Arterial versus capillary sampling for analysing blood gas pressures

Pitfalls in the diagnosis and management of acid-base disorders in humans: a laboratory medicine perspective

Diagnostic errors affect patient management, and as blood gas analysis is mainly performed without the laboratory, users must be aware of the potential pitfalls. The aim was to provide a summary of common issues users should be aware of.A narrative review was performed using online databases such as PubMed, Google Scholar and reference lists of identified papers. Language was limited to English.Errors can be pre-analytical, analytical or post-analytical. Samples should be analysed within 15 min and kept at room temperature and taken at least 15-30 min after changes to inspired oxygen and ventilator settings, for accurate oxygen measurement. Plastic syringes are more oxygen permeable if chilled. Currently, analysers run arterial, venous, capillary and intraosseous samples, but variations in reference intervals may not be appreciated or reported. Analytical issues can arise from interference secondary to drugs, such as spurious hyperchloraemia with salicylate and hyperlactataemia with ethylene glycol, or pathology, such as spurious hypoxaemia with leucocytosis and alkalosis in hypoalbuminaemia. Interpretation is complicated by result adjustment, for example, temperature (alpha-stat adjustment may overestimate partial pressure of carbon dioxide (pCO2) in hypothermia, for example), and inappropriate reference intervals, for example, in pregnancy bicarbonate, and pCO2 ranges should be lowered.Lack of appreciation for patient-specific and circumstance-specific reference intervals, including extremes of age and altitude, and transformation of measurements to standard conditions can lead to inappropriate assumptions. It is vitally important for users to optimise specimen collection, appreciate the analytical methods and understand when reference intervals are applicable to their specimen type, clinical question or patient.

Comparison of mathematically arterialised venous blood gas sampling with arterial, capillary, and venous sampling in adult patients with hypercapnic respiratory failure: a single-centre longitudinal cohort study

BACKGROUND

Accurate arterial blood gas (ABG) analysis is essential in the management of patients with hypercapnic respiratory failure, but repeated sampling requires technical expertise and is painful. Missed sampling is common and has a negative impact on patient care. A newer venous to arterial conversion method (v-TAC, Roche) uses mathematical models of acid-base chemistry, a venous blood gas sample and peripheral blood oxygen saturation to calculate arterial acid-base status. It has the potential to replace routine ABG sampling for selected patient cohorts. The aim of this study was to compare v-TAC with ABG, capillary and venous sampling in a patient cohort referred to start non-invasive ventilation (NIV).

METHODS

Recruited patients underwent near simultaneous ABG, capillary blood gas (CBG) and venous blood gas (VBG) sampling at day 0, and up to two further occasions (day 1 NIV and discharge). The primary outcome was the reliability of v-TAC sampling compared with ABG, via Bland-Altman analysis, to identify respiratory failure (via PaCO₂) and to detect changes in PaCO₂ in response to NIV. Secondary outcomes included agreements with pH, sampling success rates and pain.

RESULTS

The agreement between ABG and v-TAC/venous PaCO₂ was assessed for 119 matched sampling episodes and 105 between ABG and CBG. Close agreement was shown for v-TAC (mean difference (SD) 0.01 (0.5) kPa), but not for CBG (-0.75 (0.69) kPa) or VBG (+1.00 (0.90) kPa). Longitudinal data for 32 patients started on NIV showed the closest agreement for ABG and v-TAC (R²=0.61). v-TAC sampling had the highest first-time success rate (88%) and was less painful than arterial (p<0.0001).

CONCLUSION

Mathematical arterialisation of venous samples was easier to obtain and less painful than ABG sampling. Results showed close agreement for PaCO2 and pH and tracked well longitudinally such that the v-TAC method could replace routine ABG testing to recognise and monitor patients with hypercapnic respiratory failure.

TRIAL REGISTRATION NUMBER

NCT04072848; www.

CLINICALTRIALS

gov.

Reliability of Central Venous Blood Gas Values Compared With Arterial Blood Gas Values in Critically Ill Patients

BACKGROUND

Central venous blood gas (cVBG) values are correlated with arterial blood gas (ABG) values. However, the substitution of cVBG values for ABG values in critically ill patients remains uninvestigated. Thus, we investigated the reliability between cVBG and ABG values and sought to define the conditions that could improve the reliability of cVBG values as a substitute.

METHODS

We conducted a prospective comparison of 292 sets of cVBG values and ABG values from 82 subjects admitted to the medical ICU between October 2017-July 2018. Paired cVBG and ABG samples were collected daily during the first 5 d of ICU treatment and on days 8, 15, 22, and 29. Intraclass correlation coefficient (ICC) and Bland-Altman limits of agreement (LOA) were obtained.

RESULTS

The ICC between ABG and cVBG was 0.626 for pH, 0.696 for P_CO2 , 0.869 for bicarbonate, 0.866 for base excess, and 0.989 for lactic acid. Bland-Altman plots showed clinically unacceptable LOA between all parameters. Subgroup analysis indicated a significant increase in the ICCs of P_CO2 in samples with mechanical ventilation (0.0574-0.735, P = .02) and central venous oxygen saturation (ScvO₂) ≥ 70% (0.611-0.763, P = .008). After adjustment, the 95% LOA between ABG and cVBG was -0.06 to 0.07 for pH and -7.09 to 7.05 for P_CO2 in mechanically ventilated subjects with ScvO₂ ≥ 70%.

CONCLUSIONS

ABG and cVBG values showed clinically acceptable agreements and improved reliability in mechanically ventilated subjects with ScvO₂ ≥ 70%. cVBG analysis may be a substitute for ABG analysis in mechanically ventilated patients once tissue perfusion is restored.

Accounting for arterial and capillary blood gases for calculation of cerebral blood flow in preterm infants

One of the most feared neurological complications of premature birth is intraventricular hemorrhage, frequently triggered by fluctuations in cerebral blood flow (CBF). Although several techniques for CBF measurement have been developed, they are not part of clinical routine in neonatal intensive care. A promising tool for monitoring of CBF is its numerical assessment using standard clinical parameters such as mean arterial pressure, carbon dioxide partial pressure (pCO₂) and oxygen partial pressure (pO₂). A standard blood gas analysis is performed on arterial blood. In neonates, capillary blood is widely used for analysis of blood gas parameters. The purpose of this study was the assessment of differences between arterial and capillary analysis of blood gases and adjustment of the mathematical model for CBF calculation to capillary values. The statistical analysis of pCO₂ and pO₂ values collected from 254 preterm infants with a gestational age of 23-30 weeks revealed no significant differences between arterial and capillary pCO₂ and significantly lower values for capillary pO₂. The estimated mean differences between arterial and capillary pO₂ of 15.15 mmHg (2.02 kPa) resulted in a significantly higher CBF calculated for capillary pO₂ compared to CBF calculated for arterial pO₂. Two methods for correction of capillary pO₂ were proposed and compared, one based on the mean difference and another one based on a regression model.

