Inspiratory effort and occlusion pressure in triggered mechanical ventilation

Evaluation of the accuracy of established patient inspiratory effort estimation methods during mechanical support ventilation

There is a clinical need for monitoring inspiratory effort to prevent lung- and diaphragm injury in patients who receive supportive mechanical ventilation in an Intensive Care Unit. Different pressure-based techniques are available to estimate this inspiratory effort at the bedside, but the accuracy of their effort estimation is uncertain since they are all based on a simplified linear model of the respiratory system, which omits gas compressibility of air, and the viscoelasticity and nonlinearities of the respiratory system. The aim of this in-silico study was to provide an overview of the pressure-based estimation techniques and to evaluate their accuracy using a more sophisticated model of the respiratory system and ventilator. The influence of the following parameters on the accuracy of the pressure-based estimation techniques was evaluated using the in-silico model: 1) the patient's respiratory mechanics 2) PEEP and the inspiratory pressure of the ventilator 3) gas compressibility of air 4) viscoelasticity of the respiratory system 5) the strength of the inspiratory effort. The best-performing technique in terms of accuracy was the whole breath occlusion. The average error and maximum error were the lowest for all patient archetypes. We found that the error was related to the expansion of gas in the breathing set and lungs and respiratory compliance. However, concerns exist that other factors not included in the model, such as a changed muscle-force relation during an occlusion, might influence the true accuracy. The estimation techniques based on the esophageal pressure showed an error related to the viscoelastic element in the model which leads to a higher error than the occlusion. The error of the esophageal pressure-based techniques is therefore highly dependent on the pathology of the patient and the settings of the ventilator and might change over time while a patient recovers or becomes more ill.

Breathing pattern variability: a weaning predictor in postoperative patients recovering from systemic inflammatory response syndrome

OBJECTIVE

To investigate whether breathing pattern variability can serve as a potential weaning predictor for postoperative patients recovering from systemic inflammatory response syndrome (SIRS).

DESIGN AND SETTING

A prospective measurement of retrospectively analyzed breathing pattern variability in a surgical intensive care unit.

PATIENTS

Seventy-eight mechanically ventilated SIRS patients who had undergone abdominal surgery were included when they were ready for weaning. They were divided into success (n=57) and failure (n=21) groups based upon their weaning outcome.

MEASUREMENTS AND RESULTS

Before weaning, tidal volume, total breath duration, inspiratory time, expiratory time, and peak inspiratory flow were continuously monitored for 30 min, while patients received 5 cmH2O pressure support weaning trial. After the patients successfully completed the trial, they were extubated. Successful weaning was defined as patients free from the ventilator for over 48 h, whereas a weaning failure was considered as reinstitution of mechanical ventilation within 48 h of extubation. The coefficient of variation and two values of standard deviation (SD1 and SD2; indicators of the dispersion of data points in the plot) obtained from the Poincaré plot of five respiratory parameters in the failure group were significantly lower than those in the success group. The area under the receiver operating characteristic curve of these variability indices was within the range of 0.73-0.80, indicating the accuracy of prediction.

CONCLUSIONS

Small breathing pattern variability is associated with a high incidence of weaning failure in postoperative patients recovering from SIRS, and this variability may potentially serve as a weaning predictor.

Extubation failure: diagnostic value of occlusion pressure (P0.1) and P0.1-derived parameters

OBJECTIVE

To evaluate the ability of the new, built-in occlusion pressure (P0.1) measurement to predict extubation failure.

DESIGN AND SETTING

Prospective observational multicentre study in the ICU of five general hospitals.

PATIENTS

Hundred thirty patients on mechanical ventilation longer than 48 h when considered ready for weaning.

MEASUREMENTS AND RESULTS

Patients underwent a 30-min spontaneous breathing trial with simultaneous monitoring of occlusion pressure (P0.1) and breathing pattern (f/Vt). Sixteen patients (12%) failed the weaning trial and full ventilatory support was resumed, while 114 tolerated the trial and were extubated. Twenty-one (18%) required reintubation within 48 h. The area under the ROC curve for diagnosing extubation failure was 0.53 for f/Vt, 0.59 for P0.1 and 0.61 for P0.1*f/Vt (p=NS). Accordingly, P0.1*f/Vt more than 100 detected extubation failure with a sensitivity of 0.89, specificity of 0.35, positive predictive value of 0.21 and negative predictive value of 0.94.

CONCLUSION

During a first trial of spontaneous breathing on pressure support ventilation (PSV), bedside P0.1 and P0.1*f/Vt are of little help, if any, for predicting extubation failure.

ICU cornerstone: 'triggering effort'

In an autobiographical manner, I describe the pathophysiology of auto-PEEP (positive end-expiratory pressure) and its role as a factor that increases the work of breathing in mechanically ventilated patients.

