Terminology vs Physiology and Mechanics

Les respirateurs d'anesthésie de nouvelle génération vont-ils modifier notre pratique chez l'enfant ?

Simple SummaryMechanical ventilation is mandatory during ophthalmological surgeries when neuromuscular blocking agents are used. Ventilation of paediatric patients is more complicated because of their small size, the underdevelopment of their respiratory system and the lack of evidence-based guidelines for this age group. We presented two cases of severe hypoventilation during anaesthesia in paediatric dogs due to inadvertent ventilation mainly of the upper airways, and possible development of lung collapse. Both patients were ventilated using the same ventilator in a volume-controlled mode, and the actual level of hypercapnia was detected only when manual ventilation was used, and alveoli were properly ventilated. Adjustments in the volume-controlled ventilation failed to ventilate effectively, and manual ventilation was used for the remainder of the procedure until recovery. Both patients recovered well. Pressure-controlled ventilation, ventilators that take into consideration the compliance of the breathing system and the use of manual breathing, among other techniques, could contribute to better ventilation of those patients.A 2-month-old male 1.56 kg Yorkshire terrier (Case No. 1) and a 3-month-old male 2.3 kg Jack Russell Terrier (Case No. 2) were scheduled for ophthalmological surgery under general anaesthesia and neuromuscular blockade. For both patients, volume-controlled ventilation (VCV) was used with set tidal volumes (VT) of 13 mL/kg and 20 mL/kg for cases No. 1 and 2, respectively. The type of ventilator used did not take into account the intrinsic compliance of the breathing system; therefore, a significant part of the delivered VT was wasted in the expansion of the breathing system, and did not reach the patients, causing alveolar hypoventilation. Both cases developed low dynamic compliance (CD), and after a recruitment manoeuvre, EtCO2 of up to 116 mmHg and 197 mmHg were revealed for cases No. 1 and 2, respectively. The two cases had to be ventilated manually, using positive inspiratory pressures (PIP) of 20–25 mmHg, in order to improve alveolar ventilation and reduce the EtCO2, as adjustments to the VCV were ineffective. Both patients maintained an oxygen haemoglobin saturation between 94% and 100% throughout the procedure and they recovered well. Using a higher VT from the beginning, to compensate for the compliance of the breathing system, or the use of pressure-controlled ventilation (PCV), could have potentially helped to avoid these two incidences of severe hypercapnia.

Afferent vagal nerve activity from stretch-receptors in the lung and efferent phrenic nerve activity were recorded during spontaneous respiration and during positive-pressure ventilation with three different types of ventilators. During spontaneous respiration the efferent phrenic nerve activity slightly preceded the afferent vagal nerve activity. Volume-controlled ventilation did not alter the phrenic nerve activity when the ventilation was set at a rate equal to that during spontaneous respiration, but afferent vagal volleys increased in duration. At higher frequencies of insufflation spontaneous inspiration was inhibited. An increase in afferent vagal nerve activity and a concomitant slight decrease in efferent phrenic nerve activity were obtained during animal triggered pressure-controlled ventilation. High-frequency positive-pressure ventilation (HFPPV) gave rise to basal, non-grouped activity in vagal afferents, causing inhibition of inspiration. During HFPPV, spontaneous respiration can take place on activation of other afferents to the respiratory centre. Clinical aspects of respirator treatment from a neurophysiological standpoint are discussed.

Emergency mechanical ventilators developed during the pandemic were used to meet the high demand in intensive care units to care for COVID-19 patients. An example of such ventilators is Masi, developed in Peru and installed in more than 15 hospitals around the country. This study aimed to compare Masi's performance with other emergency mechanical ventilators manufactured during the covid-19 pandemic such as Neyün, Spiro Wave and a prototype developed by the Faculty of Engineering of the National University of Asuncion (FIUNA). Three configurations of a test lung were used, combining different values of resistance and compliance (C1, C2 and C3). Ventilators were set to volume-controlled ventilation with tidal volume = 400 mL, respiratory rate = 12 breaths/minute, and positive end-expiratory pressure (PEEP) = 8 cm H2O. These parameters were measured in a series of ten two-minute tests which then were evaluated through a two-way analysis of variance, considering the type of ventilator and test lung configuration as the two independent variables. For target values, MASI delivered VT that ranged from 319 to 432 ml (-20 to +8%), respiratory rate of 12 bpm, and PEEP from 8.4 to 9.5 cm H2O (+5 to +20%). In contrast, for instance, Neyün delivered VT that ranged from 199 to 543 ml (-50 to +35%) and PEEP from 7.05 to 9.21 cm H2O (--11 to +15%), with p<0.05. The analysis of variance showed that he differences between preset and delivered parameters were influenced by the type of ventilator and, significantly, by the test lung configuration.Clinical Relevance- This establishes the most advantageous conditions in which three emergency mechanical ventilators work and a quantitative perspective in this topic.