A prolonged postinspiratory pause enhances CO2 elimination by reducing airway dead space

Mean airway pressure as a parameter of lung-protective and heart-protective ventilation

Mean airway pressure (MAP) is the mean pressure generated in the airway during a single breath (inspiration + expiration), and is displayed on most anaesthesia and intensive care ventilators. This parameter, however, is not usually monitored during mechanical ventilation because it is poorly understood and usually only used in research. One of the main determinants of MAP is PEEP. This is because in respiratory cycles with an I:E ratio of 1:2, expiration is twice as long as inspiration. Although MAP can be used as a surrogate for mean alveolar pressure, these parameters differ considerably in some situations. Recently, MAP has been shown to be a useful prognostic factor for respiratory morbidity and mortality in mechanically ventilated patients of various ages. Low MAP has been associated with a lower incidence of 90-day mortality, shorter ICU stay, and shorter mechanical ventilation time. MAP also affects haemodynamics: there is evidence of a causal relationship between high MAP and low perfusion index, both of which are associated with poor prognosis in mechanically ventilated patients. Elevated MAP values have also been associated with high central venous pressure and lactate, which are indicative of ventilator-associated right ventricular failure and tissue hypoperfusion, respectively. MAP, therefore, is an important parameter to measure in clinical practice. The aim of this review has been to identify the determinants of MAP, the pros and cons of using MAP instead of traditional protective ventilation parameters, and the evidence that supports the use of MAP in clinical practice.

Effect of end-inspiratory pause on airway and physiological dead space in anesthetized horses

OBJECTIVE

To evaluate the impact of a 30% end-inspiratory pause (EIP) on alveolar tidal volume (V_Talv), airway (V_Daw) and physiological (V_Dphys) dead spaces in mechanically ventilated horses using volumetric capnography, and to evaluate the effect of EIP on carbon dioxide (CO₂) elimination per breath (Vco₂br^-1), PaCO_2, and the ratio of PaO₂-to-fractional inspired oxygen (PaO₂:FiO₂).

STUDY DESIGN

Prospective research study.

ANIMALS

A group of eight healthy research horses undergoing laparotomy.

METHODS

Anesthetized horses were mechanically ventilated as follows: 6 breaths minute^-1, tidal volume (V_T) 13 mL kg^-1, inspiratory-to-expiratory time ratio 1:2, positive end-expiratory pressure 5 cmH₂O and EIP 0%. Vco₂br^-1 and expired tidal volume (V_TE) of 10 consecutive breaths were recorded 30 minutes after induction, after adding 30% EIP and upon EIP removal to construct volumetric capnograms. A stabilization period of 15 minutes was allowed between phases. Data were analyzed using a mixed-effect linear model. Significance was set at p < 0.05.

RESULTS

The EIP decreased V_Daw from 6.6 (6.1-6.7) to 5.5 (5.3-6.1) mL kg^-1 (p < 0.001) and increased V_Talv from 7.7 ± 0.7 to 8.6 ± 0.6 mL kg^-1 (p = 0.002) without changing the V_TE. The V_Dphys to V_TE ratio decreased from 51.0% to 45.5% (p < 0.001) with EIP. The EIP also increased PaO₂:FiO₂ from 393.3 ± 160.7 to 450.5 ± 182.5 mmHg (52.5 ± 21.4 to 60.0 ± 24.3 kPa; p < 0.001) and Vco₂br^-1 from 0.49 (0.45-0.50) to 0.59 (0.45-0.61) mL kg^-1 (p = 0.008) without reducing PaCO₂.

CONCLUSIONS AND CLINICAL RELEVANCE

The EIP improved oxygenation and reduced V_Daw and V_Dphys, without reductions in PaCO₂. Future studies should evaluate the impact of different EIP in healthy and pathological equine populations under anesthesia.

Lung-protective properties of expiratory flow-initiated pressure-controlled inverse ratio ventilation: A randomised controlled trial

Background

Expiratory flow-initiated pressure-controlled inverse ratio ventilation (EF-initiated PC-IRV) reduces physiological dead space. We hypothesised that EF-initiated PC-IRV would be lung protective compared with volume-controlled ventilation (VCV).

