Carbon Dioxide Rebreathing Research Articles

Pulmonary Carbon Dioxide Elimination for Cardiac Output Monitoring in Peri‐operative and Critical Care Patients: History and Current Status

Minimally invasive measurement of cardiac output as a central component of advanced haemodynamic monitoring has been increasingly recognised as a potential means of improving perioperative outcomes in patients undergoing major surgery. Methods based upon pulmonary carbon dioxide elimination are among the oldest techniques in this field, with comparable accuracy and precision to other techniques. Modern adaptations of these techniques suitable for use in the perioperative and critical are environment are based on the differential Fick approach, and include the partial carbon dioxide rebreathing method. The accuracy and precision of this approach to cardiac output measurement has been shown to be similar to other minimally invasive techniques. This paper reviews the underlying principles and evolution of the method, and future directions including recent adaptations designed to deliver continuous breath-by-breath monitoring of cardiac output.

Effect of variable circuit flow rate during the expiratory phase on CO2 elimination

Effect of variable circuit flow rate during the expiratory phase on CO2 elimination Peter A Keszler,1 Pankaj Nagaraj,1 Kabir Abubakar,1 Martin Keszler, 21Georgetown University, Washington, DC, USA; Georgetown University Hospital, Washington, DC, USA; 2Brown University, Women and Infants Hospital of Rhode Island, Providence, RI, USABackground: Some continuous flow infant ventilators allow independent setting of inspiratory and expiratory circuit flow rate. In the Dräger Babylog 8000+ ventilator, this is called "variable inspiratory, variable expiratory flow" (VIVE). Some clinicians believe that lower expiratory flow decreases expiratory resistance. The minimum expiratory flow rate needed to avoid re-breathing of carbon dioxide (CO2) has never been established.Objective: We sought to determine if re-breathing becomes evident at the lowest possible expiratory flow rate setting of 1 L/min.Design/methods: We conducted a bench study using end-tidal CO2 (ETCO2) measurement and a 45 mL (90 mL for the "term" model) test lung pre-filled with 100% CO2. We previously showed that the time needed for ETCO2 to be eliminated from the lung is a highly reproducible indicator of efficiency of ventilation. Re-breathing would thus be identified by an increase in the time required for the CO2 to be washed out from the test lung at stable settings of rate and tidal volume (VT). Using a Babylog 8000+ ventilator in volume guarantee mode with VIVE and a standard ventilator circuit, we tested the effect of decreasing expiratory flow rate under conditions simulating three sizes of patients: extremely low birth weight infant, wt = 600 g (VT = 3.5 mL, respiratory rate (RR) = 60 breaths min-1, minute ventilation (MV) = 210 mL/min, expiratory flow rate = 3 L/min, 2 L/min, and 1 L/min), very low birth weight infant, wt = 1.5 kg (VT= 7 mL, RR = 60 breaths min-1, MV = 420 mL/min, expiratory flow rate = 4 L/min, 3 L/min, 2 L/min, and 1 L/min), and term infant, wt = 3.6 kg (VT = 16 mL, RR = 60 breaths min-1, MV = 960 mL/min, expiratory flow rate = 5 L/min, 4 L/min, 3 L/min, 2 L/min, and 1 L/min). Each measurement was repeated four times and the mean values were compared by analysis of variance for repeated measures.Results: The CO2 elimination times were always within 10% of each other with each repetition. There was no significant increase in the elimination time even at the lowest flows.Conclusion: Clinically significant re-breathing does not occur even with expiratory flow rate at the lowest possible setting of 1 L/min. VIVE is safe to use, although its clinical utility has not been established.Keywords: mechanical ventilation, newborn, bench study, re-breathing, expiratory flow rate, end-tidal CO2

