Occlusion pressure as a technique in evaluating respiratory control

Findings of ventilator-measured P0.1 in assessing respiratory drive in patients with severe ARDS

BACKGROUND

Providers should adjust the depth of sedation to promote lung-protective ventilation in patients with severe ARDS. This recommendation was based on the assumption that the depth of sedation could be used to assess respiratory drive.

OBJECTIVE

To assess the association between respiratory drive and sedation in patients with severe ARDS by using ventilator-measured P0.1 and RASS score.

METHODS

Loss of spontaneous breathing was observed within 48 h of mechanical ventilation in patients with severe ARDS, and spontaneous breathing returned after 48 hours. P0.1 was measured by ventilator every 12 ± 2 hours, and the RASS score was measured synchronously.

RESULTS

The RASS score was moderately correlated with P0.1 (R𝑆𝑝𝑒𝑎𝑟𝑚𝑎𝑛, 0.570; 95% CI, 0.475 to 0.637; p= 0.00). However, only patients with a RASS score of -5 were considered to have no excessive respiratory drive, but there was a risk for loss of spontaneous breathing. A P0.1 exceeding 3.5 cm H2O in patients with other RASS scores indicated an increase in respiratory drive.

CONCLUSION

RASS score has little clinical significance in evaluating respiratory drive in severe ARDS. P0.1 should be evaluated by ventilator when adjusting the depth of sedation to promote lung-protective ventilation.

Accuracy of P0.1 measurements performed by ICU ventilators: a bench study

BACKGROUND

Occlusion pressure at 100 ms (P0.1), defined as the negative pressure measured 100 ms after the initiation of an inspiratory effort performed against a closed respiratory circuit, has been shown to be well correlated with central respiratory drive and respiratory effort. Automated P0.1 measurement is available on modern ventilators. However, the reliability of this measurement has never been studied. This bench study aimed at assessing the accuracy of P0.1 measurements automatically performed by different ICU ventilators.

METHODS

Five ventilators set in pressure support mode were tested using a two-chamber test lung model simulating spontaneous breathing. P0.1 automatically displayed on the ventilator screen (P0.1_vent) was recorded at three levels of simulated inspiratory effort corresponding to P0.1 of 2.5, 5 and 10 cm H₂O measured directly at the test lung and considered as the reference values of P0.1 (P0.1_ref). The pressure drop after 100 ms was measured offline on the airway pressure-time curves recorded during the automated P0.1 measurements (P0.1_aw). P0.1_vent was compared to P0.1_ref and to P0.1_aw. To assess the potential impact of the circuit length, P0.1 were also measured with circuits of different lengths (P0.1_circuit).

RESULTS

Variations of P0.1_vent correlated well with variations of P0.1_ref. Overall, P0.1_vent underestimated P0.1_ref except for the Löwenstein^® ventilator at P0.1_ref 2.5 cm H₂O and for the Getinge group^® ventilator at P0.1_ref 10 cm H₂O. The agreement between P0.1_vent and P0.1_ref assessed with the Bland-Altman method gave a mean bias of - 1.3 cm H₂O (limits of agreement: 1 and - 3.7 cm H₂O). Analysis of airway pressure-time and flow-time curves showed that all the tested ventilators except the Getinge group^® ventilator performed an occlusion of at least 100 ms to measure P0.1. The agreement between P0.1_vent and P0.1_aw assessed with the Bland-Altman method gave a mean bias of 0.5 cm H₂O (limits of agreement: 2.4 and - 1.4 cm H₂O). The circuit's length impacted P0.1 measurements' values. A longer circuit was associated with lower P0.1_circuit values.

CONCLUSION

P0.1_vent relative changes are well correlated to P0.1_ref changes in all the tested ventilators. Accuracy of absolute values of P0.1_vent varies according to the ventilator model. Overall, P0.1_vent underestimates P0.1_ref. The length of the circuit may partially explain P0.1_vent underestimation.

