Effect of breathing pattern on the pressure-time product calculation

Background:  The pressure-time product (PTP) is often used to compare conditions with different breathing patterns. Being the pressure-time product calculated with pressures changes over a minute, mechanical load and inspiration time per minute should be its main determinants. The aim of this study was to investigate if the method of PTP computation is affected by the breathing pattern when mechanical load and inspiratory time per minute are constant.
Methods:  Respiratory mechanics and the PTP developed by the ventilator were calculated in 10 mechanically ventilated patients at three different respiratory rate/tidal volume combinations, provided that minute ventilation and inspiratory time per minute were constant.
Results:  The static elastance did not change at different tidal volumes. Despite the constant elastic load over a minute, the elastic PTP showed an increment greater than 200% from the higher to the lower respiratory rate, responsible for approximately 80% of the whole PTP increment. On the contrary a 'corrected' elastic PTP (calculated using the square root of the elastic pressure-time area), the elastic double product of the respiratory system and the mean elastic pressure per minute, did not significantly change.
Conclusions:  Changes in breathing pattern markedly affected the PTP independently by the mechanical load and the inspiratory time per minute. In these conditions it could not correctly estimate the metabolic cost of breathing. The use of a 'corrected' PTP, the mean inspiratory pressure per minute or the double product of the respiratory system, could overcome this limitation.     

Quantal ventilatory variability during spontaneous breathing anaesthesia

Background. Cardioventilatory coupling is the triggering ofinspiratory onset by preceding cardiac activity. We have observedtwo forms of coupling with a bimodal (‘quantal’)variation of respiratory period. Methods. We investigated the variables of inspiratory duration(TI), expiratory duration (TE), and tidal volume (VT) whererespiratory period variation was bimodal. In 25 anaesthetizedspontaneously breathing subjects we took 11 samples of recordingwhere the variation of respiratory period was quantal. Results. In eight of these epochs the variation in respiratoryperiod was associated with fluctuations in the number of heartbeats per breath (entrainment ratio) with a constant time intervalbetween inspiration and the immediately preceding heart beat(coupling interval), which we define as pattern II coupling.During pattern II coupling, the quantal variations in respiratoryperiod were entirely caused by variation in TE, with no associatedchanges in either TI or VT. The other three epochs with quantalvariations in respiratory period were observed in pattern IIIcoupling, where an alternating fluctuation in both entrainmentratio and coupling interval occurs. During pattern III coupling,quantal fluctuations were observed in TE, TI, and VT. Implications. Cross correlation analysis suggested that whenpattern III was present, TI was dependent upon the precedingTE, which differs markedly from traditional views on the interactionbetween inspiratory and expiratory duration. VT was linearlyrelated to TI, and so could also be determined by the precedingTE during this type of coupling. Br J Anaesth 2003; 91: 184–9     

A comparison of anaesthetic breathing systems during spontaneous ventilation

Five anaesthetic breathing systems (Magill, Lack, Humphrey ADE, enclosed Magill and Bain) were compared using spontaneous ventilation in a simple lung model. The fresh gas flow at which rebreathing occurred was determined for each system by the application of four modified definitions of rebreathing. Two were based on the measurement of minimum inspired and two on end-expired carbon dioxide. The four A systems performed similarly with each individual definition. The rebreathing points found for each individual breathing system differed markedly between definitions, with those determined by the minimum inspired CO2 occurring at low, and probably misleading, FGF/VE ratio. The Bain system demonstrated rebreathing at considerably higher fresh gas flows whichever definition was used.     

Augmented spontaneous breathing and pulmonary gas exchange during pneumoperitoneum

BACKGROUND AND OBJECTIVE: Ventilation of the lungs with positive end-expiratory pressure during pneumoperitoneum has been shown to improve the arterial partial pressure of oxygen. The implications of spontaneous breathing on pulmonary gas exchange remain unknown in this setting. We therefore sought to examine the influence of pressure-support ventilation with spontaneous breathing on gas exchange during simulated laparoscopy. METHODS: Ten pigs were subjected to pneumoperitoneum at a pressure of 15 cmH2O. Animals received, in a random order, pressure-support and pressure-controlled ventilation for 60 min per mode. Inert gas and haemodynamic measurements were performed before changing to a subsequent mode. RESULTS: Pressure-support ventilation was more efficient than pressure-controlled ventilation regarding perfusion of normal V(A)/Q lung areas (78 +/- 4% vs. 72 +/- 5%) (P < 0.05), alveolar-arterial partial pressure of oxygen difference (9.73 +/- 1.3 vs. 11.2 +/- 1.2 kPa) and arterial partial pressure of oxygen (14.93 +/- 1.6 vs. 13.7 +/- 2.0 kPa) (P < 0.05). CONCLUSIONS: Pressure-support ventilation resulted in significantly better gas exchange than pressure-controlled ventilation in this model of simulated laparoscopy.     

