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Abstracts of recently published papers in 2010  

The diagnostic role of capnography in pulmonary embolism.

Kurt OK, Alpar S, Sipit T, et al. Am J Emerg Med 2010, May, 28(4):460-5.

The authors evaluated the diagnostic contribution of alveolar dead space fraction (AVDSf) measured using capnography in patients admitted with suspected pulmonary embolism (PE). A total of 58 patients who were admitted to their hospital with suspected PE between October 2006 and January 2008 were included in this study. All patients were assessed using the Wells clinical score, capnography, computed tomographic pulmonary angiography, D-dimer measurement, lower-extremity venous Doppler ultrasonography, and V/Q scintigraphy. Forty patients (69%) had PE based on computed tomographic pulmonary angiography findings. The AVDSf value with the highest sensitivity and specificity, which was at the same time statistically significant, was 0.09. This value was consistent with the AVDSf value obtained using receiver operating characteristic analysis. In our study, the sensitivity of capnography was 70%, with a specificity of 61.1%, positive predictive value of 80%, and negative predictive value of 47.8%. The inclusion  of AVDSf in combination with other scoring systems that evaluate clinical likelihood of PE and D-dimer levels result in higher sensitivity and specificity rates for the diagnosis of PE.

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Alveolar dead-space response to activatd protein C in acute respiratory distress syndrome.

Kallet RH, Jasmer RM, Pittet JF. Respir Care 2010;55(5):617-22.

The authors report a complicated case of acute respiratory distress syndrome (ARDS) from severe sepsis, in which they measured the ratio of physiologic dead space to tidal volume (V(D)/V(T)) with volumetric capnography prior to, during, and after therapy with human recombinant activated protein C. It is known that elevated V(D)/V(T) primarily reflects increased alveolar V(D), probably caused by pronounced thrombi formation in the pulmonary microvasculature. This may be particularly true when severe sepsis is the cause of ARDS. The authors serially measured V(D)/V(T) in a 29-year-old man with sepsis-induced ARDS over the course of activated protein C therapy. Treatment with activated protein C resulted in a pronounced reduction in V(D)/V(T), from 0.55 to 0.27. Alveolar V(D) decreased from 165 mL to 11 mL (93% reduction). Activated protein C was terminated at 41 hours because of gastrointestinal bleeding. When the measurement was repeated 29 hours after therapy was discontinued, V(D)/V(T) had increased modestly, to 0.34, whereas alveolar V(D) had increased to 71 mL, or 43% of the pre-activated-protein-C baseline measurement. Alveolar V(T) rose from 260 mL to 369 mL and decreased slightly after termination of activated protein C (336 mL). Over the course of activated protein C therapy there was a persistent decrease in alveolar V(D) and increase in alveolar V(T), even while positive end-expiratory pressure was reduced and respiratory-system compliance decreased. Thus, improved alveolar perfusion persisted despite signs of alveolar de-recruitment. This suggests that activated protein C may have reduced microvascular obstruction. This report provides indirect evidence that microvascular obstruction may play an important role in elevated V(D)/V(T) in early ARDS caused by severe sepsis.

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End-tidal carbon dioxide concentration can estimate the appropriate timing of weaning off from extracorporeal membrane oxygenation for refractory circulatory failure.

Naruke T, Inomata T, et al. In Heart J. 2010;51(2);116-20

There are no reports of direct parameters indicating cardiac recovery to determine the timing of weaning off ECMO. Twenty-five patients supported by ECMO due to hemodynamic deterioration were divided into 2 groups according to their outcome: weaned ECMO (W: n = 18) or not (NW: n = 7). In the W group, the authors examined the differences in parameters between the 2 time points, ECMO introduction, and the reduction in ECMO flow to 40% of the initial setting known as the conventional recovery point (C-point). Significant differences were observed in systolic pulmonary artery pressure, the cardiac index measured by the thermodilution method, C-reactive protein, lactate, base excess, and the end-tidal CO(2) concentration (ETCO(2)). Next, by closely examining these 6 parameters measured every 12 hours, the authors found that only ETCO(2) had always changed steeply, like a 'flexion point' (E-point), in all W cases, but not in NW. The E-point was defined as an initial increase in ETCO(2) of >or= 5 mmHg over the preceding 12 hours with a continued rise over the next 12 hours. E-points appeared as much as 95 +/- 60 hours earlier than C-points and also preceded weaning off of ECMO. ETCO(2) can be a useful continuous parameter for predicting the adequate timing of weaning off of ECMO for circulatory failure at the bedside.