Methods

Twenty-eight men undergoing robot-assisted laparoscopic radical prostatectomy were enrolled in this randomised controlled trial. The EF-initiated PC-IRV group (n = 14) used pressure-controlled ventilation with the volume guaranteed mode. The inspiratory to expiratory (I:E) ratio was individually adjusted by observing the expiratory flow-time wave. The VCV group (n = 14) used the volume control mode with a 1:2 I:E ratio. The Mann–Whitney U test was used to compare differences in the serum cytokine levels.

Results

There were no significant differences in serum IL-6 between the EF-initiated PC-IRV (median 34 pg ml^-1 (IQR 20.5 to 63.5)) and VCV (31 pg ml^-1 (24.5 to 59)) groups (P = 0.84). The physiological dead space rate (physiological dead space/expired tidal volume) was significantly reduced in the EF-initiated PC-IRV group as compared with that in the VCV group (0.31 ± 0.06 vs 0.4 ± 0.07; P<0.001). The physiological dead space rate was negatively correlated with the forced vital capacity (% predicted) in the VCV group (r = -0.85, P<0.001), but not in the EF-initiated PC-IRV group (r = 0.15, P = 0.62). Two patients in the VCV group had permissive hypercapnia with low forced vital capacity (% predicted).

Conclusions

There were no differences in the lung-protective properties between the two ventilatory strategies. However, EF-initiated PC-IRV reduced physiological dead space rate; thus, it may be useful for reducing the ventilatory volume that is necessary to maintain normocapnia in patients with low forced vital capacity (% predicted) during robot-assisted laparoscopic radical prostatectomy.

An appropriate inspiratory flow pattern can enhance CO2 exchange, facilitating protective ventilation of healthy lungs

BACKGROUND

In acute lung injury, CO2 exchange is enhanced by prolonging the volume-weighted mean time for fresh gas to mix with resident alveolar gas, denoted mean distribution time (MDT), and by increasing the flow rate immediately before inspiratory flow interruption, end-inspiratory flow (EIF). The objective was to study these effects in human subjects without lung disease and to analyse the results with respect to lung-protective ventilation of healthy lungs.

METHODS

During preparation for intracranial surgery, the lungs of eight subjects were ventilated with a computer-controlled ventilator, allowing breath-by-breath modification of the inspiratory flow pattern. The durations of inspiration (TI) and postinspiratory pause (TP) were modified, as was the profile of the inspiratory flow wave (i.e. constant, increasing, or decreasing). The single-breath test for CO2 was used to quantify airway dead space (VDaw) and CO2 exchange.

RESULTS

A long MDT and a high EIF augment CO2 elimination by reducing VDaw and promoting mixing of tidal gas with resident alveolar gas. A heat and moisture exchanger had no other effect than enlarging VDaw. A change of TI from 33 to 15% and of TP from 10 to 28%, leaving the time for expiration unchanged, would augment tidal elimination of CO2 by 14%, allowing a 10% lower tidal volume.

CONCLUSIONS

In anaesthetized human subjects without lung disease, CO2 exchange is enhanced by a long MDT and a high EIF. A short TI and a long TP allow significant reduction of tidal volume when lung-protective ventilation is required.

CLINICAL TRIAL REGISTRATION

NCT01686984.

End-inspiratory pause prolongation in acute respiratory distress syndrome patients: effects on gas exchange and mechanics

Background

End-inspiratory pause (EIP) prolongation decreases dead space-to-tidal volume ratio (Vd/Vt) and PaCO₂. We do not know the physiological benefits of this approach to improve respiratory system mechanics in acute respiratory distress syndrome (ARDS) patients when mild hypercapnia is of no concern.

