96.1 3.3
T5 O2 5 L/min 25.9 1.5 a 24.5 1.2 a 141.6 8.4 *,a 120.3 11.8 *,a
T10 O2 5 L/min 26.2 2.9 a 24.6 1.7 a 134.9 5.6 *,a 127.2 13 *,a
T15 O2 10 L/min 29.8 1.2 a 27.7 5.3 a 177.3 12.4 *,a,b 153.7 13.7 *,a,b
T20 O2 10 L/min 31.3 1.3 a 27.9 4.5 a 177.7 10.3 *,a,b 155.4 12 *,a,b
T25 O2 15 L/min 36.4 1.9 *,a,b 33.8 3.3 *,a,b 207.1 15.1 *,a,b,c 177.7 14 *,a,b,c
T30 O2 15 L/min 36.8 2.9 *,a,b 34.2 2.1 *,a,b 201.6 19.7 *,a,b,c 183.8 17.5 *,a,b,c
T45 O2 15 L/min 37.4 1.7 *,a,b 34.5 2.3 *,a,b 209.7 14.3 *,a,b,c 180.7 13.8 *,a,b,c
T60 O2 15 L/min 37.7 2.4 *,a,b 34.3 3.9 *,a,b 214.6 18 *,a,b,c 184.1 11 *,a,b,c
T65 20.5 0.5 21 0 109.6 10.6 *,a 103.8 13 *
T70 20.5 0.5 20.6 0.5 95.1 5.5 97.6 8.7
T75 20.5 0.5 21 0 93 7 93.5 3.6
T90 20.3 0.5 21 0 97.1 6.3 96.3 3.1

*Significantly different between groups.

a = Significantly different to baseline (T0) within the same group. b = Significantly different to 5L/min (T5) within the same group. c = Significantly different to 10 L/min (T15) within the same group.

Oxygen supplementation resulted in a significant increase in FIO2 and PaO2 when compared with baseline (T0, no oxygen supplementation) in both groups. In group N, there was no significant increase in FIO2 or PaO2 when oxygen flow rate was increased from 5 to 10 L/min, but increasing the flow to 15 L/min resulted in significantly higher FIO2 and PaO2 values compared with 5-L/min flow rates. In group T, increasing the oxygen flow rate from 5 to 10 L/min as well as increasing from 10 to 15 L/min resulted in significant changes in FIO2 and PaO2. The PaO2 always adjusted within the first 5 minutes of flow rate changes.

When comparing the groups, the FIO2 was statistically higher in group T compared with group N at 25, 30, 45, and 60 minutes during O2 insufflation (15 L/min). The highest inspired O2 concentration in group N was 34.3% and in group T was 37.7% (P = 0.026). The PaO2 was significantly higher in group T compared with group N at all time points during O2 insufflation as well as 5 minutes after insufflation ended.

The expiratory CO2 values (mean group T, 41 ± 3 mm Hg; mean group N, 40 ± 3 mm Hg) as well as the arterial carbon dioxide partial pressure values (mean group T, 45 ± 2 mm Hg; mean group N, 45 ± 3 mm Hg) did not change over the study period and were not different between treatment groups.

Heart rate (mean, 34 bpm; range, 28 to 40 bpm) and respiratory rate (mean, 7 bpm; range, 4 to 12 bpm) were always within normal limits and not different over time or between groups.

Discussion

In this study, both supplemental oxygen delivery systems were effective in increasing FIO2 and PaO2 in standing horses. As hypothesized, the efficacy in increasing the FIO2 was affected by the location of oxygen supplementation source with intratracheal administration resulting in higher FIO2 compared with intranasal insufflation. Further, the oxygen flow rate had a significant influence on the FIO2.

Wilson et al8 showed that, in horses, oxygen delivered via a nasal cannula at 15 L/min resulted in an FIO2 of up to 50%, while in the present study nasal insufflation failed to increase FIO2 over 35%. Unless intubated and attached to a breathing circuit, regardless of what type of supplemental oxygen delivery device is used, the patient is not breathing the pure oxygen solely, but rather a combination of room air plus the oxygen fraction from the supplemental device. The final FIO2 depends on multiple factors such as respiratory rate, minute ventilation, degree of dead space, ventilation-perfusion mismatch, positioning of the nasal cannula, patient’s body weight, and the relative duration of inspiration and expiration. All of these factors are possible explanations for the differences in FIO2 seen between studies. As the minute ventilation was not measured in this study, it is possible that this population of horses had a higher minute ventilation leading to more dilution of the inspiratory oxygen supplementation.