Cardiac Output Monitoring and Fluid Responsiveness

To the Editor, I appreciate the interest that Dr. Peled has taken in this review of hemodynamic monitoring in neurocritical care and thank him for his letter. Accuracy and precision of minimally invasive cardiac output (CO) monitoring devices is an important and highly debated issue [1]. Apart from the discussion on proper statistical methodology that should be used to compare a new device to a reference method [2], it should be realized that there is even a more fundamental problem, the one relating to the inherent limitations of the reference method itself. In fact, there is no bedside ‘‘gold standard’’ when it comes to the measurement of CO [3]. The widely accepted invasive bedside standard is the pulmonary artery catheter (PAC), nevertheless higher-precision tools like indwelling flow probes when used to compare the PAC with minimally invasive tools demonstrate that the PAC is as precise or imprecise [4–6]. In addition, as Dr. Peled notes, the behavior of these monitors in different hemodynamic conditions has not been elucidated and both accuracy and precision may vary greatly. Moreover, the use of AHM within the neurocritical care literature remains sparse and guidance comes by extrapolating knowledge from medical–surgical critical care populations. A relevant meta-analysis has recently been published by Peyton and Chong [4]. The authors reviewed the literature on four different minimally invasive methods (pulse contour, esophageal Doppler, partial carbon dioxide rebreathing, and transthoracic bioimpedance) to assess their bias, precision and percentage error (PE) in agreement with thermodilution. This study found all four methods achieving a similar ±40–45 % PE in agreement with thermodilution leading the authors to challenge the ‘‘standard’’ PE of ±30 % which is often considered the accepted limits of agreement as initially suggested by Critchley and Critchley [7]. Obviously more research is needed to appropriately validate the currently available CO monitors and to also develop newer, potentially better performing devices. The second point of Dr. Peled’s letter refers to the markers of fluid responsiveness. It should be clear that the property of volume responsiveness, i.e., operating on the ascending part of a Frank-Starling curve, does not mean that a patient needs fluids. The dynamic markers of fluid responsiveness including PPV, SVV, and dIVC among others should be consulted after a clinical decision has been made in augmenting a patient’s stroke volume or mean arterial pressure and in keeping with their limitations (e.g., spontaneous breathing and atrial fibrillation). A predicted non-responder should not be given a fluid bolus since his/ her SV will not increase with additional volume, alternative physiologic interventions (for e.g., inotropes or vasopressors) may be considered. As a more general comment, these markers are not meant to be ‘‘one more number to fix’’, they are devised to complement the clinical picture and assist in choosing among interventions in patients with complex physiologic derangements.

Postural control and ventilatory drive during voluntary hyperventilation and carbon dioxide rebreathing

The present study sought to establish links between hyperventilation and postural stability. Eight university students were asked to stand upright under two hyperventilation conditions applied randomly: (1) a metabolic hyperventilation induced by 5 min of hypercapnic-hyperoxic rebreathing (CO(2)-R); and, (2) a voluntary hyperventilation (VH) of 3 min imposed by a metronome set at 25 cycles per min. Recordings were obtained with eyes open, with the subjects standing on a force plate over 20-s periods. Ventilatory response, displacements in the centre of pressure in both the frontal and sagittal planes and fluctuations in the three planes of the ground reaction force were monitored in the time and frequency domains. Postural changes related to respiratory variations were quantified by coherence analysis. Myoelectric activities of the calf muscles were recorded using surface electromyography. Force plate measurements revealed a reduction in postural stability during both CO(2)-R and VH conditions, mainly in the sagittal plane. Coherence analysis provided evidence of a ventilatory origin in the vertical ground reaction force fluctuations during VH. Electromyographic analyses showed different leg muscles strategies, assuming the existence of links between the control of respiration and the control of posture. Our results suggest that the greater disturbing effects caused by voluntary hyperventilation on body balance are more compensated when respiration is under automatic control. These findings may have implications for understanding the organisation of postural and respiratory activities and suggest that stability of the body may be compromised in situations in which respiratory demand increases and requires voluntary control.

Panic-Relevant Cognitive Processes Among Smokers

To further understand the frequent co-occurrence of smoking and panic disorder (PD), we examined panic-relevant cognitive processes among heavy smokers, half of whom were in 12-hour withdrawal, and nonsmokers. All participants (N = 85) underwent a 5-minute carbon dioxide rebreathing challenge. Prior to the challenge, participants completed questionnaires on reasons for smoking, anxiety sensitivity, and suffocation fear. Results are consistent with a model in which smokers with predisposing risk factors (high anxiety sensitivity and high suffocation fear) misappraise bodily sensations and experience panicky symptoms. No evidence was found that being in acute withdrawal heightened this risk. Overall, findings highlight (a) cognitive vulnerabilities that may place smokers at elevated risk for developing PD and hence (b) potential targets for intervention.