Device for humidification and controlled oxygenation during spontaneous breathing

A system for humidification with adjustable flows and oxygen concentrations for use in spontaneously breathing patients is described.     

Measurement of effective alveolar carbon dioxide tension during spontaneous breathing in normal subjects and patients with chronic airways obstruction.

BACKGROUND--The measurement of effective alveolar carbon dioxide tension (PA-CO2eff) is still a matter of debate. It has, however, become common practice to use arterial instead of alveolar CO2 tension for computing alveolar oxygen tension (PAO2) and physiological dead space, not only in normal subjects but also in patients. The purpose of this study was to estimate alveolar CO2 tension during spontaneous breathing with a new bedside technique which is simple and non-invasive, and to compare these values with arterial CO2 tension measured in normal subjects and patients with chronic airways obstruction. METHODS--The subjects breathed quietly through the equipment assembly (mouthpiece, monitoring ring, Fleisch transducer head) connected to a pneumotachograph and a fast response infrared CO2 analyser. The method is a computerised calculation of the volume weighted effective alveolar CO2 tension obtained from the simultaneously recorded expiratory flow and CO2 concentration versus time curves. An arterial blood sample was taken to measure PaCO2 for comparison during the study. RESULTS--The results showed a mean difference (PACO2eff-PaCO2) of -0.205 kPa in 20 normal subjects and -0.460 kPa in 46 patients. The 95% confidence interval of the bias was -0.029 to -0.379 kPa in normal subjects and -0.213 to -0.707 kPa in patients. The limits of agreement between PACO2eff and PaCO2 were 0.526 to -0.935 in normal subjects and 1.170 to -2.088 in patients. CONCLUSIONS--The volume weighted effective alveolar PCO2 in normal subjects and patients with chronic airways obstruction is lower than the arterial PCO2 and is recommended as a better estimate in the classical equations for estimating dead space and intrapulmonary shunt.     

Repeatability of airway resistance measurements made using the interrupter technique

BACKGROUND: To be able to interpret any measurement, its repeatability should be known. This study reports the repeatability of airway resistance measurements using the interrupter technique (Rint) in children with and without respiratory symptoms. METHODS: Children aged 2-10 years who were healthy, had persistent isolated cough, or who had previous wheeze were studied. On the same occasion, three Rint measurements were made 15 minutes apart, before and after placebo and salbutamol given in random order. Results from those given placebo first were analysed for within-occasion repeatability. Between-occasion repeatability measurements were made 2-20 weeks apart (median 3 weeks). RESULTS: For 85 pairs of measurements before and after placebo the limits of agreement were 20% expected resistance and were unaffected by age or health status. The change in resistance following bronchodilator in one of 18 healthy children, 12 of 28 with cough, and 22 of 39 with wheeze exceeded this threshold. For between-occasion measurements the limits of agreement were 32% in 72 healthy subjects, 49% in 57 with cough, and 53% in 95 with previous wheeze. CONCLUSION: The measurement of airways resistance by the interrupter technique is clinically meaningful when change following an intervention such as the administration of bronchodilator is greater than its within-occasion repeatability. Between-occasion repeatability is too poor to judge change confidently.     

A comparison between the Bain and Magill anaesthetic systems during spontaneous breathing

The efficiency of the Bain system has been compared with that of the Magill system in ten conscious subjects breathing spontaneously. Air was supplied at fresh flow rates of 150 ml/kg and decreased stepwise at four-minute intervals until a flow of 50 ml/kg was attained. Expired minute volume and end-tidal carbon dioxide concentrations were measured. No rebreathing could be demonstrated with the Magill stystem at flow rates above approximately 70 ml/kg. In contrast, rebreathing was evident at all flow rates with the Bain system. It is concluded that acceptable carbon dioxide levels during spontaneous breathing with the Bain circuit can only be maintained by considerable active hyperventilation when using flow rates of 150 ml/kg and less.     

