Noninvasive monitoring of carbon dioxide: A comparison of the partial pressure of transcutaneous and end-tidal carbon dioxide with the partial pressure of arterial carbon dioxide

This study compares two noninvasive techniques for monitoring the partial pressure of carbon dioxide (Pco2) in 24 anesthetized adult patients. End-tidal PCO₂ (PETCO₂) and transcutaneous Pco₂ (PtcCO₂) were simultaneously monitored and compared with arterial Pco₂ (PaCO₂) determined by intermittent analysis of arterial blood samples. PETCO₂ and PtcCO₂ values were compared with PaCO₂ values corrected to patient body temperature (PaC0_2T) and PaCO₂ values determined at a temperature of 37°C (PaCO₂). Linear regression was performed along with calculations of the correlation coefficient (r), bias, and precision of the four paired variables:PETCO₂ versus PaCO₂ and PaCO_2T (n = 211), and PtcCO₂ versus PaCO₂ and PaCO_2T (n = 233). Bias is defined as the mean difference between paired values, whereas precision is the standard deviation of the difference. The following values were found forr, bias, and ± precision, respectively.PetCO₂ versus PaCO₂: 0.67, ?7.8 mm Hg, ±6.1 mm Hg;PETCO₂ versus PaCO_2T: 0.73, ?5.8 mm Hg, ±5.9 mm Hg;PETCO₂ versus PaCO₂: 0.87, ?1.6 mm Hg, ±4.3 mm Hg; PtcCO₂ versus PaC0_2T: 0.84, +0.7 mm Hg, ±4.8 mm Hg. Although each of thesePCO ₂ variables is physiologically different, there is a significant correlation (P < 0.001) between the noninvasively monitored values and the blood gas values. Temperature correction of the arterial values (PaCO_2T) slightly improved the correlation, with respect toPETCO₂, but it had the opposite effect for PtcCO₂. In this study, the chief distinction between these two noninvasive monitors was thatPETCO₂ had a large negative bias, whereas PtcCO₂ had a small bias. We conclude from these data that PtcCO₂ may be used to estimate PaCO₂ with an accuracy similar to that ofPetCO₂ in anesthetized patients.     

Transcutaneous carbon dioxide and oxygen tension in newborn infants: Reliability of a combined monitor of oxygen tension and carbon dioxide tension

We evaluated a new combined sensor for monitoring transcutaneous carbon dioxide tension (PtcCO₂) and oxygen tension (PtcO₂) in 20 critically ill newborn infants. Arterial oxygen tension (PaO₂) ranged from 16 to 126 torr and arterial carbon dioxide tension (PaCO₂) from 14 to 72 torr. Linear correlation analysis (100 paired values) of PtcO₂ versus PaO₂ showed anr value of 0.75 with a regression equation of PtcO₂=8.59+0.905 (PaO₂), while PtcCO₂ versus PaCO₂ revealed a correlation coefficient ofr=0.89 with an equation of PtcCO₂=2.53+1.06 (PaCO₂). The bias between PaO₂ and PtcO₂ was –2.8 with a precision of ±16.0 torr (range, –87 to +48 torr). The bias between PaCO₂ and PtcCO₂ was –5.1 with a precision of ±7.3 torr (range, –34 to +8 torr). The transcutaneous sensor detected 83% of hypoxia (PaO₂ <45 torr), 75% of hyperoxia (PaO₂ >90 torr), 45% of hypocapnia (PaCO₂ <35 torr), and 96% of hypercapnia (PaCO₂ >45 torr). We conclude that the reliability of the combined transcutaneousPo ₂ andPCo ₂ monitor in sick neonates is good for detecting hypercapnia, fair for hypoxia and hyperoxia, but poor for hypocapnia. It is an improvement in that it spares available skin surface and requires less handling, but it appears to be slightly less accurate than the single electrodes.     

Continuous tracheal gas insufflation enables a volume reduction strategy in hyaline membrane disease: technical aspects and clinical results

Objective: Instrumental dead space wash-out can be used to improve carbon dioxide clearance. The aim of this study was to define, using a bench test, an optimal protocol for long-term use, and to assess the efficacy of this technique in neonates. Design: A bench test with an artificial lung model, and an observational prospective study. Dead space wash-out was performed by continuous tracheal gas insufflation (CTGI), via six capillaries molded in the wall of a specially designed endotracheal tube, in 30 preterm neonates with hyaline membrane disease. Setting: Neonatal intensive care unit of a regional hospital. Results: The bench test study showed that a CTGI flow of 0.5 l/min had the optimal efficacy-to-side-effect ratio, resulting in a maximal or submaximal efficacy (93 to 100 %) without a marked increase in tracheal and CTGI circuit pressures. In the 30 newborns, 15 min of CTGI induced a significant fall in arterial carbon dioxide tension (PaCO₂), from 45 ± 7 to 35 ± 5 mmHg (p = 0.0001), and in 14 patients allowed a reduction in the gradient between Peack inspirating pressure and positive end-expiratory pressure from 20.8 ± 4.6 to 14.4 ± 3.7 cmH₂O (p < 0.0001) while keeping the transcutaneous partial pressure of carbon dioxide constant. As predicted by the bench test, the decrease in PaCO₂ induced by CTGI correlated well with PaCO₂ values before CTGI (r = 0.58, p < 0.002) and with instrumental dead space-to-tidal volume ratio (r = 0.54, p < 0.005). Conclusion: CTGI may be a useful adjunct to conventional ventilation in preterm neonates with respiratory disease, enabling an increase in CO₂ clearance or a reduction in ventilatory pressure. Received: 11 September 1997 Accepted: 21 April 1998     

