How does positive end-expiratory pressure decrease CO2 elimination from the lung?

Intraoperative Protective Mechanical Ventilation in Dogs: A Randomized Clinical Trial

Objective

To evaluate gas exchange, respiratory mechanics, and hemodynamic impact of mechanical ventilation with low tidal volume (V_T) in dogs with the use of positive end-expiratory pressure (PEEP) or preceded by alveolar recruitment maneuver (ARM).

Study Design

Prospective randomized clinical trial.

Animals

Twenty-one healthy client-owned mesocephalic healthy dogs, 1-7 years old, weighing 10-20 kg, and body condition scores 4-6/9 admitted for periodontal treatment.

Methods

Isoflurane-anesthetized dogs in dorsal recumbency were ventilated until 1 h with a volume-controlled ventilation mode using 8 mL kg^-1 of V_T. The dogs were distributed in 2 groups: in the ARM group, PEEP starts in 0 cmH₂O, increasing gradually 5 cmH₂O every 3 min, until reach 15 cmH₂O and decreasing in the same steps until 5 cmH₂O, maintaining this value until the end; and PEEP group, in which the pressure 5 cmH₂O was instituted from the beginning of anesthesia and maintained the same level up to the end of the anesthesia. Cardiopulmonary, metabolic, oxygenation parameters, and respiratory mechanics were recorded after the anesthesia induction (baseline-BL), 15, 45, and 75 min after BL and during the recovery.

Results

The ARM increased the static compliance (C_st) (15 min after baseline) when compared with baseline moment (24.9 ± 5.8 mL cmH₂0^-1 vs. 20.7 ± 5.4 mL cmH₂0^-1-p = 0.0364), oxygenation index (PaO₂/FIO₂) (505.6 ± 59.2 mmHg vs. 461.2 ± 41.0 mmHg-p = 0.0453) and reduced the shunt fraction (3.4 ± 2.4% vs. 5.5 ± 1.6%-p = 0.062). In the PEEP group, no statistical differences were observed concerning the variables evaluated. At the beginning of the evaluation, the driving pressure (DP) before ARM was significantly greater than all other evaluation time points (6.9 ± 1.8 cmH₂0).

Conclusions and Clinical Relevance

The use of 8 mL kg^-1 of V_T and 5 cmH₂0 PEEP without ARM maintain adequate oxygenation and mechanical ventilation in dental surgeries for up to 1 h. The use of ARM slightly improved compliance and oxygenation during the maneuver.

Application of dead space fraction to titrate optimal positive end-expiratory pressure in an ARDS swine model

This study aimed to apply the dead space fraction [ratio of dead space to tidal volume (VD/VT)] to titrate the optimal positive end-expiratory pressure (PEEP) in a swine model of acute respiratory distress syndrome (ARDS). Twelve swine models of ARDS were constructed. A lung recruitment maneuver was then conducted and the PEEP was set at 20 cm H₂O. The PEEP was reduced by 2 cm H₂O every 10 min until 0 cm H₂O was reached, and VD/VT was measured after each decrement step. VD/VT was measured using single-breath analysis of CO₂, and calculated from arterial CO₂ partial pressure (PaCO₂) and mixed expired CO₂ (PeCO₂) using the following formula: VD/VT = (PaCO₂ - PeCO₂)/PaCO₂. The optimal PEEP was identified by the lowest VD/VT method. Respiration and hemodynamic parameters were recorded during the periods of pre-injury and injury, and at 4 and 2 cm H₂O below and above the optimal PEEP (Po). The optimal PEEP in this study was found to be 13.25±1.36 cm H₂O. During the Po period, VD/VT decreased to a lower value (0.44±0.08) compared with that during the injury period (0.68±0.10) (P<0.05), while the intrapulmonary shunt fraction reached its lowest value. In addition, a significant change of dynamic tidal respiratory compliance and oxygenation index was induced by PEEP titration. These results indicate that minimal VD/VT can be used for PEEP titration in ARDS.

