Fluid dynamic factors in tracheal pressure measurement

A comparison of endotracheal tube compensation techniques for the measurement of respiratory mechanical impedance at low frequencies

Measurement of respiratory impedance ([Formula: see text]) in intubated patients requires accurate compensation for pressure losses across the endotracheal tube (ETT). In this study, we compared time-domain (TD), frequency-domain (FD) and combined time-/frequency-domain (FT) methods for ETT compensation. We measured total impedance ([Formula: see text]) of a test lung in series with three different ETT sizes, as well as in three intubated porcine subjects. Pressure measurement at the distal end of the ETT was used to determine the true [Formula: see text]. For TD compensation, pressure distal to the ETT was obtained based on its resistive and inertial properties, and the corresponding [Formula: see text] was estimated. For FD compensation, impedance of the isolated ETT was obtained from oscillatory flow and pressure waveforms, and then subtracted from [Formula: see text]. For TF compensation, the nonlinear resistive properties of the ETT were subtracted from the proximal pressure measurement, from which the linear resistive and inertial ETT properties were removed in the frequency-domain to obtain [Formula: see text]. The relative root mean square error between the actual and estimated [Formula: see text] ([Formula: see text]) showed that TD compensation yielded the least accurate estimates of [Formula: see text] for the in vitro experiments, with small deviations observed at higher frequencies. The FD and TF compensations yielded estimates of [Formula: see text] with similar accuracies. For the porcine subjects, no significant differences were observed in [Formula: see text] across compensation methods. FD and TF compensation of the ETT may allow for accurate oscillometric estimates of [Formula: see text] in intubated subjects, while avoiding the difficulties associated with direct tracheal pressure measurement.

The Effects of Prone with Respect to Supine Position on Stress Relaxation, Respiratory Mechanics, and the Work of Breathing Measured by the End-Inflation Occlusion Method in the Rat

PURPOSE

The working hypothesis is that the prone position with respect to supine may change the geometric configuration of the lungs inside the chest wall, thus their reciprocal mechanical interactions, leading to possible effects on stress relaxation phenomena and respiratory mechanics.

METHOD

The effects of changing body posture from supine to prone on respiratory system mechanics, particularly on stress relaxation, were investigated in the rat by the end-inflation occlusion method.

RESULTS

In the prone with respect to supine position, an increment of the frictional resistance of the airway (from 0.13 ± 0.01 to 0.19 ± 0.02 cm H2O/l sec(-1), p < 0.05) and a decrement of the stress relaxation-linked pressure dissipation (from 0.51 ± 0.05 to 0.45 ± 0.05 cm H2O/l sec(-1), p < 0.01) were found. Respiratory system elastance and total resistive pressure dissipation did not change significantly. Accordingly, a significant increase of the frictional "ohmic" mechanical inspiratory work of breathing and a decrease of the visco-elastic work of inspiration were demonstrated, while no significant changes occurred for the total mechanical work of breathing and its total resistive and elastic components.

CONCLUSION

It is concluded that postural changes affect the visco-elastic characteristics of the respiratory system and the related stress relaxation phenomena by influencing the disposition and relation of the lungs inside the chest wall and their relative geometrical configuration, and the interaction phenomena of the constitutive parenchymal structures, i.e., elastin and collagen fibers. Since the prone position resulted in no serious or disadvantageous respiratory system mechanical derangement, it is suggested it may be usefully applied in nursing or for therapeutic goals.

A REVIEW OF THE EFFECTS OF BODY TEMPERATURE VARIATIONS ON RESPIRATORY MECHANICS: MEASUREMENTS BY THE END-INFLATION OCCLUSION METHOD IN THE RAT

The temperature of body fluids is expected to affect tissues mechanical properties, including respiratory system tissues. This is because of the changes in airway smooth muscle tone and contractile properties, influencing airway frictional resistance to airflow, and because of the temperature effects on the stress–strain relationships of elastin and collagen, which determinates the elastic behavior of the lungs as reflected by their pressure–volume relationship. Alveolar surfactant biological and physical properties have also been shown to be affected by temperature changes, suggesting influences on the respiratory system hysteretic properties.

Experimental works describing the effects of body temperature variations on respiratory mechanics are reviewed, including recent findings dealing with investigations on respiratory mechanics carried out by the end-inflation occlusion method in the rat. This method allows to determine, together with the elastance of the respiratory system, its resistive properties too. In particular, both the ohmic airway resistance due to frictional forces in the airway and the additional visco-elastic resistance exerted because of tissues stress-relaxation may be quantified.

The effects of body temperature variations were assessed, and experimentally induced temperature increments and/or decrements allowed to conclude that respiratory system tissues stiffness, both the ohmic and the stress-relaxation linked resistances, and the hysteretic behavior of the respiratory system, decrease with temperature increments. The mechanisms responsible for these effects are analyzed.

In vitro estimation of pressure drop across tracheal tubes during high-frequency percussive ventilation

Tracheal tubes (TT) are used in clinical practice to connect an artificial ventilator to the patient's airways. It is important to know the pressure used to overcome tube impedance to avoid lung injury. Although high-frequency percussive ventilation (HFPV) has been increasingly used, the mechanical behavior of TT under HFPV has not yet been described. Thus, we aimed at characterizing in vitro the pressure drop across TT (ΔPTT) by identifying the model that best fits the measured pressure-flow (P-V̇) relationships during HFPV under different working pressures (PWork), percussive frequencies and mechanical loads. Three simple models relating ΔPTT and flow (V̇) were tested. Model 1 is characterized by linear resistive [Rtube ⋅ V̇(t)] and inertial [I · V̈(t)] terms. Model 2 takes into consideration Rohrer's approach [K1· V̇(t) + K2 ⋅V̇(t)] and inertance [I ·V̈(t)]. In model 3 the pressure drop caused by friction is represented by the non-linear Blasius component [Kb· V̇(1.75)(t)] and the inertial term [I· V̈(t)]. Model 1 presented a significantly higher root mean square error of approximation than models 2 and 3, which were similar. Thus, model 1 was not as accurate as the latter, possibly due to turbulence. Model 3 presented the most robust resistance-related coefficient. Estimated inertances did not vary among the models using the same tube. In conclusion, in HFPV ΔPTT can be easily calculated by the physician using model 3.