Effects of high-frequency oscillatory ventilation on circulation in neonates with pulmonary interstitial emphysema or RDS

Objective: Mechanical ventilation may impair cardiovascular function if the transpulmonary pressure rises. Studies on the effects of high-frequency oscillatory ventilation (HFOV) on cardiovascular functions have yielded conflicting results. This study was done to compare alterations in left ventricular output and blood flow velocities in the anterior cerebral artery, internal carotid artery, and celiac artery using a Doppler ultrasound divice before and 2 h after initiating HFOV in neonates with respiratory distress syndrome (RDS) or pulmonary interstitial emphysema (PIE). Design: Prospective clinical study. Setting: Neonatal intensive care unit in a perinatal center. Patients: 18 critically ill infants (postnatal age 47 ± 12 h; mean ± SD) were studied before and during HFOV (piston oscillator). Indications for HFOV were severe respiratory failure due to PIE (n = 10) and severe surfactant deficiency (RDS, n = 8). In the RDS group, gestational age was 27 ± 6 weeks (range 26–31 weeks) and birthweight 1620 ± 380 g (range 850–1970 g). In the PIE group, gestational age was 28 ± 2 weeks (range 26–36 weeks) and birthweight 1740 ± 470 g (range 890–2760 g). Measurements and main results: During HFOV, mean airway pressure was maintained at the same level as during intermittent mandatory ventilation in both groups (RDS, 12 ± 2 cmH₂O; PIE, 10 ± 2 cmH₂O). Compared to intermittent mandatory ventilation, several of the 12 parameters studied changed significantly (p < 0.004) during HFOV. In the RDS group, the partial pressure of oxygen in arterial blood/fractional inspired oxygen (PaO₂/FIO₂) ratio increased from 56 ± 9 to 86 ± 7 and partial pressure of carbon dioxide in arterial blood (PaCO₂) decreased from 49 ± 4 to 35 ± 3 mmHg. In the PIE group, PaO₂/FIO₂ ratio increased from 63 ± 8 to 72 ± 7 and PaCO₂ decreased from 63 ± 7 to 40 ± 5 mmHg. In the PIE group, heart rate decreased (135 ± 15 before HFOV vs 115 ± 14 min^{− 1} during HFOV) and mean systolic blood pressure increased (before 43 ± 4 vs 51 ± 4 mmHg during HFOV) significantly, whereas these parameters did not change in the RDS group. Left ventricular output increased significantly in the PIE group (210 ± 34 before vs 245 ± 36 ml/kg per min during HFOV; p < 0.004), but not in the RDS group (225 ± 46 before vs 248 ± 47 ml/kg per min during HFOV; k < 0.05). Shortening fraction and systemic resistance did not change in either group. In the PIE group, mean blood flow velocities in the internal carotid artery (+ 59 %), anterior cerebral artery (+ 65 %) and celiac artery (+ 45 %) increased significantly but did not change in the RDS group. Conclusions: The results show that HFOV as used in this study, improves oxygenation, CO₂ elimination, and circulation in infants with RDS and PIE. However, systemic, cerebral, and intestinal circulation improved more in neonates with PIE than in those with RDS. This may be due to higher pulmonary compliance in infants with PIE when compared to those with RDS. Received: 15 May 1996 Accepted: 31 January 1997     

Acute effects of continuous rotational therapy on ventilation-perfusion inequality in lung injury