Optimization of positive end-expiratory pressure by volumetric capnography variables in lavage-induced acute lung injury

BACKGROUND

In the acute respiratory distress syndrome (ARDS), lung-protective ventilation strategies combine the delivery of small tidal volumes (VT) with sufficient positive end-expiratory pressure (PEEP). However, an optimal approach guiding the setting of PEEP has not been defined. Monitoring volumetric capnography is useful to detect changes in lung aeration.

OBJECTIVES

The aim of this study was to determine whether volumetric capnography may be a useful method to determine the optimal PEEP in ARDS.

METHODS

In 8 lung-lavaged piglets, PEEP was reduced from 20 to 4 cm H2O in steps of 4 cm H2O every 10 min followed by full lung recruitment. Volumetric capnography, respiratory mechanics, blood gas analysis, hemodynamic data and whole-lung computed tomography scans were obtained at each PEEP level.

RESULTS

After lung recruitment, end-expiratory lung volume progressively decreased from 1,160 ± 273 ml at PEEP 20 cm H2O to 314 ± 86 ml at PEEP 4 cm H2O. The ratio of alveolar dead space (VDalv) to alveolar VT (VTalv) and the phase III slope of volumetric capnography (SIII) reached a minimum at PEEP 16 cm H2O. At this PEEP level, overaerated lung regions were significantly reduced, nonaerated lung regions did not increase, and partial pressure of oxygen in arterial blood/fraction of inspired oxygen (P/F) and static respiratory system compliance (Crs) reached a maximum. At PEEP levels <16 cm H2O, nonaerated lung regions significantly increased, P/F and Crs deteriorated, and VDalv/VTalv and SIII began to increase.

CONCLUSIONS

In this surfactant-depleted model, PEEP at the lowest VDalv/VTalv and SIII allows an optimal balance between lung overinflation and collapse. Hence, volumetric capnography is a useful bedside approach to identify the optimal PEEP.

Year in review 2012: Critical Care--Respirology

Acute respiratory failure is a dominant feature of critical illness. In this review, we discuss 17 studies published last year in Critical Care. The discussion focuses on articles on several topics: respiratory monitoring, acute respiratory distress syndrome, noninvasive ventilation, airway management, secretion management and weaning.

A mathematical model for carbon dioxide elimination: an insight for tuning mechanical ventilation

PURPOSE

The aim is to provide better understanding of carbon dioxide (CO2) elimination during ventilation for both the healthy and atelectatic condition, derived in a pressure-controlled mode. Therefore, we present a theoretical analysis of CO2 elimination of healthy and diseased lungs.

METHODS

Based on a single-compartment model, CO2 elimination is mathematically modeled and its contours were plotted as a function of temporal settings and driving pressure. The model was validated within some level of tolerance on an average of 4.9% using porcine dynamics.

RESULTS

CO2 elimination is affected by various factors, including driving pressure, temporal variables from mechanical ventilator settings, lung mechanics and metabolic rate.

CONCLUSION

During respiratory care, CO2 elimination is a key parameter for bedside monitoring, especially for patients with pulmonary disease. This parameter provides valuable insight into the status of an atelectatic lung and of cardiopulmonary pathophysiology. Therefore, control of CO2 elimination should be based on the fine tuning of the driving pressure and temporal ventilator settings. However, for critical condition of hypercapnia, airway resistance during inspiration and expiration should be additionally measured to determine the optimal percent inspiratory time (%TI) to maximize CO2 elimination for treating patients with hypercapnia.

Prolonged inspiratory time produces better gas exchange in patients undergoing laparoscopic surgery: A randomised trial

BACKGROUND

Laparoscopic surgery performed with a patient in the Trendelenburg position is known to have adverse effects on pulmonary gas exchange and respiratory mechanics. We supposed that prolonged inspiratory time can improve gas exchange at lower airway pressure.