Methods

The investigation was conducted in an intensive care unit of a university hospital, and 13 ARDS patients were included. The study was designed in three phases. First phase, baseline measurements were taken. Second phase, the EIP was prolonged until one of the following was achieved: (1) EIP of 0.7 s; (2) intrinsic positive end-expiratory pressure ≥1 cmH₂O; or (3) inspiratory–expiratory ratio 1:1. Third phase, the Vt was decreased (30 mL every 30 min) until PaCO₂ equal to baseline was reached. FiO₂, PEEP, airflow and respiratory rate were kept constant.

Results

EIP was prolonged from 0.12 ± 0.04 to 0.7 s in all patients. This decreased the Vd/Vt and PaCO₂ (0.70 ± 0.07 to 0.64 ± 0.08, p < 0.001 and 54 ± 9 to 50 ± 8 mmHg, p = 0.001, respectively). In the third phase, the decrease in Vt (from 6.3 ± 0.8 to 5.6 ± 0.8 mL/Kg PBW, p < 0.001) allowed to decrease plateau pressure and driving pressure (24 ± 3 to 22 ± 3 cmH₂O, p < 0.001 and 13.4 ± 3.6 to 10.9 ± 3.1 cmH₂O, p < 0.001, respectively) and increased respiratory system compliance from 29 ± 9 to 32 ± 11 mL/cmH₂O (p = 0.001). PaO₂ did not significantly change.

Conclusions

Prolonging EIP allowed a significant decrease in Vt without changes in PaCO₂ in passively ventilated ARDS patients. This produced a significant decrease in plateau pressure and driving pressure and significantly increased respiratory system compliance, which suggests less overdistension and less dynamic strain.

Dead space during one-lung ventilation

PURPOSE OF REVIEW

Describe the importance of monitoring dead space during thoracic surgery, specifically during one-lung ventilation.

RECENT FINDINGS

The concept of dead space has gained renewed interest among anesthesiologists ever since breath-by-breath measurement by volumetric capnography became available. Monitoring dead space during thoracic surgery assesses the ventilatory deficiencies related to increases in instrumental, airway and/or alveolar dead space, when ventilating patients with positive pressure and double-lumen tubes. Another interesting use of such monitoring is to detect ventilator-induced lung injury due to tidal overdistension. This type of injury threatens the fragile lungs especially during one-lung ventilation and can clinically be recognized as an increase in airway and alveolar dead space above normal values. To date, lung protective ventilation is based on the use of low tidal volumes and airway pressures to decrease overdistension. It has been shown to reduce the incidence of postoperative pulmonary complications after thoracic surgeries. However, such a ventilatory strategy impairs ventilation and induces hypercapnia due to increases in dead space. Therefore, continuous assessment of dead space is helpful in guiding ventilation and avoiding overdistension while maintaining the elimination of CO(2) during thoracic surgery sufficiently high.

SUMMARY

Monitoring dead space helps anesthesiologists monitor the status of the lung and find appropriate ventilatory settings during thoracic surgeries.

Effects of different flow patterns and end-inspiratory pause on oxygenation and ventilation in newborn piglets: an experimental study

BACKGROUND

Historically, the elective ventilatory flow pattern for neonates has been decelerating flow (DF). Decelerating flow waveform has been suggested to improve gas exchange in the neonate when compared with square flow (SF) waveform by improving the ventilation perfusion. However, the superiority of DF compared with SF has not yet been demonstrated during ventilation in small infants. The aim of this study was to compare SF vs. DF, with or without end-inspiratory pause (EIP), in terms of oxygenation and ventilation in an experimental model of newborn piglets.

METHODS

The lungs of 12 newborn Landrace/LargeWhite crossbred piglets were ventilated with SF, DF, SF-EIP and DF-EIP. Tidal volume (VT), inspiratory to expiratory ratio (I/E), respiratory rate (RR), and FiO2 were keep constant during the study. In order to assure an open lung during the study while preventing alveolar collapse, a positive end-expiratory pressure (PEEP) of 6 cmH2O was applied after a single recruitment maneuver. Gas exchange, lung mechanics and hemodynamics were measured.

