Tidal Volume Measurements Research Articles

Measurement of Actual Tidal Volume Delivered by Apparatuses for Mechanical Ventilation of Lungs

Measurement of Actual Tidal Volume Delivered by Apparatuses for Mechanical Ventilation of Lungs

Supplemental inhaled gases alter tidal volume delivery and measurement

Supplemental inspired nitrogen (N(2)) or carbon dioxide (CO(2)) is commonly used to balance pulmonary blood flow in patients with single-ventricle physiology. The objective of this study was to assess if supplemental inspired gas alters delivery or measurement of tidal volume (V(T)) by a ventilator. Prospective, experimental study. Respiratory Care Laboratory, Cincinnati Children's Hospital. Using a test lung, expired V(T) measurements from Servo 300 ventilators were compared with actual delivered V(T) (true V(T)) at baseline and during supplemental N(2) or CO(2) administration to mimic clinical use in single-ventricle patients. At compliance settings simulating normal and compromised lung function, true V(T) was determined by the test lung and inline Pneumotach. True and measured V(T) were compared by repeated-measures analysis of variance with significance defined as p < .05. With normal lung compliance, supplemental gas administration increases both true and measured V(T), and expired V(T) measurements remain accurate. With poor lung compliance, supplemental gas flow disproportionately affects V(T) measurement. Poor lung compliance reduces true V(T) markedly (p < .001), causing a large discrepancy between true and measured V(T). Supplemental gas administration amplifies this discrepancy because the additional gas flow in the circuit erroneously augments expired V(T) measurements by the ventilator (p < .001). The discrepancy is greatest with higher-set V(T) and greater supplemental gas flow. The addition of supplemental inspired gas directly into the ventilator circuit can alter tidal volume delivery or measurement by a ventilator. The extent and magnitude of the alterations are determined by lung compliance. Variable effects of supplemental gas administration may confound ventilator management of patients with single-ventricle physiology.

Conformal treatment planning using 4DCT can decrease ipsilateral lung dose and improve tumor coverage: A prospective 4DCT treatment planning study

To determine the full range of lung tumor motion in non-coached, free-breathing lung cancer patients and to use the data to evaluate 3D conformal radiation treatment planning techniques. 20 patients were enrolled in a prospective study of lung tumor motion. Each patient was immobilized in the treatment position and underwent quantitative spirometry during a high resolution CT scan. The CT scanner was operated in cine mode and acquired 15 sets of twelve contiguous 1.5 mm slices. The spirometer measured tidal volume and was compared with the imaged air volume using a semiautomatic autothreshold to correct for measurement drift. Each image was associated a tidal volume measurement, subdivided by inhalation or exhalation phase. Inhalation, mid-inhalation, exhalation, and mid-exhalation image datasets were fused with a standard helical CT scan acquired during the same session. The inhalation and exhalation tumor volumes (GTV) were combined to produce an internal target volume (ITVl) that implicitly assumed linear motion. A second ITV (ITVh) was produced by combining the ITVl and the mid-inspiration and mid-exhalation GTVs. This enabled an evaluation of the effects of internal hysteresis on the ITV definition. 3D conformal radiation therapy plans were generated to 1) prescribe 70 Gy to the isocenter of the GTV defined using the standard helical scan (GTVs) using a 2 cm block margin; and 2) prescribe 66.5 Gy (95% of 70 Gy) to cover 100% of ITVl and 3) prescribe 66.5 Gy to cover 100% of ITVh using customized margins for 2) and 3). To date, 12 patients have been analyzed. 8 patients had upper lobe tumors and 8 patients had peripheral tumors. The tumors moved an average of 0.9 (range 0.3 to 1.5) cm. The ratio of ITVl to GTVs was 1.0–4.0 (mean 1.95, median 1.70) and the average ratio of ITVh to ITVl was 1.16. This indicated that the GTVs inaccurately modeled the moving tumor, while incorporation of hysteresis did not substantially impact the definition of ITV in all but one patient. Block margins required to cover the ITVh and ITVl ranged from 1.0 to 1.5 cm. The 2 cm margin around the GTVs overcompensated for tumor motion in at least one direction in all patients. This resulted in a 5% decrease (range 2% - 8%) in V20 of the ipsilateral lung for the ITV-based plans. In 3 patients, the GTVs was an inaccurate model for the ITV and even the 2 cm margin was insufficient to adequately cover the tumor (Figure 1). Motion analysis can decrease the ipsilateral lung dose in most patients by allowing margin reduction compared to standard clinical practice. It also shows inadequate tumor coverage in some patients. In certain patients, full tumor motion may not be adequately represented by inhalation and exhalation reconstructions.