Minimally Invasive Measurement of Cardiac Output during Surgery and Critical Care

When assessing the accuracy and precision of a new technique for cardiac output measurement, the commonly quoted criterion for acceptability of agreement with a reference standard is that the percentage error (95% limits of agreement/mean cardiac output) should be 30% or less. We reviewed published data on four different minimally invasive methods adapted for use during surgery and critical care: pulse contour techniques, esophageal Doppler, partial carbon dioxide rebreathing, and transthoracic bioimpedance, to assess their bias, precision, and percentage error in agreement with thermodilution. An English language literature search identified published papers since 2000 which examined the agreement in adult patients between bolus thermodilution and each method. For each method a meta-analysis was done using studies in which the first measurement point for each patient could be identified, to obtain a pooled mean bias, precision, and percentage error weighted according to the number of measurements in each study. Forty-seven studies were identified as suitable for inclusion: N studies, n measurements: mean weighted bias [precision, percentage error] were: pulse contour N = 24, n = 714: -0.00 l/min [1.22 l/min, 41.3%]; esophageal Doppler N = 2, n = 57: -0.77 l/min [1.07 l/min, 42.1%]; partial carbon dioxide rebreathing N = 8, n = 167: -0.05 l/min [1.12 l/min, 44.5%]; transthoracic bioimpedance N = 13, n = 435: -0.10 l/min [1.14 l/min, 42.9%]. None of the four methods has achieved agreement with bolus thermodilution which meets the expected 30% limits. The relevance in clinical practice of these arbitrary limits should be reassessed.

Aeration properties of a new sleeping surface for infants

Prone sleeping position, use of soft mattresses and head covering by bedclothes are known risk factors for sudden infant death syndrome (SIDS). Rebreathing carbon dioxide (CO(2) ) may be a possible mechanism or a confounding factor of SIDS. To compare the aeration properties of a new concept of infant sleeping surface (Net) to three commercial mattresses advertised to improve aeration and to two standard infant mattresses. Two experiments were performed: (I) A container (head box), filled with 7% CO(2) mixture, was opened to the mattress to allow gas mixture to passively diffuse outside and equilibrate with the surrounding room air. (II) Simulation of normal breathing of an infant, using a unidirectional reciprocal syringe, to determine CO(2) accumulation within the head box. CO(2) concentrations in the head box were continuously measured until CO(2) levels fell below 1% or for 5 min (experiment I), or until CO(2) accumulation levels plateaued or for 6 min (experiment II). The Net had a significantly faster rate of CO(2) elimination (88.5 ± 4.6 and 91.9 ± 0.9 sec, Net alone and when covered with a sheet, respectively) compared to 238.3 ± 14.2 sec to 387.8 ± 7.9 sec for the other mattresses (P < 0.001). Only the Net was able to prevent CO(2) accumulation with maximal CO(2) levels (0.56 ± 0.03% and 1.16 ± 0.05%; Net alone and when covered with a sheet, respectively) significantly lower than the range of 4.6-6.3% for the other mattresses (P < 0.001). The new sleeping surface exhibited significantly better aeration properties in dispersing CO(2) and in preventing its accumulation.

Framingham Cardiovascular Risk Profile Correlates with Impaired Hippocampal and Cortical Vasoreactivity to Hypercapnia

Vascular risk factors affect cerebral blood flow (CBF) and cerebral vascular reactivity, contributing to cognitive decline. Hippocampus is vulnerable to both Alzheimer's disease (AD) pathology and ischemia; nonetheless, the information about the impact of vascular risk on hippocampal perfusion is minimal. Cognitively, healthy elderly (NL=18, 69.9±6.7 years) and subjects with mild cognitive impairment (MCI=15, 74.9±8.1 years) were evaluated for the Framingham cardiovascular risk profile (FCRP). All underwent structural imaging and resting CBF assessment with arterial spin labeling (ASL) at 3T magnetic resonance imaging (MRI). In 24 subjects (NL=17, MCI=7), CBF was measured after a carbon dioxide rebreathing challenge. Across all subjects, FCRP negatively correlated with hippocampal (ρ=-0.41, P=0.049) and global cortical (ρ=-0.46, P=0.02) vasoreactivity to hypercapnia (VR(h)). The FCRP-VR(h) relationships were most pronounced in the MCI group: hippocampus (ρ=-0.77, P=0.04); global cortex (ρ=-0.83, P=0.02). The FCRP did not correlate with either volume or resting CBF. The hippocampal VR(h) was lower in MCI than in NL subjects (Z=-2.0, P=0.047). This difference persisted after age and FCRP correction (F([3,20])=4.6, P=0.05). An elevated risk for vascular pathology is associated with a reduced response to hypercapnia in both hippocampal and cortical tissue. The VR(h) is more sensitive to vascular burden than either resting CBF or brain volume.