METHODS

One hundred patients undergoing gynaecologic laparoscopic surgery were randomly assigned to one of four groups: conventional inspiratory-to-expiratory (I : E) ratio (Group 1 : 2), I : E ratio of 1 : 1 (Group 1 : 1), 2 : 1 (Group 2 : 1), or 1 : 2 with external positive end-expiratory pressure (PEEP) of 5 cmH2 O (Group 1 : 2 PEEP). Tidal volume was set to 6 ml/kg, and I : E ratio was adjusted at the onset of pneumoperitoneum. Arterial blood gas analysis with measurements of partial pressure of arterial oxygen/fraction of inspired oxygen (PaO2 /FiO2 ), and physiologic dead space-to-tidal volume ratio (VD /VT ) was performed 15 min after anaesthetic induction (T1), and 30 (T2) and 60 min (T3) after onset of CO2 insufflation.

RESULTS

PaO2 /FiO2 at T3 in Groups 1 : 1, 2 : 1, and 1 : 2 PEEP were higher than Group 1 : 2. The partial pressure of arterial carbon dioxide at T3 in Group 2 : 1 was lower than the other groups. The VD /VT at T2 and T3 were lower in Groups 1 : 1 and 2 : 1 than Groups 1 : 2 and 1 : 2 PEEP. Peak or plateau airway pressure was higher in Group 1 : 2 PEEP than the other groups.

CONCLUSIONS

A prolonged inspiratory time demonstrated a beneficial effect on oxygenation. Furthermore, it showed better CO2 elimination without elevating the peak or plateau airway pressure compared with applying external PEEP. In terms of gas exchange and respiratory mechanics, a prolonged inspiratory time appears to be superior to applying external PEEP in patients undergoing laparoscopic surgery in the Trendelenburg position.

How do changes in exhaled CO₂ measure changes in cardiac output? A numerical analysis model

OBJECTIVE

In a previous study in anesthetized animals, the slope of percent decreases in exhaled CO₂ versus percent decreases in cardiac output (Q(T) inflation of vena cava balloons) was 0.73. To examine the mechanisms underlying this exhaled CO₂-Q(T) relationship, an iterative numerical analysis computer model of non-steady state CO(2) kinetics was developed.

METHODS

The model consisted of a large peripheral tissue compartment connected by venous return and [Formula: see text] to a small central pulmonary compartment. Equations were developed to describe the movement of CO₂ in this system. Decreases in Q(T) were accompanied by experimentally measured increases in alveolar dead space fraction (VD: (alv)/VT: (alv)), generated by decreased pulmonary vascular pressure during the Q(T) decrease.

RESULTS

When the model was perturbed by a 40% decrease in Q(T) and an increase in VD: (alv)/VT: (alv) from 5 to 20.6%, average alveolar expired P(CO₂) (PAE(CO₂)) decreased from 37.5 to 29.4 mm Hg, similar to the animal experiments. Due to the high peripheral tissue compliance for CO₂, the computer model demonstrated that, after a decrease in Q(T), at least 1 h was required for compartment CO₂ stores to approach a new equilibrium state.

CONCLUSIONS

The numerical analysis computer model helps to delineate the mechanisms underlying how decreased Q(T) resulted in decreased exhaled CO₂. The model permitted deconvolution of the effects of simultaneous variables and the interrogation of parameters that would be difficult to measure in actual experiments.

Monitoring dead space during recruitment and PEEP titration in an experimental model

OBJECTIVE

To test the usefulness of dead space for determining open-lung PEEP, the lowest PEEP that prevents lung collapse after a lung recruitment maneuver.

DESIGN

Prospective animal study.

SETTING

Department of Clinical Physiology, University of Uppsala, Sweden.

SUBJECTS

Eight lung-lavaged pigs.