CONCLUSION

Capillary blood gas analysis with correction for pO₂ as proposed in the present work is an acceptable alternative to arterial sampling for the assessment of CBF.

WHAT IS KNOWN

• Arterial blood analysis is the gold standard in clinical practice. However, capillary blood is widely used for estimating blood gas parameters. • There is no significant difference between the arterial and capillary pCO₂ values, but the capillary pO₂ differs significantly from the arterial one.

WHAT IS NEW

• The lower capillary pO₂ values yield significantly higher values of calculated CBF compared to CBF computed from arterial pO₂ measurements. • Two correction methods for the adjustment of capillary pO₂ to arterial pO₂ that made the difference in the calculated CBF insignificant have been proposed.

Agreement between Arterial and Capillary pH, pCO2, and Lactate in Patients in the Emergency Department

Background

Blood gas analysis (BGA) is a frequent painful procedure in emergency departments. The primary objective of the study was a quantitative analysis to assess the mean difference and 95% confidence interval of the difference between capillary and arterial BGA for pH, pCO₂, and lactate. Secondary objective was to measure the sensitivity and specificity of capillary samples to detect altered pH, hypercarbia, and lactic acidosis. Adults admitted to the ED were screened for inclusion. We studied the agreement between the two methods for pH, pCO₂, and lactate with Bland-Altman bias plot analysis and receiver operating characteristic curves.

Results

One hundred ninety-seven paired analyses were included. Mean difference for pH between arterial and capillary BGA was 0.0095, and 95% limits of agreement (LOA) were −0.048 to 0.067. For pCO₂, mean difference was −0.3 mmHg, and 95% LOA were −8.5 to 7.9 mmHg. Lactate mean difference was −0.93 mmol/L, and 95% LOA were −2.7 to 0.8 mmol/L. At a threshold of 7.34, capillary pH had 98% sensitivity and 97% specificity to detect acidemia; at 45.9 mmHg, capillary pCO₂ had 89% sensitivity and 96% specificity to detect hypercarbia. At a threshold of 3.5 mmol/L, capillary lactate had 66% sensitivity to detect lactic acidosis.

Conclusion

Capillary BGA cannot replace arterial BGA despite high concordance between the two methods for pH and pCO₂ and moderate concordance for lactate. Capillary measures had good accuracy when used as a screening tool to detect altered pH and hypercarbia but insufficient sensitivity and specificity when screening for lactic acidosis.

Comparison of Simultaneous Capillary Blood Gases and End-Tidal Carbon Dioxide in Mechanically Ventilated Pediatric Patients with Acute Respiratory Failure

In the follow-up of ventilation, invasive blood gas analysis and noninvasive monitoring of end-tidal carbon dioxide (ETCO ₂ ) are used. We aimed to investigate the relationship between capillary partial pressure of carbon dioxide (PcCO ₂ ) levels and ETCO ₂ and also to investigate ETCO ₂ 's predictive feature of PcCO ₂ levels. This study included 28 female and 30 male pediatric patients; 28 patients were type-1 respiratory failure (RF), 16 patients were acute respiratory distress syndrome, and 14 patients were type-2 RF. Our results showed a significant correlation between ETCO ₂ and PcCO ₂ . Although the strength of the correlation was weak throughout the measurements, the strength of this correlation increased significantly in type-2 RF.

Predictors of Morphine Efficacy for Dyspnea in Inpatients with Chronic Obstructive Pulmonary Disease: A Secondary Analysis of JORTC-PAL 07

Objective: This study aimed to explore the predictors of morphine efficacy in the alleviation of dyspnea in COPD.

Background: Dyspnea is prevalent in patients with chronic obstructive pulmonary disease (COPD) and often persists despite conventional treatment.

Methods: A secondary analysis of a multi-institutional prospective before–after study was conducted focusing on morphine use for alleviating dyspnea in COPD patients. Subjects included COPD patients with dyspnea at seven hospitals in Japan. Patients received 12 mg/day of oral morphine (or 8 mg/day if they had low body weight or renal impairment). Univariate and multivariate logistic regression analyses were performed with numerical rating scale (NRS) score of the current dyspnea intensity in the evening of day 0, Eastern Cooperative Oncology Group Performance Status (ECOG PS; ≤2 or ≥3), age, and partial arterial pressure of carbon dioxide (PaCO₂) as independent factors; an improvement of ≥1 in the evening NRS score of dyspnea from day 0 to 2 was the dependent factor.

Results: Thirty-five patients were enrolled in this study between October 2014 and January 2018. Excluding one patient who did not receive the treatment, data from 34 patients were analyzed. In the multivariate analysis, lower PaCO₂ was significantly associated with morphine efficacy for alleviating dyspnea (odds ratio [OR] 0.862, 95% confidence interval [CI] 0.747–0.994), whereas the NRS of dyspnea intensity on day 0 (OR 1.426, 95% CI 0.836–2.433), ECOG PS (OR 4.561, 95% CI 0.477–43.565), and patients' age (OR 0.986, 95% CI 0.874–1.114) were not.

Discussion: Morphine can potentially alleviate dyspnea in COPD patients with lower PaCO₂. Trial registration: UMIN000015288 (http://www.umin.ac.jp/ctr/index.htm)

[Guideline for Long-Term Oxygen Therapy - S2k-Guideline Published by the German Respiratory Society]

Long-term oxygen therapy is of great importance both for reducing mortality and for improving performance in patients with chronic lung diseases. The prerequisites for Long-term oxygen therapy are adequate diagnostics and clearly defined indication. A causal distinction into chronic hypoxaemic and hypercapnic respiratory failure is reasonable, from which the differential indication for non-invasive ventilation results.The revised guideline covers the diagnostics and indication of chronic lung and heart diseases, the role of oxygen in terminal illness and gives a detailed description of available oxygen devices. The guideline is intended to help avoid undersupply, oversupply and false prescriptions. Furthermore, the chapter "Postacute Oxygen Therapy" discusses the procedure, relevant in everyday life, but not yet clearly defined, for prescribing oxygen therapy for the home at the end of an inpatient stay. Another important point, the correct prescription of mobile oxygen systems, is also presented in the guideline. This document is a revised version of the guideline for longterm oxygen therapy and replaces the version of 2008.

Effects of endurance exercise under hypoxia on acid-base and ion balance in healthy males

[Purpose]

This study was performed to investigate the acid-base and ion balance at rest and after exercise in healthy males under normoxia, moderate hypoxia, and severe hypoxia.

[Methods]

Ten healthy Korean males completed three different trials on different days, comprising exercise under normoxia (F_iO₂ = 20.9%, N trial), moderate hypoxia (F_iO₂ = 16.5%, MH trial), and severe hypoxia (F_iO₂ = 12.8%, SH trial). They undertook endurance exercise for 30 min on a cycle ergometer at the same relative exercise intensity equivalent to 80% maximal heart rate under all conditions. Capillary blood samples were obtained to determine acid-base and ion balance at rest and after exercise.

