Intraoperative pulmonary hyperdistention estimated by transthoracic lung ultrasound: A pilot study

INTRODUCTION

Transthoracic lung ultrasound can assess atelectasis reversal and is considered as unable to detect associated hyperdistention. In this study, we describe an ultrasound pattern highly suggestive of pulmonary hyperdistention.

METHODS

Eighteen patients with normal lungs undergoing lower abdominal surgery were studied. Electrical impedance tomography was calibrated, followed by anaesthetic induction, intubation and mechanical ventilation. To reverse posterior atelectasis, a recruitment manoeuvre was performed. Positive-end expiratory pressure (PEEP) titration was then obtained during a descending trial - 20, 18, 16, 14, 12, 10, 8, 6 and 4cmH₂O. Ultrasound and electrical impedance tomography data were collected at each PEEP level and interpreted by two independent observers. Spearman correlation test and receiving operating characteristic curve were used to compare lung ultrasound and electrical impedance tomography data.

RESULTS

The number of horizontal A lines increased linearly with PEEP: from 3 (0, 5) at PEEP 4cmH₂O to 10 (8, 13) at PEEP 20cmH₂O. The increase number of A lines was associated with a parallel and significant decrease in intercostal space thickness (p=0.001). The lung ultrasound threshold for detecting pulmonary hyperdistention was defined as the number of A lines counted at the PEEP preceding the PEEP providing the best respiratory compliance. Six A lines was the median threshold for detecting pulmonary hyperdistention. The area under the receiving operating characteristic curve was 0.947.

CONCLUSIONS

Intraoperative transthoracic lung ultrasound can detect lung hyperdistention during a PEEP descending trial. Six or more A lines detected in normally aerated regions can be considered as indicating lung hyperdistention.

TRIAL REGISTRATION

NCT02314845 Registered on ClinicalTrials.gov.

Performance of ICU ventilators during noninvasive ventilation with large leaks in a total face mask: a bench study

Objective:

Discomfort and noncompliance with noninvasive ventilation (NIV) interfaces are obstacles to NIV success. Total face masks (TFMs) are considered to be a very comfortable NIV interface. However, due to their large internal volume and consequent increased CO₂ rebreathing, their orifices allow proximal leaks to enhance CO₂ elimination. The ventilators used in the ICU might not adequately compensate for such leakage. In this study, we attempted to determine whether ICU ventilators in NIV mode are suitable for use with a leaky TFM.

Methods:

This was a bench study carried out in a university research laboratory. Eight ICU ventilators equipped with NIV mode and one NIV ventilator were connected to a TFM with major leaks. All were tested at two positive end-expiratory pressure (PEEP) levels and three pressure support levels. The variables analyzed were ventilation trigger, cycling off, total leak, and pressurization.

Results:

Of the eight ICU ventilators tested, four did not work (autotriggering or inappropriate turning off due to misdetection of disconnection); three worked with some problems (low PEEP or high cycling delay); and one worked properly.

Conclusions:

The majority of the ICU ventilators tested were not suitable for NIV with a leaky TFM.

Evaluation of manual resuscitators used in ICUs in Brazil

OBJECTIVE:

To evaluate the performance of manual resuscitators (MRs) used in Brazil in accordance with international standards.

METHODS:

Using a respiratory system simulator, four volunteer physiotherapists employed eight MRs (five produced in Brazil and three produced abroad), which were tested for inspiratory and expiratory resistance of the patient valve; functioning of the pressure-limiting valve; and tidal volume (V_T) generated when the one-handed and two-handed techniques were used. The tests were performed and analyzed in accordance with the American Society for Testing and Materials (ASTM) F920-93 criteria.

RESULTS:

Expiratory resistance was greater than 6 cmH₂O . L⁻¹ . s⁻¹ in only one MR. The pressure-limiting valve, a feature of five of the MRs, opened at low pressures (< 17 cmH₂O), and the maximal pressure was 32.0-55.9 cmH₂O. Mean V_T varied greatly among the MRs tested. The mean V_T values generated with the one-handed technique were lower than the 600 mL recommended by the ASTM. In the situations studied, mean V_T was generally lower from the Brazilian-made MRs that had a pressure-limiting valve.

CONCLUSIONS:

The resistances imposed by the patient valve met the ASTM criteria in all but one of the MRs tested. The pressure-limiting valves of the Brazilian-made MRs usually opened at low pressures, providing lower V_T values in the situations studied, especially when the one-handed technique was used, suggesting that both hands should be used and that the pressure-limiting valve should be closed whenever possible.

Experimental study on the efficiency and safety of the manual hyperinflation maneuver as a secretion clearance technique

OBJECTIVE

To evaluate, in a lung model simulating a mechanically ventilated patient, the efficiency and safety of the manual hyperinflation (MH) maneuver as a means of removing pulmonary secretions.

METHODS

Eight respiratory therapists (RTs) were asked to use a self-inflating manual resuscitator on a lung model to perform MH as if to remove secretions, under two conditions: as routinely applied during their clinical practice; and after receiving verbal instructions based on expert recommendations. In both conditions, three clinical scenarios were simulated: normal lung function, restrictive lung disease, and obstructive lung disease.

RESULTS

Before instruction, it was common for an RT to compress the resuscitator bag two times, in rapid succession. Proximal pressure (Pprox) was higher before instruction than after. However, alveolar pressure (Palv) never exceeded 42.5 cmH₂O (median, 16.1; interquartile range [IQR], 11.7-24.5), despite Pprox values as high as 96.6 cmH₂O (median, 36.7; IQR, 22.9-49.4). The tidal volume (VT) generated was relatively low (median, 640 mL; IQR, 505-735), and peak inspiratory flow (PIF) often exceeded peak expiratory flow (PEF), the median values being 1.37 L/s (IQR, 0.99-1.90) and 1.01 L/s (IQR, 0.55-1.28), respectively. A PIF/PEF ratio < 0.9 (which theoretically favors mucus migration toward the central airways) was achieved in only 16.7% of the maneuvers.