Application of the Laryngeal Mask Airway for Anesthesia in Three Chimpanzees and One Gibbon

Three pediatric chimpanzees and one pediatric gibbon were anesthetized for routine physical examination. Anesthesia was maintained with inhalant delivered via a laryngeal mask airway (LMA). The LMA was easy to insert, provided adequate control of the airway for ventilation, and caused no tracheal stimulation. No complications were observed. As compared with a face mask, the LMA has the advantage of a more secure airway; the ability to effectively ventilate the patient; less dead space, which leads to lower rebreathing of carbon dioxide; and less exposure of personnel to waste gases. As compared with an endotracheal tube, the LMA causes less airway trauma, is easier to place, and is less stimulating to the patient. The LMA should be considered for use in fasted non-human primates presented for procedures lasting less than 60 min where high peak inspiratory pressures are not needed.

An unusual cause of carbon dioxide rebreathing in a circle absorber system

To the Editor: Hypercapnia while using a circle absorber system is a known complication. The main reasons of hypercapnia due to rebreathing are exhausted soda lime, malfunctioning unidirectional valves and chamber bypass. We report an unusual cause of chamber bypass causing hypercapnia due to rebreathing. A 30-year-old male patient with a history of a road traffic accident was posted for emergency decompressive craniotomy. Routine check of the anaesthesia machine (in accordance with the recommended pre-anaesthetic check list) was performed. General anaesthesia was administered and maintained using a circle absorber system at low flows (1 l/min). Standard monitoring was performed, and levels were within normal limits. Twenty minutes later, the FiCO2 gradually increased from 0 to 15 mmHg, and the EtCO2 increased from 34 to 50 mmHg with a rebreathing pattern on the capnograph. The soda lime in the canister did not show any colour change, and the unidirectional valves were confirmed to be functioning normally. The fresh gas flows were increased (5 l/min), and manual ventilation was initiated. On closer inspection of the soda lime canister, one of the three rings on which the inner chamber of soda lime canister is seated was found to be missing (Fig. 1). With replacement of the rubber ring, the FiCO2 and EtCO2 normalized. The anaesthesia machine which was being used had been in service for 3 years, and periodic checks were carried out by a maintenance engineer authorised by the company. The circle absorber breathing system used in our patient contained a single chamber of soda lime surrounded by an outer chamber. In such a system, the exhaled gases pass from above, downwards through the soda lime, then further through the holes at bottom of the inner chamber to the space between the outer chamber and inner canister and finally below and then upwards (Fig. 2). In our case, the absence of the rubber ring caused the gases to escape to the outer chamber, thereby bypassing the soda lime and resulting in CO2 rebreathing. The rubber ring was displaced during the change of soda lime or cleaning of the canister; as such it was a human error. The operation theatre technicians have since been specifically instructed to be careful during their handling of the soda lime assembly in the future in order to prevent further potential mishaps. In addition, the anesthesia residents have been made aware of the possibility of a poorly positioned rubber ring in the soda lime canister if other common causes of CO2 rebreathing and hypercapnia has been clinically excluded. Such realignments can occur despite routine checks of the anaesthesia machine. The manufacturer of the machine has also been informed of this incident with the displaced ring in the soda lime assembly, and we have suggested the possibility of permanent fixation of the ring as this would avoid this inadvertent complication. However, permanent fixation may make cleaning of the canister tedious owing to the collection of moisture. Problems have been encountered with the double canister soda lime assembly in which the sealing ring between the two canisters was displaced [1, 2]. We report this case P. Agrawal B. Gupta N. D’souza Jai Prakash Narayan Apex Trauma Centre, All India Institute of Medical Sciences (AIIMS), New Delhi 110029, India

Monitoring in resuscitation: Comparison of cardiac output measurement between pulmonary artery catheter and NICO

