Measurement of tracheal temperature is not a reliable index of total respiratory heat loss in mechanically ventilated patients

Abstract

Minimizing total respiratory heat loss is an important goal during mechanical ventilation. The aim of the present study was to evaluate whether changes in tracheal temperature (a clinical parameter that is easy to measure) are reliable indices of total respiratory heat loss in mechanically ventilated patients. Total respiratory heat loss was measured, with three different methods of inspired gas conditioning, in 10 sedated patients. The study was randomized and of a crossover design. Each patient was ventilated for three consecutive 24-h periods with a heated humidifier (HH), a hydrophobic heat-moisture exchanger (HME) and a hygroscopic HME. Total respiratory heat loss and tracheal temperature were simultaneously obtained in each patient. Measurements were obtained during each 24-h study period after 45 min, and 6 and 24 h. < 0.01). Simultaneous measurements of maximal tracheal temperatures revealed no significant differences between the HH (35.7-35.9°C) and either HME (hydrophobic 35.3-35.4°C, hygroscopic 36.2-36.3°C). In intensive care unit (ICU) mechanically ventilated patients, total respiratory heat loss was twice as much with either hydrophobic or hydroscopic HME than with the HH. This suggests that a much greater amount of heat was extracted from the respiratory tract by the HMEs than by the HH. Tracheal temperature, although simple to measure in ICU patients, does not appear to be a reliable estimate of total respiratory heat loss.

Background:

Minimizing total respiratory heat loss is an important goal during mechanical ventilation. The aim of the present study was to evaluate whether changes in tracheal temperature (a clinical parameter that is easy to measure) are reliable indices of total respiratory heat loss in mechanically ventilated patients.

Method:

Total respiratory heat loss was measured, with three different methods of inspired gas conditioning, in 10 sedated patients. The study was randomized and of a crossover design. Each patient was ventilated for three consecutive 24-h periods with a heated humidifier (HH), a hydrophobic heat-moisture exchanger (HME) and a hygroscopic HME. Total respiratory heat loss and tracheal temperature were simultaneously obtained in each patient. Measurements were obtained during each 24-h study period after 45 min, and 6 and 24 h.

Results:

< 0.01). Simultaneous measurements of maximal tracheal temperatures revealed no significant differences between the HH (35.7-35.9°C) and either HME (hydrophobic 35.3-35.4°C, hygroscopic 36.2-36.3°C).

Conclusion:

In intensive care unit (ICU) mechanically ventilated patients, total respiratory heat loss was twice as much with either hydrophobic or hydroscopic HME than with the HH. This suggests that a much greater amount of heat was extracted from the respiratory tract by the HMEs than by the HH. Tracheal temperature, although simple to measure in ICU patients, does not appear to be a reliable estimate of total respiratory heat loss.

Introduction

]. In addition, medical gases are dried to avoid condensation damage to valves and regulators in the distribution network.

].

].

We hypothesized that tracheal temperature, which is easily measured in mechanically ventilated patients, could be a simple and reliable index of total respiratory heat loss. We therefore prospectively studied a cohort of ICU patients to determine whether changes in total respiratory heat loss could be evaluated from concomitant changes in tracheal temperature.

Patients and methods

Ten patients were included in a prospective, controlled, unblinded study. The study was randomized and of a crossover design. Each patient was ventilated for three consecutive 24-h periods. With Institutional Review Board approval and informed written consent from the patients' families, we studied tracheally intubated, mechanically ventilated patients, who were sedated with sufentanil. The patients were stable following the initial management. They needed mechanical ventilation for acute respiratory failure after neurological crisis or multiple trauma including head trauma. The ventilatory circuit consisted of inspiratory and expiratory lines connected by a Y-piece. The ventilator used was an Engström Erika (Engström Medical, Sweden). Respiratory rate, tidal volume, fractional inspired oxygen and positive end-expiratory pressure were adjusted to maintain arterial partial oxygen tension above 80 mmHg and arterial partial carbon dioxide tension between 35 and 40 mmHg, and these parameters were not modified during the study period. Ventilators used dry medical gases delivered by the hospital distribution network.