Effect of lung compliance and endotracheal tube leakage on measurement of tidal volume

IntroductionThe objective of this laboratory study was to measure the effect of decreased lung compliance and endotracheal tube (ETT) leakage on measured exhaled tidal volume at the airway and at the ventilator, in a research study with a test lung.MethodsThe subjects were infant, adult and pediatric test lungs. In the test lung model, lung compliances were set to normal and to levels seen in acute respiratory distress syndrome. Set tidal volume was 6 ml/kg across a range of simulated weights and ETT sizes. Data were recorded from both the ventilator light-emitting diode display and the CO2SMO Plus monitor display by a single observer. Effective tidal volume was calculated from a standard equation.ResultsIn all test lung models, exhaled tidal volume measured at the airway decreased markedly with decreasing lung compliance, but measurement at the ventilator showed minimal change. In the absence of a simulated ETT leak, calculation of the effective tidal volume led to measurements very similar to exhaled tidal volume measured at the ETT. With a simulated ETT tube leak, the effective tidal volume markedly overestimated tidal volume measured at the airway.ConclusionPrevious investigators have emphasized the need to measure tidal volume at the ETT for all children. When ETT leakage is minimal, it seems from our simulated lung models that calculation of effective tidal volume would give similar readings to tidal volume measured at the airway, even in small patients. Future studies of tidal volume measurement accuracy in mechanically ventilated children should control for the degree of ETT leakage.

How safe is intermittent positive pressure ventilation in preterm babies ventilated from delivery to newborn intensive care unit?

Objectives: To examine whether clinically determined ventilator settings will produce acceptable arterial blood gas values on arrival, in preterm infants ventilated from delivery to the newborn intensive care unit (NICU)....

Effect of perfluorocarbon (perfluorooctyl bromide) vapor on tidal volume measurement during partial liquid ventilation.

To compare measured tidal volumes with and without perfluorocarbon (perfluorooctyl bromide) vapor, by using tidal volumes in the range suitable for neonates ventilated with partial liquid ventilation. We also aimed to determine the correction factor needed to calculate tidal volumes measured in the presence of perfluorooctyl bromide vapor. Prospective, experimental study. Neonatal research laboratory. Reproducible tidal volumes from 5 to 30 mL were produced with a rodent ventilator and drawn from humidifier chambers immersed in a water bath at 37 degrees C. Control tidal volumes were drawn from a chamber containing oxygen and water vapor, and the perfluorocarbon tidal volumes were drawn from a chamber containing oxygen, water vapor, and perfluorooctyl bromide vapor. Tidal volumes were measured by a VenTrak respiratory mechanics monitor with a neonatal flow sensor and a Dräger pneumotachometer attached to a Dräger neonatal ventilator. All tidal volumes measured with perfluorooctyl bromide vapor were increased compared with control. The VenTrak-measured tidal volumes increased by 1.8% to 3.5% (an overall increase of 2.2%). The increase was greater with the Dräger hot-wire anemometer: from 2.4% to 6.1% (an overall increase of 5.9%). Regression equations for mean control tidal volumes (response, Y) vs. mean perfluorooctyl bromide tidal volumes (predictor, X) are as follows: for the VenTrak, Y = -0.026 + (0.978 x X), r =.9999, p <.0001; and for the Dräger, Y = 0.251 + (0.944 x X), r =.9996, p <.0001. The presence of perfluorooctyl bromide vapor in the gas flowing through pneumotachometers gives falsely high tidal volume measurements. An estimate of the true tidal volume allowing for the presence of perfluorooctyl bromide vapor can be made from regression equations. Any calculation of lung mechanics must take into account the effect of perfluorooctyl bromide vapor on the measurement of tidal volume.

