LETTERS TO THE EDITOR
Flow Sensitivity
of the Mallinckrodt
Continuous
Positive
Airway
Pressure
Device
To the Editor:
The provision of nondependent lung continuous positive airway pressure (CPAP) is an important therapy for the treatment and prevention of hypoxemia during one-lung anesthesia. Mallinckrodt Anesthesiology (Mallinckrodt Anesthesia Products, St Louis, MO) has recently introduced a CPAP device packaged with their double-lumen endotracheal tubes. This device develops oxygen flow-sensitive CPAP, a feature that may not be obvious to the user. The device is a variable orifice positive end-expiratory pressure (PEEP) valve with an attached 1 L bag, oxygen supply tubing, and a 15 mm connector to one lumen of the double-lumen tube. The use, in this device, of a variable orifice PEEP valve as opposed to a pressure limiting PEEP valve means that alterations in the oxygen flow rate will result in changes in the amount of CPAP provided.’ Analysis of Poiseuille’s equation for laminar flow reveals that the pressure developed across a resistance is proportional to the flow. However, for turbulent flow, the pressure developed is proportional to the square of the flow.’ Therefore, depending on the predominance of laminar and turbulent Aow states across the orifice, the pressure developed by this CPAP device can be described by Pressure = (ki Flow) + (k2 . Flow2), where ki and k2 are constants. This means that an accidental doubling of flow rate would result in a fourfold pressure increase, and the accidental halving of flow rate would decrease pressure fourfold. In anticipation of this, the device states that the flow rate must be set at 5 L/min, and the product information leaflet also states this effect. The product information leaflet further indicates that the delivered pressure may vary +33% to -25% from the dialed value. To determine the flow sensitivity of the Mallinckrodt CPAP device, it was attached to a water manometer and an oxygen source. The oxygen flow was determined by a calibrated rotameter flowmeter accurate to ?2% over the range of 1 to 10 Limin. For each of the available nominal CPAP pressures (1,2,5, 10 cm H20) the oxygen flow rate was varied from 1 to 10 L/min and the pressure measured at the valve. As shown in Fig 1, the pressure developed by the CPAP device is dependent on the oxygen flow through the device. Users of this device must be aware that changes in oxygen flow rate have profound effects on the developed pressure.
40 -
Fig 1. The measured pressure developed by the CPAP device for different oxygen flow rates at each of the nominal device settings. -0-, 10 cm H,O; -_*-, 5 cm H,O; -O-, -U-, 1 cm HzO.
O-
2 cm H,O;
Journalof Cardiothoracic and VascularAnesthesia,
4
6
8
10
Flow rate (L/min)
Vol6, No 6 (December), 1992: pp 771-776
771
LETTERS TO THE EDITOR
772
A better method of providing CPAP to the nondependent lung is to use a CPAP device incorporating PEEP valve. This will result in flow insensitivity of the device with a resultant improvement in safety.
