Intratracheal pulmonary ventilation and continuous positive airway pressure in a sheep model of severe acute respiratory failure

Use of a Portable Mechanical Ventilator during Cardiopulmonary Resuscitation is Feasible, Improves Respiratory Parameters, and Prevents the Decrease of Dynamic Lung Compliance

BACKGROUND

For practical and protective ventilation during cardiopulmonary resuscitation (CPR), a 150-grams mechanical ventilator (VLP2000E) that limits peak inspiratory pressure (PIP) during simultaneous ventilation with chest compressions was developed.

OBJECTIVES

To evaluate the feasibility of VLP2000E ventilation during CPR and to compare monitored parameters versus bag-valve ventilation.

METHODS

A randomized experimental study with 10 intubated pigs per group. After seven minutes of ventricular fibrillation, 2-minute CPR cycles were delivered. All animals were placed on VLP2000E after achieving return of spontaneous circulation (ROSC).

RESULTS

Bag-valve and VLP2000E groups had similar ROSC rate (60% vs. 50%, respectively) and arterial oxygen saturation in most CPR cycles, different baseline tidal volume [0.764 (0.068) vs. 0.591 (0.123) L, p = 0.0309, respectively] and, in 14 cycles, different PIP [52 (9) vs. 39 (5) cm H2O, respectively], tidal volume [0.635 (0.172) vs. 0.306 (0.129) L], ETCO2[14 (8) vs. 27 (9) mm Hg], and peak inspiratory flow [0.878 (0.234) vs. 0.533 (0.105) L/s], all p < 0.0001. Dynamic lung compliance (≥ 0.025 L/cm H2O) decreased after ROSC in bag-valve group but was maintained in VLP2000E group [0.019 (0.006) vs. 0.024 (0.008) L/cm H2O, p = 0.0003].

CONCLUSIONS

VLP2000E ventilation during CPR is feasible and equivalent to bag-valve ventilation in ROSC rate and arterial oxygen saturation. It produces better respiratory parameters, with lower airway pressure and tidal volume. VLP2000E ventilation also prevents the significant decrease of dynamic lung compliance observed after bag-valve ventilation. Further preclinical studies confirming these findings would be interesting.

Tracheobronchial injury during intratracheal pulmonary ventilation in rabbits

OBJECTIVE

We compared tracheobronchial injury following short-term intratracheal pulmonary ventilation (ITPV) and conventional mechanical ventilation (CMV) in a healthy rabbit model. ITPV, a form of tracheal gas insufflation, has been shown to decrease deadspace ventilation and increase CO2 removal and therefore may reduce ventilator-induced lung injury.

SETTING

Medical center laboratory.

SUBJECTS

Twenty-five rabbits.

INTERVENTIONS

Rabbits were randomly assigned to either ITPV or CMV (n = 15 and 10, respectively). Both groups were mechanically ventilated for 8 hrs at the same ventilator settings (FIO2, 0.4; rate, 30 breaths/min; flow, 4 L x min(-1); positive end-expiratory pressure, 4 cm H2O; tidal volume, 40 mL). Peak, mean, and end-expiratory carinal pressures, ITPV flow rate, and hemodynamic variables were continuously monitored. Tissue samples for histologic analysis were obtained postmortem from the trachea contiguous to the tip of the endotracheal tube, the distal trachea, the carina, and the main bronchus. The histologic sections were scored, in a single-blind fashion, for ciliary damage, ulceration, hemorrhage, overall inflammation, intraepithelial inflammatory infiltrate, and edema.

MEASUREMENTS AND MAIN RESULTS

ITPV was associated with significantly lower Paco and deadspace ventilation ratio than CMV. The combined tracheobronchial injury scores for all samples were significantly higher in the ITPV group compared with the CMV group (p <.005; Mann-Whitney U test). The ITPV injury scores, compared with CMV injury scores, were significantly higher at the carina and main bronchus (p <.01; Kruskal-Wallis test followed by Dunn's multiple comparison test). The area adjacent to the endotracheal tube showed the same degree of damage in both groups. Analysis of the injury scores in individual damage categories demonstrated the greatest difference in the ulceration category (p <.001).

CONCLUSIONS

In our study, ITPV, compared with CMV at the same minute ventilation, was associated with a significantly greater difference in tracheobronchial damage at the carina and main bronchus. We postulate that this difference may have been caused by the turbulence of the gas flow generated by the small-caliber ITPV catheter used in our neonatal-size animal model.