Objective: To investigate ventilation-perfusion (V_A/Q) relationships, during continuous axial rotation and in the supine position, in patients with acute lung injury (ALI) using the multiple inert gas elimination technique. Design: Prospective investigation. Setting: Eighteen-bed intensive care unit in a university hospital. Patients and interventions: Ten patients with ALI (PaO₂/FIO₂ ratio < 300 mm Hg) were mechanically ventilated in a pressure controlled mode and placed on a kinetic treatment table. Measurements and results: Distributions of V_A/Q were determined 1) during rotation (after a period of 20 min) and 2) after a resting period of 20 min in the supine position. During axial rotation, intrapulmonary shunt (19.1 ± 15 % of cardiac output) was significantly reduced in comparison with when in the supine position (23 ± 14 %, p < 0.05), areas with “low” V_A/Q were not affected by the positioning maneuver. General V_A/Q mismatch (logarithmic distribution of pulmonary blood flow) was decreased during rotation (0.87 ± 0.37) in comparison with when the patient was in the supine position (0.93 ± 0.37, p < 0.05). Arterial oxygenation was significantly improved during continuous rotation (PaO₂/FIO₂ = 217 ± 137 mm Hg) as compared with in the supine position (PaO₂/FIO₂ = 174 ± 82 mm Hg, p < 0.05). The positive response of the continuous rotation on arterial oxygenation was only demonstrated in patients with a Murray Score of 2.5 or less, indicating a “mild to moderate” lung injury, while in patients presenting with progressive ARDS (Murray Score > 2.5), the acute positive response was limited. Conclusions: Continuous axial rotation might be a method for an acute reduction of V_A/Q mismatch in patients with mild to moderate ALI, but this technique is not effective in late or progressive ARDS. Further studies including a large data collection are needed. Received: 19 June 1997 Accepted: 6 November 1997     

Transcutaneous P<Emphasis Type="SmallCaps">tc</Emphasis>CO<Subscript>2</Subscript> measurement in combination with arterial blood gas analysis provides superior accuracy and reliability in ICU patients

Hyper or hypoventilation may have serious clinical consequences in critically ill patients and should be generally avoided, especially in neurosurgical patients. Therefore, monitoring of carbon dioxide partial pressure by intermittent arterial blood gas analysis (PaCO₂) has become standard in intensive care units (ICUs). However, several additional methods are available to determine PCO₂ including end-tidal (PetCO₂) and transcutaneous (PtcCO₂) measurements. The aim of this study was to compare the accuracy and reliability of different methods to determine PCO₂ in mechanically ventilated patients on ICU. After approval of the local ethics committee PCO₂ was determined in n = 32 ICU consecutive patients requiring mechanical ventilation: (1) arterial PaCO₂ blood gas analysis with Radiometer ABL 625 (ABL; gold standard), (2) arterial PaCO₂ analysis with Immediate Response Mobile Analyzer (IRMA), (3) end-tidal PetCO₂ by a Propaq 106 EL monitor and (4) transcutaneous PtcCO₂ determination by a Tina TCM4. Bland–Altman method was used for statistical analysis; p < 0.05 was considered statistically significant. Statistical analysis revealed good correlation between PaCO₂ by IRMA and ABL (R² = 0.766; p < 0.01) as well as between PtcCO₂ and ABL (R² = 0.619; p < 0.01), whereas correlation between PetCO₂ and ABL was weaker (R² = 0.405; p < 0.01). Bland–Altman analysis revealed a bias and precision of 2.0 ± 3.7 mmHg for the IRMA, 2.2 ± 5.7 mmHg for transcutaneous, and ?5.5 ± 5.6 mmHg for end-tidal measurement. Arterial CO₂ partial pressure by IRMA (PaCO₂) and PtcCO₂ provided greater accuracy compared to the reference measurement (ABL) than the end-tidal CO₂ measurements in critically ill in mechanically ventilated patients patients.     

High survival rate in 122 ARDS patients managed according to a clinical algorithm including extracorporeal membrane oxygenation

Objective: We investigated whether a treatment according to a clinical algorithm could improve the low survival rates in acute respiratory distress syndrome (ARDS). Design: Uncontrolled prospective trial. Setting: One university hospital intensive care department. Patients and participants: 122 patients with ARDS, consecutively admitted to the ICU. Interventions: ARDS was treated according to a criteria-defined clinical algorithm. The algorithm distinguished two main treatment groups: The AT-sine-ECMO (advanced treatment without extracorporeal membrane oxygenation) group (n = 73) received a treatment consisting of a set of advanced non-invasive treatment options, the ECMO treatment group (n = 49) received additional extracorporeal membrane oxygenation (ECMO) using heparin-coated systems. Measurements and results: The groups differed in both APACHE II (16 ± 5 vs 18 ± 5 points, p = 0.01) and Murray scores (3.2 ± 0.3 vs 3.4 ± 0.3 points, p = 0.0001), the duration of mechanical ventilation prior to admission (10 ± 9 vs 13 ± 9 days, p = 0.0151), and length of ICU stay in Berlin (31 ± 17 vs 50 ± 36 days, p = 0.0016). Initial PaO₂/FIO₂ was 86 ± 27 mm Hg in AT-sine-ECMO patients that improved to 165 ± 107 mm Hg on ICU day 1, while ECMO patients showed an initial PaO₂/FIO₂ of 67 ± 28 mm Hg and improvement to 160 ± 102 mm Hg was not reached until ICU day 13. Q˙_S/Q˙_T was significantly higher in the ECMO-treated group and exceeded 50 % during the first 14 ICU days. The overall survival rate in our 122 ARDS patients was 75 %. Survival rates were 89 % in the AT-sine ECMO group and 55 % in the ECMO treatment group (p = 0.0000). Conclusions: We conclude that patients with ARDS can be successfully treated with the clinical algorithm and high survival rates can be achieved. Received: 9 April 1997 Accepted: 13 May 1997     