INTERVENTIONS

Animals were ventilated using constant flow mode with VT of 6ml/kg, respiratory rate of 30bpm, inspiratory-to-expiratory ratio of 1:2, and FiO(2) of 1. Baseline measurements were performed at 6cmH(2)O of PEEP. PEEP was increased in steps of 6cmH(2)O from 6 to 24cmH(2)O. Recruitment maneuver was achieved within 2min at pressure levels of 60/30cmH(2)O for Peak/PEEP. PEEP was decreased from 24 to 6cmH(2)O in steps of 2cmH(2)O and then to 0cmH(2)O. Each PEEP step was maintained for 10min.

MEASUREMENTS AND RESULTS

Alveolar dead space (VD(alv)), the ratio of alveolar dead space to alveolar tidal volume (VD(alv)/VT(alv)), and the arterial to end-tidal PCO(2) difference (Pa-ET: CO(2)) showed a good correlation with PaO(2), normally aerated areas, and non-aerated CT areas in all animals (minimum-maximum r(2)=0.83-0.99; p<0.01). Lung collapse (non-aerated tissue>5%) started at 12[Symbol: see text]cmH(2)O PEEP; hence, open-lung PEEP was established at 14cmH(2)O. The receiver operating characteristics curve demonstrated a high specificity and sensitivity of VD(alv) (0.89 and 0.90), VD(alv)/VT(alv) (0.82 and 1.00), and Pa-ET: CO(2) (0.93 and 0.95) for detecting lung collapse.

CONCLUSIONS

Monitoring of dead space was useful for detecting lung collapse and for establishing open-lung PEEP after a recruitment maneuver.

Non-steady state monitoring by respiratory gas exchange

Traditionally, the study of CO2 and O2 kinetics in the body has been mostly confined to equilibrium conditions. However, the peri-anesthesia period and the critical care arena often involve conditions of non-steady state. The detection and explanation of CO2 kinetics during non-steady state pathophysiology have required the development of new methodologies, including the CO2 expirogram, average alveolar expired PCO2, and CO2 volume exhaled per breath. Several clinically relevant examples of non-steady state CO2 kinetics perturbations are examined, including abrupt decrease in cardiac output, application of positive end-expiratory pressure during mechanical ventilation, and occurrence of pulmonary embolism. The lesser known area of non-steady state O2 kinetics is introduced, including the measurement of pulmonary O2 uptake per breath. Future directions include the study of the respiratory quotient per breath, where the anaerobic threshold during anesthesia is identified by increasing respiratory quotient.

Arterial blood gas and pH analysis. Clinical approach and interpretation

Arterial blood gas and pH analysis are performed during anesthesia or critical care medicine for (1) assessment of acid-base balance, (2) assessment of pulmonary oxygenation of arterial blood, and (3) assessment of alveolar ventilation by measurement of arterial blood PCO2. Total physiologic and alveolar dead spaces are evaluated by comparing the alveolar PCO2 with the mixed expired and mixed alveolar PCO2, respectively. This article provides a clinical approach and interpretation of arterial blood gas and pH analysis.

Carbon dioxide kinetics and capnography during critical care

Greater understanding of the pathophysiology of carbon dioxide kinetics during steady and nonsteady state should improve, we believe, clinical care during intensive care treatment. Capnography and the measurement of end-tidal partial pressure of carbon dioxide (PETCO2) will gradually be augmented by relatively new measurement methodology, including the volume of carbon dioxide exhaled per breath (VCO2,br) and average alveolar expired PCO2. Future directions include the study of oxygen kinetics.

How does positive end-expiratory pressure decrease pulmonary CO2 elimination in anesthetized patients?

