Relationship between arterial carbon dioxide and end-tidal carbon dioxide when a nasal sampling port is used

Autonomous control of ventilation through closed-loop adaptive respiratory pacing

Mechanical ventilation is the standard treatment when volitional breathing is insufficient, but drawbacks include muscle atrophy, alveolar damage, and reduced mobility. Respiratory pacing is an alternative approach using electrical stimulation-induced diaphragm contraction to ventilate the lung. Oxygenation and acid-base homeostasis are maintained by matching ventilation to metabolic needs; however, current pacing technology requires manual tuning and does not respond to dynamic user-specific metabolic demand, thus requiring re-tuning of stimulation parameters as physiological changes occur. Here, we describe respiratory pacing using a closed-loop adaptive controller that can self-adjust in real-time to meet metabolic needs. The controller uses an adaptive Pattern Generator Pattern Shaper (PG/PS) architecture that autonomously generates a desired ventilatory pattern in response to dynamic changes in arterial CO₂ levels and, based on a learning algorithm, modulates stimulation intensity and respiratory cycle duration to evoke this ventilatory pattern. In vivo experiments in rats with respiratory depression and in those with a paralyzed hemidiaphragm confirmed that the controller can adapt and control ventilation to ameliorate hypoventilation and restore normocapnia regardless of the cause of respiratory dysfunction. This novel closed-loop bioelectronic controller advances the state-of-art in respiratory pacing by demonstrating the ability to automatically personalize stimulation patterns and adapt to achieve adequate ventilation.

A comparative evaluation of capnometry versus pulse oximetry during procedural sedation and analgesia on room air

Objective:

Important questions remain regarding how best to monitor patients during procedural sedation and analgesia (PSA). Capnometry can detect hypoventilation and apnea, yet it is rarely used in emergency patients. Even the routine practice of performing preoxygenation in low-risk patients is controversial, as supplementary oxygen can delay the detection of respiratory depression by pulse oximetry. The purpose of this study was to determine whether the capnometer or the pulse oximeter would first detect respiratory events in adults breathing room air.

Methods:

During a randomized clinical trial comparing fentanyl with low-dose ketamine for PSA with titrated propofol, patients were monitored using pulse oximetry and continuous oral–nasal sampled capnography. Supplemental oxygen was administered only for oxygen desaturation. Sedating physicians identified prespecified respiratory events, including hypoventilation (end-tidal carbon dioxide > 50 mm Hg, rise of 10 mm Hg from baseline or loss of waveform) and oxygen desaturation (pulse oximetry < 92%). These events and their timing were corroborated by memory data retrieved from the monitors.

Results:

Of 63 patients enrolled, 57% (36) developed brief oxygen desaturation at some point during the sedation. All responded to oxygen, stimulation or interruption of propofol. Measurements of end-tidal carbon dioxide varied substantially between and within patients before study intervention. Hypoventilation (19 patients, 30%) was only weakly associated with oxygen desaturation (crude odds ratio 1.4 [95% confidence interval 0.47 to 4.3]), and preceded oxygen desaturation in none of the 12 patients in whom both events occurred (median lag 1:50 m:ss [interquartile range 0:01 to 3:24 m:ss]).

Conclusion:

During PSA in adults breathing room air, desaturation detectable by pulse oximeter usually occurs before overt changes in capnometry are identified.

A Portable Infection Screening System Designed for Onboard Entry Screening Based on Multi-Parameter Vital Signs

After outbreak of severe acute respiratory syndrome (SARS) in 2003, many international airport quarantines adopted fever-based screening to identify infected individuals using infrared thermography to control global pandemic. Unfortunately, the sensitivity of fever-based screening system did not exceed 70.4% at Narita International Airport. In order to achieve accurate onboard entry screening for highly contagious infectious diseases, the authors developed a portable system designed for onboard entry screening with linear discriminant analysis. Within several tens of seconds, the system automatically discriminates infected individuals from normal subjects using measured heart rate, respiratory rate, as well as facial surface temperature determined by thermography. The size of system is small enough to be placed on airplane tray tables. The authors tested on 68 subjects including 12 influenza patients to evaluate the system. The result showed sensitivity of 91.7% and specificity of 92.9%. The system seems to be promising for onboard infection screening to safeguard public health.