Prognosis value of partial arterial oxygen pressure in patients with septic shock subjected to pre-hospital invasive ventilation

Objective

Mechanical ventilation can help improve the prognosis of septic shock. While adequate delivery of oxygen to the tissue is crucial, hyperoxemia may be deleterious. Invasive out-of-hospital ventilation is often promptly performed in life-threatening emergencies. We propose to determine whether the arterial oxygen pressure (PaO₂) at the intensive care unit (ICU) admission is associated with mortality in patients with septic shock subjected to pre-hospital mechanical ventilation.

Transcutaneous arterial carbon dioxide pressure monitoring in critically ill adult patients

Objective To evaluate the accuracy of transcutaneous PCO₂ (PtcCO₂) as a surrogate for arterial PCO₂ (PaCO₂) in a cohort of adult critically ill patients in a medical intensive care unit (ICU). Design Prospective observational study comparing paired measures of transcutaneous and arterial PCO₂. Setting A 26-bed medical ICU. Patients Fifty ICU patients monitored with a SenTec Digital Monitor placed at the ear lobe over prolonged periods. Results A total of 189 paired PCO₂ measures were obtained. Twenty-one were excluded from analysis, because profound skin vasoconstriction was present (PCO₂ bias = −10.8 ± 21.8  mmHg). Finally, 168 were analysed, including 137 obtained during mechanical ventilation and 82 under catecholamine treatment. Body temperature was below 36°C for 27 measurements. Mean duration of monitoring was 17 ± 17 h. The mean difference between PaCO₂ and PtcCO₂ was −0.2 ± 4.6  mmHg with a tight correlation (R ² = 0.92, p > 0.01). PCO₂ bias did not significantly change among three successive measurements. Changes in PaCO₂ and in PtcCO₂ between two blood samples were well correlated (R ² = 0.78, p > 0.01). Variations of more than 8 mmHg in PtcCO₂ had 86% sensitivity and 80% specificity to correctly predict similar changes in PaCO₂ in the same direction. Catecholamine dose or respiratory support did not affect PtcCO₂ accuracy. Hypothermia has only a small effect on accuracy. No complication related to a prolonged use of the sensor was observed Conclusion Transcutaneous PCO₂ provides a safe and reliable trend-monitoring tool, provided there is no major vasoconstriction. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.     

Effects of noninvasive positive pressure ventilatory support in non-COPD patients with acute respiratory insufficiency after early extubation

Objective: To investigate the effects of noninvasive positive pressure ventilation (NPPV) on pulmonary gas exchange, breathing pattern, intrapulmonary shunt fraction, oxygen consumption, and resting energy expenditure in patients with persistent acute respiratory failure but without chronic obstructive pulmonary disease (COPD) after early extubation. Design: Prospective study. Setting: Multidisciplinary intensive care unit of a university hospital. Patients: 15 patients after prolonged mechanical ventilation (> 72 h) with acute respiratory insufficiency after early extubation. Interventions: Criteria for early extubation were arterial oxygen tension (PaO₂) L 40 mm Hg (fractional inspired oxygen 0.21), arterial carbon dioxide tension (PaCO₂) K 55 mm Hg, pH > 7.32, respiratory rate K 40 breaths per min, tidal volume (V_T) L 3 ml/kg, rapid shallow breathing index K 190 and negative inspiratory force L 20 cmH₂O. After extubation, two modes of NPPV were applied [continuous positive airway pressure (CPAP) of 5 cmH₂O and pressure support ventilation (PSV) with 15 cmH₂O pressure support]. Measurements and main results: Oxygenation and ventilatory parameters improved during both modes of NPPV (p < 0.05): increase in PaO₂ of 11 mm Hg during CPAP and 21 mm Hg during PSV; decrease in intrapulmonary shunt fraction of 7 % during CPAP and 12 % during PSV; increase in tidal volume of 1 ml/kg during CPAP and 4 ml/kg during PSV; decrease in respiratory rate 6 breaths/min during CPAP and 9 breaths/min during PSV. Oxygen consumption (15 % during CPAP, 22 % during PSV) and resting energy expenditure (12 % during CPAP, 20 % during PSV) were reduced (p < 0.05). PaCO₂ decreased, whereas minute ventilation and pH increased during PSV (p < 0.05). The median duration of NPPV was 2 days. Two patients had to be reintubated. Conclusions: In non-COPD patients with persistent acute respiratory failure after early extubation, NPPV improved pulmonary gas exchange and breathing pattern, decreased intrapulmonary shunt fraction, and reduced the work of breathing. Received: 14 May 1999 Final revision received: 25 June 1999 Accepted: 6 July 1999     