The design of the study was as follows. Changes in respiratory heat exchanges were induced by using three different methods of gas conditioning. In a randomized order and in a crossover manner, patients were ventilated for 24-h periods with a HH (Bennett Cascade 2; Nellcor-Puritan Bennett, CA, USA) and one HME (hydrophobic HME [Pall BB100; Pall Europe Limited, UK] or hygroscopic HME [Bact-HME; Pharma System AB, Sweden]).

Total respiratory heat exchanges of ventilated gases were computed by summing the algebraic values of the convective or sensible heat exchanges (Wcv) and the evaporative, latent, or insensible heat exchange (WEV):

) (1)

) (2)

].

Tracheal temperature was measured from a thermal probe placed 3-5 cm above the carina and displayed on a chart recorder (Yokogawa T, 4153 HA 323928, Tokyo, Japan) Maximum and minimum temperatures were obtained during a given respiratory cycle. Maximum tracheal temperature was the highest temperature measured during expiration, and the minimum temperature was the lowest temperature measured during inspiration.

). In the inspiratory limb of the device, two thermal probes -one wet and one dry - were inserted. The psychrometric method is based on comparing the temperatures obtained with the two thermal probes. The upstream probe measures the actual gas temperature. The downstream probe is coated by sterile cotton that is wet with sterile water. Evaporation in the inspiratory limb is directly proportional to the dryness of the inspired gas. The temperature gradient measured between the two probes increases when inspired gas humidity decreases. When the inspired gas is fully saturated with water (100% RH), no thermal gradient is measured. No probe was inserted in the expiratory line. Both the inspiratory and the expiratory lines were equipped with two directional valves to avoid inadvertent mixing of inspiratory and expiratory gases. The internal volume of the device was 75 ml (30 ml for the inspiratory line and 45 ml for the expiratory line). Temperatures recorded by the two probes were measured and displayed on the chart recorder (Yokogawa T, 4153 HA 323928).

With the use of a psoriometric diagram, RH was obtained. Then, AH at saturation point was obtained using the following formula:

(3)

O/l) = absolute humidity at saturation point (100% of RH), and T (°C) = dry probe temperature.

AHs was used to calculate AH from the following formula:

O/l) (4)

RH, AH, average gas temperature in the inspiratory limb, maximum and minimum tracheal temperatures, and total respiratory heat loss were obtained for each device after a 45-min period for optimal stabilization. Then, the same parameters were collected after 24 h of ventilation with the HH, and at 6 and 24 h with the tested HMEs.

< 0.05 was considered statistically significant.

Results

O) and fractional inspired oxygen was 0.50 ± 0.2 (0.3-0.7).

< 0.01).

shows the evolution of tracheal (maximum and minimum), room and body temperatures. No significant differences were observed in either maximal or minimal tracheal temperatures with any of the three devices tested. Thus, total respiratory heat loss could not be predicted from measurements of tracheal temperature. Tracheal temperatures were well maintained with each device over the 24-h study periods. Room and body temperatures did not show any significant differences over the study periods.

< 0.001). The two HMEs achieved the same levels of RH, and no significant changes were observed with time. With regard to AH, the hygroscopic HME achieved a significantly better performance than the hydrophobic HME during all of the three periods. No significant modifications in AH were observed after 24 h of use with either HME.

This prospective, randomized, controlled study clearly shows some variations in the ability of the HH and the two tested HMEs to preserve the heat of the ventilatory gases when used in mechanically ventilated patients.

Discussion

].

O/l). In addition to achieving adequate levels of RH, AH and tracheal temperature, adequate gas conditioning should also be aimed at minimizing total respiratory heat loss. The optimal level of total respiratory heat loss is not known in ICU patients, but it should be kept as low as possible.