Implementation of a respiratory drive monitor on a Servo Ventilator

OBJECTIVE

To design and evaluate a clinical monitor of respiratory drive (P0.1) and other respiratory variables in a simple way, using a commercial ventilator.

METHODS

Nine healthy males were studied as they were breathing spontaneously in a Servo 900C Ventilator, at rest and during light exercise (50 W). The ventilator was slightly modified to improve its mechanical performance during spontaneous breathing, and was used as a measuring instrument. All the relevant information was retrieved, calculated and monitored by a PC. Respiratory drive was assessed as occlusion pressures from the inspiratory airway pressure signal. The equipment was compared with a two-way non-rebreathing laboratory system. Furthermore, negative and positive inspiratory pressures were applied from the ventilator, to study respiratory responses to mechanical loads.

RESULTS

At rest, the ventilator introduced a minor influence on inspiratory time and P0.1, but not in ventilation, tidal volume, expiratory duration and respiratory frequency. During exercise, the influence was more evident. This effect could also be noticed in the coefficients of variation. The responses to mechanical loads were easily recorded and can be used as a simple test of central load-compensating mechanisms. CONCLUSIONS. The ventilator, with limitations, may be an alternative to conventional techniques, especially in clinical studies of the central inspiratory activity with and without respiratory loading.

A mathematical and physiological evaluation of the different hypoxic response models in normal man

The purpose was: (1) to investigate which hypoxic response (HR) model(s) might be most suitable to measure the HR taking into account 'goodness of fit' to the individual data, clinical applicability and information obtained from each model; and (2) to investigate if the models are mutually exchangeable. The ventilatory (VE) and occlusion pressure (P0.1) HR were measured in 33 healthy volunteers (37+/-12 years). We found that the exponential, linear on oxygen saturation (LINSAT), hyperbolic with either a fixed (HYPV) or a variable position of the vertical asymptote (HYPH) models were comparable with respect to 'goodness of fit' to the individual data, but the correlation between the HR models was poor, except for the correlation between LINSAT and HYPV. Comparing HR data is justified when they are either provided by the same HR model, or by LINSAT or HYPV. Within each model the VE HR can be replaced by the P0.1 HR. LINSAT has the advantage of an easy applicable linear relationship, HYPH provides additional information about the O2-CO2 interaction component of the HR, but the model is more complex because two variables have to be determined by computerized iteration.

[A new device for measuring occlusion pressure]

Mouth pressure in the first 100 ms of inspiration with the airway occluded (P0.1) is an indirect measure of central airways activity. Few adequately validated devices are available on the market for measuring this variable automatically, impeding wider use of this measurement. Our aim was to determine the validity and precision of a new automatized manometer for measuring P0.1 and compare the device to the apparatus normally used in our laboratory. The newly manufactured device consisted of a semiconductor type pressure transduce connected to an analog-to-digital converter. The usual apparatus used was constructed in our laboratory from a Honewell transducer, an analog-to-digital card (Data Translation DT2801) for an IBM-compatible PC and a standard spread sheet program. Both devices recorded a continuous signal at a sample frequency of 100 Hz. To determine linearity, accuracy, sensitivity and precision, two columns of 5 and 25 cmH2O were used. For clinical assessment, 33 patients scheduled for lung function analysis were tested. Five measurements of P0.1 were performed on each. The sensitivity of the new device was 0.1 cmH2O; precision expressed as a variation coefficient was 0%, and linearity expressed as a correlation coefficient was r = 0.999 between the two columns. The mean difference between the two devices was 0.11 (0.46) cmH2O (p < 0.0001). Analysis of concordance confirmed a reading bias for the new device in comparison with the usual one, although the difference was not clinically significant (0.08 to 0.14 cmH2O). We conclude that the new device gives good results and its availability on the market may allow more frequent clinical application of occlusion pressure measurement.