Clinical evaluation of a computer-controlled pressure support mode

We have designed a computerized system providing closed-loop control of the level of pressure support ventilation (PSV). The system sets itself at the lowest level of PSV that maintains respiratory rate (RR), tidal volume (VT), and end-tidal CO(2) pressure (PET(CO(2))) within predetermined ranges defining acceptable ventilation (i.e., 12 < RR < 28 cycles/min, VT > 300 ml [> 250 if weight < 55 kg], and PET(CO(2)) < 55 mm Hg [< 65 mm Hg if chronic CO(2) retention]). Ten patients received computer-controlled (automatic) PSV and physician-controlled (standard) PSV, in random order, during 24 h for each mode. An estimation of occlusion pressure (P(0.1)) was recorded continuously. The average time spent with acceptable ventilation as previously defined was 66 +/- 24% of the total ventilation time with standard PSV versus 93 +/- 8% with automatic PSV (p < 0.05), whereas the level of PSV was similar during the two periods (17 +/- 4 cm H(2)O versus 19 +/- 6 cm H(2)O). The time spent with an estimated P(0.1) above 4 cm H(2)O was 34 +/- 35% of the standard PSV time versus only 11 +/- 17% of the automatic PSV time (p < 0.01). Automatic PSV increased the time spent within desired ventilation parameter ranges and apparently reduced periods of excessive workload.

Respiratory drive and pulmonary mechanics during haemodialysis with ultrafiltration in ventilated patients

The improvements of respiratory drive and pulmonary mechanics which follow haemodialysis with ultrafiltration in mechanically ventilated renal failure patients seem predictable but have not been studied before. In this study, 14 renal failure patients with stable haemodynamics mechanically ventilated with pressure support ventilation (PSV) were enrolled. Respiratory drive (represented as P0.1), pulmonary mechanics, breathing pattern, arterial blood gas and haemodynamics were measured according to the time schedule: pre-dialysis (Time 0), and at 60, 120, 180, 240 minutes thereafter. Following the removal of excess lung water during haemodialysis, auto-PEEP and patient's work of breathing (WOBp) decreased gradually. P0.1 lessened progressively along with the improvement in pulmonary mechanics. The changes in auto-PEEP and WOBp correlated closely to the pre- and post-dialysis decline of P0.1 (delta P0.1). There was a negative, moderately significant correlation between the amount of fluid ultrafiltrated during dialysis (delta UF) and the delta P0.1 (R = -0.54). The breathing pattern remained stable during dialysis. No hypoventilation or hypoxaemia occurred despite the development of metabolic alkalosis induced by bicarbonate dialysate. We have shown that respiratory drive decreases gradually during bicarbonate haemodialysis. The improvements of pulmonary mechanics, rather than the rapid alkalization of body fluids, responds to the decrease of P0.1 in renal failure patients ventilated with PSV.

Influence of inspired oxygen concentration on deadspace, respiratory drive, and PaCO2 in intubated patients with chronic obstructive pulmonary disease

OBJECTIVES

To investigate the response of CO2-retaining chronic obstructive pulmonary disease (COPD) patients to an increase in FIO2 following a period of mechanical ventilation with PaO2 in the normal range. The administration of a high FIO2 to chronic obstructive pulmonary disease (COPD) patients may result in hypercapnia. Recent evidence indicates that the hypercapnia may be due to reversal of preexisting regional hypoxic pulmonary vasoconstriction resulting in a greater deadspace. This effect would be more pronounced in patients whose initial PaO2 was < 60 torr (< 7.9 kPa).

DESIGN

Single blinded, prospective study.

SETTING

A medical surgical intensive care until in a tertiary care, teaching hospital.

PATIENTS

COPD CO2-retaining patients.

INTERVENTIONS

FIO2 increased to 0.7.

MEASUREMENTS AND MAIN RESULTS

Twelve intubated COPD patients weaned from mechanical ventilation were studied both at their baseline FIO2 (0.3 to 0.4), and following a 20-min period of exposure to an FIO2 of 0.7. Mean baseline values were: PaO2 of 85 torr (11.3 kPa), PCO2 of 56 torr (7.5 kPa), deadspace of 73%, and respiratory drive normal, as measured by P0.1. Statistical analysis using the paired Student's t-test showed that the PaO2 increased significantly when the FIO2 was increased to 0.7, but there was no significant change in PaCO2, deadspace, or respiratory drive.

CONCLUSION

These results show that following a period of mechanical ventilation with an FIO2 sufficient to maintain a normal PaO2, a further increase in FIO2 does not result in an increased PaCO2 in this group of CO2-retaining COPD patients.

Cardiorespiratory dynamics during successful and unsuccessful trials of weaning from mechanical ventilation following cardiac surgery

The purpose of this study was to examine the effects of a weaning trial with pressure support ventilation (PSV) on the cardiorespiratory dynamics in 20 cardiac surgery patients. In the patients who failed a weaning trial (failure group,n=6), the mean duration of cardiopulmonary bypass was 270±83 min and the mean postoperative lung-thorax compliance was 38±5 ml·cmH₂O^-1, whereas in successfully weaned patients (success group,n=14) they were 145±30 min and 55±10 ml·cm H₂O^-1 respectively (mean±SD). Significant differences were recognized in those values between the two groups. Preoperative cardiac function, intraoperative blood loss, and postoperative fluid balance were similar in both groups. Cardiac index (CI) increased similarly in both groups. Pao₂/FIo₂ and percentage intrapulmonary shunt were constant in the success group, and these variables worsened in the failure group. Oxygen consumption ([Formula: see text]) increased and mixed venous O₂ tension ([Formula: see text]) decreased in the failure group, whereas[Formula: see text] remained constant and[Formula: see text] increased in the success group. These data suggest that prolonged cardiopulmonary bypass might have produced acute lung injury. Decreased lung compliance may be responsible for rapid shallow breathing and an increase in oxygen consumption during a weaning trial, and may lead to weaning failure from mechanical ventilation.