RESULTS

The inspiratory flow waveform had no effect on arterial oxygenation pressure (PaO2) (276 vs. 278 mmHg, p = 0.77), alveolar dead space to alveolar tidal volume (VDalv/VTalv) (0.21 vs. 0.19 ml, p = 0.33), mean airway pressure (Pawm) (13.1 vs. 14.0 cmH2O, p = 0.69) and compliance (Crs) (3.5 vs. 3.5 ml cmH2O(-1), p = 0.73) when comparing SF and DF. A short EIP (10%) did not produce changes in the results.

CONCLUSION

The present study showed that there are no differences between SF, DF, SF-EIP and DF-EIP in oxygenation, ventilation, lung mechanics, or hemodynamics in this experimental model of newborn piglets with healthy lungs.

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.

Dead space and CO₂ elimination related to pattern of inspiratory gas delivery in ARDS patients

INTRODUCTION

The inspiratory flow pattern influences CO₂ elimination by affecting the time the tidal volume remains resident in alveoli. This time is expressed in terms of mean distribution time (MDT), which is the time available for distribution and diffusion of inspired tidal gas within resident alveolar gas. In healthy and sick pigs, abrupt cessation of inspiratory flow (that is, high end-inspiratory flow (EIF)), enhances CO₂ elimination. The objective was to test the hypothesis that effects of inspiratory gas delivery pattern on CO₂ exchange can be comprehensively described from the effects of MDT and EIF in patients with acute respiratory distress syndrome (ARDS).

METHODS

In a medical intensive care unit of a university hospital, ARDS patients were studied during sequences of breaths with varying inspiratory flow patterns. Patients were ventilated with a computer-controlled ventilator allowing single breaths to be modified with respect to durations of inspiratory flow and postinspiratory pause (TP), as well as the shape of the inspiratory flow wave. From the single-breath test for CO₂, the volume of CO₂ eliminated by each tidal breath was derived.

RESULTS

A long MDT, caused primarily by a long TP, led to importantly enhanced CO₂ elimination. So did a high EIF. Effects of MDT and EIF were comprehensively described with a simple equation. Typically, an efficient and a less-efficient pattern of inspiration could result in ± 10% variation of CO₂ elimination, and in individuals, up to 35%.

CONCLUSIONS

In ARDS, CO₂ elimination is importantly enhanced by an inspiratory flow pattern with long MDT and high EIF. An optimal inspiratory pattern allows a reduction of tidal volume and may be part of lung-protective ventilation.

Protective ventilation in experimental acute respiratory distress syndrome after ventilator-induced lung injury: a randomized controlled trial

Background

Low tidal volume (V_T), PEEP, and low plateau pressure (P_PLAT) are lung protective during acute respiratory distress syndrome (ARDS). This study tested the hypothesis that the aspiration of dead space (ASPIDS) together with computer simulation can help maintain gas exchange at these settings, thus promoting protection of the lungs.

Methods

ARDS was induced in pigs using surfactant perturbation plus an injurious ventilation strategy. One group then underwent 24 h protective ventilation, while control groups were ventilated using a conventional ventilation strategy at either high or low pressure. Pressure–volume curves (P_el/V), blood gases, and haemodynamics were studied at 0, 4, 8, 16, and 24 h after the induction of ARDS and lung histology was evaluated.

Results

The P_el/V curves showed improvements in the protective strategy group and deterioration in both control groups. In the protective group, when respiratory rate (RR) was ≈60 bpm, better oxygenation and reduced shunt were found. Histological damage was significantly more severe in the high-pressure group. There were no differences in venous oxygen saturation and pulmonary vascular resistance between the groups.

Conclusions

The protective ventilation strategy of adequate pH or Pa_CO2 with minimal V_T, and high/safe P_PLAT resulting in high PEEP was based on the avoidance of known lung-damaging phenomena. The approach is based upon the optimization of V_T, RR, PEEP, I/E, and dead space. This study does not lend itself to conclusions about the independent role of each of these features. However, dead space reduction is fundamental for achieving minimal V_T at high RR. Classical physiology is applicable at high RR. Computer simulation optimizes ventilation and limiting of dead space using ASPIDS. Inspiratory P_el/V curves recorded from PEEP or, even better, expiratory P_el/V curves allow monitoring in ARDS.