Decomposition and solubility of H2O2: implications in exhaled breath condensate

Hydrogen peroxide (H2O2) is one of the metabolic end products present in exhaled breath. High levels of H2O2 found in breath condensate are an indicator of airway inflammation and could be used for monitoring the condition of patients with chronic obstructive pulmonary disease. However, sampling conditions such as breath temperature, condensing temperature, flow rate and collection time can affect the intrinsic properties of H2O2-its solubility, volatility, and decomposition rate. Sudden decreases to H2O2 concentration may be due to the sampling conditions instead of the patient's health status. The decomposition rate and Henry's law constant for saturated H2O2 vapor (RH > 95%) within 22-42 °C, which correlates to room temperature and range of human breath temperatures, are needed for better understanding and standardization of breath collection. In this study, we determined the effects of initial H2O2 concentration, temperature, and sampling time on the decomposition rate by comparing electrochemical measurements of H2O2 in simulated breath samples. The experimental results showed the decomposition rate of H2O2 increased as the breath temperature and sampling time increased and the solubility of H2O2 increased with increasing flow rate and condensing temperature during sampling. Prediction models for H2O2 sensing in exhaled breath sample were developed that could be used in the standardization of exhaled breath condensate collection. These experimental findings need to be further verified with human/animal breath samples.

The effect of body cooling on respiratory system mechanics and hysteresis in rats

Literature reports and theoretical considerations suggest that body cooling may affect respiratory mechanics in vivo. To examine this hypothesis, healthy rats were studied using the end-inflation occlusion method under control conditions and after total body cooling. Respiratory mechanics parameters, hysteresis areas, the inspiratory work of breathing, and its elastic and resistive components, were calculated. After body cooling (mean rectal temperature from 36.6 ± 0.25 to 32.1 ± 0.26 °C), the ohmic and the additional visco-elastic respiratory system resistances, the hysteresis, the total inspiratory work of breathing, and its resistive components, were all increased. No significant changes were detected for the static and dynamic respiratory system elastance mean values, and the related elastic component of the work of breathing. These data indicate that body cooling increases the mechanical inspiratory work of breathing by increasing the resistive pressures dissipation. This effect is evident even for limited temperature variations, and it is suggested that it may occur in the event of accidental or therapeutic hypothermia.

Effects of the pneumoperitoneum and Trendelenburg position on respiratory mechanics in the rats by the end-inflation occlusion method

PURPOSE:

To describe the consequences of the cranial displacement of the diaphgram occurring during pneumoperitoneum (Pnp) and/or Trendelenburg (Tnd) position on respiratory mechanics. Possible addictive effects and the changes of the viscoelastic respiratory system resistance were studied, which were not extensively described before.

METHODS:

The end-inflation occlusion method was applied on eight rats. It allows us to determine mechanical parameters such as respiratory system static elastance, the ohmic resistance due to frictional forces in the airways, and the additional viscoelastic impedance due to tissues deformation. Measurements during mechanical ventilation were taken in controls (supine position), after 20–25° head-down tilting (Tnd), after abdominal air insufflation up to 12 mmHg abdominal pressure in the supine position (Pnp), and combining Tnd + Pnp. Tnd and Pnp modalities were similar to those commonly applied during surgical procedures in humans.

RESULTS:

We confirmed the previously described detrimental effects on respiratory mechanics due to the diaphgram displacement during both Pnp and Tnd. The increment in the total resistive pressure dissipation was found to depend primarily on the effects on the viscoelastic characteristics of the respiratory system. Data suggesting greater effects of Pnp compared to those of Tnd were obtained.

CONCLUSION:

The cranial displacement of the diaphgram occurring as a consequence of Pnp and/or Tnd, for example during laparoscopic surgical procedures, causes an increment of respiratory system elastance and viscoelastic resistance. The analysis of addictive effects show that these are more likely to occur when Pnp + Tnd are compared to isolated Tnd rather than to isolated Pnp.

Erythropoietin acutely decreases airway resistance in the rat

While some experimental data suggest that erythropoietin (EPO) influences respiratory mechanics, reports on scientific trials are lacking. In the present work, respiratory mechanics were measured using the end-inflation occlusion method in control and EPO treated anaesthetised and positive-pressure ventilated rats. Causing an abrupt inspiratory flow arrest, the end-inflation occlusion method makes it possible to measure the ohmic airway resistance and the respiratory system elastance. It was found that EPO induces a significant decrement in the ohmic airway resistance, not noted in control animals, 20 and 30 min after intraperitoneal EPO injection. The elastic characteristics of the respiratory system did not vary. Hypotheses about the mechanism (s) explaining these results were addressed. In particular, additional experiments have indicated that the decrement in airway resistance could be related to an increase in nitric oxide production induced by EPO. Spontaneous increments in plasmatic erythropoietin levels, such as those that take place in association with hypoxia and/or blood loss, appear to be related to the decrement in airway resistance, allowing pulmonary ventilation to increase without altering respiratory mechanics leading to deleterious increments in energy dissipation during breathing.

The effect of angiotensin-converting enzyme inhibition by captopril on respiratory mechanics in healthy rats

CONTEXT

Angiotensin stimulates smooth-muscle contraction. Accordingly, angiotensin-converting enzyme (ACE) inhibition is expected to decrease airway resistance.

OBJECTIVES

To measure the effects of ACE inhibition on respiratory mechanics in healthy mammals.

MATERIALS AND METHODS

We measured respiratory mechanics before and after i.p. ACE inhibitor captopril (100 mg/kg) in normal anaesthetised rats. The end-inflation occlusion method allowed the measurements of respiratory system elastance and ohmic and viscoelastic pressure dissipations. Respiratory system hysteresis and the elastic and resistive work of breathing were calculated.

RESULTS

Captopril induced a reduction of the ohmic and the total respiratory system resistances, while respiratory system hysteresis and elastance did not change. Accordingly, a reduction of the resistive and of the total work of breathing was observed.

CONCLUSIONS

The captopril-induced reduction of airway resistance indicates that angiotensin modulates bronchomotor tone in basal conditions. ACE inhibition may positively affect respiratory system mechanics and work of breathing.

Effects of cysteinyl-leukotriene receptors' antagonism by montelukast on lung mechanics and olfactory system histology in healthy mice

CONTEXT

At variance with steroid administration, the possible effects of leukotrienes inhibition on basal respiratory mechanics and olfactory system function are still unclear.

OBJECTIVE

To investigate if interference with the leukotrienes activity may influence basal lung mechanics in healthy mammals, as well as the olfactory system.

MATERIALS AND METHODS

We measured lung mechanics by the end-inflation occlusion method in control and in montelukast i.p. treated anaesthetised healthy mice (10 mg/kg/die for a week). A study of olfactory system histology was also conducted.

RESULTS

Elastance and resistive properties of the lung were not affected by montelukast, while a significant increment of lung hysteresis was observed. The analysis of olfactory system histology revealed no significant effects of montelukast compared to controls.

DISCUSSION AND CONCLUSIONS

Leukotrienes' antagonism does not affect respiratory mechanics in basal conditions, except for a hysteresis increment, which might counteract the increase in expiratory flow in asthmatic subjects assuming montelukast.