The horses in this study had a mean body weight of approximately 545 kg, and horses in this weight range are reported to have a minute ventilation of approximately 50 to 60 L/min.14,15 Therefore, a supplementation of 15 L of pure oxygen/min would have contributed approximately 25% of the total minute ventilation, which should have resulted in an FIO2 of approximately 40%.16 However, given the dynamics of ventilation and the nonlinearity of inspiration, a 15-L/min oxygen flow rate results in approximately 0.25 L/s, compared with the mean inspiratory flow rate of approximately 3 L/s in a normal horse at rest.16 For this and other reasons, oxygen delivery from a nasal cannula is considered unsatisfactory in human medicine if precise control of inspired oxygen is desired.17

In this study, intratracheal administration of O2 resulted in higher FIO2 values and consequently higher PaO2, compared with intranasal administration. These findings are in alignment with studies previously published in hypoxemic foals and human patients.10,18 The higher FIO2 values are attributed to the location of the transtracheal catheter tip at the level of the carina, thus bypassing a significant volume of dead space.10 Further, insufflation of oxygen near the carina and reduction of the anatomic dead space can improve alveolar ventilation and thereby decrease the work of breathing.9,18 Even though calculation of venous admixture was not performed in this study, a linear increase in PaO2 was observed as FIO2 increased. Additionally, the results of PaO2:FIO2 ratio, when O2 was administered, showed values > 500. These findings are consistent with what is reported for healthy horses, in which venous admixture is normally less than 5%.8,19 This is another indicator that the higher PaO2 values in group T horses was due to the higher FIO2 achieved.

No complications occurred during or after placement of the catheter and administration of oxygen from the beginning until the end of the experiment. Although placement of the catheter was initially more invasive than nasal insufflation, it was well tolerated by all the horses and only mild bleeding, swelling, and subcutaneous emphysema occurred following insertion of the tracheal catheters, all of which were self-limiting.

In agreement with previous studies that investigated the equilibration time for PaO2 after changes in FIO2 in horses,8,20,21 the rise in FIO2, following oxygen administration, occurred rapidly and did not persist when oxygen delivery changed or stopped. In this study, equilibration time for the PaO2 was less than 5 minutes after changing the FIO2. This was demonstrated by the lack of difference between arterial samples obtained and analyzed 5 and 10 minutes after changing the flow rates and the FIO2, respectively. This is particularly relevant for clinical scenarios in which a rapid response to oxygen therapy is desired or when patients are weaned off oxygen insufflation therapy.22

This study had several limitations. Only systemically healthy horses were used in this study. It has been shown that horses affected by recurrent airway obstruction have a higher respiratory frequency and higher peak inspiratory flow rates than healthy control horses; therefore, the FIO2 of recurrent airway obstruction–affected animals is lower than that of healthy animals when breathing room air.8 Further, human patients with chronic obstructive pulmonary disease experience equilibration times at a slower rate.22 Evaluation of the PaO2/FIO2 relationship for a prolonged period and in a broader range of patients will be necessary to improve the understanding of diagnostic tools and treatment end points.11

We chose to administer oxygen with rates up to 15 L/min. As shown by Wilson et al,8 oxygen flow rates up to 30 L/min can be used to reduce dilution of minute ventilation with room air and further increase FIO2. However, in the same study, occasional coughing and gagging became evident with high oxygen flow rate, likely due to irritation of the upper airways. It is, however, likely that with higher flow rates, higher FIO2 would have been achieved, which might have overcome the disadvantages of the intranasal administration.

This study protocol included a stepwise increase in oxygen flow rate every 10 minutes, maintaining the highest flow rate (15 L/min) for 40 minutes for a total oxygen insufflation time of 60 minutes. When oxygen flow rates were changed, equilibrium in FIO2 values was reached after 5 minutes. This is in alignment with other studies looking at the equilibration time during O2 administration.8,20,23 However, it cannot be excluded that longer insufflation periods would have impacted ventilation on the alveolar level due to resorption atelectasis formation induced by over-physiological oxygen concentrations.24

And at last, sedation with α2 agonists was necessary for the catheter placement in all our horses, which could have had an effect on gas exchange as xylazine has been shown to have temporal effects on the ventilatory function in horses, inducing mild hypoventilation and changes in alveolar perfusion.25 However, a low dose of xylazine was used for sedation and the time between sedation and the first measurement was approximately 45 minutes. Xylazine has a relatively short elimination half-life as well as duration of action in horses.26,27 Therefore, although we cannot rule out that the sedation affected the respiratory response to oxygen therapy, this is unlikely for our study setup.

In conclusion, both the intranasal as well as the intratracheal oxygen administration resulted in a significant increase in FIO2 and PaO2 in healthy, spontaneously breathing horses. Intratracheal oxygen administration resulted in higher FIO2 and consequently higher PaO2 values and should therefore be considered in horses with severe respiratory compromise. As in previous studies, less than 10 minutes was sufficient for PaO2 to equilibrate to new FIO2.

Acknowledgments

Funded by the Raymond Firestone Trust and Tamworth Research Grant.

The authors declare that there were no conflicts of interest.

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