Hypercapnia resulting from a faulty co-axial (Bain) circuit.

The Bain co-axial circuit is fully established in general anaesthesia practice. A major concern is the potential malfunctioning of the circuit due to avulsion of the inner fresh gas delivery tube at the machine end of the circuit. The following case report presents a case in which a patient connected to the Bain circuit developed severe hypercapnia in the early intraoperative period due to the above mentioned defect.

INTRODUCTION

The Bain circuit was introduced by Bain and Spoerel in 1972.[1] The co-axial system is fully established in general anaesthesia practice. There are many published reports of malfunction of the inner tube of the Bain co-axial circuit, with potentially lethal complications for the patient.[2] Any defect in the inner tube may result in large apparatus dead space.[34] The following case report describes a case where profound hypercapnia occurred consequent to avulsion of the inner tube of the co-axial circuit at the machine end of the circuit, which was neither apparent nor visible to the anaesthesiologist. This report therefore emphasizes the need for testing the co-axial circuit for any circuit malfunction before each use.

CASE REPORT

A 30 year-old male presented as an emergency for repair of a crush injury of the left hand. Induction was done with thiopentone sodium 5 mg/kg and fentanyl citrate 1.5 mg/kg. After achieving muscle relaxation with succinylcholine 1 mg/kg, the patient was intubated with a 7.5-mm cuffed Portex® endotracheal tube. Anaesthesia was maintained with oxygen, nitrous oxide, isoflurane, and intermittent doses of vecuronium bromide. The vital signs monitored were blood pressure, oxygen saturation, electrocardiogram, temperature, and end-tidal carbon dioxide (ETCO2). The initial parameters were normal, with an ETCO2 of 38 mmHg. After 5 minutes, ETCO2 increased to 50 mm Hg. The gas flows, airway pressure, temperature, chest expansion and bilateral air entry in to lungs were all checked and found to be normal. We changed the D-Fend of our side-stream capnograph for a new one but the ETCO2 continued to rise. Simultaneously, the inspiratory carbon dioxide baseline shifted upwards, indicating rebreathing. ETCO2 rose to 80 mmHg over the next 20 minutes. The heart rate rose from the baseline of 80 per minute to 110 per minute and blood pressure rose from a baseline of 110/70 mmHg to 140/94 mmHg; however, the oxygen saturation was 100%. Not finding any cause for the rise of ETCO2, we decided to change the circuit. Following this, the ETCO2 curve began to fall and reached the normal value of 40 mmHg over the next couple of minutes and the inspiratory carbon dioxide baseline returned to zero. Simultaneously, the patient's heart rate and blood pressure also started declining.

Close examination of the original circuit revealed that the inner tubing of the co-axial tube had become disconnected from its seat at the machine end of the circuit [Figure 1]. This was a previously unused Bain circuit and we had not checked the integrity of the inner tubing before inducing the patient. The surgery was completed and the patient was extubated with no adverse event.

Figure 1

Diconnected inner tubing of the co-axial circuit

DISCUSSION

The widely used Bain anaesthetic breathing system requires an intact inner tube. The inner coloured tube carries the inspiratory gases. If this tube should become disconnected or develop any breach in its integrity there will be a huge increase in dead space, with consequent hypercapnia and its complications.[56]

Four main causes for development of hypercapnia during anaesthesia have been described:

Reduced alveolar ventilation, with rise in partial pressure of CO2

Inhalation of exhaled CO2 (as has been noted in association with a defective fresh gas flow tube of a Bain circuit)

Inhalation of exogenous CO2

Increased metabolic rate (as in malignant hyperthermia)

Hypercapnia leads to sympathetic system stimulation, with tachycardia, hypertension, arrhythmias, excessive sweating, and peripheral vasodilatation, which may lead to excessive intraoperative blood loss.[7] Our patient had significant increase in heart rate and blood pressure, which was initially erroneously attributed to the light plane of anaesthesia.

A number of tests have been described to assess any co-axial circuit malfunction; these include:

Visual inspection of tubing for any obvious disruption or obstruction.[89]

Pethick test: This tests the low-pressure system and the integrity of the inner tube. The procedure is as follows: Occlude the patient end of the circuit. Close the adjustable pressure limiting valve. Fill the circuit using the oxygen flush valve. Release the occlusion at the patient end and flush. Collapse of the reservoir bag due to the creation of venturi effect in the outer tube is an indication that the inner tube is intact.[8] This test will not detect a system in which inner tube is omitted or does not extend to the patient port or one that has holes at the patient end of the inner tube.

Foex-Crampton-Smith manoeuvre: This manoeuvre assesses the gas flow line from the flowmeters of the machine to the patient end of the circuit. With an oxygen flow at 2 litres/minute, the patient end of the inner tube is occluded briefly for 2–3 seconds using the forefinger.[10] A positive test is indicated by descent of the rotameter bobbin due to back pressure; with removal of finger, the bobbin ascends to its original position. Ghani suggested the use of the plunger of a 3-ml syringe to occlude the inner tube more precisely.[9]

CONCLUSION

This case report highlights the possibility of severe hypercapnia due to dead-space rebreathing as a result of disconnection of the inner tube of the co-axial circuit. We present this as a warning and reminder that the integrity of the co-axial circuit must be always checked visually as well as mechanically. In this regard, the Foex-Crampton-Smith manoeuvre with the Ghani modification appears to be most satisfactory method of assessing the integrity of the gas line from the flowmeter up to the patient end of the Bain circuit.[9]