Endotracheal tube resistance and inertance in a model of mechanical ventilation of newborns and small infants-the impact of ventilator settings on tracheal pressure swings

Resistive properties of endotracheal tubes (ETTs) are particularly relevant in newborns and small infants who are generally ventilated through ETTs with a small inner diameter. The ventilation rate is also high and the inspiratory time (ti) is short. These conditions effectuate high airway flows with excessive flow acceleration, so airway resistance and inertance play an important role. We carried out a model study to investigate the impact of varying ETT size, lung compliance and ventilator settings, such as peak inspiratory pressure (PIP), positive end expiratory pressure (PEEP) and inspiratory time (ti) on the pressure-flow characteristics with respect to the resistive and inertive properties of the ETT. Pressure at the Y piece was compared to direct measurement of intratracheal pressure (P(trach)) at the tip of the ETT, and pressure drop (ΔP(ETT)) was calculated. Applying published tube coefficients (Rohrer's constants and inertance), P(trach) was calculated from ventilator readings and compared to measured P(trach) using the root-mean-square error. The most relevant for ΔP(ETT) was the ETT size, followed by (in descending order) PIP, compliance, ti and PEEP, with gas flow velocity being the principle in common for all these parameters. Depending on the ventilator settings ΔP(ETT) exceeded 8 mbar in the smallest 2.0 mm ETT. Consideration of inertance as an additional effect in this setting yielded a better agreement of calculated versus measured P(trach) than Rohrer's constants alone. We speculate that exact tracheal pressure tracings calculated from ventilator readings by applying Rohrer's equation and the inertance determination to small size ETTs would be helpful. As an integral part of ventilator software this would (1) allow an estimate of work of breathing and implementation of an automatic tube compensation, and (2) be important for gentle ventilation in respiratory care, especially of small infants, since it enables the physician to estimate consequences of altered ventilator settings at the tracheal level.

Respiratory mechanics in 1-day old chicken hatchlings and effects of prenatal hypoxia

This study examined the static and dynamic properties of the respiratory system in avian hatchlings, and the effects of incubation in hypoxia (15% O(2)) on these variables. In 1-day old chicken (Gallus gallus) hatchlings killed by an anesthetic overdose the static compliance of the respiratory system (C(rs)) was measured from the pressure-volume curve, constructed by step changes in lung volume. The dynamic compliance, C(rs)(dyn), resistance, R(rs), and time constant, τ(rs), were measured during mechanical ventilation at rates up to 90 cpm. The results indicated that (1) static C(rs) in hatchlings is several folds higher than in neonatal mammals of similar size, (2) during mechanical ventilation the respiratory system becomes hyperinflated and much stiffer; at 65 cpm (which is the respiratory frequency of hatchlings spontaneously breathing at rest) C(rs)(dyn) was about one tenth of the static value, (3) after prenatal hypoxia static C(rs), C(rs)(dyn), R(rs) and τ(rs) were similar to controls; only the magnitude of the hyperinflation was slightly decreased. It is concluded that in avian hatchlings (a) despite the large respiratory volume of the air sacs, expiration can occur passively because the hyperinflation greatly decreases C(rs)(dyn) and shortens τ(rs), and (b) prenatal hypoxia of the level tested has no major effects on the mechanical properties of the respiratory system.

Effects of Tityus serrulatus scorpion venom on lung mechanics and inflammation in mice

The present study evaluated the effects of an intramuscular injection of Tityus serrulatus venom (TsV) (0.67 miocrog/g) on lung mechanics and lung inflammation at 15, 30, 60 and 180 min after inoculation. TsV inoculation resulted in increased lung elastance when compared with the control group (p < 0.001); these values were significantly higher at 60 min than at 15 and 180 min (p < 0.05). Resistive pressure (DeltaP1) values decreased significantly at 30, 60 and 180 min after TsV injection (p < 0.001). TsV inoculation resulted in increased lung inflammation, characterised by an increased density of mononuclear cells at 15, 30, 60 and 180 min after TsV injection when compared with the control group (p < 0.001). TsV inoculation also resulted in an increased pulmonary density of polymorphonuclear cells at 15, 30 and 60 min following injection when compared to the control group (p < 0.001). In conclusion, T. serrulatus venom leads to acute lung injury, characterised by altered lung mechanics and increased pulmonary inflammation.

Pulmonary mechanic and lung histology injury induced by Crotalus durissus terrificus snake venom

In the present work we investigated the effects of Crotalus durissus terrificus venom (CdtV) on the pulmonary mechanic events [static and dynamic elastance, resistive (DeltaP1) and viscoelastic pressures (DeltaP2)] and histology after intramuscular injection of saline solution (control) or venom (0.6 microg/g). The static and dynamic elastance values were increased significantly after 3 h of venom inoculation, but were reduced at control values in the other periods studied. The DeltaP1 values that correspond to the resistive properties of lung tissue presented a significant increase after 6h of CdtV injection, reducing to basal levels 12h after the venom injection. In DeltaP2 analysis, correspondent to viscoelastic components, an increase occurred 12 h after the venom injection, returning to control values at 24 h. CdtV also caused an increase of leukocytes recruitment (3-24 h) to the airways wall as well as to the lung parenchyma. In conclusion, C. durissus terrificus rattlesnake venom leads to lung injury which is reverted, after 24 h of inoculation.

Calculation of intratracheal airway pressure in ventilated neonatal piglets with endotracheal tube leaks

OBJECTIVE

In ventilated neonates, only the applied pressure of the ventilator is adjusted and monitored. When an endotracheal tube leaks, intratracheal pressure decreases depending on the size of the endotracheal tube and of the leak. Furthermore, an increase in resistance and/or compliance might delay the increase of intratracheal pressure during inspiration and its decline during expiration. Short inspiratory time can cause insufficient ventilation, because intratracheal pressure peak might not be reached. Short expiratory time may lead to air trapping, because intratracheal pressure could not return to baseline. The aim of this study was to develop a mathematical algorithm to calculate intratracheal pressure continuously during ventilation and to evaluate the accuracy of this method.

DESIGN

Prospective, animal study.

SETTING

University research laboratory.

SUBJECTS

To verify the mathematical algorithm, eight neonatal piglets (1600-2600 g) were studied under different endotracheal tube leak conditions (45% to 98%). The median compliance and resistance were 1.06 mL/cm H2O/kg and 123 cm H2O/L/sec, respectively.

INTERVENTIONS

Pressure decreases caused by the different endotracheal tubes were measured in a model while air flow was increased stepwise. Based on these results, a mathematical method was developed to calculate intratracheal pressure under leak conditions continuously in relation to the flow through the endotracheal tube as well as to calculate the values of resistance, compliance, and applied pressure of the ventilator.

MEASUREMENTS AND MAIN RESULTS

The intratracheal pressure calculated was compared with the measured intratracheal pressure over time. The differences between measured and calculated intratracheal pressure related to peak applied pressure of the ventilator did not exceed 10%. The medians of absolute amounts of differences between measured and calculated intratracheal pressure were <1 cm H2O.