[Results]

Exercise-induced blood lactate elevations were significantly increased as hypoxic conditions became more severe; SH > MH > N trials (P = 0.003). After exercise, blood glucose levels were significantly higher in the SH trial than in the N and MH trials (P = 0.001). Capillary oxygen saturation (S_CO₂) levels were significantly lowered as hypoxic conditions became more severe; SH > MH > N trials (P < 0.001). The pH levels were significantly lower in the MH trial than that in the N trial (P = 0.010). Moreover, H_CO₃- levels were significantly lower in the SH trial than in the N trial, with significant interaction (P = 0.003). There were no significant differences in blood Na⁺, K⁺, and Ca²⁺ levels between the trials.

[Conclusion]

MH and SH trials induced greater differences in glucose, lactate, S_CO₂, pH, and H_CO₃- levels in capillary blood compared to the N trial. Additionally, lactate, S_CO₂, and H_CO₃- levels showed greater changes in the SH trial than in the MH trial. However, there were no significant differences in Na⁺, K⁺, and Ca²⁺ levels in MH and SH trials compared to the N trial.

Comparison of pulse oximetry and earlobe blood gas with CO-oximetry in children with sickle cell disease: a retrospective review

OBJECTIVES

To investigate the agreement between pulse oximetry (SpO₂) and oxygen saturation (SaO₂) measured by CO-oximetry on arterialised earlobe blood gas (EBG) in children and adolescents with sickle cell disease (SCD).

DESIGN AND SETTING

We retrospectively reviewed 39 simultaneous and paired SaO₂ EBG and SpO₂ measurements from 33 ambulatory patients with SCD (32 subjects with Haemoglobin SS and one with Haemoglobin Sß⁺, 52% male, mean±SD age 11.0±3.6, age range 5-18). Measurements were performed between 2012 and 2015 when participants were asymptomatic. Hypoxaemia was defined as SaO₂ ≤93%. A Bland-Altman analysis was performed to assess the accuracy of SpO₂ as compared with EBG SaO₂.

RESULTS

The mean±SD SpO₂ and SaO₂ values in the same patients were, respectively, 93.6%±3.7% and 94.3%±2.9%. The bias SpO₂-SaO₂ was -0.7% (95% limits of agreement from -5.4% to 4.1%) and precision was 2.5%. In 9/39 (23%) cases, the difference in SpO₂-SaO₂ was greater than the expected error range ±2%, with SaO₂ more often underestimated by SpO₂ (6/9), especially at SpO₂values ≤93%. Thirteen participants (33%) were hypoxaemic. The sensitivity of SpO₂ for hypoxaemia was 100%, specificity 85% and positive predictive value 76%.

CONCLUSIONS

Pulse oximetry was inaccurate in almost a quarter of measurements in ambulatory paediatric patients with SCD, especially at SpO₂values ≤93%. In these cases, oxygen saturation can be confirmed through EBG CO-oximetry, which is easier to perform and less painful than traditional arterial blood sampling.

Agreement between arterial and non-arterialised fingertip capillary blood gas and acid-base values

Background: Arterial puncture is considered the gold standard for obtaining blood gas and acid-base values and facilitates the assessment of acutely and critically ill patients, as well as control of patients in long-term oxygen therapy (LTOT). Substitutional capillary sampling has been proposed, as researchers cite lower complication rates, physician independence, lower degree of invasiveness and higher degree of patient comfort. An arterialised earlobe is considered the method of choice to obtain capillary blood sampling, but in an acute setting, the need for vasodilating pastes may be time-consuming and impractical. The aim of this study is to examine whether accurate blood gas and acid-base measurements can be obtained using non-arterialised fingertip blood. Materials and methods: Consecutive arterial punctures and non-arterialised capillary blood samples were drawn from 62 patients with stable-phase chronic obstructive pulmonary disease (COPD), and subsequently analysed. Agreement between arterial and capillary blood gas values was compared using the method recommended by Bland and Altman. Results: Results show that limits of agreement (LoA) regarding PO₂ (LoA: -1.27-4.45 kPa); Base Excess (LoA: -1.35-0.55); lactate (LoA: -0.77-0.20 mmol/l) and SO₂ (LoA: -0.02-0.06) are wider than what would be applicable for clinical use. However, clinically acceptable LoA were obtained regarding PCO₂ (LoA: -0.64-0.38 kPa); pH (LoA: -0.02-0.03), and HCO₃ ^- (LoA: -1.06-0.55 mmol/l). Conclusion: LoA for PCO₂, pH and HCO₃ ^- indicate that measurement of these parameters in non-arterialised capillary blood may be useful in clinical practice/an acute setting. What this paper adds: Capillary blood sampling provides a fast, non-invasive means of obtaining blood gas-values;Traditionally, capillary blood sampling for blood gas analysis is obtained from the earlobe using arterialisation;The present study presents accurate measurements of PCO₂, HCO₃ ^- and pH using non-arterialised fingertip capillary blood;The present study is the first to show this in a population of stable-phase COPD patients.

Mathematical arterialisation of peripheral venous blood gas for obtainment of arterial blood gas values: a methodological validation study in the clinical setting

Arterial blood gas (ABG) analysis is an essential tool in the clinical assessment of acutely ill patients. Venous to arterial conversion (v-TAC), a mathematical method, has been developed recently to convert peripheral venous blood gas (VBG) values to arterialized VBG (aVBG) values. The aim of this study was to test the validity of aVBG compared to ABG in an emergency department (ED) setting. Twenty ED patients were included in this study. ABG and three aVBG samples were collected from each patient. The aVBG samples were processed in three different ways to investigate appropriate sample handling. All VBG samples were arterialized using the v-TAC method. ABG and aVBG samples were compared using Lin's concordance correlation coefficient (CCC), Bland-Altman plots and misclassification analysis. Clinical acceptable threshold of aVBG value deviance from ABG values were ± 0.05 pH units, ± 0.88 kPa pCO₂ and ± 0.88 kPa pO₂. CCC revealed an agreement in pH and pCO₂ parameters for both aVBG in comparison to ABG. In all aVBG samples, an overestimation of pO₂ compared to ABG was observed. Bland-Altman plot revealed clinically acceptable mean difference and limits-of-agreement intervals between ABG and aVBG pH and pCO₂, but not between ABG and aVBG pO₂. Arterialization of VBG using v-TAC is a valid method for measuring pH and pCO₂, but not for pO₂. Larger clinical studies are required to evaluate the applicability of v-TAC in different patient subpopulations.

Arterialized and venous blood lactate concentration difference during different exercise intensities

Objective

The purpose of this study was to investigate the difference between arterialized and venous blood lactate concentrations [La] during constant-load exercises at different intensities.

Methods

Fifteen physically active men cycled for 30 minutes (or until exhaustion) at the first lactate threshold (LT₁), at 50% of the difference between the first and second lactate threshold (TT_50%), at the second lactate threshold (LT₂), and at 25% of the difference between LT₂ and maximal aerobic power output (TW_25%). Samples of both arterialized and venous blood were collected simultaneously at rest and every 5 minutes during the exercise.