The management of chronic lung disease

Lung injury leading to chronic respiratory problems is a consequence of ventilation of the immature, surfactant deficient lung, and starts as soon as the baby is born. Strategies to minimize lung damage are an important part of the management of chronic lung disease (CLD). Prevention of overdistension of the lung, application of positive end expiratory pressure (PEEP) during resuscitation and prophylactic surfactant have been shown to reduce lung injury. The treatment of maternally acquiredUreaplasma urealyticum has not been shown to affect CLD.Ventilator strategies must aim to reduce lung injury and continuous on-line flow measurements, particularly measurement of tidal volume, can help. Oxygen is essential but can also be toxic and there is uncertainty about optimum saturation levels. Diuretics, bronchodilators and methylxanthines have short-term effects but have not been shown to affect outcome. Postnatal steroids improve lung function but they have been associated with cerebral palsy. Their use should be limited to very severe cases, and only after discussion with parents. The place of routine prophylaxis against respiratory syncytial virus (RSV) remains controversial.

Pulmonary function assessment by whole-body plethysmography in restrained versus unrestrained mice

Introduction: The evaluation of pulmonary physiological measurements in laboratory animals is an essential tool in many biomedical and toxicological research areas. Recently, an unrestrained single chambered whole-body plethysmograph that utilizes a barometric analysis technique to quantify pulmonary physiological values has gained widespread use. However, results generated with the single chamber plethysmograph have come under increased scrutiny because airflow in the lung is indirectly measured. The purpose of the present study was to use mice with known interstrain differences in pulmonary physiology (A/J, BALB/c, CD-1, and B6C3F1) and compare the physiological data generated with a single chamber plethysmograph to data obtained in the widely accepted double chamber noninvasive airway mechanics (NAM) plethysmograph in which the animals are restrained. Methods: Animals were placed into the plethysmographs and baseline physiological data acquired. The mice were then subjected to challenge with aerosols generated from isotonic saline (control) and methacholine solutions of increasing concentration (2.5–320 mg/ml) for 3 min for determination of the concentration of methacholine that induced a 200% increase in airway resistance (PC 200R). Results: Repeated physiological measurements on the same animals in both the single and double chamber plethysmographs demonstrated that each instrument generated reproducible baseline physiological data. However, comparison of physiological data generated with the double-chambered instrument to that generated with the single chamber plethysmograph revealed several significant differences. While the single chamber plethysmograph appeared to give inaccurate measurements of tidal volume, it provided much better analysis of airway reactivity based on PC 200R results. In contrast, the double chamber plethysmograph provided accurate physiological data such as tidal volume and respiratory rate, but provided inaccurate and irreproducible airway reactivity results based on PC 200R. Discussion: Our results indicate that the choice of single or double chamber plethysmograph for physiological measurements should be linked to the study objectives and the type of data required.

Effect of jaw-thrust and continuous positive airway pressure on tidal breathing in deeply sedated infants

Objectives: To examine the physiologic impact of the jaw-thrust maneuver or the administration of continuous positive airway pressure (CPAP) on tidal breathing in deeply sedated infants. Study design: Prospective, non-randomized study of infants undergoing elective fiberoptic bronchoscopy while sedated with intermittent doses of propofol. Methods: Spontaneous tidal breathing was measured in the supine position by means of a spirometer attached to a bronchoscopy face mask. Tidal breaths were recorded under the following conditions: (1) neutral sniffing position, (2) jaw-thrust, (3) neutral sniffing position, and (4) CPAP of 5 cm H2O. Improvement was defined as a change of more than twice the coefficient of variation of repeated measurements of tidal volume and flows from baseline. Results: Jaw-thrust increased tidal volume, minute ventilation, and peak tidal inspiratory and expiratory flows significantly in all 13 infants studied (mean ± SEM age = 8 ± 2 months). CPAP increased peak tidal inspiratory and expiratory flows by more than twice the coefficient of variation of baseline measurements in 6 patients and tidal volume and minute ventilation in 5 of 10 patients studied. Conclusion: Jaw-thrust and CPAP are effective techniques to improve ventilation of sedated infants undergoing interventions that compromise upper airway patency. (J Pediatr 2001;138:826-30)

Chest and abdominal surface motion measurement for continuous monitoring of respiratory function.

The spirometric devices currently available are not well suited for continuous or ambulatory monitoring of subjects. The proposed solution details a method of assessing lung function that avoids the need for flow measurement at the mouth and provides more detailed information of the dynamics of chest motion while allowing the subject to move freely. The solution to this problem comprises a distributed sensor system that allows regional motions during breathing manoeuvres to be investigated as well as a measurement of tidal respiratory volume. Preliminary studies demonstrate performance benefits of an array of sensors over two-compartment respiratory measurements.