Reduction of ventilator settings allowed by intravenous oxygenator (IVOX) in ARDS patients

Objective To evaluate the possibility of reducing ventilator settings to “safe” levels by extrapulmonary gas exchange with IVOX in ARDS patients. Design Uncontrolled open clinical study. Setting Medical Intensive Care Unit of a University Hospital. Patients 6 patients with ARDS who entered into IVOX phase II clinical trials. Interventions The end-point of this study was to reduce ventilator settings from the initial values, recorded on the day of inclusion, to the following: peak inspiratory pressure <40 cmH₂O, mean airway pressure <25 cmH₂O and tidal volume <10 ml/kg. Trials to achieve this goal were made on volume-controlled ventilation within the 24h before and after IVOX insertion. Comparison of the results achieved during these trials used wilcoxon test. Results Before IVOX implantation reduction of ventilator settings was not possible in the 6 patients, despite a non-significant increase in PaO₂/FIO₂ was achieved. IVOX permitted significant decrease in PaCO₂ (from 60.5±15 to 52±11 mmHg;p=0.02) before any modification of the ventilatory mode. After IVOX insertion, a significant decrease of the ventilator settings was performed: peak and mean airway pressures dropped from 44±10 to 36.8±6.7;p=0.02 and from 26.3±5.6 to 22.5±3.9 cmH₂O;p=0.02, respectively. Concommitantly, PaCO₂ remained unchanged and PaO₂/FIO₂ increased significantly from 93±28 to 117±52;p=0.04. The interruption of oxygen flow on IVOX was associated with a slight decrease of the oxygen variables. Tolerance of IVOX was satisfactory. However, a significant decrease both in cardiac index and in pulmonary wedge pressures (from 4.5±1.2 to 3.4±9;p=0.03 and from 16±5 to 11±2;p=0.04, respectively) was observed. Conclusion Gas exchange achieved by IVOX allowed reduction of ventilator settings in 6 ARDS patients in whom previous attempts have failed. CO₂ removal by the device, may explain these results. Efficacy of IVOX on arterial oxygenation was uncertain.     

Novel Use of a Gas Analyzer Can Reliably Predict the Arterial Oxygen among Emergency Department Patients Undergoing Rapid Sequence Intubation

BackgroundTo our knowledge, no study has assessed the correlation of fraction of inspired oxygen (FiO₂) and end-tidal oxygen (EtO₂) values obtained from a gas analyzer during the preoxygenation period of rapid sequence intubation (RSI) to predict partial pressure of oxygen (PaO₂) among patients requiring intubation in the emergency department (ED).ObjectiveThe purpose of this study was to determine whether a simple equation using EtO₂ and FiO₂ at time of induction could reliably estimate minimal PaO₂ in ED patients undergoing RSI.MethodsWe conducted an observational pilot study performed in an adult ED utilizing a gas analyzer to obtain EtO₂ and FiO₂ values in ED patients undergoing RSI from data collectors blinded to our objective. The Pearson correlation coefficient was calculated between the equation's predicted PaO₂ and the PaO₂ drawn from an arterial blood gas shortly after intubation. A Bland-Altman plot analysis was performed to identify any additional bias.ResultsSeventy-five patients were enrolled. The equation's mean predicted minimal PaO₂ and mean PaO₂ from an arterial blood gas within 3 min after intubation was 178 mm Hg (95% confidence interval [CI] 145–211 mm Hg) and 209 mm Hg (95% CI 170–258 mm Hg), respectively. The Pearson correlation coefficient between the predicted minimal PaO₂ and post-intubation PaO₂ demonstrated a strong correlation (r² = 0.89). The Bland-Altman plot indicated no bias affecting the correlation between the predicted and actual PaO₂.ConclusionsAmong ED patients undergoing RSI, the use of a gas analyzer to measure EtO₂ and FiO₂ can provide a reliable measure of the minimal PaO₂ at the time of induction during the RSI phase of preoxygenation.     

Comparison of different methods for dead space measurements in ventilated newborns using CO2-volume plot