Results

The arterialized blood [La] was higher at minute 5 than venous blood [La] for all exercise intensities (p < 0.05). After this period, the arterialized and venous [La] samples became similar until the end of the exercise (p > 0.05). The arterialized-venous difference during the first 10 minutes was greater for the two highest exercise intensities (LT₂ and TW_25%) compared with the two lowest (LT₁ and TT_50%, p < 0.05). Thereafter, arterialized-venous difference decreased progressively, reaching values close to zero for all exercise intensities (p > 0.05).

Conclusion

These results suggest a delayed lactate appearance in the venous blood, which is accentuated at higher exercise intensities. The lactate measured in arterialized and venous blood is interchangeable only when blood samples are collected at least 10 minutes after the exercise starts.

Capillary PO2 does not adequately reflect arterial PO2 in hypoxemic COPD patients

Purpose

To compare arterial (P_aO₂) with capillary (P_cO₂) partial pressure of oxygen in hypoxemic COPD patients because capillary blood gas analysis (CBG) is increasingly being used as an alternative to arterial blood gas analysis (ABG) in a non-intensive care unit setting, although the agreement between P_cO₂ and P_aO₂ has not been evaluated in hypoxemic COPD patients.

Patients and methods

Bland–Altman comparison of P_aO₂ and P_cO₂ served as the primary outcome parameter if P_cO₂ values were ≤60 mmHg and the secondary outcome parameter if P_cO₂ values were ≤55 mmHg. Pain associated with the measurements was assessed using a 100-mm visual analog scale.

Results

One hundred and two P_aO₂/P_cO₂ measurement pairs were obtained. For P_cO₂ values ≤60 mmHg, the mean difference between P_aO₂ and P_cO₂ was 5.99±6.05 mmHg (limits of agreement: −5.88 to 17.85 mmHg). For P_cO₂ values ≤55 mmHg (n=73), the mean difference was 5.33±5.52 mmHg (limits of agreement: −5.48 to 16.15 mmHg). If P_aO₂ ≤55 (≤60) mmHg was set as the cut-off value, in 20.6% (30.4%) of all patients, long-term oxygen therapy have been unnecessarily prescribed if only P_cO₂ would have been assessed. ABG was rated as more painful compared with CBG.

Conclusions

P_cO₂ does not adequately reflect P_aO₂ in hypoxemic COPD patients, which can lead to a relevant number of unnecessary long-term oxygen therapy prescriptions.

Home oxygen therapy: evidence versus reality

INTRODUCTION

LTOT is a well-established treatment option for hypoxemic patients. Scientific evidence for its benefits of LTOT dates back to the 1980s, when two randomized controlled trials showed prolonged survival in COPD-patients undergoing LTOT for at least 15 hours/day. In contrast, the potential benefits of LTOT in non-COPD-patients has not been well researched and the recommendations for its application are primarily extrapolated from trials on COPD-patients. Recently, a large trial confirmed that COPD-patients who don't meet classic indication criteria, and have moderate desaturation at rest or during exercise, do not benefit from oxygen therapy. Also the significant technical evolution of LTOT devices has improved its application. Areas covered: A literature research was performed in pubmed regarding home oxygen therapy (terms: LTOT, ambulatory oxygen therapy, short burst oxygen therapy, nocturnal oxygen therapy). Expert commentary: LTOT proved a survival benefit for COPD patients about 30 years ago. Whether the results of these trials are still valid for patients under modern treatment guidelines remains unknown. Nevertheless, the classic indication criteria for LTOT still persist in guidelines, since there is a lack of updated evidence for the effects of LTOT in more severe hypoxemic patients.

The Effect of 4 Weeks Fixed and Mixed Intermittent Hypoxic Training (IHT) on Respiratory Metabolic and Acid-base Response of Capillary Blood During Submaximal Bicycle Exercise in Male Elite Taekwondo Players

[Purpose]

The purpose of our study was to determine the effectiveness of 4 weeks fixed and mixed intermittent hypoxic training (IHT) and its difference from exercise training at sea-level on exercise load, respiratory metabolic and acid-base response of capillary blood during 80% maximal heart rate (HRmax) bicycle exercise in male elite Taekwondo players.

[Methods]

Male elite Taekwondo players (n = 25 out of 33) were randomly assigned to training at sea-level (n = 8, control group), training at 16.5%O2 (2000 m) simulated hypoxic condition (n = 9, fixed IHT group), and training at 14.5%O2 (3000 m) up to 2 weeks and 16.5%O2 (2000 m) simulated hypoxic condition (n = 8, mixed IHT group) for 3 weeks. We compared their average exercise load, respiratory metabolic, and acid-base response of the capillary blood during 80% HRmax submaximal bicycle exercise before and after 4 weeks training.

[Results]

Fixed and mixed IHT groups showed positive improvement in respiratory metabolic and acid-base response of the capillary blood during 80% HRmax submaximal bicycle exercise after 4 weeks training. However, all dependent variables showed no significant difference between fixed IHT and mix IHT.

[Conclusion]

Results suggested that mixed and fixed IHT is effective in improving respiratory metabolic and acid-base response of capillary blood in male elite Taekwondo players. Thus, IHT could be a novel and effective method for improving exercise performance through respiratory metabolic and acid-base response.

Improving the sampling technique of arterialized capillary samples to obtain more accurate PaO2 measurements

Arterialized earlobe capillary blood samples (ELCS) have been used as a measurement of blood gas status for over 20 years. There is general acceptance that there is a strong correlation and limits of agreement between arterial and arterialized blood samples with respect to pH and PaCO2. Although the correlation between the arterial and arterialized PaO2 is good, the limits of agreement poor. Our aim was to improve the accuracy of this technique in the measurement of PaO2 by simultaneously monitoring the oxygen saturation by pulse oximetry whilst taking an ELCS. We hypothesize that significant discrepancies between the SaO2 and SpO2 highlight either a poorly arterialized sample or an over aerated sample from air bubbles. We compared the SpO2 with the SaO2 of an arterial sample from 27 inpatients. We used the limits of agreement between these samples to define the degree of discordance we would accept between SaO2 and SpO2 before repeat ELCS. Subsequently, 252 consecutive patients attending our respiratory physiology unit over a six-month period had an ELCS and simultaneous SpO2.If there was a discrepancy between SaO2 and SPO2 of > 2% the ELCS was repeated. There was a good correlation and limits of agreement between the SPO2 and arterial SaO2 (r = 0.97, mean difference + 95% limits of agreement: 0.34 + 2.68). A difference of more than 2% between arterialized SaO2 and SpO2 was identified in 21 patients out of 252 (8.3%) with SaO2 higher in two and lower in 19 (r = 0.96, mean difference +95% limits of agreement: 0.66 + 3.1). Repeat ELCS of these 21 samples reduced this discrepancy improving the concordance of the measurements (r = 0.98, mean difference + 95% limits of agreement: 0.47 + 1.0). In one case a difference of 3% remained between the saturations. We conclude that the addition of simultaneous pulse oximetry with ELCS will identify rogue measurements in about 8% of cases highlighting the need for repeat samples and thus increasing the accuracy of the measurement of PaO2 by ELCS.