Fog-induced Respiratory Responses Are Attenuated by Nedocromil Sodium in Humans

Fog inhalation induces cough and bronchoconstriction in patients with asthma, but only cough in normal subjects; whether it also influences the pattern of breathing is unclear. Nedocromil sodium (NCS) inhibits the cough response to inhalation of several pharmacological agents but its effects on fog-induced cough and changes in the pattern of breathing are unknown. We evaluated the effects of no drug, placebo, and 4- and 8-mg NCS administration on the cough threshold and changes in the pattern of breathing during fog inhalation in 14 healthy subjects. Measurements of tidal volume (VT), duration of inspiratory and expiratory times (TI and TE, respectively), total duration of the respiratory cycle (TT), mean inspiratory flow (VT/TI), duty cycle (TI/TT), respiratory frequency (f, 60/TT), and inspiratory minute ventilation (V I) were obtained by inductive plethysmography. Median cough threshold values were unaffected by placebo, but were increased (p < 0.01) by both NCS doses. In no-drug and placebo trials, inhalation of the threshold fog concentration caused increases in both VT/TI and V I (p always < 0.05) due to selective increases (p < 0.01) in VT. These changes were markedly attenuated by both NCS doses administration. Thus, fog induces coughing and increases in VT, VT/ TI, and V I in healthy subjects; NCS possesses antitussive effects and attenuates fog-induced changes in the pattern of breathing, possibly through inhibition of rapidly adapting "irritant" receptors.

Accuracy and consistency of respiratory inductive plethysmography for overnight tidal volume measurement.

To validate the accuracy and consistency of respiratory inductive plethysmography (RIP) in measuring tidal volume after an overnight sleep, tidal volumes of 18 patients with suspected sleep-disordered breathing and 8 normal volunteers were measured simultaneously with RIP (VTRIP) and with an ultrasonic airflow meter (VTUFM) before and after an unstrained overnight sleep on supine and lateral decubitus. The bias of the VTRIP was expressed as (VTRIP-VTUFM)/ VTUFM.100%, limits of agreement between VTRIP and VTUFM was measured by averaged bias +/- 2 s. Results showed that in normal subjects, the bias of RIP before and after overnight sleep was precise and consistent in both supine (0.7% and -1.6%) and lateral decubitus (3.7% and -0.56%). In these patients, the bias of RIP before and after sleep in supine also remained small (1.9% and 1.7%), but it became larger in lateral decubitus (24.5% and 20.4%) and 11.5% exceeded the limits of agreement observed in the evening. The patients' body mass indices (BMI) were higher than those of normal subjects (median 34.2 vs. 27.8 kg/m2). Pooled data showed that the bias of VTRIP in the morning on lateral decubitus but not on supine was correlated to BMI (Spearman R = 0.32, n = 52, P = 0.02). Thus, we were led to conclude that the accuracy of VTRIP overnight was precise and consistent in normal subjects, but the deviation of VTRIP measured on lateral decubitus in patients especially in those with excessive obesity was greater, thus, the method should not be used for quantitative determination.

Influence of jacket placement on respiratory compliance during raised lung volume measurements in infants.

SUMMARY. Recent introduction of the raised lung volume rapid thoraco-abdominal compression (RVRTC) technique for measuring forced expiratory maneuvers in infants provides the potential opportunity to assess respiratory mechanics simultaneously by using multiple linear regression (MLR) of the relaxed breaths preceding jacket inflation to force expiration. This study was undertaken to investigate whether data obtained from raised lung volume are influenced by placement of the rapid thoraco-abdominal compression (RTC) squeeze jacket. Paired measurements of tidal volume (V(T)) and respiratory rate (RR) during tidal breathing, and of inflation volume (V(inf)), respiratory system compliance (C(rs)), and resistance (R(rs)) during passive lung inflations were made in 60 (30 male) healthy term infants with and without a fastened, but uninflated RTC jacket in place. Jacket placement was associated with a significant reduction (P < 0.0001) in weight-corrected V(inf) [-1.86 (95% confidence interval, -2.46, -1.27) mL.kg(-1)] and C(rs) [-0.77 (-1.04, -0.49) mL.kPa(-1).kg(-1)]. This represented a reduction in weight-corrected C(rs) from 9.00 to 8.24 mL.kPa(-1).kg(-1), with the fall being >10% in 42% of infants studied. There was no significant change in R(rs) or weight-corrected V(T). If passive respiratory mechanics are to be measured during raised lung volume maneuvers, they should be performed prior to the jacket being fastened, unless considerable care is taken with each infant to ensure that the jacket does not restrict chest wall movement during maximum inflation.