Ventilatory parameters in children during propofol anaesthesia: a comparison with halothane

PURPOSE

The purpose of this study was to compare the effects of propofol on ventilation with those of halothane.

METHODS

Respiration was studied in 20 spontaneously breathing children undergoing elective dental restoration randomized to receive either propofol (Group P) or halothane (Group H) anaesthesia. Data were recorded at different inspired concentrations of halothane (F1H) or propofol (RivP) during a washout of the anaesthetic agent. The F1H 2%, 1% and 0% corresponded to an end-tidal halothane concentration of 1.38 +/- 0.06%, 0.857 +/- 0.03% and 0.191 +/- 0.01% respectively. The RivP were 18, 15, 12, 9 and 0 mg.kg-1.hr-1. The inspiratory flow waveform, the CO2 waveform and the occluded inspiratory pressure waveform were recorded. The flow waveform was analyzed for minute ventilation (Vi), and tidal volume (VT), parameters of breath Timing [Total time (Ttot), Inspiratory time (Ti)] and parameters of breath Amplitude [mean inspiratory flow (VT/Ti)]. The slope of the initial 100 msec (dP/dt0.1) of an occluded inspiration, together with the occluded inspiratory time (Tiocc) and the ratio of the occluded to unoccluded inspiratory time (Tiocc/Ti) were obtained.

RESULTS

There were intergroup differences in the preemergence values of Ttot, dP/dt0.1 and Tiocc/Ti. In group P the Vi, VT and Ttot increased and PETCO2 decreased during a washout of propofol. The values of dP/dt0.1 in Group P at all RivP were half the values of those in Group H. The ratio Tiocc/Ti did not change in Group P and increased in Group H during a washout of halothane.

CONCLUSION

Propofol anaesthesia was associated with a decrease in Vi whereas during halothane anaesthesia, Vi did not change. Ventilation in Group P differed from Group H in parameters of both breath Drive and Timing.

Pattern of ventilation during halothane anaesthesia in infants less than two months of age

PURPOSE

To examine the breathing pattern of infants aged less than two months in order to understand better the effect of halothane on ventilation in infants.

METHODS

The inspiratory flow waveform, the CO2 waveform and occluded inspiratory pressure waveform were recorded at different inspired concentrations of halothane using a washout of halothane in two groups of infants undergoing elective herniorrhaphy. Data were analyzed for minute ventilation (Vi) and tidal volume (VT), parameters of timing of the breath [Total time (Ttot), Inspiratory time (Ti), and the ratio of the occluded to unoccluded inspiratory time (TiOCC/Ti)], parameters of Amplitude of the neural output [mean inspiratory flow (VT/Ti)] and parameters of the Shape of the inspiratory breath profile [the inspiratory flow centroid (Ci/Ti), the inspiratory duty cycle (Ti/Ttot)]. The airway was occluded at end expiration and the slope of the initial 100 msec of occlusion (dP/dt) together with the maximal negative pressure (PMAX) and occluded inspiratory time (TiOCC) were obtained. We studied ten infants < 48 wk post-conceptional age (PCA) and ten infants > 48 wk. PCA Flow (V), pressure (Pao) and carbon dioxide tension (PCO2) were recorded at three concentrations of inspired halothane (FiH): 0%, 1% and 2% which corresponded to an end-tidal halothane concentration of about 0.2%, 0.8% and 1.2% respectively.

RESULTS

In both groups Vi, VT and VT/Ti decreased whereas dP/dt, did not, suggesting that the respiratory pump was impaired. The parameters of breath Shape did not change. Importantly the parameters of Timing showed different tendencies. In infants > 48 wk PCA TiOCC/Ti decreased. In infants < 48 wk PCA, TiOCC/Ti did not change.

CONCLUSIONS

The different response in the timing parameter TiOCC/Ti is consistent with a different effect of halothane on parameters of ventilatory timing in infants < 48 wk PCA and this may represent a maturational effect.