Distal projection of insufflated gas during tracheal gas insufflation

Tracheal gas insufflation (TGI) flushes expired gas from the ventilator circuitry and central airways, augmenting CO2 clearance. Whereas a significant portion of this washout effect may occur distal to the injection orifice, the penetration and mixing behavior of TGI gas has not been studied experimentally. We examined the behavior of 100% oxygen TGI injected at set flow rates of 1-20 l/min into a simulated trachea consisting of a smooth-walled, 14-mm-diameter tube. Models incorporating a separate coaxial TGI injector, a rough-walled trachea, and a bifurcated trachea were also studied. One-hundred percent nitrogen, representing expiratory flow, passed in the direction opposite to TGI at set flow rates of 1-25 l/min. Oxygen concentration within the "trachea" was mapped as a function of axial and radial position. Three consistent findings were observed: 1) mixing of expiratory and TGI gases occurred close to the TGI orifice; 2) the oxygenated domain extended several centimeters beyond the endotracheal tube, even at high-expiratory flows, but had a defined distal limit; and 3) more distally from the site of gas injection, the TGI gas tended to propagate along the tracheal wall, rather than as a central projection. We conclude that forward-directed TGI penetrates a substantial distance into the central airways, extending the compartment susceptible to CO2 washout.

Intratracheal pulmonary ventilation versus conventional mechanical ventilation: continuous carinal pressure monitoring at low and high flows and frequencies

We continuously measured proximal and carinal pressures at low and high flow rates and frequencies during conventional mechanical ventilation (CMV) and intratracheal pulmonary ventilation (ITPV), using an artificial lung. The proximal peak inspiratory pressure (PIP), carinal PIP, proximal positive end expiratory pressure (PEEP), and carinal PEEP, or negative end expiratory pressure (NEEP), were measured during simulated CMV and ITPV. Two levels of frequency (30 and 90 per min) and two gas flow rates (3 and 6 L/min) were examined, in both dry and humid states (four combinations of gas flow and frequency at each state). The gas flow and inspiratory time were held constant throughout the CMV and ITPV trials. Humidification of the ventilatory circuit during ITPV prevented the accurate measurement of carinal pressures. This problem was solved by introducing a continuous "bias flow" of 11 ml/min into the pressure monitoring line. A combination of low gas flow and low frequency with CMV showed no significant differences between the proximal and carinal PIP, as well as the proximal and carinal PEEP. The same combination with ITPV, however, resulted in a significantly lower carinal PIP and PEEP, compared to proximal PIP and PEEP. Carinal PIP and PEEP during ITPV were also significantly lower than those observed during CMV with a low flow and low frequency rates. During both CMV and ITPV, using a combination of a high flow rate with a high breathing frequency, carinal PIPs were significantly lower than proximal PIPs. ITPV, however, generated much larger differences between proximal and carinal PIPs than the CMV. A significant NEEP was generated at the carinal level during ITPV with high flow rates, both with high and low frequencies. The NEEP did not occur with a low gas flow, in combination with either a low frequency or a high frequency. The "bias flow" had no significant effect on carinal pressures. In conclusion, ITPV, compared with CMV, generates a significantly lower carinal PIP, but it may also generate carinal NEEP. For safety reasons, therefore, it is essential to monitor carinal pressures continuously in patients treated with ITPV.

Intratracheal pulmonary ventilation in a rabbit lung injury model: continuous airway pressure monitoring and gas exchange efficacy

OBJECTIVES

To compare carinal pressures vs. proximal airway pressures, and gas exchange efficacy with a constant minute volume, in lung-injured rabbits during conventional mechanical ventilation (CMV) and intratracheal pulmonary ventilation (ITPV); and to evaluate performance of a prototype ITPV gas delivery and continuous airway pressure monitoring system.

DESIGN

Prospective controlled study.

SETTING

Animal research laboratory at a teaching hospital.

SUBJECTS

Sixteen adult female rabbits.

INTERVENTIONS

Anesthetized rabbits were tracheostomized with a multilumen endotracheal tube. Anesthesia and muscle relaxation were maintained continuously throughout the study. Proximal airway pressures and carinal pressures were recorded continuously. The injection port of the multilumen endotracheal tube was used for the carinal pressure monitoring. To prevent obstruction of the port, it was flushed with oxygen at a rate of 11 mL/min. CMV was initiated with a pressure-limited, time-cycled ventilator set at an FiO2 of 1.0 and at a flow of 1.0 L/kg/min. The pressure limit of the ventilator was effectively disabled. A normal baseline for arterial blood gases was achieved by adjusting the inspiratory/expiratory time ratios. ITPV was established using a flow of 1.0 L/kg/min through a reverse thrust catheter, at the same baseline and inspiratory/expiratory ratio. Carinal positive end-expiratory pressure was maintained at a constant value of 2 cm H2O by adjusting the expiratory resistance of the ventilator circuit Lung injury was achieved over a 30-min period by three normal saline lavages of 5 mL/kg each. After lung injury, all animals were consecutively ventilated for 1 hr with CMV, for 1 hr with ITPV, and again for 1 hr with CMV. Six rabbits were ventilated at 30 breaths/min (group 1), and ten rabbits were ventilated at 80 breaths/min (group 2). Four rabbits in group 2 were subjected, 1 hr after return to CMV from ITPV, to another session of ITPV, with positive end-expiratory pressure gradually being increased to 4, 6, and 8 cm H2O for 15 mins each.