CONCLUSIONS

The accuracy of the calculation of intratracheal pressure ensures adequate monitoring of artificial ventilation, even in the presence of endotracheal tube leaks. This might decrease the risk of barotrauma and improve the effectiveness of ventilation.

Flow-dependent resistance of nasal masks used for non-invasive positive pressure ventilation

OBJECTIVE AND BACKGROUND

Endotracheal tube resistance is known to be flow-dependent and this understanding has improved the application of invasive ventilation. However, similar physiological studies on the interface between patients and non-invasive positive pressure ventilation (NPPV) have not been performed. Therefore, this study was aimed at investigating the resistance of nasal masks used for NPPV.

METHODOLOGY

The flow-dependent pressure drop of the small (S), medium-small (MS) and medium (M) Contour Nasal Mask (Respironics Inc., Murrysville, PA, USA) was measured with and without a connecting tube (length 18 cm, internal diameter 1.5 cm) in a laboratory study. The resistance was calculated by Rohrer's equation using the standard least-squares-fit technique. The present study explicitly differentiated between the resistance of the nasal mask alone when measured against atmosphere and the additional resistance caused by the nasal mask when airtightly fitted to a model head (interaction with the face).

RESULTS

Higher flow rates resulted in a non-linearly increasing pressure drop across the interface. This flow-dependent resistance of the S/MS/M mask was comparably low when not interacting with the face, but increased when interacting with the face. This flow-dependent resistance of the mask was several-fold higher when adding the connection tube and tended to be higher during expiration.

CONCLUSION

There is a non-linear flow-dependent pressure drop across the nasal mask which is low and independent of its size, but increases when interacting with the face. The connecting tube is the major determinant of the resistance originating from facial appliances used for NPPV.

Inertance measurements by jet pulses in ventilated small lungs after perfluorochemical liquid (PFC) applications

Perfluorochemical liquid (PFC) liquids or aerosols are used for assisted ventilation, drug delivery, lung cancer hyperthermia and pulmonary imaging. The aim of this study was to investigate the effect of PFC liquid on the inertance (I) of the respiratory system in newborn piglets using partial liquid ventilation (PLV) with different volumes of liquid. End-inspiratory (I(in)) and end-expiratory (I(ex)) inertance were measured in 15 ventilated newborn piglets (age < 12 h, mean weight 724 +/- 93 g) by brief flow pulses before and 80 min after PLV using a PFC volume (PF5080, 3 M) of 10 ml kg(-1) (N = 5) or 30 ml kg(-1) (N = 10). I was calculated from the imaginary part of the measured respiratory input impedance by regression analysis. Straight tubes with 2-4 mm inner diameter were used to validate the equipment in vitro by comparison with the analytically calculated values. In vitro measurements showed that the measuring error of I was <5% and that the reproducibility was better than 1.5%. The correlation coefficient of the regression model to determine I was >0.988 in all piglets. During gas ventilation, I(in) and I(ex) (mean +/- SD) were 31.7 +/- 0.8 Pa l(-1) s(2) and 33.3 +/- 2.1 Pa l(-1) s(2) in the 10 ml group and 32.4 +/- 0.8 Pa l(-1) s(2) and 34.0 +/- 2.5 Pa l(-1) s(2) in the 30 ml group. However, I of the 3 mm endotracheal tube (ETT) used was already 26.4 Pa l(-1) s(2) (about 80% of measured I). During PLV, there was a minimal increase of I(in) to 33.1 +/- 2.5 Pa l(-1) s(2) in the 10 ml group and to 34.5 +/- 2.7 Pa l(-1) s(2) in the 30 ml group. In contrast, the increase of I(ex) was dramatically larger (p < 0.001) to 67.7 +/- 13.3 Pa l(-1) s(2) and to 74.8 +/- 9.3 Pa l(-1) s(2) in the 10 ml and 30 ml groups, respectively. Measurements of I by jet pulses in intubated small animals are reproducible. PFC increases the respiratory inertance, but the magnitude depends considerably on its spatial distribution which changes during the breathing cycle. Large differences between I(in) and I(ex) are an indicator for liquid in airways or the ETT.

Positive end-expiratory pressure prevents lung mechanical stress caused by recruitment/derecruitment

This study tests the hypotheses that a recruitment maneuver per se yields and/or intensifies lung mechanical stress. Recruitment maneuver was applied to a model of paraquat-induced acute lung injury (ALI) and to healthy rats with (ATEL) or without (CTRL) previous atelectasis. Recruitment was done by using 40-cmH2O continuous positive airway pressure for 40 s. Rats were, then, ventilated for 1 h at zero end-expiratory pressure (ZEEP) or positive end-expiratory pressure (PEEP; 5 cmH2O). Atelectasis was generated by inflating a sphygmomanometer around the thorax. Additional groups did not undergo recruitment but were ventilated for 1 h under ZEEP. Lung resistive and viscoelastic pressures and static elastance were computed before and immediately after recruitment, and at the end of 1 h of ventilation. Lungs were prepared for histology. Type III procollagen (PCIII) mRNA expression in lung tissue was analyzed by RT-PCR. Lung mechanics improved after recruitment in the CTRL and ALI groups. One hour of ventilation at ZEEP increased alveolar collapse, static elastance, and lung resistive and viscoelastic pressures. Alveolar collapse was similar in ATEL and ALI, and recruitment opened the alveoli in both groups. ALI showed higher PCIII expression than ATEL or CTRL groups. One hour of ventilation at ZEEP did not increase PCIII expression but augmented it significantly in the three groups when applied after recruitment. However, PEEP ventilation after recruitment avoided any increment in PCIII expression in all groups. In conclusion, recruitment followed by ZEEP was more deleterious in ALI than in mechanical ATEL, although ZEEP alone did not elevate PCIII expression. Ventilation with 5-cmH2O PEEP prevented derecruitment and aborted the increase in PCIII expression.

The effects of tracheostomy cuff deflation during continuous positive airway pressure

Continuous flow positive pressure devices bridge the gap between mechanical and unsupported ventilation in patients recovering from critical illness. At this point, patients are often fully awake, yet the inflated tracheostomy cuff prevents them from speaking or swallowing. The aim of this study was to investigate the effects of cuff deflation. After ethics committee approval and informed consent, we recorded airway pressures with catheters placed 3 cm beyond the distal tracheostomy tip, respiratory rate, heart rate and peripheral oxygen saturation with continuous positive airway pressures set at 5, 7.5 and 10 cmH(2)O with the cuff inflated and deflated. Sixteen patients completed the study. There were small falls in end expiratory pressure on cuff deflation. The median (interquartile range) pressure drop with set airway pressure of 5 cmH(2)O was 0.25 (0-1.4) mmHg, which increased to 1 (0-3) mmHg at 7.5 cmH(2)O and 1.5 (0-4) mmHg at 10 cmH(2)O. These changes were not clinically significant and cardiopulmonary parameters remained stable. All patients were able to vocalise following cuff deflation. Twelve patients passed a blue dye swallow screen within a day of tolerating cuff deflation. These results suggest that pressures fall slightly following cuff deflation but this is associated with respiratory stability and may allow patients to talk and swallow.