[Peripheral muscle fatigue and ventilatory response to effort in chronic airflow limitation]

We aimed to analyze peripheral muscle fatigue and ventilatory pattern in a group of 28 individuals (12 health and 16 with chronic air flow limitation) performing incremental exercise. The level of exercise reached was significantly less for patients than for healthy subjects (107 +/- 30 W vs. 234 +/- 44 W). Minute ventilation (VE) was 54 +/- 15 l/min evolving linearly with no breaking pint, and the respiration pattern was tachypneic with high frequency (f) and low circulating volumen (VT) in patients. The ration Ti/Ttot did not change during exercise in either of the groups. Occlusion pressure (P0.1) was always higher in the patient group (p < 0.001) while mean inspiratory flow was higher at rest and at moderate levels of exercise (P < 0.05) but significantly lower at high levels (1.95 +/- 0.6 vs. 3.98 +/- 1 l.s-1). Muscle fatigue, defined as the fall in the H/L index in the electromyogram, appeared in 11/12 healthy subjects (H/L: 71 +/- 11%) and in 9/16 patients (H/L: 67 +/- 17%). There were no physical differences between the 2 groups of patients (those with and without fatigue). Patients with fatigue showed a more moderate degree of obstruction (FEV1 68 +/- 12% vs. 42 +/- 13% v. ref) with less airways impedance (p < 0.001) and hypoxia (SaO2 91.3% vs. 87%), and a better ventilatory response to exercise (VE 61 +/- 14 vs. 45 +/- 10 l/min) with a higher mean inspiratory flow (2.25 +/- 0.54 vs. 1.57 +/- 0.54 l.s-1) in spite of there being no differences in P0.1. The restricting factor was ventilatory limitation, although muscle fatigue appeared in 53% of the patients. Patients who experienced muscle fatigue had less obstruction and better ventilatory response to exercise.

Ventilatory depression by halothane in infants and children

The purpose of this study was to extend previous observations of a greater decrease in tidal volume in infants than in children during halothane anaesthesia. We analyzed the inspiratory flow waveform recorded during spontaneous ventilation in: infants, two to six months of age, and children, one to five years of age. In addition we analyzed the CO2 signal and the pressure waveform during an occluded inspiration. The pressure generated during the initial 100 msec of inspiratory occlusion, an index of respiratory drive, was analyzed to give some insight into the aetiology of the age-related differences. In 15 infants and 15 children, Flow (V), pressure (Pao) and PCO2 were recorded at three concentrations of inspired halothane (FIH): 0%, 1% and 2% which correspond to an endtidal halothane concentration of about 0.3%, 0.9% and 1.3% respectively. Data were analyzed for minute ventilation (Vi) and parameters of timing (Total time (Ttot), Inspiratory time (Ti)), the amplitude of the neural output (mean inspiratory flow (VT/Ti), tidal volume (VT)) and the shape of the inspiratory breath profile (the inspiratory centroid flow (Ci/Ti), the inspiratory duty cycle (Ti/Ttot)). In some, the airway was occluded at end expiration and the slope of the initial 100 msec of occlusion (dP/dt) together with the maximal negative pressure (PMAX) were measured. Estimates of respiratory mechanics E'rs (PMAX/VT) and (VT/Ti)/(dP/dt) were obtained. The VT and Ttot decreased with increasing FIH in both infants and children (P < 0.05). The PETCO2 increased in both groups and the % increase was greater in infants.(ABSTRACT TRUNCATED AT 250 WORDS)

Ventilatory chemosensitivity in parents of infants with sudden infant death syndrome