Noninvasive capnometry for end-tidal carbon dioxide monitoring via nasal cannula in nonintubated neonates

BACKGROUND

Arterial blood gas analysis is the gold standard for assessing the adequacy of ventilation. However, arterial blood sampling may be associated with serious complications in neonates. The aim of the study was to utilize the side-stream capnometry measurement of end-tidal carbon dioxide (PetCO₂) via nasal cannula circuits and to verify the reliability of PetCO₂ in reflecting the arterial blood carbon dioxide(PaCO₂) level in nonintubated neonates.

METHODS

A retrospective medical record review analysis was performed in nonintubated neonates admitted to the neonatal ward in a medical center. Simultaneous arterial PaCO₂ and PetCO₂ levels were evaluated. PaCO₂ and PetCO₂ levels were compared by paired t test and were correlated using Pearson's correlation. The PetCO₂ bias was defined as the difference between PaCO₂ and PetCO₂, and was assessed by Bland-Altman plot analysis.

RESULTS

A total of 34 neonates were recruited, and data of 54 pairs of PaCO₂ and PetCO₂ levels were available for comparison. The average (mean ± SD) gestational age was 32.5 ± 4.2 weeks, and the average birth weight was 1881 ± 1077 g. There was a good correlation between PetCO₂ and PaCO₂ levels among all paired samples (r = 0.809, p < 0.001). When the data were divided into those with respiratory disease (n = 34) and those without (n = 20), significant correlation between PetCO₂ and PaCO₂ levels were both noted in the former group (r = 0.823, p < 0.001) and the latter group (r = 0.770, p < 0.001). The overall average mean value of PetCO₂ was lower than that of PaCO₂ (39.4 ± 8.8 mmHg vs. 41.3 ± 9.2 mmHg, p = 0.014). The difference between PetCO₂ and PaCO₂ levels was significant only among those with respiratory disease (38.8 ± 9.8 mmHg vs. 41.2 ± 10.3 mmHg, p = 0.027), but not among those without (40.5 ± 7.0 mmHg vs. 41.6 ± 7.2 mmHg, p = 0.289).

CONCLUSIONS

End-tidal CO₂ measurement by side-stream capnometry through nasal cannula could provide an accurate and noninvasive estimate of PaCO₂ levels in nonintubated neonates.

Physiologic effects of nasopharyngeal administration of supplemental oxygen at various flow rates in healthy neonatal foals

OBJECTIVE

To evaluate the effects of various flow rates of oxygen administered via 1 or 2 nasal cannulae on the fraction of inspired oxygen concentration (FIO2) and other arterial blood gas variables in healthy neonatal foals.

ANIMALS

9 healthy neonatal (3- to 4-day-old) foals.

PROCEDURES

In each foal, a nasal cannula was introduced into each naris and passed into the nasopharynx to the level of the medial canthus of each eye; oxygen was administered at 4 flow rates through either 1 or both cannulae (8 treatments/foal). Intratracheal FIO2, intratracheal end-tidal partial pressure of carbon dioxide, and arterial blood gas variables were measured before (baseline) and during unilateral and bilateral nasopharyngeal delivery of 50, 100, 150, and 200 mL of oxygen/kg/min.

RESULTS

No adverse reactions were associated with administration of supplemental oxygen except at the highest flow rate, at which the foals became agitated. At individual flow rates, significant and dose-dependent increases in FIO2, PaO2, and oxygen saturation of hemoglobin (SaO2) were detected, compared with baseline values. Comparison of unilateral and bilateral delivery of oxygen at similar cumulative flow rates revealed no differences in evaluated variables.

CONCLUSIONS AND CLINICAL RELEVANCE

Results indicated that administration of supplemental oxygen via nasal cannulae appeared to be a highly effective means of increasing FIO2, PaO2, and SaO2 in neonatal foals. These findings may provide guidance for implementation of oxygen treatment in hypoxemic neonatal foals.