Rebreathing and end-tidal CO2 during spontaneous breathing with the Bain circuit

In order to examine the relationship between end-tidal CO₂ (Fet_{co 2}) and inspired CO₂ (Fi_{co 2}) in anaesthetized patients breathing spontaneously with a Bain breathing circuit and afresh gasvolume ( \(\dot VF\) ) of 100 ml · kg^-1 · min^-1, the respiratory rate (f) and minute ventilation ( \(\dot VE\) ) was changed in two groups of six patients each by the induction or reversal of narcotic respiratory depression. During light nitrous oxide-halothane anaesthesia (Group I), the intravenous injection of 0.1 mg · kg^-1 of alphaprodine caused arapid fall in F_{co 2} from 2.3 ± 0.5 per cent to 0.7 ± 0.1 per cent concomitant with the reduction inf(37 ± 5 to 16 ± 4) breath -min^-1 and \(\dot VE\) (137 ± 29 to 55 ± 13 ml · kg^-1 · min^-1), while the Fet_{co 2} rose gradually from 5.2 ± 0.9 percent to 6.4 ± 0.9 per cent over a ten-minute period. During light nitrous oxide-halothane anaesthesia supplemented by alphaprodine (Group II), 0.2 mg of naloxone intravenously caused a rise in Fi_{co 2} from 0.5 ± 0.3 per cent to 2.9 ± 0.6 per cent simultaneous with arise in f (11 ± 2 to 25 ± 7 breath · min^-1) and \(\dot VE\) (70 ± 25 to 133 ± 34 ml · kg^-1 · min^-1), while the FET_{co 2} declined gradually over a ten-minute period from 7.6 ± 0.7 per cent to 6.4 ± 0.4 per cent. The change in Fl_{co 2} always occurred exactly at the same time as the drug-induced change in respiration. It was associated with acorresponding change in the degree of mixing of fresh gas and expired gas within the breathing system and appeared to correlate with the change in the ratio \(\dot VE/\dot VF\) . There was no indication that the Fl_{co 2} or the distribution of CO₂ within the system had any effect onFet _{co 2} or CO₂ elimination. Under these conditions theFl _{co 2} and the volume of rebreatked CO₂ can not be the cause but must be regarded as apassive change consequent to the altered pattern of breathing.     

Fresh gas flow in coaxial Mapleson A and D circuits during spontaneous breathing

In a lung model simulating spontaneously breathing halothane anaesthesia, the rebreathing characteristics of the coaxial Mapleson A (Lack circuit) and D (Bain circuit) systems were tested. Using decreasing fresh gas flows (VF), the end-tidal carbon dioxide fraction (FACO2) was monitored and the point of rebreathing (R.P.) detected. The effects of changes in minute volume (VE), dead-space to tidal volume ratio (VD/VT) and carbon dioxide elimination (VCO2) were studied. The effect of increased tidal volumes (VT) on FACO2 was investigated for some different fresh gas flows (VF). The VF/VE ratio for R.P. in the Bain circuit was approximately 2 and in the Lack circuit 0.88. In both circuits an increase in VE and a decrease in the VD/VT ratio resulted in higher demands on VF if rebreathing was to be avoided. The latter effect was much more pronounced in the Lack circuit. In neither system did any changes in VCO2 affect the rebreathing characteristics. The conclusion was drawn that the Lack system is a much better choice concerning the fresh gas flows for anaesthesia with spontaneous breathing than the Bain system. It was also concluded that the fresh gas flows recommended by Humphrey for the Lack system (i.e. 51 ml X min-1 X kg b.w.-1) and by the manufacturers for the Bain system (i.e. 100 ml X min-1 X kg b.w.-1) are inadequate and should be increased if a considerable degree of rebreathing is to be avoided.     

Accuracy of prediction of fresh gas flow requirements during spontaneous breathing with the T-piece

In 40 spontaneously breathing children (7.3-47.9 kg) anaesthetized with halothane for minor surgical procedures the fresh gas flow (FGF) at onset of rebreathing (FGFr) was determined and end-tidal CO2 concentration (ETCO2), minute ventilation (VE), tidal volume (VT) and respiratory rates (f) were registered. The accuracy of predicting the FGFr from 2 X VE, 3 X VE and from two formulae (FGF = 15 X kg X f and FGF = 3 X (1000 + 100 X kg) was evaluated. FGFr ranged from 3.5 to 10 l min-1. FGF calculated from 2 X VE was inadequate. Calculations of FGF from 3 X VE and with the two formulae gave an adequate FGF in more than 80% of the children. No serious under-estimations were found. In a few cases FGF level was overestimated by more than 150%. It is suggested that when the theoretical calculation of FGF results in flow rates well over 10 l min-1 an upper flow rate limit of 10 l min-1 may be used in children weighing less than 30 kg since no child required a FGF over this rate.