Pulmonary mechanics and lung histology in acute lung injury induced by Bothrops jararaca venom

Pulmonary mechanics [static (Est) and dynamic (Edyn) elastances, resistive (DeltaP1) and viscoelastic pressures (DeltaP2)], histology, and bronchoalveolar lavage fluid (BALF) from BALB/c mice were analysed 1, 24, 48 and 72 h after intravenous injection of saline or Bothrops jararaca crude venom [0.3 (V0.3) or 1 (V1) microg.g(-1)]. Est, Edyn, and DeltaP2 increased at 1 h in both V groups, being significantly higher in V1 than in V0.3, decreasing progressively, reaching control values at 48 h in V0.3, but remaining altered in V1 at 72 h. DeltaP1 augmented in V1 at 1 h, returning to normal at 72 h. Histological changes in V0.3 group included interstitial oedema, alveolar collapse, and increased cellularity, which returned to normal at 48 h. These changes were more intense in V1 group, with alveolar oedema and haemorrhage. BALF showed time-dependent neutrophil influx in V0.3. In conclusion, venom led to time- and dose-dependent pulmonary mechanical changes, together with moderate inflammation in V0.3 and acute lung injury in V1.

Evaluation of respiratory mechanics and lung histology in a model of atelectasis

To develop a reproducible model of atelectasis, 15 mechanically ventilated Wistar rats were wrapped around the thorax/abdomen with a sphygmomanometer. The cuff was inflated to transpulmonary pressures (PL) of -4 cmH2O (group A) and -8 cmH2O (group B) for 5 sec. Group C was not compressed. Airflow, volume, tracheal and oesophageal pressures were registered. Respiratory system (rs), lung (L), and chest wall resistive (DeltaP1), viscoelastic/inhomogeneous pressures (DeltaP2), DeltaPtot (=DeltaP1 + DeltaP2), static (Est) and dynamic (Edyn) elastances, and DeltaE (=Edyn - Est) were determined before and after compression. In A, respiratory mechanics remained unaltered. In B, Est,rs (+99%), Est,L (+111%), DeltaE,rs (+41%), DeltaE,L (+73%), DeltaP1,rs (+45%), DeltaP1,L (+44%), DeltaP2,rs (+41%), DeltaP2,L (+69%), DeltaPtot,rs (+40%), and DeltaPtot,L (+58%) increased after compression. Mean alveolar diameter and bronchiolar lumen decreased in A, and were even smaller in B. In conclusion, chest wall compression with PL of -8 cmH2O yielded a reproducible alveolar collapse, which resulted in increased elastic, resistive and viscoelastic/inhomogeneous pressures.

Good short-term agreement between measured and calculated tracheal pressure

BACKGROUND

Tracheal pressure (P(tr)) is required to measure the resistance of the tracheal tube and the breathing circuit. P(tr) can either be measured with a catheter or, alternatively, calculated from the pressure-flow data available from the ventilator.

METHODS

Calculated P(tr) was compared with measured P(tr) during controlled ventilation and assisted spontaneous breathing in 18 healthy and surfactant-depleted piglets. Their lungs were ventilated using different flow patterns, tidal volumes (V(T)) and levels of positive end-expiratory pressure.

RESULTS

In terms of the root mean square error (RMS), indicating the average deviation of calculated from measured P(tr), the difference between calculated and measured P(tr) was 0.6 cm H(2)O (95%CI 0.58-0.65) for volume-controlled ventilation; 0.73 cm H(2)O (0.72-0.75) for pressure support ventilation; and 0.78 cm H(2)O (0.75-0.80) for bi-level positive airway pressure ventilation.

CONCLUSION

The good agreement between calculated and measured P(tr) during varying conditions, suggests that calculating P(tr) could help setting the ventilator and choosing the appropriate level of support.

A program based on a 'selective' least-squares method for respiratory mechanics monitoring in ventilated patients

This paper proposes a program for continuous estimation of respiratory mechanics parameters in ventilated patients. This program can be used with any ventilator providing airway pressure and flow signals without additional equipment. Overall breathing resistance, dynamic elastance (E) and positive end expiratory pressure (P(0)) are periodically estimated by multiple linear regression on selected parts of breathing cycles. Experimental validation together with justification of the selection procedure are based on signals obtained while ventilating a lung mechanical analogue with various intensive care ventilators. Clinical validity has been tested on 12 ventilated patients. The quality of estimation has been assessed by mean square difference between measured and reconstituted pressure (MSE), coefficient of determination (R(2)) and the condition number (a confidence index), and by comparison of E and P(0) with corresponding static values. The high R(2) and the low MSE obtained on most clinical cycles indicate that selected parts of cycles obey closely the model underlying parameter estimation. Agreement between static and dynamic parameters demonstrates the clinical validity of our program.

Effects of PEEP on inspiratory and expiratory mechanics in adult respiratory distress syndrome

The purpose of the present study was to assess the mechanical behavior of the respiratory system separately during inspiration and expiration in adult respiratory distress syndrome (ARDS) and the influence of PEEP on any phasic variations ofthe mechanical respiratory parameters. Airways pressure (P), flow (V), and volume (V) signals were recorded in nine patients with ARDS and 10 patients without known respiratory disorder (control group). All patients were artificially ventilated at three levels of positive end-expiratory pressure (PEEP): 0, 5, and 10 hPa. Data were analyzed separately for inspiratory and expiratory records using multiple linear regression analysis (MLRA) according to the equation: P=Ers V+Rrs V'+P0, where Ers and Rrs represent, respectively, the intubated respiratory system elastance and resistance, and P0 the end-expiratory pressure. In the ARDS group expiratory Ers (ErsEXP=45.58 +/- 4.24 hPa/L) was substantially higher (p<0.01) than inspiratory Ers (ErsINSP=36.76 +/- 2.55) with a marked effect of applied PEEP in diminishing the difference between ErsEXP and ErsINSP (p<0.01). For the ARDS group inspiratory Rrs (RrsINSP) decreased significantly with increasing PEEP (PEEP=0: RrsINSP=16.43, PEEP=10: RrsINSP=13.28, p<0.01). The found differences between ErsEXP and ErsINSP could be attributable to an influence of mechanical ventilation by positive airway pressure on pulmonary edemaand interstitial fluid during the inspiratory phase of the respiratory cycle.