Objective: The aim of the study was to test the applicability of Ventrak 1550/Capnogard 1265 (V-C) for respiratory dead space (Vd) measurement and to determine anatomic (V_Dana), physiologic (V_Dphys), and alveolar dead spaces (V_Dalv) in ventilated neonates. Design: Prospective study. Setting: Neonatal intensive care unit. Patients: 33 investigations in 22 ventilated neonates; median gestational age 34.5 weeks (range 27–41), median birthweight 2658 g (range 790–3940). Method: The single-breath CO₂ test (SBT-CO₂) and transcutaneous partial pressure of carbon dioxide (PCO₂) were recorded simultaneously and Vd was determined (1) automatically (V-C software), (2) by interactive analysis of the PCO₂ volume plot, and (3) manually by Bohr/Enghoff equations using data obtained by V-C. Results: Vd measurements were possible in all cases by method 3 but not possible by methods 1 and 2 in 22 of 33 investigations (67 %), especially in preterm neonates, because of disturbed signals. V_Dana/kg (1.6 ± 0.6 ml/kg, mean ± SD), V_Dana/tidal volume (Vt) (0.36 ± 0.09) were lower compared to published data in spontaneously breathing infants, whereas V_Dphys/kg (2.3 ± 0.9 ml/kg) and V_Dphys/Vt (0.50 ± 0.12) are comparable to data obtained from the literature. Five minutes after insertion of the sensor (dead space 2.6 ml) into the ventilatory circuit, the transcutaneous PCO₂ rose above baseline for 3.2 % (patients > 2500 g) and 5.7 % (patients < 2500 g). The time necessary for one analysis was 50–60 min. Conclusion: In ventilated newborns, dead space measurements were possible only in one-third by SBT-CO₂, but in all cases by Bohr/Enghoff equations. Improved software could further reduce the time needed for one analysis. Received: 8 December 1998 Accepted: 19 April 1999     

Progress in the development of a fluorescent intravascular blood gas system in man

In vitro and in vivo animal studies have shown accurate measurements of arterial blood pH (pHa), carbon dioxide tension (PaCO₂), and oxygen tension (PaO₂) with small intravascular fluorescent probes. Initial human clinical studies showed unexplained intermittent large drops in sensor oxygen tension (PiO₂). Normal volunteers were studied to elucidate this problem. In the first part of this study, the probe and cannula were manipulated and the probe configuration and its position within the cannula were varied. The decreases in PiO₂ were judged to be primarily due to the sensor touching the arterial wall. Retraction of the sensor tip within the cannula eliminated the problem. In the second part of this study, the accuracy of the retracted probe was evaluated in 4 subjects who breathed varying fractions of inspired oxygen and carbon dioxide. The arterial ranges achieved were 7.20 to 7.59 for pH, 22 to 70 mm Hg for PaCO₂, and 46 to 633 mm Hg for PaO₂. Linear regression of 48 paired sensor (i) versus arterial values showed pHi = 0.896 pHa + 0.773 (r = 0.98, SEE = 0.017); PiCO₂ = 1.05 PaCO₂-1.33 (r = 0.98, SEE = 2.4 mm Hg); and PiO₂ = 1.09 PaO₂-20.6 (r = 0.99, SEE = 21.2 mm Hg). Bias (defined as the mean differences between sensor and arterial values) and precision (SD of differences) were, respectively, -0.003 and 0.02 tor pHi, 0.77 and 2.44 mm Hg for PiCO₂, and -2.9 and 25.4 mm Hg for PiO2. The mean in vivo 90% response times for step changes in inspired gas were 2.64, 3.88, and 2.60 minutes, respectively, for pHi, PiCO₂, and PiO₂.     

Bradykinin Production and Increased Pulmonary Endothelial Permeability during Acute Respiratory Failure in Unanesthetized Sheep

To investigate mechanisms of pulmonary edema in respiratory failure, we studied unanesthetized sheep with vascular catheters, pleural balloons, and chronic lung lymph fistulas. Animals breathed either a hypercapnic-enriched oxygen (n = 5) or a hypercapnic-hypoxic (n = 5) gas mixture for 2 h. Every 15 min blood gases, pressures, cardiac output, lymph flow (Qlym), plasma and lymph albumin (mol wt, 70,000), IgG (mol wt, 150,000), IgM (mol wt, 900,000), and blood bradykinin concentrations were determined. In both groups, cardiac output and pulmonary arterial pressures increased, whereas left atrial pressures were unchanged. Acidosis alone (arterial pH = 7.16, PaCO₂ = 81 mm Hg, PaO₂ = 250 mm Hg) resulted in a doubling of lymph flow, a small increase in protein flux, and a decrease in lymph to plasma protein concentration (L/P) ratio for all three proteins. Acidotic-hypoxic animals (arterial pH = 7.16, PaCO₂ = 84 mm Hg, PaO₂ = 48 mm Hg) tripled Qlym. In these animals the increase in lymphatic flux of albumin, IgG, and IgM was significantly (P < 0.05) greater than that seen in either the acidosis alone group or in animals where left atrial pressures were elevated (n = 5; P < 0.05). Also, their percent increase in flux of the large protein (IgM) was greater than for the small protein (albumin) (P < 0.05). With acidosis alone, only pulmonary arterial bradykinin concentration increased (1.27±0.25 ng/ml SE), whereas acidosis plus hypoxia elevated both pulmonary arterial bradykinin concentrations (4.83±1.14 ng/ml) and aortic bradykinin concentration (2.74±0.78 ng/ml). These studies demonstrate that hypercapnic acidosis stimulates in vivo production of bradykinin. With superimposed hypoxia, and therefore decreased bradykinin degradation, there is an associated sustained rise in Qlym with increased lung permeability to proteins.     