Influence of jacket placement on respiratory compliance during raised lung volume measurements in infants

Summary. Recent introduction of the raised lung volume rapid thoraco-abdominal compression (RVRTC) technique for measuring forced expiratory maneuvers in infants provides the potential opportunity to assess respiratory mechanics simultaneously by using multiple linear regression (MLR) of the relaxed breaths preceding jacket inflation to force expiration. This study was undertaken to investigate whether data obtained from raised lung volume are influenced by placement of the rapid thoraco-abdominal compression (RTC) squeeze jacket. Paired measurements of tidal volume (VT) and respiratory rate (RR) during tidal breathing, and of inflation volume (Vinf), respiratory system compliance (Crs), and resistance (Rrs) during passive lung inflations were made in 60 (30 male) healthy term infants with and without a fastened, but uninflated RTC jacket in place. Jacket placement was associated with a significant reduction (P < 0.0001) in weight-corrected Vinf [−1.86 (95% confidence interval, −2.46, −1.27) mL·kg−1] and Crs [−0.77 (−1.04, −0.49) mL ·kPa−1 ·kg−1]. This represented a reduction in weight-corrected Crs from 9.00 to 8.24 mL ·kPa−1 ·kg−1, with the fall being >10% in 42% of infants studied. There was no significant change in Rrs or weight-corrected VT. If passive respiratory mechanics are to be measured during raised lung volume maneuvers, they should be performed prior to the jacket being fastened, unless considerable care is taken with each infant to ensure that the jacket does not restrict chest wall movement during maximum inflation. Pediatr Pulmonol. 2001; 31:51–58. © 2001 Wiley-Liss, Inc.

Room A, 10/17/2000 9: 00 AM - 11: 00 AM (PS) Delivery of Heliox by Anesthesia Machine Produces Error in Fresh Gas Flow and Tidal Volume Measurements

<b>Room A, 10/17/2000 9: 00 AM - 11: 00 AM (PS) Delivery of Heliox by Anesthesia Machine Produces Error in Fresh Gas Flow and Tidal Volume Measurements</b>

The effect of perfluorocarbon vapour on the measurement of respiratorytidal volume during partial liquid ventilation

During partial liquid ventilation perfluorocarbon vapour is present in theexhaled gases. The volumes of these gases are measured by pneumotachometers.Error in measuring tidal volumes will give erroneous measurement of lungcompliance during partial liquid ventilation. We aim to compare measured tidalvolumes with and without perfluorocarbon vapour using tidal volumes suitablefor use in neonates.Tidal volumes were produced with a 100 ml calibrationsyringe from 20 to 100 ml and with a calibrated Harvard rodent ventilator from2.5 to 20 ml. Control tidal volumes were drawn from a humidifier chambercontaining water vapour and the PFC tidal volumes were drawn from a humidifierchamber containing water and perfluorocarbon (FC-77) vapour. Tidal volumeswere measured by a fixed orifice, target, differential pressure flowmeter(VenTrak) or a hot-wire anenometer (Bear Cub) placed between the calibrationsyringe or ventilator and the humidifier chamber.All tidal volumes measuredwith perfluorocarbon vapour were increased compared with control (ANOVA p < 0.001 and post t-test p < 0.0001). Measured tidal volume increased from 7to 16% with the fixed orifice type flow-meter, and from 35 to 41% with thehot-wire type. In conclusion, perfluorocarbon vapour flowing throughpneumotachometers gives falsely high tidal volume measurements. Calculation oflung compliance must take into account the effect of perfluorocarbon vapour onthe measurement of tidal volume.

Thoracoabdominal asynchrony failed to grade airway obstructions in foals.

Respiratory inductive plethysmography (RIP) can be used to obtain a valid measure of tidal volume in humans. This device also compares the contributions to ventilation of the thorax and abdomen. Although thoracoabdominal asynchrony is a prominent clinical feature for patients with airway obstruction, the accuracy of the RIP device to assess the severity of obstruction is unclear. This study analyzes how well RIP variables reflect the degree of a fixed external inspiratory plus expiratory resistive load in foals. Foals were employed because the species and age group are commonly afflicted with respiratory disease. Eight conscious, sedated (xylazine 1.25 mg/kg body wt) foals were subjected to randomly ordered resistive loads at the airway opening and, on a separate day, to histamine aerosol challenge. During resistive loading, phase angle changed significantly, as did phase relation (P < or = 0.05). However, no significant correlation was found between the degree of change in resistive load and the degree to which phase angle or relation was altered (r(s) = 0.41 and 0.25, respectively). In addition, neither phase angle nor relation changed significantly with histamine challenge. We conclude that, although RIP variables changed markedly with fixed upper airway resistive loading, the degree to which they changed was erratic and therefore not useful for grading these obstructions. Furthermore, RIP variables were insensitive measures of histamine-induced bronchoconstriction.