Respiratory mechanics and lung histology in normal rats anesthetized with sevoflurane

Respiratory system, lung, and chest wall mechanical properties were subdivided into their resistive, elastic, and viscoelastic/inhomogeneous components in normal rats, to define the sites of action of sevoflurane. In addition, we aimed to determine the extent to which pretreatment with atropine modified these parameters. Twenty-four rats were divided into four groups of six animals each: in the P group, rats were sedated (diazepam) and anesthetized with pentobarbital sodium; in the S group, sevoflurane was administered; in the AP and AS groups, atropine was injected 20 min before sedation/anesthesia with pentobarbital and sevoflurane, respectively. Sevoflurane increased lung viscoelastic/inhomogeneous pressures and static elastance compared with rats belonging to the P group. In AS rats, lung static elastance increased in relation to the AP group. In conclusion, sevoflurane anesthesia acted not at the airway level but at the lung periphery, stiffening lung tissues and increasing mechanical inhomogeneities. These findings were supported by the histological demonstration of increased areas of alveolar collapse and hyperinflation. The pretreatment with atropine reduced central and peripheral airway secretion, thus lessening lung inhomogeneities.

Suture or prosthetic reconstruction of experimental diaphragmatic defects

OBJECTIVE

Diaphragmatic reconstruction may cause several respiratory changes. The aims of the present study were to evaluate the respiratory changes induced by two methods of diaphragmatic reconstruction.

METHODS

Two groups of rats with an experimental diaphragmatic defect were studied. In one group (n = 5), diaphragmatic resection was followed by stitching together the borders of the wound (SUT); in another group (n = 5), the defect was repaired by suturing in a polytetrafluoroethylene (PTFE) patch. All animals were sedated, anesthetized, paralyzed, and mechanically ventilated. Spirometry, respiratory mechanics, and thoracoabdominal morphometry were evaluated before and after diaphragmatic reconstruction.

RESULTS

The suture of the diaphragm significantly decreased FVC and FEV(1), and increased respiratory system, lung, and chest wall static and dynamic elastances and viscoelastic/inhomogeneous pressures in relation to their respective control values. On the other hand, diaphragmatic reconstruction with PTFE increased only respiratory system, lung, and chest wall static elastances. In addition, respiratory system, pulmonary, and chest wall viscoelastic/inhomogeneous pressures and dynamic elastances, as well as respiratory system and lung elastances, were significantly greater in SUT than in PTFE. Lateral diameter at the level of the xiphoid and cephalocaudal pulmonary diameter diminished only in the SUT group.

CONCLUSIONS

The reconstruction of the diaphragm with PTFE might be preferred to simple suture for surgical repair of large diaphragmatic defects, at least from a mechanical standpoint.

Continuous calculation of intratracheal pressure in the presence of pediatric endotracheal tubes

OBJECTIVE

To measure the pressure-flow relationship of pediatric endotracheal tubes (ETTs) in trachea models, to mathematically describe this relationship, and to evaluate in trachea/lung models a method for calculation of pressure at the distal end of the ETT (Ptrach) by subtracting the flow-dependent pressure drop across the ETT from the airway pressure measured at the proximal end of the ETT.

DESIGN

Trachea models and trachea/lung models.

SETTING

Research laboratory in a university medical center.

INTERVENTIONS

The pressure-flow relationship of pediatric ETTs (inner diameter, 2.5-6.5 mm) was determined using a physical model consisting of a tube connector, an anatomically curved ETT, and an artificial trachea. The model was ventilated with sinusoidal gas flow (12-60 cycles/min). The coefficients of an approximation equation considering ETT resistance and inertance were fitted separately to the measured pressure-flow curves for inspiration and expiration. Calculated Ptrach was compared with directly measured Ptrach in mechanically ventilated physical trachea/lung models.

MEASUREMENTS AND MAIN RESULTS

The pressure-flow relationship was considerably nonlinear and showed hysteresis around the origin caused by the inertia of accelerated gas. ETT inertance ranged from 0.1 to 0.4 cm H2O/L x sec2 (inner diameter, 6-2.5 mm). The abrupt change in cross-sectional area at the tube connector caused an inspiration-to-expiration asymmetry. Calculated and measured Ptrach were within +/- 1 cm H2O. Correspondence between measured and calculated Ptrach is improved even further when the ETT inertance is taken into account.

CONCLUSIONS

Ptrach can continuously be monitored in the presence of pediatric ETT by combining ETT coefficients and the flow and airway pressure continuously measured at the proximal end of the ETT.

Temporal evolution of pneumothorax: respiratory mechanical and histopathological study

Respiratory mechanics, chest wall configuration, and lung morphometry were determined in rats before and at 30 (PTX.30) and 60 (PTX.60) min after pneumothorax induction (intrathoracic injection of 8 ml of room air; 50% collapse). Pneumothorax increased respiratory system and lung elastances and viscoelastic/inhomogeneous pressures in both groups, but respiratory system and lung resistive pressures increased only in PTX.60 group. Antero-posterior diameters at the third intercostal space and xiphoid levels, circumference at xiphoid level, and thoracic cephalo-caudal diameter increased significantly after pneumothorax induction independently of temporal evolution. In both groups lung collapse, hyperinflation, and interstitial and alveolar edema were present. Additionally, in PTX.60 group the central airways calibre diminished in relation to PTX.30. In conclusion pneumothorax yields changes in respiratory system and lung elastic and viscoelastic parameters, which are related to alveolar collapse and edema, respectively. Temporal evolution of pneumothorax also leads to changes in lung resistive pressure, probably because of airway narrowing.

Effect of I/E ratio on mean alveolar pressure during high-frequency oscillatory ventilation

This study investigated factors contributing to differences between mean alveolar pressure (PA) and mean pressure at the airway opening (Pao) during high-frequency oscillatory ventilation (HFOV). The effect of the inspiratory-to-expiratory time (I/E) ratio and amplitude of oscillation on the magnitude of - Pao (Pdiff) was examined by using the alveolar capsule technique in normal rabbit lungs (n = 4) and an in vitro lung model. The effect of ventilator frequency and endotracheal tube (ETT) diameter on Pdiff was further examined in the in vitro lung model at an I/E ratio of 1:2. In both lung models, fell below Pao during HFOV when inspiratory time was shorter than expiratory time. Under these conditions, differences between inspiratory and expiratory flows, combined with the nonlinear relationship between resistive pressure drop and flow in the ETT, are the principal determinants of Pdiff. In our experiments, the magnitude of Pdiff at each combination of I/E, frequency, lung compliance, and ETT resistance could be predicted from the difference between the mean squared inspiratory and expiratory velocities in the ETT. These observations provide an explanation for the measured differences in mean pressure between the airway opening and the alveoli during HFOV and will assist in the development of optimal strategies for the clinical application of this technique.