CONCLUSIONS

Under the conditions tested, MH produced safe Palv levels despite high Pprox. However, the MH maneuver was often performed in a way that did not favor secretion removal (PIF exceeding PEF), even after instruction. The unfavorable PIF/ PEF ratio was attributable to overly rapid inflations and low VT.

Evaluation of resistance in 8 different heat-and-moisture exchangers: effects of saturation and flow rate/profile

INTRODUCTION

When endotracheal intubation is required during ventilatory support, the physiologic mechanisms of heating and humidifying the inspired air related to the upper airways are bypassed. The task of conditioning the air can be partially accomplished by heat-and-moisture exchangers (HMEs).

OBJECTIVES

To evaluate and compare with respect to imposed resistance, different types/models of HME: (1) dry versus saturated, (2) changing inspiratory flow rates.

MATERIALS AND METHODS

Eight different HMEs were studied using a lung model system. The study was conducted initially by simulating spontaneous breathing, followed by connecting the system directly to a mechanical ventilator to provide pressure-support ventilation.

RESULTS

None of the encountered values of resistance (0.5\N3.6 cm H(2)O/L/s) exceeded the limits stipulated by the previously described international standard for HMEs (International Standards Organization Draft International Standard 9360-2) (not to exceed 5.0 cm H(2)O with a flow of 1.0 L/s, even when saturated). The hygroscopic HME had less resistance than other types, independent of the precondition status (dry or saturated) or the respiratory mode. The hygroscopic HME also had a lesser increase in resistance when saturated. The resistance of the HME was little affected by increases in flow, but saturation did increase resistance in the hydrophobic and hygroscopic/hydrophobic HME to levels that could be important at some clinical conditions.

CONCLUSIONS

Resistance was little affected by saturation in hygroscopic models, when compared to the hydrophobic or hygroscopic/hydrophobic HME. Changes in inspiratory flow did not cause relevant alterations in resistance.

Mask mechanics and leak dynamics during noninvasive pressure support ventilation: a bench study

OBJECTIVE

To study the mask mechanics and air leak dynamics during noninvasive pressure support ventilation.

SETTING

Laboratory of a university hospital.

DESIGN

A facial mask was connected to a mannequin head that was part of a mechanical respiratory system model. The mask fit pressure (P(mask-fit)) measured inside the mask's pneumatic cushion was adjusted to 25 cmH(2)O using elastic straps. Pressure support (PS) was set to ensure a maximal tidal volume distal to the mask (VT(distal)) but avoiding failure to cycle to exhalation.

MEASUREMENTS

Airway pressure (P(aw)), P(mask-fit), mask occlusion pressure (P(mask-occl)=P(mask-fit)-P(aw)), VT proximal (VT(prox)), distal to the mask (VT(distal)), air leak volume ( Leak=VT(prox)-VT(distal)), and inspiratory air leak flow rate (difference between inspiratory flow proximal and distal to the mask) were recorded.

RESULTS

PS 15 cmH(2)O was the highest level that could be used without failure to cycle to exhalation (VT(distal) of 585+/-4 ml, leak of 32+/-1 ml or 5.2+/-0.2% of VT(prox), and a minimum P(mask-occl) of 1.7+/-0.1 cmH(2)O). During PS 16 cmH(2)O the P(mask-occl) dropped to 1.1+/-0.1 cmH(2)O, and at this point all flow delivered by the ventilator leaked around the mask, preventing the inspiratory flow delivered by the ventilator from reaching the expiratory trigger threshold.

CONCLUSION

P(mask-fit) and P(mask-occl) can be easily measured in pneumatic cushioned masks and the data obtained may be useful to guide mask fit and inspiratory pressure set during noninvasive positive pressure ventilation.

Hypomagnesemia in critically ill cancer patients: a prospective study of predictive factors

Hypomagnesemia is the most common electrolyte disturbance seen upon admission to the intensive care unit (ICU). Reliable predictors of its occurrence are not described. The objective of this prospective study was to determine factors predictive of hypomagnesemia upon admission to the ICU. In a single tertiary cancer center, 226 patients with different diagnoses upon entering were studied. Hypomagnesemia was defined by serum levels <1.5 mg/dl. Demographic data, type of cancer, cause of admission, previous history of arrhythmia, cardiovascular disease, renal failure, drug administration (particularly diuretics, antiarrhythmics, chemotherapy and platinum compounds), previous nutrition intake and presence of hypovolemia were recorded for each patient. Blood was collected for determination of serum magnesium, potassium, sodium, calcium, phosphorus, blood urea nitrogen and creatinine levels. Upon admission, 103 (45.6%) patients had hypomagnesemia and 123 (54.4%) had normomagnesemia. A normal dietary habit prior to ICU admission was associated with normal Mg levels (P = 0.007) and higher average levels of serum Mg (P = 0.002). Postoperative patients (N = 182) had lower levels of serum Mg (0.60 +/- 0.14 mmol/l compared with 0.66 +/- 0.17 mmol/l, P = 0.006). A stepwise multiple linear regression disclosed that only normal dietary habits (OR = 0.45; CI = 0.26-0.79) and the fact of being a postoperative patient (OR = 2.42; CI = 1. 17-4.98) were significantly correlated with serum Mg levels (overall model probability = 0.001). These findings should be used to identify patients at risk for such disturbance, even in other critically ill populations.