Continuous circadian measurements of ventilation in behaving adult rats

Body temperature (Tb) and oxygen consumption ( V ̇ O 2 ) are important determinants of ventilation ( V ̇ e). While the circadian rhythms in Tb and V ̇ O 2 have been well described, the daily pattern of V ̇ e has not due to limitations in the available methods for measuring V ̇ e. Here we describe an adaptation of the barometric method using a chamber in which a large flow through very small restrictions was generated by the combined action of a positive pressure pump on the entrance and a negative pressure pump at the outlet. In this way the chamber effectively behaved as a closed system, despite having a high enough flow for long-term recording in freely moving, undisturbed small animals. This system was then used to test the hypothesis that V ̇ e oscillates with a circadian pattern similar to that of Tb . Measurements of tidal volume (V t) and breathing rate (f), in combination with Tb and activity by telemetry, were made in eight adult rats over 4–6 days under 12:12 light:dark conditions. Both V t, f, and thus V ̇ e, showed a circadian pattern similar to that of Tb and activity; that is, values were higher during the dark compared to the light phase. The differences in V ̇ e levels according to the time of the day suggest that mechanisms involved in the control of breathing may also have circadian patterns.

Effect of cricoid pressure on insertion of and ventilation through the cuffed oropharyngeal airway

We have assessed the effect of cricoid pressure on insertion of and ventilation through the cuffed oropharyngeal airway (COPA) in 53 patients, in a double-blind, randomized study. Two anaesthetists assessed adequacy of ventilation in anaesthetized and paralysed patients at the same time but using different methods. The first assessed ventilation clinically, by observing synchronized chest expansion with gentle manual ventilation and the second noted measurements of tidal volume (VT) and peak inspiratory pressure (PIP). Five mask ventilated breaths ('baseline') were assessed as above. Patients were then allocated randomly to receive cricoid pressure (group A, n = 28) or no cricoid pressure (group B, n = 25). Five further mask ventilated breaths ('after manoeuvre') were again assessed. A COPA was then inserted and five further breaths ('after COPA') were assessed. A COPA was inserted at the first attempt in all patients except for one in group A who required two attempts. COPA placement was difficult in one patient in group B who had a small distance between the incisor teeth. Ventilation was clinically 'adequate' in all patients except for one in the cricoid pressure group. There were no significant differences in measured VT or PIP between 'baseline' and 'after manoeuvre' breaths. Significant differences in VT and PIP were found after COPA insertion in the group that received cricoid pressure, with a mean decrease in VT of 108 ml (P = 0.0049) and a mean increase in PIP of 5.2 cm H2O (P = 0.0111).

Automatic control of pressure support mechanical ventilation using fuzzy logic.

There is currently no universally accepted approach to weaning patients from mechanical ventilation, but there is clearly a feeling within the medical community that it may be possible to formulate the weaning process algorithmically in some manner. Fuzzy logic seems suited this task because of the way it so naturally represents the subjective human notions employed in much of medical decision-making. The purpose of the present study was to develop a fuzzy logic algorithm for controlling pressure support ventilation in patients in the intensive care unit, utilizing measurements of heart rate, tidal volume, breathing frequency, and arterial oxygen saturation. In this report we describe the fuzzy logic algorithm, and demonstrate its use retrospectively in 13 patients with severe chronic obstructive pulmonary disease, by comparing the decisions made by the algorithm with what actually transpired. The fuzzy logic recommendations agreed with the status quo to within 2 cm H(2)O an average of 76% of the time, and to within 4 cm H(2)O an average of 88% of the time (although in most of these instances no medical decisions were taken as to whether or not to change the level of ventilatory support). We also compared the predictions of our algorithm with those cases in which changes in pressure support level were actually made by an attending physician, and found that the physicians tended to reduce the support level somewhat more aggressively than the algorithm did. We conclude that our fuzzy algorithm has the potential to control the level of pressure support ventilation from ongoing measurements of a patient's vital signs.