Capnometry in the prehospital setting: are we using its potential?

Capnometry is a non-invasive monitoring technique which allows fast and reliable insight into ventilation, circulation, and metabolism. In the prehospital setting it is mainly used to confirm correct tracheal tube placement. In addition it is a useful indicator of efficient ongoing cardiopulmonary resuscitation due to its correlation with cardiac output, and successful resuscitation. It helps to confirm the diagnosis of pulmonary thromboembolism and to sustain adequate ventilation in mechanically ventilated patients. In patients with haemorrhage, capnometry provides improved continuous haemodynamic monitoring, insight into adequacy of tissue perfusion, optimisation within current hypotensive fluid resuscitation strategy, and prevention of shock progression through controlled fluid administration.

The Utility of Supplemental Oxygen During Emergency Department Procedural Sedation and Analgesia With Midazolam and Fentanyl: A Randomized, Controlled Trial

STUDY OBJECTIVE

To determine whether supplemental oxygen reduces the incidence of hypoxia by 20% in study patients receiving midazolam and fentanyl for emergency department procedural sedation and analgesia.

METHODS

Patients were randomized to receive either supplemental oxygen or compressed air by nasal cannula at 2 L per minute. Physicians were blinded to the gas used and end-tidal carbon dioxide (ETCO2) data. Respiratory depression was defined a priori as oxygen saturation less than 90%, ETCO2 level greater than 50 mm Hg, an absolute change from baseline of 10 mm Hg, or loss of the ETCO2 waveform.

RESULTS

Of the 80 patients analyzed, 44 received supplemental oxygen and 36 received compressed air. Twenty supplemental oxygen patients and 19 compressed air patients met at least 1 criterion for respiratory depression. Six supplemental oxygen patients and 5 compressed air patients experienced hypoxia (P=.97; effect size 0%; 95% confidence interval -15% to +15%). Fourteen patients in each group met ETCO2 criteria for respiratory depression but were not hypoxic. Physicians identified respiratory depression in 8 of 11 patients who became hypoxic and 0 of 28 patients who met ETCO2 criteria for respiratory depression but who did not become hypoxic. There were no adverse events.

CONCLUSION

Supplemental oxygen did not reduce (or trend toward reducing) the incidence of hypoxia in patients moderately sedated with midazolam and fentanyl. However, our lower-than-expected rate of hypoxia limits the power of this comparison. Blinded capnography frequently identified respiratory depression undetected by the treating physicians.

Concordance between capnography and arterial blood gas measurements of carbon dioxide in acute asthma

STUDY OBJECTIVE

We examine the concordance between end-tidal partial pressure of CO2 (PetCO2) measured by capnography and arterial partial pressure of carbon dioxide (PaCO2) obtained by arterial blood gas in acute asthmatic patients presenting to the emergency department.

METHODS

This was a prospective observational cohort study of acutely ill adult asthmatic patients undergoing an arterial blood gas measurement as part of their evaluation. PetCO2 was recorded during exhalation into a capnograph while arterial blood was pulsing in the arterial blood gas tubing. Concordance between PetCO2 and PaCO2 was displayed as a Bland-Altman matrix, using prespecified limits of agreement of +/-5 mm Hg difference between PetCO2 and PaCO2 in each patient.

RESULTS

The mean difference between the PetCO2 and PaCO2 levels was 1.0 mm Hg (95% confidence interval -0.1 to 2.0 mm Hg), with a median of 0 mm Hg. Of the 39 patients enrolled, 37 (95%) fell within the a priori limits of agreement.

CONCLUSION

In adult asthmatic patients with acute exacerbations, concordance between PetCO2 measured by capnography and PaCO2 measured by arterial blood gas was high. These findings must be validated before capnography replacement of arterial blood gas as an accurate means of assessing alveolar ventilation in acute asthma is recommended.