Monitoring in the intensive care

In critical care, the monitoring is essential to the daily care of ICU patients, as the optimization of patient's hemodynamic, ventilation, temperature, nutrition, and metabolism is the key to improve patients' survival. Indeed, the decisive endpoint is the supply of oxygen to tissues according to their metabolic needs in order to fuel mitochondrial respiration and, therefore, life. In this sense, both oxygenation and perfusion must be monitored in the implementation of any resuscitation strategy. The emerging concept has been the enhancement of macrocirculation through sequential optimization of heart function and then judging the adequacy of perfusion/oxygenation on specific parameters in a strategy which was aptly coined “goal directed therapy.” On the other hand, the maintenance of normal temperature is critical and should be regularly monitored. Regarding respiratory monitoring of ventilated ICU patients, it includes serial assessment of gas exchange, of respiratory system mechanics, and of patients' readiness for liberation from invasive positive pressure ventilation. Also, the monitoring of nutritional and metabolic care should allow controlling nutrients delivery, adequation between energy needs and delivery, and blood glucose. The present paper will describe the physiological basis, interpretation of, and clinical use of the major endpoints of perfusion/oxygenation adequacy and of temperature, respiratory, nutritional, and metabolic monitorings.

Effects of inspiratory pause on CO2 elimination and arterial PCO2 in acute lung injury

A high respiratory rate associated with the use of small tidal volumes, recommended for acute lung injury (ALI), shortens time for gas diffusion in the alveoli. This may decrease CO(2) elimination. We hypothesized that a postinspiratory pause could enhance CO(2) elimination and reduce Pa(CO(2)) by reducing dead space in ALI. In 15 mechanically ventilated patients with ALI and hypercapnia, a 20% postinspiratory pause (Tp20) was applied during a period of 30 min between two ventilation periods without postinspiratory pause (Tp0). Other parameters were kept unchanged. The single breath test for CO(2) was recorded every 5 min to measure tidal CO(2) elimination (VtCO(2)), airway dead space (V(Daw)), and slope of the alveolar plateau. Pa(O(2)), Pa(CO(2)), and physiological and alveolar dead space (V(Dphys), V(Dalv)) were determined at the end of each 30-min period. The postinspiratory pause, 0.7 +/- 0.2 s, induced on average <0.5 cmH(2)O of intrinsic positive end-expiratory pressure (PEEP). During Tp20, VtCO(2) increased immediately by 28 +/- 10% (14 +/- 5 ml per breath compared with 11 +/- 4 for Tp0) and then decreased without reaching the initial value within 30 min. The addition of a postinspiratory pause significantly decreased V(Daw) by 14% and V(Dphys) by 11% with no change in V(Dalv). During Tp20, the slope of the alveolar plateau initially fell to 65 +/- 10% of baseline value and continued to decrease. Tp20 induced a 10 +/- 3% decrease in Pa(CO(2)) at 30 min (from 55 +/- 10 to 49 +/- 9 mmHg, P < 0.001) with no significant variation in Pa(O(2)). Postinspiratory pause has a significant influence on CO(2) elimination when small tidal volumes are used during mechanical ventilation for ALI.

Computer simulation allows goal-oriented mechanical ventilation in acute respiratory distress syndrome

Introduction

To prevent further lung damage in patients with acute respiratory distress syndrome (ARDS), it is important to avoid overdistension and cyclic opening and closing of atelectatic alveoli. Previous studies have demonstrated protective effects of using low tidal volume (V_T), moderate positive end-expiratory pressure and low airway pressure. Aspiration of dead space (ASPIDS) allows a reduction in V_T by eliminating dead space in the tracheal tube and tubing. We hypothesized that, by applying goal-orientated ventilation based on iterative computer simulation, V_T can be reduced at high respiratory rate and much further reduced during ASPIDS without compromising gas exchange or causing high airway pressure.

