If tracheal extubation of a patient with a known difficult airway is followed by respiratory distress, then tracheal reintubation and ventilation may be difficult or impossible . Tracheal extubation over a long, hollow, small internal diameter (ID), semirigid catheter (hereafter referred to as a jet stylet [JS]) greatly reduces the risk of inadequate ventilation and failed reintubation in such a circumstance. A JS is inserted into an in-situ endotracheal tube (ETT) prior to extubation. After the ETT is withdrawn over the JS, the small ID catheter may then be used as a means of postextubation ventilation (by intermittent jetting of a high-pressure oxygen source through the hollow catheter) and/or as an intratracheal stylet/guide for reintubation .
If reintubation is considered necessary, then the new ETT is inserted over the JS. Determination of proper intratracheal location of the new ETT is essential prior to removal of the JS. To determine whether the new ETT is correctly positioned, the new ETT should be connected to the breathing circuit by an elbow adaptor that has a self-sealing diaphragm (i.e., the bronchoscopy port of a bronchoscopy elbow adapator). The JS should exit the new ETT through the self-sealing diaphgragm in the elbow connector, thus theoretically allowing ventilation and CO2 detection through the annular space between the JS and ETT. The purpose of this study was to determine the functional size equivalent of the annular space between the JS and ETT for all combinations of variously sized JSs and ETTs and to determine whether this annular space will permit detection of exhaled CO2 within a clinically acceptable period of time.
Our experiment consisted of two parts. One model measured the airflow resistance of variously sized test catheters alone and for the annular space between variously sized ETTs containing variously sized JSs (ETT/JS). The other model investigated CO2 detection in the gas coming from the annular space between the JS and ETT for all possible JS/ETT combinations. A medium JS does not fit into a 4.0 ETT and a large JS does not fit into a 5.0 ETT; therefore these ETT/JS combinations were not studied.
To obtain the pressure versus annular space flow measurements, a flowmeter (supplied by a wall air source) was connected to the proximal end of a test ETT or ETT/JS by a bronchoscopy elbow connector (Instrument Industries, Bethal Park, PA) Figure 1. The jet stylet was sealed into the bronchoscopy port of the elbow connector. A mercury manometer was interposed between the flowmeter and the ETT or ETT/JS.
The CO2 detection experimental model consisted of a compliant 3.0-L bag that was sequentially attached to a tank of 5.0% [CO2] with 95.0% [O2], a pressure transducer, and an electronically triggered solenoid valve (ASCA, Florham Park, NJ) Figure 2. The other end of the solenoid valve was attached to an ETT by way of a 15-mm male connector glued to the distal external end of the ETT. In the ETT/JS combinations, the JS exited the proximal end of the ETT through the bronchoscopy port of the elbow connector and was sealed into place as above. The proximal end of the elbow connector was connected to a capnograph by a T-piece that was left open. The pressure transducer, the electronic valve, and the capnograph were connected to a thermal recorder to simultaneously record the pressure within the bag, valve opening, and CO2 detection.
The control data for the pressure versus annular space flow measurements was obtained using intravenous catheter sizes 14-, 16-, and 18-gauge and ETT sizes 2.5, 3.0, 3.5, 4.0, 5.0, 6.0, 7.0, 8.0, and 9.0 mm ID (hereafter referred to as 2.5 ETT, 3.0 ETT, etc.). Pressure versus flow data were also gathered for all the combinations of small, medium, and large JS (hereafter referred to as JSS, JSM, and JSL) within the 4.0-9.0 ETTs. The 4.0 ETT cannot accommodate the JSM or JSL and the 5.0 ETT cannot accommodate the JSL; therefore, these combinations are physically impossible. The pressure at the proximal end of the ETT and ETT/JS was obtained at constant flow rates of 2.0, 3.0, 4.0, and 5.0 L/min and at intervals of 5.0 L/min thereafter up to 70 L/min. All pressure versus flow measurements were at steady state.