[Monitoring of pCO2 during ventilation]

Respiratory insufficiency type 2 (ventilatory failure) is characterized by hypercapnia due to alveolar hypoventilation. Therefore, the monitoring of pCO2 is essential for diagnostic and surveillance purposes. Various techniques which differ in the way of measurement (e.g., invasive/noninvasive, continuous/noncontinuous) and their indication are available. Arterial blood gas analysis (ABG) as an invasive procedure is the gold standard procedure and is mostly used in emergency medicine or intensive care units (ICUs). Another method to evaluate pCO2 is capillary blood gas analysis (CBG). Furthermore, endtidal pCO2-(PetCO2) and transcutaneous CO2-measurement (PtcCO2) are able to continuously and noninvasively monitor pCO2. PetCO2 is mostly used in the field of anesthesiology during general anesthesia and is integrated in many ventilators, also in ICUs. However, PetCO2 is limited in monitoring pCO2 in patients with lung disease and it is only reasonably usable in invasively ventilated patients. Transcutaneous pCO2 (PtcCO2) is available as an alternative, especially in chronic respiratory failure and to diagnose hypoventilation in sleep-related breathing disorders, and it has substantial advantages in these indications compared to discontinuous measurements, e.g., blood gas analysis. The various methods to monitor pCO2 are generally used synergistically in clinical practice.

Using venous blood gas analysis in the assessment of COPD exacerbations: a prospective cohort study

Introduction

Identifying acute hypercapnic respiratory failure is crucial in the initial management of acute exacerbations of COPD. Guidelines recommend obtaining arterial blood samples but these are more difficult to obtain than venous. We assessed whether blood gas values derived from venous blood could replace arterial at initial assessment.

Methods

Patients requiring hospital treatment for an exacerbation of COPD had paired arterial and venous samples taken. Bland–Altman analyses were performed to assess agreement between arterial and venous pH, CO₂ and . The relationship between SpO₂ and SaO₂ was assessed. The number of attempts and pain scores for each sample were measured.

Results

234 patients were studied. There was good agreement between arterial and venous measures of pH and (mean difference 0.03 and −0.04, limits of agreement −0.05 to 0.11 and −2.90 to 2.82, respectively), and between SaO₂ and SpO₂ (in patients with an SpO₂ of >80%). Arterial sampling required more attempts and was more painful than venous (mean pain score 4 (IQR 2–5) and 1 (IQR 0–2), respectively, p<0.001).

Conclusions

Arterial sampling is more difficult and more painful than venous sampling. There is good agreement between pH and values derived from venous and arterial blood, and between pulse oximetry and arterial blood gas oxygen saturations. These agreements could allow the initial assessment of COPD exacerbations to be based on venous blood gas analysis and pulse oximetry, simplifying the care pathway and improving the patient experience.

A Randomized Controlled Trial on the Effect of Needle Gauge on the Pain and Anxiety Experienced during Radial Arterial Puncture

Background

Arterial punctures for assessment of arterial blood-gases can be a painful procedure. Lidocaine can be used to reduce pain prior to needle insertion but it is not a widely accepted practice. The purpose of this study was to determine whether a large size needle induces more pain compared to a smaller size needle for radial arterial puncture and to assess the anxiety associated with radial arterial punctures.

Methods

We conducted a prospective, double-blind, randomized, controlled, monocentric study including all outpatients who had a planned assessment of arterial blood gas analysis. Patients were randomized to have the arterial puncture performed with a 23 or a 25 G needle. The main judgement criteria was pain during arterial puncture. Visual analogue scale for pain (VAS-P) and visual analogue scale for anxiety (VAS-A) were used to assess pain and anxiety during radial arterial puncture.

Results

Two hundred consecutive patients were randomized. The 25 G needle was as painful as the 23 G needle (6.63 mm [0–19 mm] vs. 5.21 mm [0–18.49 mm], respectively, p = 0.527). Time for arterial puncture was longer with the 25 G needle than with the 23 G needle (42 s [35–55 s] vs. 33 s [24.5–35 s], respectively, p = 0.002). There was a correlation between the level of anxiety prior to the arterial puncture and the pain experienced by the patients (p: 0.369, p<0.0001). There was a correlation between the pain experienced by patients and the anxiety experienced in anticipation of another arterial puncture (p: 0.5124, p<0.0001).

Conclusions

The use of 23 G needle allows quicker arterial sampling and is not associated with increased pain and symptoms. Anxiety was correlated with the pain experienced by patients during arterial punctures.

Trial Registration

Clinicaltrials.gov: NCT02320916

Blood microsampling from the ear capillary in non-human primates

Blood sampling from awake non-human primates (NHPs) is classically performed under constraint in the cephalic or saphenous vein. It is a challenging, potentially harmful and stressful procedure which may lead to biased results and raises ethical concerns. Laboratory NHPs undergo a head-restrained procedure allowing for a safer procedure of collecting blood from their ears. Using regular capillary blood collection devices 500 µL of blood can be easily withdrawn per puncture point, which is sufficient for performing most of the usual modern biological assays. This procedure has been validated by measuring total proteins, cortisol and vasopressin concentrations from concomitant blood samples taken from the saphenous vein and the ear capillary vessels of macaques (n = 16). We observed strong correlations between the blood concentrations of total proteins, cortisol and vasopressin (r = 0.72, r = 0.63, r = 0.83, respectively; all P values <0.01) taken from the saphenous vein and from the ear capillary. There were no significant differences between blood concentrations taken from the saphenous vein and the ear capillary. Our alternative to the classical blood collection procedure is harmless and can be routinely performed, which can therefore improve scientific results while increasing animal welfare in accordance with the 3R (replacement, reduction and refinement) principles.

Arterialised earlobe capillary blood gases in the COPD population

Arterialised ear lobe capillary blood (ELCB) gas sampling is a widely used clinical procedure undertaken across both primary and secondary care settings. The prevalence of this sampling method has grown among health professionals, coupled with a growing demand for domiciliary oxygen therapy in the UK, in particular for those who have chronic obstructive pulmonary disease (COPD). Research studies supporting arterialised ELCB gas sampling show inconsistencies in technique, and a survey of respiratory nurses' current practice demonstrated wider inconsistencies. In the absence of national clinical guidelines to direct this practice, and an acknowledged and accepted under-calculation of partial pressure of oxygen, this article investigates the sampling method used to obtain arterialised ELCB gas sampling and consequently questions its reliability in practice.