Accuracy of end-tidal carbon dioxide tension analyzers

Substantial mean differences between arterial carbon dioxide tension (PaCO₂) and end-tidal carbon dioxide tension (PetCO₂) in anesthesia and intensive care settings have been demonstrated by a number of investigators. We have explored the technical causes of error in the measurement ofPetCO₂ that could contribute to the observed differences. In a clinical setting, the measurement ofPetCO₂ is accomplished with one of three types of instruments, infrared analyzers, mass spectrometers, and Raman spectrometers, whose specified accuracies are typically ±2, ±1.5, and ±0.5 mm Hg, respectively. We examined potential errors inPetCO₂ measurement with respect to the analyzer, sampling system, environment, and instrument. Various analyzer error sources were measured, including stability, warm-up time, interference from nitrous oxide and oxygen, pressure, noise, and response time. Other error sources, including calibration, resistance in the sample catheter, pressure changes, water vapor, liquid water, and end-tidal detection algorithms, were considered and are discussed. On the basis of our measurements and analysis, we estimate the magnitude of the major potential errors for an uncompensated infrared analyzer as: inaccuracy, 2 mm Hg; resolution, 0.5 mm Hg; noise, 2 mm Hg; instability (12 hours), 3 mm Hg; miscalibration, 1 mm Hg; selectivity (70% nitrous oxide), 6.5 mm Hg; selectivity (100% oxygen), −2.5 mm Hg; atmospheric pressure change, <1 mm Hg; airway pressure at 30 cm H₂O, 2 mm Hg; positive end-expiratory pressure or continuous positive airway pressure at 20 cm H₂O, 1.5 mm Hg; sampling system resistance, <1 mm Hg; and water vapor, 2.5 mm Hg. In addition to these errors, other systematic mistakes such as an inaccurate end-tidal detection algorithm, poor calibration technique, or liquid water contamination can lead to gross inaccuracies. In a clinical setting, unless the user is confident that all of the technical error sources have been eliminated and the physiologic factors are known, depending onPetCO₂ to determine PaCO₂ is not advised. An erratum to this article is available at .     

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

End-tidal carbon dioxide (ETCO₂) values obtained from awake nonintubated patients may prove to be useful in estimating a patient’s ventilatory status. This study examined the relationship between arterial carbon dioxide tension (PaCO₂) and ETCO₂ during the preoperative period in 20 premedicated patients undergoing various surgical procedures. ETCO₂ was sampled from a 16-gauge intravenous catheter pierced through one of the two nasal oxygen prongs and measured at various oxygen flow rates (2, 4, and 6 L/min) by an on-line ETCO₂ monitor with analog display. Both peak and time-averaged values for ETCO₂ were recorded. The results showed that the peak ETCO₂ values (mean = 38.8 mm Hg) correlated more closely with the PaCO₂ values (mean = 38.8 mm Hg; correlation coefficient r = 0.76) than did the average ETCO₂ values irrespective of the oxygen flow rates. The time-averaged PaCO₂-ETCO₂ difference was significantly greater than the PaCO₂-peak ETCO₂ difference (P < 0.001). Values for subgroups within the patient population were also analyzed, and it was shown that patients with minute respiratory rates greater than 20 but less than 30 and patients age 65 years or older did not differ from the overall studied patient population with regard to PaCO₂-ETCO₂ difference. A small subset of patients with respiratory rates of 30/ min or greater (n = 30) did show a significant increase in the PaCO₂-ETCO₂ difference (P < 0.001). It was concluded that under the conditions of this study, peak ETCO₂ values did correlate with PaCO₂ values and were not significantly affected by oxygen flow rate. However, obtaining peak ETCO₂ values is clinically more difficult, especially when partial air-way obstruction is present.     

Accuracy of expiratory carbon dioxide measurements using the coaxial and circle breathing circuits in small subjects

Mass spcctrometry is widely used to measure the end-tidal concentrations of inhalation anesthetics and other gases during surgery in order to estimate their arterial concentrations. When certain breathing circuits are used in newborns, however, fresh gas or ambient air may contaminate the expired sample, introducing a systematic error in the measurement of any end-tidal gas concentration. We estimated this error in newborn piglets using carbon dioxide as an indicator substance of expired gas. The capnograms and the difference between arterial carbon dioxide tension (PaCO₂) and peakexpired carbon dioxide tension (PeCO₂) were compared when either a coaxial (Bain) or circle breathing circuit was used. Gas was sampled from the proximal airway and distal trachea. No combination of circuit and sampling site produced a flat alveolar phase until the circle circuit was modified with diversion valves to reduce gas mixing. The mean PaCO₂-PeCO₂ gradients using the coaxial/proximal sampling, coaxial/distal sampling, and modified circle/proximal sampling circuits were 12.4, 9.2, and 8.8 mm Hg, respectively. The mean PeCO₂ in each of these combinations was significantly different from the corresponding mean PaCO₂ (p<0.05). Using the modified circle circuit with distal sampling, mean PeCO₂ was not significantly different from mean PaCO₂: the mean PaCO₂-PcCO₂ gradient was 2.2 ± 0.2 mm Hg (SEM), range, 0 to 6 mm Hg, with 95% confidence limits ⩽ 8 mm Hg. When a coaxial breathing circuit is used in small subjects, PaCO₂ may be significantly underestimated regardless of sampling site, although the circle breathing circuit with distal tracheal sampling yields accurate results. Supported in part by BRS Grant SO RR05507-20 from the Biomedical Research Support Grant Program, Division of Research Resources, National Institutes of Health, and by the American Heart Association, Lancaster, PA Chapter. The authors thank Robert Hirsch, PhD, for his statistical advice, and Greg Harris and Perkin-Elmer, Inc for loaning the mass spectrometer.     