Relationship between arterial and end-tidal carbon dioxide pressures during anesthesia using a laryngeal tube

BACKGROUND

The Laryngeal Tube (LT), (VBM Medizintechnik, Sulz, Germany) is a relatively new supraglottic device for controlling the airway. Arterial carbon dioxide tension (PaCO(2)) can be estimated by monitoring the end-tidal tension of carbon dioxide (PETCO(2)). The relationship between PETCO(2) and PaCO(2) during controlled ventilation via the LT has not been reported.

METHODS

During general anesthesia, 45 patients were mechanically ventilated using an LT. PETCO(2) and PaCO(2) were measured once PETCO(2) had reached a steady state. The LT was then removed and the trachea intubated using an endotracheal tube (ETT), and the identical ventilatory variables were resumed. Following stabilization, PETCO(2) was again determined and PaCO(2) estimated.

RESULTS

The mean PETCO(2) and PaCO(2) values were 4.43 +/- 0.26 kPa and 4.67 +/- 0.32 kPa, respectively, during LT ventilation, and 4.36 +/- 0.23 kPa and 4.61 +/- 0.26 kPa, respectively, during ETT ventilation. Analysis of differences between the PETCO(2) and PaCO(2) values using the Bland and Altman method revealed a bias +/- precision of 0.24 +/- 0.15 kPa for LT and 0.27 +/- 0.15 kPa for ETT. The root mean square error was 0.28 for the LT and 0.30 for the ETT.

CONCLUSION

This study suggests that for healthy adult patients mechanically ventilated via the LT, the PETCO(2) value reflects the PaCO(2) value as closely as when patients are ETT ventilated, allowing capnometry to be used to evaluate the adequacy of ventilation.

Abnormal ventilatory responses to hypoxia in Type 2 diabetes

AIMS

The incidence of Type 2 diabetes is increasing, along with its associated micro- and macrovascular disease manifestations. Previous studies indicate that patients with Type 2 diabetes exhibit abnormal cardiopulmonary reflex responses to various stimuli, although the impact of hypoxia, a common physiological stimulus, on ventilatory responses has not previously been studied in humans with Type 2 diabetes.

METHODS

Minute ventilation (V(E)) breathing pattern responses (total breath time, T(TOT); expiratory time, T(E); inspiratory time, T(I); inspiratory duty cycle, T(I)/T(TOT)) were measured during 5 min each of normoxia and isocapnic hypoxia (arterial O2 saturation approximately 85%) in eight subjects with Type 2 diabetes and seven age- and body mass index-matched healthy subjects.

RESULTS

During normoxia, V(E) was similar in control and diabetic subjects (6.4+/-1.2, 6.4+/-1.1 l/min, respectively). In response to hypoxia, V(E) significantly increased in both groups (to 17.0+/-5.0 and 9.5+/-2.0 l/min, respectively, P<0.05), but the magnitude of increase in V(E) was significantly less in diabetic than in control subjects (P<0.05). In addition, the breathing pattern response to hypoxia differed between groups in terms of T(I)/T(TOT) and T(TOT) (P<0.05), with control subjects significantly decreasing T(TOT) and T(E) (P<0.05) while diabetic subjects tended to increase both.

CONCLUSIONS

Relative to matched control subjects, Type 2 diabetic subjects exhibit blunted V(E) responses to acute isocapnic hypoxia, suggesting that this group of diabetic subjects possesses a chemoreflex ill-equipped to respond homeostatically to hypoxic challenge.

[Method for measuring respiration in sleep: capnography for determining ventilation]