For the CO2 detection measurements, the solenoid valve was turned off (closed) and the reservoir bag was filled from the tank of 5% [CO2]/95% [O2] until the bag reached one of three test pressures: 5.0, 7.5, and 10.0 mm Hg. The 4.0-9.0 ETT, the 4.0-9.0 ETT/JSS,M,L combinations, and the ETT attachment end of the solenoid valve were rinsed with CO2-free air from a pressurized source prior to each measurement. The distal end of the ETT was attached to the solenoid valve and the proximal end of the ETT was attached to the capnograph. The thermal strip chart recorder was then turned on at a speed of 10.0 cm/s. The solenoid valve was then electronically opened thereby releasing the CO2 toward the ETT-jet stylet combination and capnograph. Once the [CO2] detected by the capnograph equaled 5%, the solenoid valve closed electronically.
Each CO2 detection measurement was completed three times and the average of the three measurements was calculated. For each capnograph tracing, the time to first detection of CO2, the time to 70% maximum (max) [CO2] detection, and the time from first detection to 70% max [CO2] was measured Figure 3; the latter time is the difference between time to 70% max [CO2] and time to first detection of CO2 and is inversely proportional to the rate of increase of CO2. Regression analysis was used to describe the relationship between net annular area (internal area of the ETT minus the external area of the JS) and the various CO2 detection times.
(Figure 4A) shows the relationship between flow rates and pressure observed at the proximal end of the various control (empty) catheters. The curves in Figure 4A are the control data (background dotted line/open circle grid) for Figure 4 B-D. Figure 4 B-D show the pressure flow data for the 4.0-9.0 ETT/JS (S),M,L combinations. The slope of the curves at any point in Figure 4 A-D is equal to the resistance of the conduit; as flow rate increased and/or the size of net conducting area of the conduit decreased (i.e., decreasing size of the ETT and/or increasing size of the JS), resistance increased.
For all driving pressures, as the size of the ETT decreased and the size of the JS increased, the time of first CO2 detection increased (range 1.4-1.8 s), the time to detection of 70% max [CO2] increased (range 1.6-4.1 s), and the time from first detection to 70% max [CO2] increased (ranged 0.3-2.3 s). No CO2 was detected with a medium JS in a 5.0 ETT and with a large JS in a 6.0 ETT, despite a detectable decrease in the initial driving pressure (i.e., the 3.0-L bag emptied a small amount, presumably through the lumen of the JS). Decreasing driving pressure (from 10 to 5 mm Hg) also slightly increased all CO2 detection times for each of the ETT/JS combinations by approximately 0.1-0.2 s for most ETT/JS combinations.
(Figure 5) displays CO2 detection data using the net annular space conducting area (internal area of the ETT minus the external area of the JS) at the three driving pressures. The resultant graphs exhibit a logarithmic relationship between the net conducting area and time to first detection of CO2, time to detection of 70% max [CO2], and time from initial detection to 70% max [CO2], regardless of which ETT or JS was used. The r valve for the curves ranged from 0.797 to 0.902 with an n of 19.
We found that some ETT/JS combinations had markedly increased resistance to air flow and markedly prolonged time for CO2 detection. Before developing a rule of acceptability for clinical use, consideration should be given to possible limitations of our experimental method.