Departments
a pressure-limited
Simon C’.Body, MBChB, FFARACS Theresu Casieti, MS Christina De Mur, MS George P. Topulos, MD of Anesthesia and Biomedical Engineering Brigham and Womens Hospital Harvard Medical School Boston, MA
REFERENCES 1. Topulos GP, Butler JP: Correction of a recurrent error. Anesthesiology 63563.564, 1985
Regional
Analgesia for Carotid Artery Endarterectomy by Winnie’s Single-Injection Technique Using a Nerve Detector
To the Editor: Regional analgesia offers a great advantage for carotid endarterectomy (CEA) as compared to general anesthesia. It enables monitoring of cerebral function of the conscious patient throughout the procedure, I.2 helps to identify the patient who needs an indwelling shunt, decreases the magnitude of operative stress, allows faster recovery, and avoids complications related to general anesthesia. This letter reports the advantage of using a nerve detector for deep cervical plexus block by Winnie’s single-injection technique in patients scheduled for CEA. We have been using regional analgesia for CEA at this institution. Not satisfied with the quality of the block and frequent use of additional doses of sedatives during surgery that led to overmedication. we tried the use of the nerve detector to identify the site of injection of local anesthetic. Nineteen patients were evaluated; 10 patients were given the block with the help of the nerve detector (ND Group), and the other 9 were given the block without use of the nerve detector (non-ND group). All patients had a unilateral CEA performed. Both groups were comparable for age, coronary artery disease, hypertension, diabetes mellitus, and neurologic status. Three patients in each group had a history of transient ischemic attacks and there was one case of hemiparesis in each group. All patients were premeditated with oral lorazepam, 0.04 - 0.06 mgikg, and intramuscular morphine sulfate, 0.1 mgikg. A unilateral deep cervical plexus block was given.” The patient was placed in the dorsal recumbent position with the head turned to the opposite side. The index and middle fingers were placed on the posterior border of the sternocleidomastoid muscle (SCM) at the level of the fourth cervical vertebra and rolled laterally over the anterior border of the scalenus anterior muscle to feel the groove between the scalenus anterior and medius muscles. An l&gauge venflon needle (Viggo. Spectramed, BOC Health Care Company, Helsingborg, Sweden) was then inserted in the medial, caudal, and dorsal directions, perpendicular to the skin to a depth of 2 to 3 cm. Parasthesias were elicited with a Myotest nerve detector (DBS. Biometer International, Odense, Denmark) in the ND group. One of the two electrodes of the nerve detector was placed on the corresponding shoulder and the other connected to the venflon needle through the hub and a stimulus given. Twitches of the neck muscles and parasthesias over the shoulder and upper arm were then elicited. Ten to 1.5 mL of 0.5% bupivicaine was then injected and the cannula withdrawn. In the non-ND group, parasthesias were elicited by the needle itself. A superficial cervical plexus block was performed with 10 mL of the same solution between the skin and muscle along the posterior border of the SCM near its midpoint in both groups. Effect of the block was then confirmed by testing with a needle. Quality of analgesia was assessed by questioning the surgeon, anesthesiologist, and the patient. Local anesthetic was neither injected in the carotid sheath nor in the line of incision in either group. Doses of morphine sulfate and diazepam were used as required. Surgery time averaged 1.5 to 2 hours in both groups. Excellent analgesia resulted in all patients of the ND group. Top-up doses of sedatives and analgesics were not required in any of the ND group. In the non-ND group, however, they were used in 7 of the 9 cases, and nalorphine had to be used in 1 case in the postoperative period. A Javid shunt was used in 1 patient of the ND group following development of slurring of speech during the period of clamping. This recovered on restoration of blood flow. No neurologic complications were observed except for hoarseness in one case in the ND group, which recovered within 72 hours. Hypertension developed in 1 patient in the ND group and in 2 cases of the non-ND group and had to be treated with vasodilators. Local hematoma developed in 1 of the ND group patients who had been on aspirin until the day of surgery.