Effects of prosthetic reconstruction of the abdominal wall on respiratory mechanics in rats

Respiratory mechanics and thoracoabdominal morphometry were determined in four sets of animal experiments before and after surgery. In group RRA the rectus abdominus muscles were removed; in RRAH rats the muscle resection was followed by lung hyperinflation; in PPM animals the defect was repaired by suturing a polypropylene mesh (Marlex); and in PPMH lung hyperinflation was performed after abdominal wall reconstruction. Lung and chest wall elastances, and chest wall viscoelastic/inhomogeneous pressures increased in RRA, RRAH and PPM groups. Static lung elastance was progressively smaller in the following order: RRA, PPM, and PPMH. In conclusion, removal of the rectus abdominus muscles and abdominal wall reconstruction could account for higher energy losses against viscoelastic and elastic forces acting on the chest wall, and these are related to a cephalad deviation of the diaphragm. Furthermore, hyperinflation reverses lung elastic modification after abdominal wall reconstruction with PPM, without beneficial effects in the presence of abdominal wall defect.

Respiratory mechanics after prosthetic reconstruction of the chest wall in normal rats

OBJECTIVE

Prosthetic reconstruction of the chest wall may yield several respiratory changes. Nevertheless, to our knowledge, no comprehensive analysis of respiratory mechanics under this condition has been hitherto performed.

METHODS

Respiratory mechanics were evaluated in two groups of rats. In one group (n=8), a polytetrafluoroethylene (PTFE) patch was used; in another group (n=8), a polypropylene mesh (Marlex) associated with methylmethacrylate (PPMM) was employed. All animals were sedated, anesthetized, paralyzed, and mechanically ventilated before and after the prosthetic reconstruction of the chest wall. After airway occlusion at end inspiration, respiratory system, pulmonary, and chest wall resistive pressures (deltaP1rs, deltaP1L, and deltaP1cw, respectively) and viscoelastic/inhomogeneous pressures (deltaP2rs, deltaP2L, and deltaP2cw, respectively) were determined. Respiratory system, lung, and chest wall static (Est(rs), EstL, and Est(cw), respectively), and dynamic elastances (Edyn(rs), EdynL, and Edyn(cw), respectively), and the corresponding delta elastances (deltaE, calculated as Edyn-Est) were also obtained.

RESULTS

In both groups, significant increases in deltaP2rs, deltaP2cw, deltaErs, deltaEcw, Est(rs), EstL, and Est(cw) were observed after chest wall reconstruction. However, deltaP2rs, deltaP2cw, deltaErs, deltaEcw, Est(rs), and EstL were significantly higher in the PPMM group than in the PTFE group.

CONCLUSIONS

Prosthetic reconstruction of the chest wall yields not only elastic changes, but also there is also an important increase of pressure dissipated against viscoelastic/inhomogeneous segments of the chest wall. Furthermore, taking into account respiratory mechanics, the PTFE patch might be preferred to the PPMM patch.

Flow resistances of disposable double-lumen, single-lumen, and Univent tubes

OBJECTIVE

To compare the airflow resistances of modern double-lumen, single-lumen, and Univent (Fuji Systems Corp; Tokyo, Japan) tubes.

DESIGN

A laboratory bench study.

SETTING

A university hospital laboratory.

MEASUREMENTS

Pressure differentials (Pd) were measured across study tubes at 10 L/min airflow (V) increments from 0 to 60 L/min in a tracheal model. Coefficients of resistance k1 (linear) and k2 (nonlinear) were calculated for each tube by the method of least squares using the Rohrer equation Pd/V = k1 + k2V. Data were assessed by analysis of variance (ANOVA) for the effects of tube design, circumference, and manufacturer on k1 and k2.

MAIN RESULTS

Calculated combined mean k1 and k2 were significantly lower for single-lumen tubes compared with double-lumen or Univent tubes. There were no significant differences for k1 values between double-lumen or Univent tubes. The values for k2 were significantly lower for double-lumen tubes compared with Univent tubes. The k2 values were significantly lower for Rusch (Duluth, GA) or Sheridan (Argyle, NY) double-lumen tubes compared with Mallinckrodt (St Louis, MO) double-lumen tubes. This difference was because of the Y-connectors of the Mallinckrodt tubes.

CONCLUSIONS

Flow resistances of modern disposable double-lumen tubes are lower than commonly perceived. In most clinical situations, there will be no decrease in flow resistance when a Rusch or Sheridan double-lumen tube is replaced by a single-lumen tube.

[Correction to the airflow measurement in the presence of a leak between the trachea and endotracheal tube]

PURPOSE

To estimate the leak between the endotracheal tube and the trachea in newborns in order to compensate for errors in airflow measurement and to monitor mechanical variables from pressure and flow signals.

METHODS

Assuming that the leak resistance (Rf) is constant during a respiratory cycle, the resistive properties of the endotracheal tube were evaluated. The method was validated in the intensive care unit with a mechanical test lung and assessed on recordings of three newborns during mechanical ventilation for RDS. We have used a least squares method for the estimation of positive end expiratory pressure (PEEP) on both newborns and simulated data.

RESULTS

Direct measurements of simulated leak resistances on the mechanical lung are in agreement with our estimation of leak resistances. In newborns, the success of flow correction is evidenced on end inspiratory pauses: corrected flow drops to zero while raw data show a constant nonzero flow. On the simulated lung, the PEEP underestimation with uncorrected flow ranges from 10 to 20 cm H20 while the corresponding, underestimation with corrected flow is less than 2 cm H2O. In newborns, the flow correction shifts the estimated PEEP from negative values (-0.3 +/- 1.3 cm H2O before correction) to positive values (3.6 +/- 0.7 cm H2O after correction) higher than the imposed PEEP (2 cm H2O).

CONCLUSIONS

The efficiency of this simple method has been demonstrated. It could be used successfully on adult patients, as there will not be flow correction in the absence of leaks.

Respiratory mechanics and morphometry after progressive intraperitoneal effusion

Respiratory mechanics and thoraco-abdominal morphometry were evaluated in anesthetized, paralyzed, mechanically ventilated rats before and after controlled intraperitoneal injection of warm (37 degrees C) saline. Respiratory system resistances and static elastance were determined in 9 animals using the end inflation occlusion method. Chest wall configuration at both functional residual capacity (FRC) and end inspiration (FRC + VT) was evaluated in: (a) 6 rats by measurements of lateral and anteroposterior diameters, and circumferences at four levels: 3rd intercostal space, xiphoid, subcostal plane and crista iliaca; and (b) 8 rats by measurements of thoracic cephalo-caudal diameter. In addition, FRC changes were measured in 6 rats. Resistances were not altered but static elastance increased progressively. Morphometric changes were similar at both FRC and FRC + VT: cephalo-caudal diameter diminished whereas all other diameters augmented; FRC decreased. In conclusion, intraperitoneal infusion of saline in rats augments elastance, and this is related to a cephalad deviation of the diaphragm plus an increase of the circumferences and diameters of the lower thorax.