Aim The cardiac output and coronary perfusion pressure generated from chest compressions during resuscitation manoeuvres can predict effectiveness and successful outcome. Until now, there is no good method for haemodynamic monitoring during resuscitation. Noninvasive partial carbon dioxide rebreathing system (NICO, Novametrix Medical Systems, Inc., Wallingford, CT, USA) is a relatively new non-invasive alternative to thermodilution for measuring cardiac output. The accuracy of the NICO system has not been evaluated during resuscitation. The aim of this study is to compare thermodilution cardiac output method with NICO system and to assess the utility of NICO during resuscitation. Methods and design Experimental study in 24 Yorkshire pigs. Paired measurements of cardiac output were determined during resuscitation (before ventricular fibrillation and after 5, 15, 30 and 45 min of resuscitation) in the supine position. The average of 3 consecutive thermodilution cardiac output measurements (10 ml 20 °C saline) was compared with the corresponding NICO measurement. Results Bland and Altman plot and Lin's concordance coefficient showed a high correlation between NICO and thermodilution cardiac output measurements although NICO has a tendency to underestimate cardiac output when compared to thermodilution at normal values of cardiac output. Conclusions There is a high degree of agreement between cardiac output measurements obtained with NICO and thermodilution cardiac output during resuscitation. The present study suggests that the NICO system may be useful to measure cardiac output generated during cardiopulmonary resuscitation.

Residual Neuromuscular Blockade and Upper Airway Muscles

Assistance Publique, Hospitaux de Paris et Universite Paris XII, Créteil, Cedex, France. philippe.duvaldestin@hmn.aphp.frI read with great interest the article by Herbstreit et al. 1describing the effect of residual neuromuscular blockade on upper airway collapsibility in humans. The authors observed that upper airway integrity was impaired when the train-of-four ratio was maintained at the level of 0.5 or 0.8 with rocuronium. In a former study in volunteers, I and others could not demonstrate any susceptibility to upper airway obstruction when the train-of-four ratio was maintained to 0.5 with an infusion of vecuronium.2In our study, decreased inspiratory negative pressure, which is known to exaggerate the possibility of upper airway collapse, was elicited by increasing ventilation during carbon dioxide rebreathing or by adding an inspiratory resistance. Breathing with pressure at airway opening held at a pressure from −5 to −40 cm H2O was also tested. No effect of partial neuromuscular blockade maintained at a train-of-four ratio of 0.5 on upper airway could be observed. When the upper airway pressure was progressively decreased to −40 cm H2O, no participant showed evidence of upper airway collapse or flow limitation at any time period. The main difference between the two studies is that subjects were breathing through a mouthpiece in our study2instead of a nasal mask in the study by Herbstreit et al. 1Therefore, the flow limitation observed by Herbstreit et al. 1may be related to resistance at the velopharynx rather than to an impaired compensatory response of the genioglossus muscle.I agree with Herbstreit et al. 1when they state that the upper airway muscles are more susceptible to neuromuscular blocking agents than the diaphragm. However, because the diaphragm is the most resistant muscle to neuromuscular blocking agents among all skeletal muscles, this comparison is not valid to point out a particular susceptibility of upper airway muscles to neuromuscular blocking agents. For example, the geniohyoid muscle, which also contributes to upper airway integrity, was shown to be more susceptible than the diaphragm but less susceptible than the adductor pollicis to mivacurium in humans.3Finally, as suggested by Herbstreit et al. ,1further work on the function of other airway openers should be performed. In the future, for a rigorous evaluation of the effect of residual neuromuscular blockade on the function of upper airway muscles, the investigator should be blinded to the level of neuromuscular blockade.Assistance Publique, Hospitaux de Paris et Universite Paris XII, Créteil, Cedex, France. philippe.duvaldestin@hmn.aphp.fr

Pulmonary Capillary Blood Flow and Cardiac Output Measurement by Partial Carbon Dioxide Rebreathing in Patients with Acute Respiratory Distress Syndrome Receiving Lung Protective Ventilation

Pulmonary Capillary Blood Flow and Cardiac Output Measurement by Partial Carbon Dioxide Rebreathing in Patients with Acute Respiratory Distress Syndrome Receiving Lung Protective Ventilation

Pulmonary Capillary Blood Flow and Cardiac Output Measurement by Partial Carbon Dioxide Rebreathing in Patients with Acute Respiratory Distress Syndrome Receiving Lung Protective Ventilation