Pulse oximeter oxygen saturation in prediction of arterial oxygen saturation in liver transplant candidates

BACKGROUND

Liver transplant is the only definitive treatment for many patients with end stage liver disease. Presence and severity of preoperative pulmonary disease directly affect the rate of postoperative complications of the liver transplantation. Arterial blood gas (ABG) measurement, performed in many transplant centers, is considered as a traditional method to diagnose hypoxemia. Because ABG measurement is invasive and painful, pulse oximetry, a bedside, noninvasive and inexpensive technique, has been recommended as an alternative source for the ABG measurement.

OBJECTIVES

The aim of this study was to evaluate the efficacy of pulse oximetry as a screening tool in hypoxemia detection in liver transplant candidates and to compare the results with ABGs.

PATIENTS AND METHODS

Three hundred and ninety transplant candidates (237 males and 153 females) participated in this study. Arterial blood gas oxyhemoglobin saturation (SaO2) was recorded and compared with pulse oximetry oxyhemoglobin saturation (SpO2) results for each participants. The area under the curve (AUC) of receiver operating characteristic (ROC) curves was calculated by means of nonparametric methods to evaluate the efficacy of pulse oximetry to detect hypoxemia.

RESULTS

Roc-derived SpO2 threshold of ≤ 94% can predict hypoxemia (PaO2 < 60 mmHg) with a sensitivity of 100% and a specificity of 95%. Furthermore, there are associations between the ROC-derived SpO2 threshold of ≤ 97% and detection of hypoxemia (PaO2 < 70 mmHg) with a sensitivity of 100% and a specificity of 46%. The accuracy of pulse oximetry was not affected by the severity of liver disease in detection of hypoxemia.

CONCLUSIONS

Provided that SpO2 is equal to or greater than 94%, attained from pulse oximetry can be used as a reliable and accurate substitute for the ABG measurements to evaluate hypoxemia in patients with end stage liver disease.

Analysis of physiological variables during acute hypoxia and maximal stress test in adolescents clinically diagnosed with mild intermittent or mild persistent asthma

OBJECTIVE

To analyze adolescents clinically diagnosed with asthma, in terms of the physiological changes occurring during acute hypoxia and during a maximal stress test.

METHODS

This was a descriptive, cross-sectional study involving 48 adolescents (12-14 years of age) who were divided into three groups: mild intermittent asthma (MIA, n = 12); mild persistent asthma (MPA, n = 12); and control (n = 24). All subjects were induced to acute hypoxia and were submitted to maximal stress testing. Anthropometric data were collected, and functional variables were assessed before and after the maximal stress test. During acute hypoxia, the time to a decrease in SpO2 and the time to recovery of SpO2 (at rest) were determined.

RESULTS

No significant differences were found among the groups regarding the anthropometric variables or regarding the ventilatory variables during the stress test. Significant differences were found in oxygen half-saturation pressure of hemoglobin prior to the test and in PaO2 prior to the test between the MPA and control groups (p = 0.0279 and p = 0.0116, respectively), as was in the oxygen extraction tension prior to the test between the MIA and MPA groups (p = 0.0419). There were no significant differences in terms of the SpO2 times under any of the conditions studied. Oxygen consumption and respiratory efficiency were similar among the groups. The use of a bronchodilator provided no significant benefit during the hypoxia test. No correlations were found between the hypoxia test results and the physiological variables.

CONCLUSIONS

Our findings suggest that adolescents with mild persistent asthma have a greater capacity to adapt to hypoxia than do those with other types of asthma.

Prediction of arterial blood gas values from arterialized earlobe blood gas values in patients treated with mechanical ventilation

Background/Objective:

Arterial blood gas (ABG) analysis is useful in evaluation of the clinical condition of critically ill patients; however, arterial puncture or insertion of an arterial catheter may sometimes be difficult and cause many complications. Arterialized ear lobe blood samples have been described as adequate to gauge gas exchange in acute and chronically ill pediatric patients.

Purpose:

This study evaluates whether pH, partial pressure of oxygen (PO₂), partial pressure of carbon dioxide (PCO₂), base excess (BE), and bicarbonate (HCO₃) values of arterialized earlobe blood samples could accurately predict their arterial blood gas analogs for adult patients treated by mechanical ventilation in an intensive care unit (ICU).

Setting:

A prospective descriptive study

Methods:

Sixty-seven patients who were admitted to ICU and treated with mechanical ventilation were included in this study. Blood samples were drawn simultaneously from the radial artery and arterialized earlobe of each patient.

Results:

Regression equations and mean percentage-difference equations were derived to predict arterial pH, PCO₂, PO₂, BE, and HCO₃-values from their earlobe analogs. pH, PCO₂, BE, and HCO₃ all significantly correlated in ABG and earlobe values. In spite of a highly significant correlation, the limits of agreement between the two methods were wide for PO₂. Regression equations for prediction of pH, PCO₂, BE, and HCO3- values were: arterial pH (pHa) = 1.81+ 0.76 × earlobe pH (pHe) [r = 0.791, P < 0.001]; PaCO₂ = 1.224+ 1.058 × earlobePCO₂ (PeCO₂) [r = 0.956, P < 0.001]; arterial BE (BEa) = 1.14+ 0.95 × earlobe BE (BEe) [r= 0.894, P < 0.001], and arterial HCO₃- (HCO₃-a) = 1.41+ earlobe HCO₃(HCO₃-e) [r = 0.874, P < 0.001]. The predicted ABG values from the mean percentage-difference equations were derived as follows: pHa = pHe × 1.001; PaCO₂ = PeCO₂ × 0.33; BEa = BEe × 0.57; and HCO₃-a = HCO₃-e × 1.06.

Conclusions:

Arterialized earlobe blood gas can accurately predict the ABG values of pH, PCO₂, BE, and HCO₃- for patients who do not require regular continuous blood pressure measurements and close monitoring of arterial PO₂ measurements.

Assessing PaCO2 in acute respiratory disease: accuracy of a transcutaneous carbon dioxide device

BACKGROUND

Pulse oximetry non-invasively assesses the arterial oxygen saturation of patients with acute respiratory disease; however, measurement of the arterial partial pressure of carbon dioxide (PaCO(2)) requires an arterial blood gas. The transcutaneous partial pressure of carbon dioxide (PtCO(2) ) has been used in other settings with variable accuracy. We investigated the accuracy of a PtCO(2) device in the assessment of PaCO(2) in patients with asthma and suspected pneumonia attending the emergency department.

METHODS

Patients with severe asthma (FEV(1) < 50% predicted) or suspected pneumonia (fever, cough and respiratory rate >18/min) were enrolled. Subjects were excluded if they had a history of chronic obstructive pulmonary disease or other conditions associated with respiratory failure. Arterial blood gases were taken at the discretion of the investigator according to clinical need, and paired with a simultaneous reading from the PtCO(2) probe.