There are five aspects of our experimental model that may have affected the results or impacted on their interpretation. First, it is possible that contaminates, such as blood, pus, secretions, talcum powder, grease, and particulate matter, may be present in the space between the ETT and JS. Contamination of the ETT/JS space would obviously increase the resistance to air flow. There was no such problem in our study. Clinically the possibility of contamination is minimized by using ETTs and JSs that are taken directly from their sterile packaging just before use and not exposed to excessive lubrication and/or powders. Second, the level of quality control of the products may affect the consistency of results expected. The outside diameters (ODs) of the JSs varied slightly from one to another (the ODs of the JS (S), JSM, JSL are close to 3.5, 4.8, and 5.8 mm, respectively) and variation in OD would obviously cause variation in the resistance of the ETT/JS. To minimize this problem in our experimental model, the same stylet was used for each of the measurements. Although the dilemma of unpredictable variations in resistance due to variation in JS size still exists, clinically these variations are likely to be very small and insignificant. Third, in the CO2 detection model there was some slight variation in the speed of valve opening. In theory, the measurements for a specific ETT/JS combination at a given driving pressure should be identical when repeated. Because small variations in these measurements were detected (5%-10% of the mean value and were thought to be due to slight degrees of valve sticking), each measurement was completed three times and the average used for data presentation. Fourth, the driving pressures used in the CO2 detection model (5.0, 7.5, and 10.0 mm Hg), which are similar to the elastic recoil pressures observed at 70%-90% of total lung capacity in normal people aged 35-60 yr , may not reflect those observed in all situations. It is important to note that some of the ETT/JS combinations found to be adequate under normal conditions may not be adequate in situations with decreased elastic recoil pressures (i.e., emphysema and underinflation of the lungs) where the time to first detection of CO2, the time to 70% max [CO2], and the time from first to 70% max [CO2] may be increased. The severity of the decreased elastic recoil pressure should be judged, and adjustments to the ETT/JS combination should be considered (i.e., using a JS one size smaller than the largest size recommended). Fifth, the test catheters and ETTs differed in length between the various sizes which was not accounted for in analyzing the resistance curves or the CO2 detection data. Since resistance is proportional to the radius to the fourth power for laminar tube flow, to the square of the width for annular flow, and to the length to the first power, it is obvious that a change in the radius (or width) affects the resistance to a much greater degree than a difference in tube length.
Our data permit generation of a simple rule of clinical performance acceptability, provided logical cut points can be determined. With respect to air flow, it is very clear that in adults, exhalation time becomes markedly prolonged with an ETT less than 4.0 mm ID , and it would be very difficult or impossible to use such air-flow-dependent clinical signs, such as auscultation of breath sounds and observation of tidal chest movement, with an ETT less than 4.0 mm ID. Therefore, for adults, we have set the equivalent surface area cut point for clinical performance acceptability for ETT/JS combinations at 4.0 mm ID. With respect to CO2 detection, the clinical performance acceptability cut point was set at a maximum time to initial CO2 detection of 3.0 s and time to 70% max [CO2] of 4.0 s. If complete exhalation occurred in 4 s at an inspiratory-expiratory ratio of 1:2, then the respiratory rate would be 10 breaths/min. In view of the fact that complete exhalation might take longer, these cut points minimize the danger of hyperinflation.
Using the above clinical performance acceptability cut points, there are several more ETT/JS combinations that are acceptable using the CO2 detection criteria than using functional size equivalent greater than 4 mm ID criteria Figure 6. Thus, the CO2 detection method is more sensitive than the ventilation signs in determining intratracheal placement of the ETT.
A rule for clinical performance acceptability is as follows: first, designate a numerical value to each JS: JSS = 4, JSM = 5, and JSL = 6. To determine whether an ETT/JS combination is satisfactory, subtract this JS number from the ETT size (i.e., 4.0 ETT = 4, 5.0 ETT = 5, etc.). If the result is 1 or less, the annular space around ETT/JS combination has a functional size less than 4 mm ID and is unacceptable. If the result equals 0, the ETT/JS combination is unacceptable for CO2 detection in less than 3 s (except for 4.0 ETT/JSS). For example, in the 6.0 ETT/JSM, the Equation becomes6 (for the 6.0 ETT) minus 5 (for the JSM) which equals 1. Therefore, this combination is unacceptable for ventilation (1 or less) but is acceptable for CO2 detection (not equal to 0).
It is important to realize that inhalation may be by conventional positive pressure through the JS/ETT annular space or by jet ventilation through the lumen of the JS. If inhalation is by jet ventilation through the lumen of the JS, one must know whether the lungs are exhaling through the annular space. If exhalation does not appear to have occurred, another jet ventilation breath should not be delivered in order to avoid hyperinflation and barotrauma.
Because it is advantageous to use an ETT that fits snugly over the JS to increase the success rate of the new ETT in following the guide into the trachea, the dilemma of inadequate conducting area within the ETT and the JS exists. It is therefore recommended that CO2 detection be the standard procedure for assessing ETT placement, as it requires a conducting area less than 4 mm ID for effectiveness.