Methods

ARDS was induced in eight pigs by surfactant perturbation and ventilator-induced lung injury. Ventilator resetting guided by computer simulation was then performed, aiming at minimal V_T, plateau pressure 30 cmH₂O and isocapnia, first by only increasing respiratory rate and then by using ASPIDS as well.

Results

V_T decreased from 7.2 ± 0.5 ml/kg to 6.6 ± 0.5 ml/kg as respiratory rate increased from 40 to 64 ± 6 breaths/min, and to 4.0 ± 0.4 ml/kg when ASPIDS was used at 80 ± 6 breaths/min. Measured values of arterial carbon dioxide tension were close to predicted values. Without ASPIDS, total positive end-expiratory pressure and plateau pressure were slightly higher than predicted, and with ASPIDS they were lower than predicted.

Conclusion

In principle, computer simulation may be used in goal-oriented ventilation in ARDS. Further studies are needed to investigate potential benefits and limitations over extended study periods.

Pattern of inspiratory gas delivery affects CO2 elimination in health and after acute lung injury

OBJECTIVE

To avoid ventilator induced lung injury, tidal volume should be low in acute lung injury (ALI). Reducing dead space may be useful, for example by using a pattern of inspiration that prolongs the time available for gas distribution and diffusion within the respiratory zone, the mean distribution time (MDT). A study was conducted to investigate how MDT affects CO2 elimination in pigs at health and after ALI.

DESIGN AND SETTING

Randomised crossover study in the animal laboratory of Lund University Biomedical Center.

SUBJECTS AND INTERVENTION

Healthy pigs and pigs with ALI, caused by surfactant perturbation and lung-damaging ventilation were ventilated with a computer-controlled ventilator. With this device each breath could be tailored with respect to insufflation time and pause time (TI and TP) as well as flow shape (square, increasing or decreasing flow).

MEASUREMENTS AND RESULTS

The single-breath test for CO2 allowed analysis of the volume of expired CO2 and the volume of CO2 re-inspired from Y-piece and tubes. With a long MDT caused by long TI or TP, the expired volume of CO2 increased markedly in accordance with the MDT concept in both healthy and ALI pigs. High initial inspiratory flow caused by a short TI or decreasing flow increased the re-inspired volume of CO2. Arterial CO2 increased during a longer period of short MDT and decreased again when MDT was prolonged.

CONCLUSIONS

CO2 elimination can be enhanced by a pattern of ventilation that prolongs MDT. Positive effects of prolonged MDT caused by short TI and decreasing flow were attenuated by high initial inspiratory flow.

CO2 elimination at varying inspiratory pause in acute lung injury

Previous studies have indicated that, during mechanical ventilation, an inspiratory pause enhances gas exchange. This has been attributed to prolonged time during which fresh gas of the tidal volume is present in the respiratory zone and is available for distribution in the lung periphery. The mean distribution time of inspired gas (MDT) is the mean time during which fractions of fresh gas are present in the respiratory zone. All ventilators allow setting of pause time, T(P), which is a determinant of MDT. The objective of the present study was to test in patients the hypothesis that the volume of CO(2) eliminated per breath, V(T)CO(2), is correlated to the logarithm of MDT as previously found in animal models. Eleven patients with acute lung injury were studied. When T(P) increased from 0% to 30%, MDT increased fourfold. A change of T(P) from 10% to 0% reduced V(T)CO(2) by 14%, while a change to 30% increased V(T)CO(2) by 19%. The relationship between V(T)CO(2) and MDT was in accordance with the logarithmic hypothesis. The change in V(T)CO(2) reflected to equal extent changes in airway dead space and alveolar PCO(2) read from the alveolar plateau of the single breath test for CO(2). By varying T(P), effects are observed on V(T)CO(2), airway dead space and alveolar PCO(2). These effects depend on perfusion, gas distribution and diffusion in the lung periphery, which need to be further elucidated.

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, Vdphys, 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 Vdphys 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.