In anesthetized, mechanically ventilated patients, 10 cm H2O positive end-expiratory pressure (PEEP10) immediately decreased the CO2 volume exhaled per breath (V(CO2,br)) by 96%, as exhaled tidal volume (VT) decreased to expand functional residual capacity during the first 8 breaths after PEEP10 began. Then, the sustained decrease in V(CO2,br) for over 10 min was due to the 19% decrease in cardiac output (QT, decreased CO2 delivery from tissues to lung) and to the decrease in alveolar ventilation (VA). In turn, decreased VA resulted from decreased VT (loss of inspired volume into the compressible volume of the ventilating circuit) and possibly from increased physiological dead space, due to the potential for new high alveolar ventilation-to-perfusion (VA/Q) lung regions. V(CO2,br) increased and recovered to baseline by 20 min of PEEP10 ventilation because QT increased to augment the CO2 delivery to the lung and alveolar P(CO2) increased (increased mixed venous P(CO2) and tissue CO2 retention) to increase V(CO2,br) while alveolar VT remained depressed. End-tidal P(CO2) (PET(CO2) progressively increased during PEEP10 and did not detect the decrease in V(CO2,br) during PEEP10 ventilation because PET(CO2) does not account for exhaled volume.

Can capnography detect bronchial flap-valve expiratory obstruction?

OBJECTIVE

We have previously shown in a mechanical lung model [1] that bronchial flap-valve expiratory obstruction results in sequential lung expiration, best detected by prolonged and low magnitude tracheal expired flow (V) from the obstructed lung. However, the normal expiratory resistance of clinical ventilation circuits might also generate prolonged, low value exhaled V, that could be confused with bronchial flap-valve obstruction. We reasoned that bronchial flap-valve obstruction would also cause sequential CO2 unloading from each lung and result in a biphasic tracheal capnogram.

METHODS

To test this hypothesis, we ventilated (VT, 650 ml; f, 10 br/min) a dual mechanical test lung, with each side connected to a separate alcohol-burning chamber. An airway adapter-monitor system measured airway V, P, PCO2, and FO2. The circumference of the diaphragm in a respiratory one-way valve was trimmed to generate unidirectional resistance to expiratory V. Measurement sequences were repeated after this flap-valve was interposed in the left "main-stem bronchus."

RESULTS AND DISCUSSION

During moderate or severe left bronchial flap-valve obstruction, left bronchial V was delayed so that the left lung anatomical dead space (devoid of CO2) mixed with normal right exhalate to depress the expiratory upstroke or early plateau of the tracheal capnogram. During severe obstruction, decreased perfusion of the left lung caused lower alveolar PCO2. Then, prolonged low V from the left bronchus also resulted in depression of the end of the tracheal alveolar plateau. In general, the low magnitude of bronchial V from the obstructed lung limited its effect on the tracheal capnogram and the best marker of sequential lung emptying during bronchial flap-valve obstruction may be late exhaled V without reduction in total tidal volume.

Carbon dioxide spirogram (but not capnogram) detects leaking inspiratory valve in a circle circuit

Expiratory valve incompetence in the circle circuit is diagnosed by using capnography (PCO2 versus time) when significant CO2 is present throughout inspiration. However, inspiratory valve incompetence will allow CO2-containing expirate to reverse flow into the inspiratory limb. CO2 rebreathing occurs early during the next inspiration, generating a short extension of the alveolar plateau and decreased inspiratory downslope of the capnogram, which may be indistinguishable from normal. We hypothesized that CO2 spirography (PCO2 versus volume) would correctly measure inspired CO2 volume (VCO2) during inspiratory valve leak. Accordingly, a metabolic chamber (alcohol combustion) was connected to a lung simulator, which was mechanically ventilated through a standard anesthesia circle circuit. By multiplying and integrating airway flow and PCO2, overall, expired, and inspired VCO2 (VCO2,br = VCO2,E - VCO2,I) were measured. When the inspiratory valve was compromised (by placing a wire between the valve seat and diaphragm), VCO2,I increased from 2.7 +/- 1.7 to 5.7 +/- 0.2 mL (P < 0.05), as measured by using CO2 spirography. In contrast, the capnogram demonstrated only an imperceptible lengthening of the alveolar plateau and did not measure VCO2,I. To maintain effective alveolar ventilation and CO2 elimination, increased VCO2,I requires a larger tidal volume, which could result in pulmonary barotrauma, decreased cardiac output, and increased intracranial pressure.