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A retrospective observational study examining the admission arterial to end-tidal carbon dioxide gradient in intubated major trauma patients.

Hiller J, Silvers, A, Mclliroy DR, Niggemeter L, White S.Anaesth Intensive Care 2010;38(2)302-6.

Major trauma patients who are intubated and ventilated are exposed to the potential risk of iatrogenic hypercapnic and hypocapnic physiological stress. In the pre-hospital setting, end-tidal capnography is used as a practical means of estimating arterial carbon dioxide concentrations and to guide the adequacy of ventilation. In this study, potentially deleterious hypercapnia (mean 47 mmHg, range 26 to 83 mmHg) due to hypoventilation was demonstrated in 49% of 100 intubated major trauma patients arriving at a major Australian trauma centre. A mean gradient of 15 mmHg arterial to end-tidal carbon dioxide concentration difference was found, highlighting the limitations of capnography in this setting. Moreover, 80% of the patients in the study had a head injury. Physiological deadspace due to hypovolaemia in these patients is commonly thought to contribute to the increased arterial to end-tidal carbon dioxide gradient in trauma patients. However in this study, scene and arrival patient hypoxia was more predictive of hypoventilation and an increased arterial to end-tidal carbon dioxide gradient than physiological markers of shock. Greater vigilance for hypercapnia in intubated trauma patients is required. Additionally, a larger study may confirm that lower end-tidal carbon dioxide levels could be safely targeted in the pre-hospital and emergency department ventilation strategies of the subgroup of major trauma patients with scene hypoxia.

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Capnography is superior to pulse oximetry for the detection of respiratory depression during colonoscopy.

Cacho G, Perez-Calle JL, Barbado A, Liedo JL, et al. Rev Esp Enferm Dig. 2010;102(2):86-9.

Pulse oximetry is a widely accepted procedure for ventilatory monitoring during gastrointestinal endoscopy, but this method provides an indirect measurement of the respiratory function. In addition, detection of abnormal ventilatory activity can be delayed, especially if supplemental oxygen is provided. Capnography offers continuous real-time measurement of expiratory carbon dioxide. The authors aimed at prospectively examining the advantages of capnography over the standard pulse oximetry monitoring during sedated colonoscopies. Fifty patients undergoing colonoscopy were simultaneously monitored with pulse oximetry and capnography by using two different devices in each patient. Several sedation regimens were administered. Episodes of apnea or hypoventilation detected by capnography were compared with the occurrence of hypoxemia. Twenty-nine episodes of disordered respiration occurred in 16 patients (mean duration 54.4 seconds). Only 38% of apnea or hypoventilation episodes were detected by pulse oximetry. A mean delay of 38.6 seconds was observed in the events detected by pulse oximetry (two episodes of disturbed ventilation were simultaneously detected by capnography and pulse oximetry). Apnea or hypoventilation commonly occurs during colonoscopy with sedation. Capnography is more reliable than pulse oximetry in early detection of respiratory depression in this setting.

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The use of capnography and the availability of airway equipment on intensive care units in the UK and the Republic of Ireland.

Gerogiou AP, Gouldson S, Amphiett AM. Anaesthesia 2010;65(5);462-7.

At least 20% of reported major adverse airway events occur on the intensive care unit. This study surveyed 315 (96%) of all general, satellite, hepatobiliary, cardiac and neuro-intensive care units in the UK and the Republic of Ireland, finding that only 100 (32%) units always use capnography for tracheal intubation while only 80 (25%) always use capnography for continuous monitoring of patients requiring controlled ventilation. Three hundred and ten (98%) units utilise a checklist of airway equipment, 311 (99%) check its functionality on a daily basis and 296 (94%) units have access to a bronchoscope. Whilst 297 (94%) ICUs have an airway trolley, sufficient equipment for unanticipated difficult intubation was only seen on 33 (10%) of units. Guidelines addressing minimum standards for monitoring and airway safety on ICU are not being met and remain below the standard expected.

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A nasal catheter for measurement of end-tidal carbon dioxide in spontaneously breathing patients: A preliminary evaluation.

Raheem MS, Wahba OM. Anesth Analg 2010;110(4):1039-42.