RESULTS

Twenty-five patients were studied with one set of data excluded because of poor PtCO(2) signal quality. The remaining 24 paired samples comprised 12 asthma and 12 pneumonia patients. The range of PaCO(2) was 19-64 mmHg with a median of 36.5 mmHg. Bland-Altman analysis showed a mean (SD) PaCO(2) - PtCO(2) difference of -0.13 (1.9) mmHg with limits of agreement of plus or minus 3.8 mmHg (-3.9 to +3.7).

CONCLUSION

A PtCO(2) device was accurate in the assessment of PaCO(2) in patients with acute severe asthma and suspected pneumonia when compared with an arterial blood gas. These bedside monitors have the potential to improve patient care by non-invasively monitoring patients with acute respiratory disease at risk of hypercapnia.

An audit of the patient's experience of arterial blood gas testing

Arterial puncture is the most common method used to obtain a sample for the measurement of arterial blood gases (ABGs) and is essential to guide the prescription of long-term oxygen therapy (LTOT) in patients with chronic hypoxic lung disease. However, this procedure is often reported by patients as a painful and unpleasant experience, which to date has not been explored. This audit specifically examines the subjective views of a small group of patients (n = 41) who are receiving LTOT who have experienced repeated ABGs. Results demonstrated that 49% (n = 20) were poorly informed regarding what the procedure involved, almost half the patients 49% (n = 20) recalled pain levels of 5 and above on a visual analogue scale and 66% (n = 27) were totally unaware that the test could make a considerable difference to their treatment. While highlighting the deficits in current practice locally, this audit concludes that the respiratory nurse specialist is in an ideal position to implement changes to improve the patient's experience of chronic disease management.

Magnitude and Time-Course of Arterio-Venous Differences in Blood-Alcohol Concentration in Healthy Men

BACKGROUND AND OBJECTIVE

Human studies of arterio-venous (AV) differences in drug concentrations and the consequences for pharmacokinetic modelling and concentration-effect relationships are very limited. We therefore investigated the intravenous and intra-arterial concentrations of alcohol (ethanol) during the absorption, distribution and elimination stages of alcohol metabolism in healthy men.

STUDY PARTICIPANTS AND METHODS

Nine male volunteers aged 26-67 years drank 0.6 g alcohol/kg bodyweight in 2-15 minutes. The drink was prepared from 95% v/v alcohol, which was diluted with an alcohol-free beverage to 20% v/v. Before the start of drinking and for 6-7 hours post-administration, blood samples were drawn at 15- to 20-minute intervals from indwelling catheters in a radial artery and a cubital vein on the same arm. The blood-alcohol concentration (BAC) was determined by headspace gas chromatography, and blood-water content was measured by desiccation.

RESULTS

The peak concentration (Cmax) of alcohol in arterial blood was 0.98 g/L (SD 0.209) compared with 0.84 g/L (SD 0.176) for venous blood (p < 0.001), whereas median time to reach Cmax (tmax) was the same (35 minutes). The AV difference was greatest at 10 minutes after the end of drinking (mean 0.20 g/L [range 0.09-0.40 g/L]), decreasing as the absorption of alcohol continued. At a median time of 90 minutes post-administration (range 45-105 minutes), the AV difference was momentarily zero. At later times, the AV differences became increasingly negative and at 280 minutes post-administration the mean was -0.051 g/L (range -0.025 to -0.078 g/L). The slope of the post-absorptive phase (k0) was 0.116 g/L/h (SD 0.0167) for arterial blood compared with 0.109 g/L/h (SD 0.0185) for venous blood (p < 0.001). The extrapolated time to reach zero BAC was 391 minutes (SD 34) for arterial blood and 420 minutes (SD 41) for venous blood; the difference of 29 minutes was statistically highly significant (p < 0.001). The apparent volume of distribution of alcohol, the area under the concentration-time curves (AUC) and the water content of arterial and venous blood samples were not significantly different for the two sampling compartments.

CONCLUSION

The arterial and venous blood-alcohol profiles were shifted in time owing to the time it takes for alcohol to equilibrate between arterial blood and tissue water. Alcohol is metabolised in the liver but not in muscle tissue, which acts as a reservoir for alcohol. The concentrations of alcohol in arterial and venous blood were the same at only one timepoint, which signifies complete equilibration of alcohol in total body water. During the entire post-absorptive phase, the concentration of alcohol in venous blood draining skeletal muscles was slightly greater than the arterial blood concentration; therefore, the AV differences were negative.

Unexpected Nocturnal Hypoxia in Patients With Acute Stroke

Background and Purpose— Patients who have had a stroke are at risk of hypoxia through alterations in the central regulation of respiration, through aspiration, and through respiratory muscle weakness. Sleep-related breathing disorders are common and may lead to episodes of nocturnal hypoxia even when daytime oxygenation is normal. The aim of this study was to assess the prevalence of unexpected nocturnal hypoxia in stroke patients.

Methods— Consecutive adult patients with stroke were recruited within 72 hours of admission to hospital. Patients with indications for oxygen treatment were excluded. Older adults from the local community were recruited as control subjects. Oxygenation was assessed by pulse oximetry (Minolta 3i) for 5 minutes when awake before bedtime and continuously from 11 pm until 7 am .

Results— Of the 238 potentially eligible stroke patients, 120 were excluded because they required oxygen, 118 were recruited, and 100 had adequate pulse oximetry data. The mean±SD age was 74±8 years for stroke patients and 72±8 years for control subjects (n=85). Mean awake oxygen saturation (S o 2 ) was 94.5±1.7% for the stroke group and 95.8±1.7% for the control group ( P <0.001). Mean nocturnal S o 2 was 93.5±1.9% in stroke patients and 94.3±1.9% in control subjects ( P <0.01). Stroke patients had a higher oxygen desaturation index (ODI 4%) (8.9 versus 2.1, P <0.001). In addition, 23% of stroke patients spent >30 minutes with S o 2 <90% during the night.

Conclusions— Oxygen saturation at night is ≈1% lower than when awake. Almost a quarter of stroke patients who are normoxic at screening during the day spend >30 minutes with an oxygen saturation <90%.

The clinical utility of arterialized earlobe capillary blood in the assessment of patients for long-term oxygen therapy

The prescription of long-term oxygen (LTOT) is underpinned by the measurement of arterial PO2, generally obtained by radial artery puncture. This test is commonly associated with patient discomfort and a test that is reliable, well-tolerated and non-invasive would be advantageous. Cutaneous oximetry has not proved sufficiently accurate. Arterialized earlobe capillary sampling has been proposed, with some authors stating that it is under-utilized. However, to date studies have yielded conflicting results and the clinical utility remains uncertain. Our regional oxygen service based at a specialist respiratory hospital undertook a prospective study of consecutive patients with chronic respiratory disease undergoing assessment for LTOT. Simultaneous radial artery and arterialized earlobe sampling was performed. Rigorous steps were taken to ensure optimal arterialization of the earlobe samples. Agreement between arterial and arterialized PO2 and PCO2 was compared using the Bland-Altman method. One hundred patients were studied. Procedural difficulties (insufficient sample or air in sample) were similar for both procedures, however clotting occurred more frequently in arterialized earlobe samples. Sixty-four sample pairs were available for comparison. The bias and limits of agreement between arterialized and arterial PO2 were wide, mean (+/- 2 SD), -048 (-2.05-1.09) kPa. The bias and limits of agreement for PCO2 were smaller. Using the absolute criterion (arterial PO2 < 7.3 kPa), 9/55 (16%) patients would receive oxygen inappropriately based on the arterialized earlobe sample. Conversely, no patients would have been denied LTOT. Radial artery puncture gave rise to significantly greater discomfort (P < 0.0001) and level of concern (P < 0.0001). Patient preference strongly favoured arterialized earlobe sampling. However, despite rigorous attention to arterialization earlobe sampling was insufficiently accurate to replace radial artery puncture in the prescription of LTOT.