RESULTS

No significant differences were observed in carinal peak inspiratory pressure between CMV and ITPV modes, at both low and high frequencies of breathing, indicating that the inspired tidal volume remained constant during both modes of ventilation. Significant gradients were noted between proximal airway and carinal peak inspiratory pressure during ITPV but not during CMV. Initiation of ITPV, at a flow of 1.0 L/kg/min, required an increase in the ventilator expiratory resistance to maintain a constant level of positive end-expiratory pressure (2 cm H2O) as measured at the carina. During ITPV, the PaCO2 was significantly reduced by 20% at 30 breaths/min (p < .05) and by 22% at 90 breaths/min (p < .01), compared with CMV. Arterial oxygenation was significantly enhanced with a positive end-expiratory pressure of 6 and 8 cm H2O (p < .05 and .001, respectively), compared with a positive end-expiratory pressure of 2 cm H2O during ITPV. All components of the new prototype gas delivery and airway pressure monitoring system functioned without failure, at least for 3 hrs of the CMV, ITPV, and CMV trials.

CONCLUSIONS

ITPV in saline-lavaged, lung-injured rabbits at breathing frequencies of 30 and 80 breaths/min, compared with CMV at the same minute ventilation, can improve CO2 exchange. During ITPV, significant pressure gradients can develop between carinal and proximal airway pressures. Continuous carinal pressure monitoring is therefore necessary for the safe clinical application of ITPV. Reliable carinal pressure monitoring can be achieved by adding a small bias flow through the carinal pressure monitoring port. Although ITPV can remove CO2 from injured lungs efficiently, simultaneous addition of positive end-expiratory pressure can further improve arterial oxygenation.

Efficacy of tracheal gas insufflation in spontaneously breathing sheep with lung injury

Tracheal gas insufflation (TGI) decreases dead space (V D) and can be combined with continuous positive airway pressure (CPAP) to decrease minute volume (VE) and effort of breathing. In 11 anesthetized sheep, we induced acute lung injury (ALI) through oleic acid (OA) infusion and studied the effects of TGI combined with CPAP (CPAP-TGI) at different TGI flows and with catheters of different designs. Sheep were randomized to two groups: Group A (n = 7) was placed on CPAP and CPAP-TGI at 10 and 15 L/min of insufflation flow delivered through a reverse thrust catheter (RTC). Group B (n = 4) was placed on CPAP and CPAP-TGI at a flow of 10 L/min delivered through a RTC, and through a straight flow catheter (SFC). Compared with CPAP alone, CPAP-TGI resulted in significantly lower VD, VE, pressure time product, and work of breathing. We found no additional benefit from TGI flow of 15 L/min, compared with 10 L/min, and no statistically significant difference between the SFC and the RTC. In conclusion, TGI can be combined with CPAP in this model of ALI to reduce ventilation and effort of breathing.

A new ventilator improves CO2 removal in newborn lambs with congenital diaphragmatic hernia

OBJECTIVES

To demonstrate improved ventilation with intratracheal pulmonary ventilation (ITPV) in new-born lambs with congenital diaphragmatic hernia, using a new microprocessor controlled ITPV-specific ventilator.

DESIGN

Prospective study, with each animal serving as its own control (paired data).

SETTING

Large animal research laboratory.

SUBJECTS

Diaphragmatic hernias were created surgically in seven fetal sheep on gestational day 100 (term = 145 days).

INTERVENTIONS

Lambs (2.7 to 5.0 kg) were delivered by cesarean section anywhere between gestational days 136 and 140. Arterial and venous catheterizations, bilateral chest tube thoracostomies, and tracheostomies were performed while the lambs received placental bypass. Initially, congenital diaphragmatic hernia lambs were supported on conventional pressure control mechanical ventilation to achieve steady state with measurements of baseline vital signs, arterial blood gases, and ventilatory settings. ITPV was instituted while maintaining constant peak carinal pressures and oxygen saturations. Statistical comparisons were made using the paired t-test.

MEASUREMENTS AND MAIN RESULTS

Postductal Paco2 decreased from 110+/-21 (SD) torr (14.7+/-2.8 kPa) to 52+/-24 torr (6.93+/-3.2 kPa; p= .0014) on ITPV. Simultaneously, pH improved from 7.04+/-0.07 to 7.31+/-0.15 (p = .0012) and minute ventilation increased from 0.66+/-0.40 to 4.00+/-1.35 L/min (p = .0016). Peak carinal pressures and postductal Pao2 were unchanged.

CONCLUSIONS

ITPV significantly improved CO2 removal in newborn lambs with diaphragmatic hernias without increasing airway pressures or changing oxygenation. Based on these results, we are conducting human clinical trials.