We measured ventilatory responses to progressive isocapnic hypoxia and to hyperoxic hypercapnia (CO2) using rebreathing techniques in 16 parents of infants with autopsy-confirmed sudden infant death syndrome (SIDS) and 18 control parents matched for age, sex, and body size. Response to ventilatory loading was assessed by repeating the CO2 test with an inspiratory flow-resistive load (16 cm H2O/L/sec). During loaded and unloaded CO2 tests, respiratory effort was also assessed by measuring the pressure generated in the first 0.1 second (P0.1) of the subsequent inspiratory effort after brief manual occlusion of the inspiratory line. Ventilatory responses of the parents of victims of SIDS to chemical and mechanical stimulation were not significantly different from those of control parents. Responses in both groups were similar to previously reported normal values. There was a linear increase in ventilation (VE) in response to hypercapnia and hypoxia and in P0.1 in response to hypercapnia. We found expected increases in P0.1/PCO2 and decreases in VE/PCO2 slopes during loaded breathing in all subjects, but no difference between groups. We conclude that parents of SIDS victims have normal ventilatory chemosensitivity and respiratory drive.

Evaluating control of breathing utilizing an on-line microcomputer

This paper describes a system that was designed to evaluate ventilatory responses to hypercapnia, airway occlusion pressure (P100), as well as measuring ventilatory drive and timing. Parameters measured include minute ventilation (VE), breathing rate (f), tidal volume (VT), inspiratory time (TI), fractional inspiratory time (TI/TTot), mean inspiratory flow (VT/TI), and end tidal partial pressure of carbon dioxide (PETCO2). These measurements allow for a thorough analysis of abnormalities of neural and neuromuscular ventilatory drive. The system's architecture was based primarily on currently available microcomputer components designed for the IEEE 696 bus.

Postoperative analgesia by nicomorphine intramuscularly versus high thoracic epidural administration. Effects on ventilatory and airway occlusion pressure responses to CO2

In this study the effects of nicomorphine, administered either intramuscularly or by high thoracic epidural route, on the ventilatory and airway occlusion pressure response to CO2 were investigated and compared. Twenty-four patients scheduled for thoracic surgery were allocated randomly to postoperative pain relief by i.m. nicomorphine or by high thoracic epidural nicomorphine. The ventilatory response to 5% carbon dioxide was measured in all patients: first 1 day before operation, secondly on the first day after surgery immediately before nicomorphine administration and finally after the administration, at the moment when no further rise in end-tidal PCO2 (PETCO2) was measured. Respiratory response was assessed in two ways, by measuring minute ventilation (VE) and mouth occlusion pressure (P0.1). There was a significant depression in ventilatory response to CO2 in the intramuscular group (P = 0.03) due to nicomorphine as assessed by the slope of VE vs PETCO2. No significant depression was found in the epidural group, irrespective of measurement of VE or P0.1. No significant shift of apnoeic threshold-PETCO2 was observed in either group.

Physiologic effects of doxapram in idiopathic apnea of prematurity

Twelve premature infants with significant apnea of prematurity while receiving therapeutic doses of aminophylline were given an intravenous infusion of doxapram, 2 or 2.5 mg/kg/hr. The ventilatory effects of the medication were monitored by means of face mask spirometry and airway occlusion studies. Doxapram therapy was associated with significant increases in minute ventilation, tidal volume, mean inspiratory flow, and airway pressure 100 msec after occlusion. Respiratory frequency and the relative duration of inspiration and expiration were unchanged. Paco2 decreased significantly during the infusion. The apnea attack rate, monitored by continuous recording, was significantly reduced after the first 6 hours of therapy. Six hours after starting doxapram, mean arterial blood pressure was significantly elevated, and continued to increase during the 24 hours of therapy. Doxapram is effective in treatment of apnea of prematurity refractory to aminophylline, and appears to act by increasing respiratory center output.

Hypnotics and the control of breathing: a review

Hypnotics are central depressants. In sufficient doses, they suppress respiration, and so their effects on respiration are important considerations in their safety. The paper reviews mechanisms of respiratory control and methods of assessment, the effects of hypnotics on control of breathing and new methods of non-invasive respiratory monitoring.