Ventilation serves the exchange of gases between the organism and the environment. Oxygen uptake and CO2 elimination are controlled by feedback loops, that keep fluctuations in arterial CO2 pressure (PaCO2) within narrow limits Disorders in the central regulation of breathing, or impairment of the respiratory apparatus, may result in a mismatch between metabolic CO2 production and ventilatory CO2, elimination and thus in fluctuations in the PaCO2: inappropriately increased ventilation (hyperventilation) causes hypocapnia, and reduced ventilation (hypoventilation) causes hypercapnia. In order to detect such disorders during sleep, PCO2 measurement is of great importance, but direct and continuous measurement of the PaCO2 is invasive and thus unsuitable in the clinical setting. An alternative is capnography, the continuous measurement of PCO2 in inhaled and exhaled air on the basis of ultrared light absorption. This paper reviews the method, its features and limitations, and the possibilities of improving capnography to better detect sleep-related breathing disorders. In addition, data obtained from 57 patients with predominantly normal lung function, but suspected sleep disordered breathing are presented. Simultaneous measurements of capnography PETCO2) and capillary PaCO2 revealed a PETCO2 difference of +0.63 +/- 3.3 (SD) Torr. PaCO2 (38.8 +/- 4.1 Torr) and PETCO2 (38.1 +/- 4.3 Torr) were not significantly different with a correlation coefficient of r = 0.68 (p < 0.001). Thus 46% of the variation in PETCO2 was explained by changes in PaCO2. Currently the literature contains few further data on capnography during sleep. It is concluded that, provided the limitations of the method are respected and comparison with the PETCO2 is made, capnography may be a useful, noninvasive and continuous measuring method for assessing ventilation during sleep in patients with suspected sleep related breathing disorders.

End-tidal PCO2 monitoring via nasal cannulae in pediatric patients: accuracy and sources of error

OBJECTIVE

To assess the correlation and accuracy of end-tidal PCO2 (PetCO2) sampled via nasal cannulae in pediatric patients by comparison to the criterion standard PaCO2, and to identify sources of error during PetCO2 monitoring via nasal cannulae.

METHODS

PetCO2 was monitored continuously by sampling end-tidal gas through nasal cannulae that had been designed and manufactured for this purpose in spontaneously breathing children undergoing conscious or deep sedation during either cardiac catheterization (n = 43) or critical care (n = 54). When both the capnographic wave form and the PetCO2 value had been stable for at least 10 minutes, the PetCO2 value was recorded while blood was drawn from an indwelling arterial line for PaCO2 measurement. The effects of age, weight, respiratory rate, oxygen delivery system, airway obstruction, mouth breathing, and cyanotic heart disease were evaluated by linear regression analysis and calculation of absolute bias (PaCO2-PetCO2).

RESULTS

Mouth breathing, airway obstruction, oxygen delivery through the ipsilateral nasal cannula, and cyanotic heart disease adversely affected accuracy. In patients without those factors, PetCO2 correlated well with PaCO2 (R2 = 0.994), and absolute bias was 3.0 +/- 1.8 mmHg.

CONCLUSIONS

Several factors-some controllable and all recognizable-affect the accuracy of PetCO2 monitored via nasal cannulae in pediatric patients. When these factors are not present, PetCO2 correlates well with PaCO2 and appears to be a useful monitor of ventilatory status during conscious or deep sedation.

Evaluation of capnography in nonintubated emergency department patients with respiratory distress

OBJECTIVE

To evaluate the ability of noninvasive capnographic measurement of end-tidal CO2 tension (PetCO2) to predict arterial CO2 tension (PaCO2) in nonintubated ED patients with respiratory distress.

METHODS

A prospective, nonblind study was performed in a level I trauma center/community teaching hospital ED. Participants included all nonintubated adult patients with respiratory distress requiring measurement of arterial blood gases (ABGs); 29 patients were enrolled. PetCO2 was measured with a capnography monitor, using both baseline tidal volumes and forced expiratory volumes. The bias between PetCO2 values and simultaneous measurements of PaCO2 by ABG was assessed.

RESULTS

PetCO2, measured with forced expiration, and PaCO2 agreed well, with bias (i.e., average difference) = 0.44 +/- 0.52 kPa (3.3 +/- 3.9 torr). PetCO2 measured with the tidal volume breath produced an unacceptably high bias of 0.82 +/- 0.70 kPa (6.1 +/- 5.2 torr). Levels of agreement between PaCO2 were similar for smokers and nonsmokers and for men and women. The arterial-end-tidal CO2 tension (Pa-etCO2) difference was not related to PaCO2. Pa-etCO2 correlated with age (r = 0.473; p = 0.01), and was significantly higher in patients with pulmonary disease (1.32 +/- 0.56 kPa; 9.9 +/- 4.2 torr) than it was in those without pulmonary disease (0.46 +/- 0.55 kPa; 3.5 +/- 4.1 torr; p < 0.001).