Heat and moisture exchangers and heated humidifiers in acute lung injury/acute respiratory distress syndrome patients. Effects on respiratory mechanics and gas exchange

Objective To compare, in acute lung injury/acute respiratory distress syndrome (ALI/ARDS) patients, the short-term effects of heat and moisture exchangers (HME) and heated humidifiers (HH) on gas exchange, and also on respiratory system mechanics when isocapnic conditions are met. Design Prospective open clinical study. Setting Intensive Care Service. Patients Seventeen invasively ventilated ALI/ARDS patients. Intervention The study was performed in three phases: (1) determinations were made during basal ventilatory settings with HME; (2) basal ventilatory settings were maintained and HME was replaced by an HH; (3) using the same HH, tidal volume (Vt) was decreased until basal PaCO₂ levels were reached. FiO₂, respiratory rate and PEEP were kept unchanged. Measurements and results Respiratory mechanics, Vd_phys, gas exchange and hemodynamic parameters were obtained at each phase. By using HH instead of HME and without changing Vt, PaCO₂ decreased from 46 ± 9 to 40 ± 8 mmHg (p < 0.001) and Vd_phys decreased from 352 ± 63 to 310 ± 74 ml (p < 0.001). Comparing the first phase with the third, Vt decreased from 521 ± 106 to 440 ± 118 ml (p < 0.001) without significant changes in PaCO₂, Vd/Vt decreased from 0.69 ± 0.11 to 0.62 ± 0.12 (p < 0.001), plateau airway pressure decreased from 25 ± 6 to 21 ± 6 cmH₂O (p < 0.001) and respiratory system compliance improved from 35 ± 12 to 42 ± 15 ml/cmH₂O (p < 0.001). PaO₂ remained unchanged in the three phases. Conclusions Reducing dead space with the use of HH decreases PaCO₂ and more importantly, if isocapnic conditions are maintained by reducing Vt, this strategy improves respiratory system compliance and reduces plateau airway pressure     

Improvement in oxygenation by prone position and nitric oxide in patients with acute respiratory distress syndrome

Objective: Inhaled nitric oxide (NO) and prone position improve arterial oxygenation in patients with the acute respiratory distress syndrome. This study was undertaken to assess the combined effects of NO and prone position in these patients. Design: Prospective clinical study. Setting: General intensive care service in a community teaching hospital. Patients: 14 mechanically ventilated adult patients with the acute respiratory distress syndrome (mean lung injury score 3.23 ± 0.27). Measurements and results: We measured hemodynamic and oxygenation parameters in the supine position and 2 h later in the prone position, before and during inhalation of 10 ppm NO. A positive response in oxygenation was defined as a ≥ 20 % increment in the arterial oxygen tension/fractional inspired oxygen ratio (PaO₂/FIO₂). In the prone position PaO₂/FIO₂ increased significantly (from 110 ± 55 to 161 ± 89 mmHg, p < 0.01) and venous admixture decreased (from 38 ± 12 to 30 ± 7 %, p < 0.01) compared to the supine position. Ten of the 14 patients were responders in the prone position. In the supine position, inhalation of NO improved oxygenation to a lesser extent, increasing PaO₂/FIO₂ to 134 ± 64 mmHg (p < 0.01) and decreasing venous admixture to 35 ± 12 %, (p < 0.01). Five of the 14 patients responded to NO inhalation supine and 8 of 14 responded prone (p = 0.22). The combination of NO therapy and prone positioning was additive in increasing PaO₂/FIO₂ (197 ± 92 mmHg) and decreasing venous admixture (27 ± 8 %) (p < 0.01). This combination also showed a positive oxygenation response on compared to the supine value without NO in 13 of the 14 patients (93 %). NO-induced changes in PaO₂/FIO₂ were correlated to changes in pulmonary vascular resistance only in the prone position. Conclusions: In patients with the acute respiratory distress syndrome, the combination of NO and prone position is a valuable adjunct to mechanical ventilation. Received: 15 June 1998 Final revision received: 13 October 1998 Accepted: 30 October 1998