Assessment of lung function in the intensive care unit

Tests of pulmonary function have become more accurate and less invasive in recent years. Our ability to monitor patients continuously with pulse oximetry, transcutaneous and end-tidal CO2, and intraarterial blood gas monitors has greatly enhanced ICU care. In intubated patients in the PICU detailed lung function studies can be performed, and in general they can be carried out with minimal disruption of routine management. Much work remains to be done to define the changes seen in various disease processes and the effects of therapeutic interventions on functional parameters. Many of the available techniques have already been developed to a point that allows them to be employed in clinical decision making. We expect that assessment of lung volumes, compliance, and resistance will become a routine part of management in children with life-threatening pulmonary diseases in the near future, and that a more intimate knowledge of the pathophysiology of respiratory disorders treated in PICU will lead to improved outcomes.

Low-frequency vs. high-frequency respiratory mechanics after methacholine challenge in artificially ventilated rabbits

Respiratory system resistance (Rrs) and elastance (Ers) were estimated by two methods, before and after methacholine in six anesthetized, paralyzed, and artificially ventilated rabbits. Rrs and Ers were obtained (1) by multiple linear regression analysis of the relationship between tracheal pressure and tidal volume and flow [Rrs(mlr)], Ers(mlr), and (2) by analysis of the Fourier transforms of tracheal pressure and flow resulting from 4 to 30 Hz pseudorandom pressure oscillations delivered by an Infant Star respirator [Rrs(os), Ers(os)]. Rrs(os) was significantly lower than Rrs(mlr). For instance, Rrs(os) at 20 Hz [Rrs(os)20] was (mean +/- SD) 17.3 +/- 3.5 vs 21.4 +/- 3.6 cm H2O x L-1 x s (P < 0.01) for Rrs(mlr). Ers(os) was significantly higher than the respective value obtained by multiple linear regression (718.2 +/- 81.0 vs 403.7 +/- 43.0 cm H2O L-1; P < 0.01). After methacholine, the changes of respiratory mechanics were similar with both methods. Rrs(mlr) and Rrs(os)20 increased respectively by 131 +/- 45 and 134 +/- 76%, and Ers(mlr) and Ers(os) increased respectively by 63 +/- 7 and 54 +/- 13%. A significant correlation was observed between Rrs(mlr) and Rrs(os)20 (r = 0.97) and between Ers(mlr) and Ers(os) (r = 0.96). We conclude that positive response to methacholine may be detected by forced oscillation as well as by multiple linear regression. However, the identified physiological components of the lung response (alteration in lung viscoelastic properties, increased lung inhomogeneity or increased intrathoracic airway shunt) are likely to be different with each method.

The minilink breathing system: resistance and suitability for spontaneous ventilation

The Portex infant breathing/ventilation systems with 8.5 mm and 15 mm internal diameter connectors were compared with a standard T-piece which had a 15 mm connector and 22 mm internal diameter tubing. The differential pressures across each system were measured at constant fresh gas flows up to 30 l.min-1 dry air. Resistance was calculated at flows compatible with quiet respiration and peak inspiratory flow. Flow resistance of the 3.0 mm internal diameter tracheal tube in conjunction with the minilink breathing systems were similar to those previously reported for tracheal tubes alone. However, the minilink breathing system assumed a greater influence on resistance when tracheal tubes of larger internal diameter were used. It added considerably more resistance than the standard tubing. This may have a deleterious effect during spontaneous ventilation in older children.

Measurement of tracheal static pressure in exercising horses

A nasotracheal catheter for measuring tracheal static pressure in exercising horses was designed according to aerodynamic engineering principles. Small ports near the end of the catheter transmitted pressure fluctuations to the recording apparatus. Accuracy was determined by the size, number, and location of pressure sensing holes on the catheter, and by the position of the catheter in the trachea. The catheter had adequate frequency response to 33 Hz, was insensitive to movement artifacts, was easily introduced, was tolerated well by horses, and resulted in small ventilatory impairment at maximal exertion.

Evaluation of the multiple linear regression method to monitor respiratory mechanics in ventilated neonates and young children

A potentially useful method to monitor respiratory mechanics in artificially ventilated patients consists of analyzing the relationship between tracheal pressure (P), lung volume (V), and gas flow (V) by multiple linear regression (MLR) using a suitable model. Contrary to other methods, it does not require any particular flow waveform and, therefore, may be used with any ventilator. This approach was evaluated in three neonates and seven young children admitted into an intensive care unit for respiratory disorders of various etiologies. P and V were measured and digitized at a sampling rate of 40 Hz for periods of 20-48 s. After correction of P for the non-linear resistance of the endotracheal tube, the data were first analyzed with the usual linear monoalveolar model: P = PO + E.V + R.V where E and R are total respiratory elastance and resistance, and PO is the static recoil pressure at end-expiration. A good fit of the model to the data was seen in five of ten children. PO, E, and R were reproducible within cycles, and consistent with the patient's age and condition; the data obtained with two ventilatory modes were highly correlated. In the five instances in which the simple model did not fit the data well, they were reanalyzed with more sophisticated models allowing for mechanical non-homogeneity or for non-linearity of R or E. While several models substantially improved the fit, physiologically meaningful results were only obtained when R was allowed to change with lung volume. We conclude that the MLR method is adequate to monitor respiratory mechanics, even when the usual model is inadequate.

Volume-cycled decelerating flow. An alternative form of mechanical ventilation

The linearly decelerating flow waveform for volume-cycled mechanical ventilation is an option on many modern ventilators. We have developed mathematical models for two available forms of volume-cycled decelerating-flow ventilation (VCDF). These equations use clinician-chosen ventilator settings as inputs (frequency, tidal volume, peak inspiratory flow or inspiratory time fraction, and end-inspiratory pause), and patient-determined inputs which describe the patient's ventilatory impedance (inspiratory [RI] and expiratory [RE] resistance and respiratory system compliance [C]. The equations predict key outcome variables: mean airway pressure; and peak, mean, and end-expiratory alveolar pressures. The mathematical expressions were validated in a mechanical lung analog. Values observed in the test lung were compared to values predicted by the mathematical models for a wide range of ventilator settings and impedance combinations (RI and RE, 5 to 40 cm H2O.s/L; C, 0.02 to 0.10 L/cm H2O). The correspondence between observed and predicted values was generally excellent across the broad range of inputs tested (r greater than or equal to 0.98). Outcome variables were quite sensitive to clinician-chosen inputs over certain critical ranges. Carefully applied, VCDF offers several theoretic advantages for the clinical setting; however, appropriate caution must be exercised to avoid the application of tissue-injuring pressure.