IMPLICATIONS

Circle circuit inspiratory valve leak will allow CO2-containing expirate to reverse flow into the inspiratory limb, with subsequent rebreathing during the next inspiration. This CO2 rebreathing causes imperceptible lengthening of the alveolar plateau of the capnogram and is detected only by using the CO2 spirogram (PCO2 versus volume).

Bymixer provides on-line calibration of measurement of CO2 volume exhaled per breath

The measurement of CO2 volume exhaled per breath (VCO2.br) can be determined during anesthesia by the multiplication and integration of tidal flow (V) and PCO2. During side-stream capnometry, PCO2 must be advanced in time by transport delay (TD), the time to suction gas through the sampling tube. During ventilation, TD can vary due to sample line connection internal volume or flow rate changes. To determine correct TD and measure accurate VCO2.br during actual ventilation. TD can be iteratively adjusted (TDADJ) until VCO2-br/tidal volume equals PCO2 measured in a mixed expired gas collection (PECO2) (J Appl. Physiol. 72:2029-2035, 1992). However. PECO2 is difficult to measure during anesthesia because CO2 is absorbed in the circle circuit. Accordingly, we implemented a bypass flow-mixing chamber device (bymixer) that was interposed in the expiration limb of the circle circuit and accurately measured PECO2 over a wide range of conditions of ventilation of a test lung-metabolic chamber (regression slope = 1.01: R2 = 0.99). The bymixer response (time constant) varied from 18.1 +/- 0.03 sec (12.5 l/min ventilation) to 66.7 +/- 0.9 sec (2.5 l/min). Bymixer PECO2 was used to correctly determine TDADJ (without interrupting respiration) to enable accurate measurement of VCO2.br over widely changing expiratory flow patterns.

How does experimental pulmonary embolism decrease CO2 elimination?

To test how large pulmonary embolism changes non-steady state CO2 kinetics, the right pulmonary artery (RPA) was occluded in 5 anesthetized, ventilated, thoracotomized dogs. By 1 min after RPA occlusion, CO2 volume exhaled per breath (VCO2,br) decreased from 9.3 +/- 2.8 to 7.0 +/- 2.6 ml and end-tidal PCO2 (PETCO2) decreased from 28.7 +/- 4.2 to 21.8 +/- 3.3 Torr. During the ensuing 70 min, VCO2,br increased back to baseline but PETCO2 was still 13% less than baseline. Both PaCO2 (41.5 +/- 1.7 to 55.1 +/- 8.1 Torr) and PvCO2 (48.2 +/- 1.9 to 62.8 +/- 6.5 Torr) steadily increased and approached equilibrium by 45 min of RPA occlusion. Cardiac output did not significantly change. In summary, RPA occlusion immediately decreased VCO2,br by 25%, due mostly to increased alveolar VD (VDalv). Then, VCO2,br recovered back to baseline as CO2 accumulated in tissues and lung. In contrast, elevated VDalv caused persistent decreased PETCO2, which did not detect recovery of VCO2,br nor increase in PaCO2 during RPA occlusion.

Carbon dioxide elimination measures resolution of experimental pulmonary embolus in dogs