Partial carbon dioxide rebreathing noninvasively measures the pulmonary capillary blood flow and estimates the cardiac output with the use of a predicted shunt value. It has been reported that the accuracy of the method is decreased in patients with high pulmonary shunt. The aim of this study was to investigate the agreement between partial rebreathing and thermodilution for the determination of pulmonary capillary blood flow and cardiac output in the setting of acute respiratory distress syndrome. Twenty consecutive patients with the acute respiratory distress syndrome were enrolled. Ventilator settings include low tidal volume (6 ml x kg(-1)) and positive end-expiratory pressure + 2 cm H2O higher than the lower inflection point if present or 10 cm H2O if not. Seven pairs of cardiac output and pulmonary capillary blood flows were recorded every 20 min over a 2-h period. The authors determined bias, SD, limit of agreement (95% confidence interval) and percentage error. Bias and agreement for cardiac output measurement were 0.8 +/- 1.2 l x min(-1) (-2.1 to 3.7 l x min(-1)), and percentage error was 36%. Bias and agreement for pulmonary capillary blood flow measurement were -0.1 +/- 0.8 l x min(-1) (-2.1 to 1.9 l x min(-1)), and percentage error was 35%. Dead space, arteriovenous oxygen content difference, mean pulmonary arterial pressure, and baseline cardiac output were independently associated with differences between methods. In patients with the acute respiratory distress syndrome, partial rebreathing cannot yet replace thermodilution for measuring pulmonary capillary blood flow or cardiac output. However, accuracy of the method is close to the boundary of clinical relevance.

Critical Care Nephrology: Core Curriculum 2009

In the inpatient setting, a high proportion of nephrology consultations are requested for patients in the Intensive Care Unit (ICU). These patients may have acute kidney injury (AKI) or may be critically ill and have end-stage renal disease (ESRD). Thus, nephrologists need to understand recent evidence-based advances in the field of critical care, as well as areas of ongoing controversy and investigation. In this article, we summarize the diagnosis and management of shock, as well as the management of sepsis, acute lung injury/acute respiratory distress syndrome (ALI/ARDS), and fulminant hepatic failure, all of which are associated with high mortality in the critical care setting. We discuss infectious complications of critical care, including ventilator associated pneumonia and catheter related infections. Supportive care, including nutrition, insulin therapy and anemia management are reviewed. Dialysis considerations in critically ill patients are discussed. Lastly, we review the management of several life-threatening intoxications, many of which require early recognition and consideration of dialysis.

Comparison of three noninvasive cardiac output monitors in patients undergoing therapeutic hypothermia after cardiac arrest

We planned to observe the performance of the LiDCOplus (LiDCO Systems, UK), PiCCO (Pulsion Medical Systems, Germany) and NICO (Novametrix-Respironics, USA) continuous cardiac output monitors during therapeutic hypothermia and rewarming post cardiac arrest. PiCCO calibration failure during hypothermia has been reported [1,2]. Use of the LiDCOplus and the NICO monitor during therapeutic hypothermia has not been reported. The LiDCOplus pulse power analysis is calibrated using a bolus of lithium chloride instead of transpulmonary thermodilution. Function of the NICO is based on the Fick principle and on partial rebreathing of carbon dioxide. LiDCOplus and NICO performance should not be affected by hypothermia.

Helmet ventilation and carbon dioxide rebreathing: effects of adding a leak at the helmet ports

We examined whether additional helmet flow obtained by a single-circuit and a modified plateau valve applied at the helmet expiratory port (open-circuit ventilators) improves CO(2) wash-out by increasing helmet airflow. Randomized physiological study in a university research laboratory. Ten healthy volunteers. Helmet continuous positive airway pressure and pressure support ventilation delivered by an ICU ventilator (closed-circuit ventilator) and two open-circuit ventilators equipped with a plateau valve placed either at the inspiratory or at the helmet expiratory port. We measured helmet air leaks, breathing pattern, helmet minute ventilation (Eh)), minute ventilation washing the helmet (Ewh)), CO(2) wash-out, and ventilator inspiratory assistance. Air leaks were small and similar in all conditions. Breathing pattern was similar among the different ventilators. Inspiratory and end-tidal CO(2) were lower, while (Eh) and (Ewh) were higher only using open-circuit ventilators with the plateau valve placed at the helmet expiratory port. This occurred notwithstanding these ventilators delivered a lower inspiratory assistance. Additional helmet flow provided by open-circuit ventilators can lower helmet CO(2) rebreathing. However, inspiratory pressure assistance significantly decreases using open-circuit ventilators, still casting doubts on the choice of the optimal helmet ventilation setup.