CONCLUSIONS

Noninvasive PetCO2 monitoring may adequately predict PaCO2 in nonintubated ED patients with respiratory distress who are able to produce a forced expiration. PetCO2 is less accurate for PaCO2 with tidal volume breathing and in patients with pulmonary disease.

Oxygraphy in spontaneously breathing subjects

BACKGROUND

Continuous monitoring of O2 and CO2 in the airways of spontaneously breathing patients can be carried out by sampling air to a gas monitor through a catheter placed in the upper airway. The graphical display of O2 (oxygraphy) is a rather new facility.

OBJECTIVE

To describe the photo-acoustic and magneto-acoustic technique for CO2 and O2 monitoring in the open unintubated airway, to evaluate the efficacy of oxygen therapy by oxygraphy and to determine alveolar gas tensions and alveolar-arterial partial pressure gradients.

DATA SOURCES

O2 and CO2 fractions in the airways were monitored in 9 healthy subjects. Blood samples were drawn from the radial artery.

METHODS

The Multigas Monitor 1,304 (Brüel and Kjaer, Naerum, Denmark) was used; end-expiratory measurements were considered as representative for the alveolar gas composition. Arterial blood was analysed by ABL520 (Radiometer Medical A/S, Copenhagen, Denmark).

RESULTS

Reliable tracings of gas fractions (FCO2 and FO2) were obtained during the respiratory cycle in all subjects. When oxygen was supplied, FO2 of the airway varied considerably during the inspiratory phase whereas it remained almost constantly during the expiratory phase. The end-expiratory FO2 increased from 0.15 breathing atmospheric air to 0.41 breathing oxygen 15 L/min through a Hudson mask. Alveolar-arterial partial pressure differences were: pO2(A-a): 1.07 +/- 0.85 kPa and pCO2(A-a): -0.04 +/- 0.33 kPa during normoventilation in atmospheric air.

CONCLUSION

Continuous monitoring of CO2 and O2 in the airway gives information about the pulmonary gas exchange and the efficacy of oxygen supply. Combined with arterial blood gas analysis the method allows determination of alveolar-arterial CO2 or O2 gradients.

The effect of nasal oxygen flow and catheter position on the accuracy of end-tidal carbon dioxide measurements by a pharyngeal catheter in unintubated, spontaneously breathing subjects

Reliable recordings of carbon dioxide concentrations during spontaneous respiration can be obtained from a catheter positioned in the hypopharynx. The present study investigated the possible influence on end-tidal carbon dioxide measurement of nasal oxygen administration, position of the sampling catheter and mouth breathing. The study demonstrated that not only can reliable capnographic tracings be obtained from a thin catheter placed in the unintubated airway, but the subject may also receive up to 6 l.min-1 of oxygen via the nasal route without interference with the accuracy of the measurements. Furthermore, the exact position of the sampling catheter tip in the airway is not critical, which means that it may be placed where it causes the patient least discomfort. Finally, mouth breathing caused a mean decrease in end-tidal carbon dioxide of 0.25 kPa compared to nose breathing.

Reliability of CO2 measurements from the airway by a pharyngeal catheter in unintubated, spontaneously breathing subjects