Several devices have been proposed to monitor end-tidal carbon dioxide tension (Petco(2)) in spontaneously breathing patients; however, many have been reported to be inaccurate.The authors designed this study to investigate the accuracy of a balloon-tipped nasal catheter in measuring Petco(2) in nontracheally intubated, spontaneously breathing patients. The catheter was assembled using a 14-F rubber Foley catheter, a tracheal tube pilot balloon, and the plastic sheath from an 18-gauge needle. The catheter was connected to the sampling tube of a gas analyzer. Petco(2) and Paco(2) were determined simultaneously in 20 otherwise healthy postsurgical patients while receiving oxygen. The mean Petco(2) - Paco(2) difference was -4.4 +/- 1.6 (SD) mm Hg with a correlation coefficient r = +0.87 (P < 0.001). Our results suggest that a balloon-tipped nasal catheter can provide a simple, easy, and reliable method for Petco(2) measurement in nontracheally intubated, spontaneously breathing patients.

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End-tidal and arterial carbon dioxide measurements correlate across all levels of physiologic dead space.

McSwain SD, Hamel DS, Smith PB, Gentile MA, et al. Respir Care 2010;55(3):288-93.

 End-tidal carbon dioxide (P(ETCO(2))) is a surrogate, noninvasive measurement of arterial carbon dioxide (P(aCO(2))), but the clinical applicability of P(ETCO(2)) in the intensive care unit remains unclear. Available research on the relationship between P(ETCO(2)) and P(aCO(2)) has not taken a detailed assessment of physiologic dead space into consideration. The hypothesis here is that P(ETCO(2)) would reliably predict P(aCO(2)) across all levels of physiologic dead space, provided that the expected P(ETCO(2))-P(aCO(2)) difference is considered. Fifty-six mechanically ventilated pediatric patients (0-17 y old, mean weight 19.5 +/- 24.5 kg) were monitored with volumetric capnography. For every arterial blood gas measurement during routine care, the authors measured P(ETCO(2)) and calculated the ratio of dead space to tidal volume (V(D)/V(T)). They assessed the P(ETCO(2))-P(aCO(2)) relationship with Pearson's correlation coefficient, in 4 V(D)/V(T) ranges. V(D)/V(T) was 0.7 for 54 measurements (11%). The correlation coefficients between P(ETCO(2)) and P(aCO(2)) were 0.95 (mean difference 0.3 +/- 2.1 mm Hg) for V(D)/V(T) 0.7. The authors concluded that there were strong correlations between P(ETCO(2)) and P(aCO(2)) in all the V(D)/V(T) ranges. The P(ETCO(2))-P(aCO(2)) difference increased predictably with increasing V(D)/V(T).

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Correlations between capnographic waveforms and peak flow meter measurement in emergency department management of asthma.

Nik Hisamuddin NA, Rashidi A, Chew KS, Kamaruddin J, Idzwan Z, Teo AH. Int j Emerg Med 2009;24(2):83-9.

The usual method for initial assessment of an acute asthma attack in the emergency room includes the use of peak flow measurement and clinical parameters. Both methods have their own disadvantages such as poor cooperation/effort from patients (peak flow meter) and lack of objective assessment (clinical parameters).The authors in this study were looking into other methods for the initial asthma assessment, namely the use of capnography. The normal capnogram has an almost square wave pattern comprising phase 1, slope phase 2, plateau phase 3, phase 4 and angle alpha (between slopes 2 and 3). The changes in asthma include decrease in slope of phase 2, increase in slope 3 and opening of angle alpha. The oobjective of the authors was to compare and assess the correlation between the changes in capnographic indices and peak flow measurement in non-intubated acute asthmatic patients attending the emergency room. They carried out a prospective study in a university hospital emergency department (ED). One hundred and twenty eight patients with acute asthma were monitored with peak flow measurements and then had a nasal cannula attached for microstream sampling of expired carbon dioxide. The capnographic waveform was recorded onto a PC card for indices analysis. The patients were treated according to departmental protocols. After treatment, when they were adjudged well for discharge, a second set of results was obtained for peak flow measurements and capnographic waveform recording. The pre-treatment and post-treatment results were then compared with paired samples t-test analysis. Simple and canonical correlations were performed to determine correlations between the assessment methods. A p value of below 0.05 was taken to be significant.  Peak flow measurements showed significant improvements post-treatment (p < 0.001). On the capnographic waveform, there was a significant difference in the slope of phase 3 (p < 0.001) and alpha angle (p < 0.001), but not in phase 2 slope (p = 0.35). Correlation studies done between the assessment methods and indices readings did not show strong correlations either between the measurements or the magnitude of change pre-treatment and post-treatment. The authors concluded that peak flow measurements and capnographic waveform indices can indicate improvements in airway diameter in acute asthmatics in the ED. Even though the two assessment methods did not correlate statistically, capnographic waveform analysis presents several advantages in that it is effort independent and provides continuous monitoring of normal tidal respiration. The authors, therefore,  propose capnography can be used for the monitoring of asthmatics in the ED.

