To the Editor:-A prolonged downstroke on the capnograph during mechanical ventilation may be attributed to malfunction of the inspiratory valve, slow ventilation, or a leak via chest tube. [1,2]We report an unusual cause of a prolonged downstroke.
A 22-yr-old, 65-kg man underwent elective discectomy of L4/5 for a prolapsed intervertebral disc. The anesthesia machine (Narkomed 4, North American Drager, Telford, PA) that incorporated a circle carbon dioxide absorber system was checked preoperatively. No leaks were detected. The patient was intubated orally with a size 8.0 cuffed Ring-Adair-Elwyn tube (Mallinkrodt, Athlone, Ireland). The endotracheal tube was connected to a straight-expired gas sampling adapter (Straight T adapter, Datex Instrumentarium, Helsinki, Finland), and this was, in turn, connected to the circle system. The carbon dioxide sampling line attached to the adapter was connected to a built-in infrared side-stream analyzer with a sampling rate of 200 ml/min.
Mechanical ventilation was commenced with a fresh gas flow of 1.5 1/min (900 ml nitrous oxide, 600 ml oxygen, isoflurane, 1%), a tidal volume of 600 ml/min, a rate of 10 breaths/min, and an inspired-to-expired ratio of 1:3. This yielded a peak airway pressure of 22 cm water and an end-tidal pressure of carbon dioxide of 38 mmHg. However, the capnograph trace showed a prolonged downstroke corresponding to a shortened phase III. A check of the circuit revealed a soft hissing sound during inspiration. A crack was then noted on the inner sleeve of the straight adapter (Figure 1). This adapter was used previously and may have cracked as a result of its reuse. Replacement of the straight adapter corrected the abnormal capnograph trace (Figure 2) but had little effect on the other parameters (tidal volume, 610 ml; peak pressure, 23 cm water: end-tidal pressure of carbon dioxide, unchanged).
The patient-end of the straight adapter can be used either as a male port for attachment of a face mask or as a female port for attachment of a endotracheal tube. To facilitate this, the straight adapter was designed with 15-mm inner sleeves and 22-mm outer sleeves, without obliterating the space in between. If there is a fracture of the inner sleeve, as in our case, entrainment of air into the circuit or leak of gases from the circuit may occur through the space between the sleeves. The dilution of expired carbon dioxide with entrained air produced the shortened phase III on the capnograph. Entrainment of air resulted from the continuous aspiration of the side-stream analyzer, because the use of a mainstream carbon dioxide analyzer does not reproduce the trace. This dilution, together with superimposed small tidal volumes from cardiac activity, also produced cardiogenic oscillations. These oscillations were readily observed because of the use of a low respiratory rate and an inspired-to-expired ratio of 1:3. In addition, a small leak occurred when peak inspiratory pressure was reached, producing the hissing sound.
This leak would not be detected with established anesthetic circuit checkout because both sleeves of the adapter would be occluded when the patient port is occluded, either by hand or with a test lung. A breakage of the inner sleeve in a similarly designed Y-connector has been reported. A careful visual inspection of adapters of this design should therefore be incorporated into routine preanesthetic checkout.
Lian Kah Ti, M.Med.
Sasanka Sekhar Dhara, F.F.A.R.C.S.(I)., F.A.N.Z.C.A.
Associate Professor; Department of Anaesthesia; National University Hospital; Singapore; email@example.com