Although several short communications have appeared describing attempts to record the concentrations of carbon dioxide (cCO2) from the unintubated airway by a catheter placed in the nose, so far only few reports have documented the reliability of the method. To evaluate the reliability of CO2 measurements by a catheter in the open, unintubated airway during spontaneous respiration, a 12 CH PVC catheter was forwarded through the nostril to the hypopharynx and connected to a capnograph in nine healthy volunteers. Another capnograph was connected to a tightly fitting face mask and simultaneous CO2 recordings were attained from the two parts of the airway during normoventilation, hyperventilation and rebreathing. A corresponding blood sample was drawn from the radial artery for blood gas analysis. The configurations of the capnograms recorded from the pharyngeal catheter were similar to those recorded from the face mask. The results were analysed by a multifactor analysis of variance. The carbon dioxide tension (pCO2) was significantly influenced by degree of ventilation (P < 0.0001), subject (P < 0.0001), measurement site (P = 0.030) and interaction subject-ventilation (P = 0.015). In spite of the significant influence of the measurement site, the difference between end tidal carbon dioxide tension (pCO2(ET)) and carbon dioxide tension in arterial blood (pCO2(a)) was small. The mean differences between paired measurements (pCO2(ET)-pCO2(a)) were -0.10 kPa +/- 0.41 kPa (mean +/- SD) for the catheter and -0.20 kPa +/- 0.43 kPa for the face mask. The study demonstrates that reliable recordings of CO2 concentrations during spontaneous respiration can be obtained by a thin catheter positioned in the hypopharynx.

Comparison of the end-tidal arterial PCO2 gradient during exercise in normal subjects and in patients with severe COPD

We undertook the present study with the following objectives: (1) to compare the difference between the end-tidal and the arterial carbondioxide concentration (P[ETa] CO2) gradients at rest and during exercise in normal subjects and patients with COPD; and (2) to analyze the factors contributing to this gradient. We studied seven normal subjects and seven patients with COPD using a symptom-limited exercise test on a cycle ergometer. Our results show that the P(ET-a)CO2 increased progressively as the individuals went from rest to higher workloads in both the normal group and in the COPD group. The P(ET-a)CO2 in patients with COPD both at rest (-3.24 +/- 2.78 mm Hg) and during exercise (1.03 +/- 2.23 mm Hg) is significantly lower than that in normal individuals at rest (1.84 +/- 3.68 mm Hg) and during exercise (10.3 +/- 6.5 mm Hg) (p < 0.01). However, the slope for the relationship between the P(ET-a)CO2 and the workload is actually significantly steeper in the patients with COPD. Although the P(ET-a)CO2 correlated significantly with the workload in both normal subjects (r = 0.63, p < 0.001) and patients (r = 0.55, p < 0.005), the P(ET-a)CO2 was much more closely correlated with the ratio of dead space to tidal volume (VD/VT) (r values of -0.86 and -0.77, respectively). Moreover, when multiple regression analysis was performed, addition of any other physiologic measure (eg, oxygen consumption [VO2], carbon dioxide production [VCO2], minute ventilation [VE], or workload) as a second independent variable after the VD/VT did not improve the correlation. This indicates that the correlation between the P(ET-a)CO2 and the workload is probably related to the dependence of the VD/VT on the workload. The PaCO2 in normal subjects and in the COPD group correlated significantly with the partial pressure of end-tidal carbon dioxide (PETCO2). Using multiple regression analysis, with the PaCO2 as the dependent variable and the PETCO2 (along with other physiologic measures) as the independent variables, we found that the standard error of the estimate was still above 2.1 mm Hg in normal subjects and in patients with COPD. We conclude that (1) during exercise, the P(ET-a)CO2 in normal subjects and in patients with COPD increases significantly, (2) the P(ET-a)CO2 gradient is more closely correlated with the VD/VT than any other physiologic variable, and (3) changes in the PETCO2 during exercise are not correlated closely with changes in the PaCO2.

Practical uses of end-tidal carbon dioxide monitoring in the emergency department

Qualitative and quantitative measurement of the carbon dioxide (CO2) concentration in respiratory gases is readily available with current technology. End-tidal CO2 (PetCO2) monitoring, whether by qualitative colorimetric methods or by solid-state spectrophotometric techniques, is becoming increasingly valuable in the Emergency Department (ED). These techniques offer a practical adjunct to the ED management of critical interventions including endotracheal intubation, conscious sedation, and cardiopulmonary resuscitation.