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Volumeteri capnography for the evalualtion of pulmonary disease in adult patients with cystic fibrosis and noncystic fibrosis bronchiectasis.

Veronez L, Moreira MM, Soares ST, et al. Lunc 2010;188(3):263-8. 

This study was designed to use volumetric capnography to evaluate the breathing pattern and ventilation inhomogeneities in patients with chronic sputum production and bronchiectasis and to correlate the phase 3 slope of the capnographic curve to spirometric measurements. Twenty-four patients with cystic fibrosis (CF) and 21 patients with noncystic fibrosis idiopathic bronchiectasis (BC) were serially enrolled. The diagnosis of cystic fibrosis was based on the finding of at least two abnormal sweat chloride concentrations (iontophoresis sweat test). The diagnosis of bronchiectasis was made when the patient had a complaint of chronic sputum production and compatible findings at high-resolution computed tomography (HRCT) scan of the thorax. Spirometric tests and volumetric capnography were performed. The 114 subjects of the control group for capnographic variables were nonsmoker volunteers, who had no respiratory symptoms whatsoever and no past or present history of lung disease. Compared with controls, patients in CF group had lower SpO(2) (P < 0.0001), higher respiratory rates (RR) (P < 0.0001), smaller expiratory volumes normalized for weight (V(E)/kg) (P < 0.028), smaller expiratory times (Te) (P < 0.0001), and greater phase 3 Slopes normalized for tidal volume (P3Slp/V(E)) (P < 0.0001). Compared with controls, patients in the BC group had lower SpO(2) (P < 0.0001), higher RR (P < 0.004), smaller V(E)/kg (P < 0.04), smaller Te (P < 0.007), greater P3Slp/V(E) (P < 0.0001), and smaller VCO(2) (P < 0.0002). The pooled data from the two patient groups compared with controls showed that the patients had lower SpO(2) (P < 0.0001), higher RR (P < 0.0001), smaller V(E)/kg (P < 0.05), smaller Te (P < 0.0001), greater P3Slp/V(E) (P < 0.0001), and smaller VCO(2) (P < 0.0003). All of the capnographic and spirometric variables evaluated showed no significant differences between CF and BC patients. Spirometric data in this study reveals that the patients had obstructive defects with concomitant low vital capacities and both groups had very similar abnormalities. The capnographic variables in the patient group suggest a restrictive respiratory pattern (greater respiratory rates, smaller expiratory times and expiratory volumes, normal peak expiratory flows). Both groups of patients showed increased phase III slopes compared with controls, which probably indicates the presence of diffuse disease of small airways in both conditions leading to inhomogeneities of ventilation.

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Dynamic model inversions techniques for breath-by-breath measurement of carbon dioxide from low bandwidth sensors.

Sivaramakrishnan S, Rajamani R, Johnson BD. Conf Proc IEEE Med Biol Soc. 2009;3039-42.

Respiratory CO(2) measurement (capnography) is an important diagnosis tool that lacks inexpensive and wearable sensors. This paper develops techniques to enable use of inexpensive but slow CO(2) sensors for breath-by-breath tracking of CO(2) concentration. This is achieved by mathematically modeling the dynamic response and using model-inversion techniques to predict input CO(2) concentration from the slow-varying output. Experiments are designed to identify model-dynamics and extract relevant model-parameters for a solidstate room monitoring CO(2) sensor. A second-order model that accounts for flow through the sensor's filter and casing is found to be accurate in describing the sensor's slow response. The resulting estimate is compared with a standard-of-care respiratory CO(2) analyzer and shown to effectively track variation in breath-by-breath CO(2) concentration. This methodology is potentially useful for measuring fast-varying inputs to any slow sensor.

 

 

 

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