Patients with severe pulmonary embolism can suffer progressive hypercapnia refractory to supramaximal mechanical ventilation, and may require open-thoracic or transvenous emergency embolectomy in addition to anticoagulation and/or thrombolysis. The functional recovery of gas exchange would be signaled by an increase in pulmonary CO2 elimination and decrease in CO2 retention; such data could guide the course of operative embolectomy. Accordingly, we studied five chloralose-urethane anesthetized, mechanically ventilated dogs with open thoraces in which the right pulmonary arteries (RPAs) were reversibly occluded with cloth snares. After waiting for steady state, we abruptly released the snare to restore RPA perfusion and experimentally simulate resolution of pulmonary embolism. For 70 min we serially measure the CO2 volume exhaled per breath (VCO2,br), arterial, mixed venous, and end-tidal PCO2 (PACO2, PVCO2, PETCO2), cardiac output (QT), and the alveolar dead space fraction (VDalv/VTalv = [PaCO2 - PETCO2/PaCO2). RPA reperfusion caused VCO2,br to significantly and abruptly increase from 8.9 +/- 2.7 to 11.6 +/- 3.6 mL; 70 min later VCO2,br had returned to baseline. PaCO2 and PVCO2 steadily decreased during 70 min of RPA reperfusion. PETCO2 increased from 25 +/- 5 to 33 +/- 5 mm Hg immediately after RPA reperfusion, as VDalv/VTalv decreased from 54% +/- 10% to 32% +/- 12%, but PETCO2 was still significantly greater than baseline at 70 min of RPA reperfusion. QT did not significantly change. We conclude that intraoperative measurement of VCO2,br should immediately detect and follow the resolution of CO2 retention in the lung and peripheral tissues after RPA reperfusion. PETCO2 could not detect the decrease of VCO2,br back to baseline because PETCO2 does not measure exhaled volume or the PCO2 waveform.

Measurement of pulmonary CO2 elimination must exclude inspired CO2 measured at the capnometer sampling site

OBJECTIVE

The pulmonary elimination of the volume of CO2 per breath (VCO2/br, integration of product of airway flow (V) and PCO2 over a single breath) is a sensitive monitor of cardio-pulmonary function and tissue metabolism. Negligible inspired PCO2 results when the capnometry sampling site (SS) is positioned at the entry of the inspiratory limb to the airway circuit. In this study, we test the hypothesis that moving SS lungward will result in significant inspired CO2 (VCO2[I]), that needs to be excluded from VCO2/br.

METHODS

We ventilated a mechanical lung simulator with tidal volume (VT) of 800 mL at 10 breaths/min. CO2 production, generated by burning butane in a separate chamber, was delivered to the lung. Airway V and PCO2 were measured (Capnomac Ultima, Datex), digitized (100 Hz for 60 s), and stored by microcomputer. Then, computer algorithms corrected for phase differences between V and PCO2 and calculated expired and inspired VCO2 (VCO2[E] and VCO2[I]) for each breath, whose difference equalled overall VCO2/br. The lung and Y-adapter (where the inspiratory and expiratory limbs of the circuit joined) were connected by the SS and a connecting tube in varying order.

RESULTS

During ventilation of the lung model (VT = 800 ml) with SS adjacent to the inspiratory limb, VCO2[E] was 16.8 +/- 0.4 ml and VCO2[I] was 1.1 +/- 0.1 ml, resulting in overall VCO2/br (VCO2[E] - VCO2[I]) of 15.7 +/- 0.4 ml. If VCO2[I] was ignored in the determination of VCO2/br, then the %error that VCO2[E] overestimated VCO2/br was 7.2 +/- 0.3%. This %error significantly increased (p < 0.05, Student's t-test) when VT was decreased to 500 mL (%error = 12.4 +/- 0.8%) or when SS was moved to the lungward side of a 60 mL connecting tube (VCO2[I] = 2.8 +/- 0.2, %error = 18.2 +/- 1.6) or a 140 mL tube (VCO2[I] = 5.9 +/- 0.3 mL, %error = 37.5 +/- 3.3).

CONCLUSIONS

When the SS was moved lungward from the inspiratory limb, instrumental dead space (VDINSTR) increased and, at end-expiration, contained exhaled CO2 from the previous breath. During the next inspiration, this CO2 was rebreathed relative to SS (i.e. VCO2[I]), and contributed to VCO2[E]. Thus, VCO2[E] overestimated VCO2/br (%error) by the amount of rebreathing, which was exacerbated by larger VDINSTR (increased VCO2[I]) or smaller VT (increased VCO2[I]-to-VCO2/br ratio).