Carbon dioxide rebreathing during non-invasive ventilation delivered by helmet: a bench study

To define how to monitor and limit CO(2) rebreathing during helmet ventilation. Physical model study. Laboratory in a university teaching hospital. We applied pressure-control ventilation to a helmet mounted on a physical model. In series 1 we increased CO(2) production (V'CO(2)) from 100 to 550 ml/min and compared mean inhaled CO(2) (iCO(2),mean) with end-inspiratory CO(2) at airway opening (eiCO(2)), end-tidal CO(2) at Y-piece (yCO(2)) and mean CO(2) inside the helmet (hCO(2)). In series 2 we observed, at constant V'CO(2), effects on CO(2) rebreathing of inspiratory pressure, respiratory mechanics, the inflation of cushions inside the helmet and the addition of a flow-by. In series 1, iCO(2),mean linearly related to V'CO(2). The best estimate of CO(2) rebreathing was provided by hCO(2): differences between iCO(2),mean and hCO(2), yCO(2) and eiCO(2) were 0.0+/-0.1, 0.4+/-0.2 and -1.3+/-0.5%. In series 2, hCO(2) inversely related to the total ventilation (MVtotal) delivered to the helmet-patient unit. The increase in inspiratory pressure significantly increased MVtotal and lowered hCO(2). The low lung compliance halved the patient:helmet ventilation ratio but led to minor changes in MVtotal and hCO(2). Cushion inflation, although it decreased the helmet's internal volume by 33%, did not affect rebreathing. A 8-l/min flow-by effectively decreased hCO(2). During helmet ventilation, rebreathing can be assessed by measuring hCO(2) or yCO(2), but not eiCO(2). It is directly related to V'CO(2), inversely related to MVtotal and can be lowered by increasing inspiratory pressure or adding a flow-by.

Fetal Heart Rate Response to Maternal Hypocapnia and Hypercapnia in Late Gestation

ObjectiveTo examine the effects of acute maternal hypocapnia and hypercapnia on electronic fetal heart rate (FHR) patterns in late gestation. MethodsThirty-five women with healthy singleton pregnancies performed a modified carbon dioxide (CO2) rebreathing procedure between 34 and 38 weeks of pregnancy. Prior to rebreathing, subjects hyperventilated for five minutes to reduce end-tidal CO2 tensions (PETco2) below 23 Torr (hypocapnia). During rebreathing, PETco2 progressively increased from hypocapnia to hypercapnia (PETTco2 = 40–60 Torr) at a constant hyperoxic end-tidal O2 tension of 150 Torr. FHR responses were classified using standardized guidelines over four periods: 20 minutes before rebreathing (pretest), during hypocapnia and hypercapnia, and 20 minutes after rebreathing (post-test). ResultsMean baseline FHR measures (SD) over the four test periods were 138(8), 144(10), 132(11), and 137(9) beats per minute (bpm). All pairwise comparisons were statistically significant except the pretest versus post-test comparison (P < 0.05, Tukey-Kramer multiple comparisons test). A single tachycardia episode of 170 bpm was recorded in the post-test period. In 20 subjects FHR variability changed from moderate in the pretest period to minimal during hypocapnia and/or hypercapnia. All but two returned to moderate FHR variability in the post-test period. One other fetus with minimal post-test variability had moderate values in the three preceding test periods. ConclusionElectronic FHR parameters remained within normal limits for third-trimester fetuses with the exception of one fetus that experienced tachycardia. Acute maternal hypocapnia and hypercapnia over the range studied had no adverse effects on fetal well-being. These results support the safety of the modified CO2 rebreathing procedure for research in healthy, low-risk pregnancy.

Fear reactivity to bodily sensations among heavy smokers and nonsmokers.

Individuals who smoke are more likely to experience panic attacks and develop panic disorder than those in the general population. One possible explanation is that smokers may experience a heightened fear response to somatic disturbances. To date, few laboratory studies have tested this hypothesis directly. The present study examined 24 adult heavy smokers (10 females) in 12-hr nicotine withdrawal and 24 adult nonsmokers (12 females) on subjective and physiological reactivity to a 4-min carbon dioxide rebreathing challenge. Results indicate that, despite an attenuated acceleration in respiration during the challenge, smokers experienced a significantly greater increase in self-reported panic symptoms than nonsmokers. In addition, smokers reported significantly greater trait levels of suffocation fear prior to the challenge. Findings are discussed with respect to the role of smoking in panic vulnerability.