Evaluation of two commercially available carbon dioxide sampling nasal cannulae

Our study compared two commercially available carbon dioxide sampling nasal cannulae for efficacy of oxygenation and relationship of end-tidal carbon dioxide (PETCO2) to arterial carbon dioxide (PaCO2). The two-prong nasal cannula (2PNC) has one prong dedicated to delivering O2 via one naris and the second prong dedicated to sampling exhaled gases via the other naris. The four-prong nasal cannula (4PNC) delivers O2 via a prong in each naris, and samples exhaled gases via another set of prongs in each naris. Forty six patients were divided into three groups, which received either 2 (n = 15), 3 (n = 16), or 4 (n = 15) L/min O2, respectively, and were studied sequentially with standard nasal cannula (SNC), the 2PNC, and then the 4PNC. At each O2 flow rate, PaO2 was equivalent regardless of whether the SNC, 2PNC, or 4PNC was used. Seventy-four percent (34/46) of the 2PNC and 0% (0/46) of the 4PNC PETCO2 values were within +/- 4 torr of the PaCO2 value. The authors conclude that the 2PNC and 4PNC are equally effective compared with an SNC in oxygenating patients, but the PETCO2 measured by the 2PNC provides a superior quantitative estimate of the PaCO2 than that obtained by the 4PNC.

End-tidal carbon dioxide measurement in infants and children during and after general anaesthesia

We have examined the reliability of end-tidal carbon dioxide (PetCO2) monitoring as an estimate of arterial carbon dioxide tension (PaCO2) in spontaneously breathing infants and children. Forty patients were studied in the post-anaesthetic care unit; 20 < 12 kg and 20 > or = 12 kg. The PetCO2 was sampled via a 5 cm 16 gauge catheter taped below an external naris and this measurement was compared with the PaCO2 of a sample drawn from an indwelling arterial line. Twenty additional patients were studied during inhalational anaesthesia. The PetCO2 was measured both from the proximal end of the elbow connector and from a 5 cm cannula inserted through the elbow. An arterial blood gas sample was obtained simultaneously. The arterial to end-tidal (Pa-et) differences were compared between the two sites. Patients studied in the post-anaesthetic care unit showed good correlation between PetCO2 and PaCO2 regardless of weight: Pa-etCO2 of -0.6 +/- 3.6 (< 12 kg) and -1.1 +/- 2.8 mmHg (> or = 12 kg). Patients studied during mask anaesthesia showed better correlation between PetCO2 and PaCO2 when PetCO2 was sampled from the cannula: Pa-etCO2 of 3.5 +/- 4.8 mmHg (cannula), 8.6 +/- 4.5 (elbow) (P < 0.05). These results suggest that end-tidal CO2 monitoring is a useful and reliable method for assessing adequacy of ventilation in spontaneously breathing children weighing between 5.2 and 35 kg.

Conscious sedation in the emergency department: the value of capnography and pulse oximetry

STUDY OBJECTIVE

The purpose of this observational study was to describe the use of nasal capnography and pulse oximetry in monitoring heavily sedated emergency department patients.

DESIGN

Prospective, nonblinded, nonrandomized, noncontrolled clinical trial.

SETTING

The study was conducted in a tertiary-care hospital with 36,000 annual ED visits.

TYPE OF PARTICIPANTS

Twenty-seven patients requiring sedation with benzodiazepines and/or narcotics for painful procedures.

INTERVENTIONS

The ventilatory status of each patient was monitored with a capnometer by nasal cannula as well as a pulse oximeter before, during, and after administration of the sedative agents.

MEASUREMENTS

Vital signs, nasal end-tidal CO2 (PETCO2) measurements, and oxygen saturation were measured at baseline, during the procedure, and for a two-hour observation period after the procedure.

MAIN RESULTS

The average PETCO2 increased from 35.9 to 42.1 mm Hg during the procedure while the oxygen saturation dropped from an average of 98% to 94.3%. One patient developed clinically significant apnea after the procedure that was picked up by the apnea alarm, and eight additional patients developed clinically silent hypoxemia and increased PETCO2 during the procedure.

CONCLUSION

The use of pulse oximetry is recommended for the detection of unrecognized hypoxemia during conscious sedation. Capnography by nasal cannula appears to be a useful modality in monitoring during conscious sedation, but further research and clinical experience are required before routine use can be recommended.