The excessive loss of moisture and heat that may occur during mechanical ventilation predisposes patients to serious airways damage. Ventilation with dry and cold gases is complicated by epithelial cell disorders, increased mucus viscosity and restriction of the mucociliary function, the clinical consequences of which are hypothermia, atelectasis and hypoxaemia. On the other hand, over-humidification or ventilation with hyperthermic gases may lead to tracheal burning, alteration in surfactant and epithelial cell disorders, the clinical consequences of which are hyperthermia, hyponatraemia, atelectasis and hypoxaemia. Thus, it is of crucial importance to monitor ventilatory gas conditioning closely, especially in ICU patients subjected to prolonged mechanical ventilation.

] that showed better efficacy of HHs over HMEs.

With regard to total respiratory heat loss, major differences were observed with the three methods of gas conditioning tested. Total respiratory heat loss was significantly less with the HH. Twice as much heat loss was measured with either HME than with the HH.

]. However, tracheal temperature, either maximum or minimum, was not modified over time with any of the devices tested in the present study, and values obtained in these patients were similar regardless of inspired gas conditioning system. Thus, changes in total respiratory heat loss were not accompanied by similar changes in tracheal temperature, and could not be predicted from the measurements of tracheal temperature, either maximum or minimum.

). Also, no significant differences were observed when maximum tracheal temperatures were compared after 24 h of ventilation with each ventilatory gas-conditioning system. The same lack of difference was observed with regard to minimal tracheal temperatures when they were compared.

The specific heat of gases is relatively low, whereas the latent heat of water is much higher. Heat would therefore tend to be lost when respiratory gases have a low absolute humidity. Because the absolute humidity with a hot water humidifier is considerably higher than with a HME, respiratory heat loss is also higher. The upper airway works as a counter-current exchange mechanism, so that at any point the temperature will vary depending on external and internal conditions. The complexity of this relationship is not fully understood, it may be that humidity rather than temperature is the driving force.

We do not believe that the present results were influenced by technical factors. Because the response time of the probes was relatively short, we correctly estimated the true breath-by-breath changes in temperature that occurred. Also, the study patients were randomly exposed to the same three ventilatory gas-conditioning systems, minimizing the risk of possible technical problems. One explanation for the present results could be that, in the case of inadequate ventilatory gas conditioning, heat was extracted from the whole respiratory tract by the inspired air-stream; heat loss would therefore have been spread over a wide exchange surface, leading to minimal or even nonmeasurable changes at the tracheal level.

Based on the findings of the present study in ICU patients, we conclude the following. First, total respiratory heat loss was significantly higher with either tested HME than with the HH, suggesting that a greater amount of heat was extracted from the respiratory tract during inhalation phase with the two tested HMEs than with the HH. Second, despite significant differences in total respiratory heat loss, no concomitant changes in tracheal temperature were observed and no significant differences were observed in the values of tracheal temperatures obtained with the three devices. Thus, tracheal temperature, although easy to measure, is not a reliable index of total respiratory heat loss.

Figures and Tables

The ventilatory circuit consisted of inspiratory and expiratory lines. Measurements were performed using two ceramic electrodes, and inspiratory and expiratory flows were separated by four one-way valves. Patients were consecutively ventilated with the HH and the HMEs.

Clinical characteristics of the study patients

Values are expressed as mean ± standard deviation.

Total respiratory heat exchanges

< 0.01 versus hydrophobic and hygroscopic HME.

Maximum and minimum tracheal temperatures

Room temperature was measured at the patients' bedsides. Body temperature was obtained in the oesophagus. Values are expressed as mean ± standard deviation (range).

Inspired relative and absolute humidity

< 0.001 versus hydrophobic HME.

Keywords

• humidification of inspired gases
• mechanical ventilation
• total respiratory heat loss
• tracheal temperature