Accuracy of cyclosporin measurements made in capillary blood samples obtained by skin puncture

International consensus guidelines suggest that cyclosporin should be measured in whole blood. In some instances it may be advantageous to collect capillary blood, by a finger or ear prick method. However, drug concentrations in skin-puncture blood may not necessarily correlate with those measured in venous blood. This study compared cyclosporin concentrations in blood collected from the fingertip or earlobe with blood collected by standard venipuncture. Patient preference for each of the blood collection methods was also assessed. Specimens were obtained from organ transplant patients receiving cyclosporin, using each of the three methods: venipuncture, finger prick, and earlobe prick. The samples were assayed using a specific radioimmunoassay and the results were compared. In the 102 sets of samples collected, the mean difference (+/- standard deviation) in cyclosporin concentration between finger prick and venipuncture and ear prick and venipuncture was 2.6% (+/- 9.5%) and 2.7% (+/- 12.1%), respectively, while the comparable median (IQR) differences were 1.9% (-3.4% to +6.6%) and -1.1% (-2.8% to +7.2%), respectively. A high degree of correlation was observed between finger prick and venipuncture or ear prick and venipuncture or ear prick and finger prick (r2 > 0.86). Of the three methods of blood collection, finger prick was the patients' preferred method (P < 0.01). These data suggest that capillary blood collected by skin puncture is suitable for use in cyclosporin blood monitoring and acceptable to patients.

Effect of gastric feeding on intragastric P(CO2) tonometry in healthy volunteers

PURPOSE

The tonometric detection of a high intragastric regional P(CO2) (PrCO2) reflecting an elevated intramucosal P(CO2) can be helpful to diagnose mucosal ischemia, if acid secretion is suppressed to avoid intragastric CO2 production through buffering of acid by bicarbonate in the stomach. It is recommended to perform tonometry in the fasting state, but this may hamper feeding of the critically ill. On the other hand, postfeeding tonometry could serve as a diagnostic stress test because feeding increases mucosal blood flow demand, provided that the meal itself does not hamper diffusion of CO2 from mucosa to tonometer balloon and does not generate intragastric CO2, independently from intramucosal P(CO2). We therefore studied the effect of a standard meal on intragastric PrCO2 tonometry in healthy volunteers with suppression of meal-stimulated gastric acid secretion and, presumably, with an adequate mucosal blood flow reserve.

MATERIAL AND METHODS

The gastric juice pH and tonometric PrCO2 were measured in 14 human volunteers, after gastric acid secretion suppression by either ranitidine (100-mg bolus, followed by 25 mg/h i.v., n = 7) or by ranitidine plus pirenzepine (10-mg bolus, followed by 3 mg/h i.v., n=7) to suppress any residual meal-stimulated gastric acid secretion, before and at 30-minute intervals until 120 minutes after oral ingestion of a standard liquid test meal (Pulmocare [Abbott, the Netherlands]; 500 mL, 750 kcal, P(CO2) 5 mm Hg, pH 7.50).

RESULTS

The gastric juice pH, which was >4.0 in all individuals throughout the study, and the PrCO2 did not depend on the regimen for gastric acid secretion suppression, and therefore the data were pooled. The PrCO2 (median [range]) after feeding was 69% (56% to 170%) of baseline (42 [37-51] mm Hg) from 0 to 30 minutes (P < .001), 85% (72% to 167%) of baseline from 30 to 60 (P < .05), 97% (57% to 193%) from 60 to 90 minutes, and 112% (97% to 189%) of baseline from 90 to 120 minutes with a rise above baseline in 10 of 14 patients. In vitro, the liquid test meal generated CO2 after adding bicarbonate but not after hydrochloric acid.

CONCLUSION

We recommend intragastric tonometry to be performed in the fasting state and discourage tonometry after feeding as a stress test, because a single test meal changes tonometric PrCO2 in a time-dependent manner until 2 hours after gastric feeding of healthy volunteers. The fall in PrCO2 directly after feeding can be attributed to dilution, whereas a rise above baseline in some patients may have been caused, as supported by CO2 production after adding bicarbonate to the test meal in vitro, by CO2 production through buffering of meal-derived acid by gastric bicarbonate, in the absence of stimulated gastric acid secretion by feeding.

Arterial lines: an analysis of good practice

The use of arterial line monitoring in critically ill children provides an indispensable foundation of critical care. The information provided by their use ensures accurate monitoring and blood gas analysis and subsequent modifications to prescribed treatment. The invasive nature of intra-arterial catheters may lead to complications of haemorrhage, thrombosis and infection. The value of arterial cannulation in relation to alternative less invasive methods of monitoring is discussed. The literature reveals that in many areas nurses are failing to unite theory and practice. This has implications for the safety of children and the accountability of nurses caring for them.

Transcutaneous monitoring of blood gases: is it comparable with arterialized earlobe sampling?

Researchers are increasingly looking for reliable non-invasive methods of assessing blood gas concentrations, and several new techniques have recently become available. Values derived using arterialized earlobe samples have been found to be comparable with conventional arterial samples, and recent studies have compared transcutaneous blood gas analysis with the traditional arterial samples and found a reasonable level of agreement in particular for the partial pressure of carbon dioxide. There are no data comparing oxygen and carbon dioxide partial pressures (pO2, pCO2) derived from arterialized samples with one of the newer transcutaneous techniques. We therefore simultaneously studied arterialized earlobe blood gas samples and values for pO2 and pCO2 obtained by a transcutaneous monitor (TINA, Radiometer, Copenhagen) in 26 subjects with varying blood gas values. There was a close agreement between the two methods for assessment of pCO2 [mean difference (95% C.I.) between transcutaneous and earlobe values 0.25 kPa (-0.004, 0.5 kPa)], but not for pO2 [1.71 kPa (0.35, 3.07 kPa)]. Similarly, the limits of agreement were narrow for pCO2 compared to those for pO2 (-0.98, 1.47 kPa and -6.44, 3.02 kPa respectively). We conclude that transcutaneous measurement of pCO2 using the TINA is acceptable in the research setting, whereas assessment of pO2 cannot reliably be made using this technique.