Flow Sensitivity
of the Mallinckrodt
Continuous
Positive
Airway
Pressure
Device
To the Editor:
The provision of nondependent lung continuous positive airway pressure (CPAP) is an important therapy for the treatment and prevention of hypoxemia during one-lung anesthesia. Mallinckrodt Anesthesiology (Mallinckrodt Anesthesia Products, St Louis, MO) has recently introduced a CPAP device packaged with their double-lumen endotracheal tubes. This device develops oxygen flow-sensitive CPAP, a feature that may not be obvious to the user. The device is a variable orifice positive end-expiratory pressure (PEEP) valve with an attached 1 L bag, oxygen supply tubing, and a 15 mm connector to one lumen of the double-lumen tube. The use, in this device, of a variable orifice PEEP valve as opposed to a pressure limiting PEEP valve means that alterations in the oxygen flow rate will result in changes in the amount of CPAP provided.’ Analysis of Poiseuille’s equation for laminar flow reveals that the pressure developed across a resistance is proportional to the flow. However, for turbulent flow, the pressure developed is proportional to the square of the flow.’ Therefore, depending on the predominance of laminar and turbulent Aow states across the orifice, the pressure developed by this CPAP device can be described by Pressure = (ki Flow) + (k2 . Flow2), where ki and k2 are constants. This means that an accidental doubling of flow rate would result in a fourfold pressure increase, and the accidental halving of flow rate would decrease pressure fourfold. In anticipation of this, the device states that the flow rate must be set at 5 L/min, and the product information leaflet also states this effect. The product information leaflet further indicates that the delivered pressure may vary +33% to -25% from the dialed value. To determine the flow sensitivity of the Mallinckrodt CPAP device, it was attached to a water manometer and an oxygen source. The oxygen flow was determined by a calibrated rotameter flowmeter accurate to ?2% over the range of 1 to 10 Limin. For each of the available nominal CPAP pressures (1,2,5, 10 cm H20) the oxygen flow rate was varied from 1 to 10 L/min and the pressure measured at the valve. As shown in Fig 1, the pressure developed by the CPAP device is dependent on the oxygen flow through the device. Users of this device must be aware that changes in oxygen flow rate have profound effects on the developed pressure.
40 -
Fig 1. The measured pressure developed by the CPAP device for different oxygen flow rates at each of the nominal device settings. -0-, 10 cm H,O; -_*-, 5 cm H,O; -O-, -U-, 1 cm HzO.
O-
2 cm H,O;
Journalof Cardiothoracic and VascularAnesthesia,
4
6
8
10
Flow rate (L/min)
Vol6, No 6 (December), 1992: pp 771-776
771
LETTERS TO THE EDITOR
772
A better method of providing CPAP to the nondependent lung is to use a CPAP device incorporating PEEP valve. This will result in flow insensitivity of the device with a resultant improvement in safety.
Departments
a pressure-limited
Simon C’.Body, MBChB, FFARACS Theresu Casieti, MS Christina De Mur, MS George P. Topulos, MD of Anesthesia and Biomedical Engineering Brigham and Womens Hospital Harvard Medical School Boston, MA
REFERENCES 1. Topulos GP, Butler JP: Correction of a recurrent error. Anesthesiology 63563.564, 1985
Regional
Analgesia for Carotid Artery Endarterectomy by Winnie’s Single-Injection Technique Using a Nerve Detector
To the Editor: Regional analgesia offers a great advantage for carotid endarterectomy (CEA) as compared to general anesthesia. It enables monitoring of cerebral function of the conscious patient throughout the procedure, I.2 helps to identify the patient who needs an indwelling shunt, decreases the magnitude of operative stress, allows faster recovery, and avoids complications related to general anesthesia. This letter reports the advantage of using a nerve detector for deep cervical plexus block by Winnie’s single-injection technique in patients scheduled for CEA. We have been using regional analgesia for CEA at this institution. Not satisfied with the quality of the block and frequent use of additional doses of sedatives during surgery that led to overmedication. we tried the use of the nerve detector to identify the site of injection of local anesthetic. Nineteen patients were evaluated; 10 patients were given the block with the help of the nerve detector (ND Group), and the other 9 were given the block without use of the nerve detector (non-ND group). All patients had a unilateral CEA performed. Both groups were comparable for age, coronary artery disease, hypertension, diabetes mellitus, and neurologic status. Three patients in each group had a history of transient ischemic attacks and there was one case of hemiparesis in each group. All patients were premeditated with oral lorazepam, 0.04 - 0.06 mgikg, and intramuscular morphine sulfate, 0.1 mgikg. A unilateral deep cervical plexus block was given.” The patient was placed in the dorsal recumbent position with the head turned to the opposite side. The index and middle fingers were placed on the posterior border of the sternocleidomastoid muscle (SCM) at the level of the fourth cervical vertebra and rolled laterally over the anterior border of the scalenus anterior muscle to feel the groove between the scalenus anterior and medius muscles. An l&gauge venflon needle (Viggo. Spectramed, BOC Health Care Company, Helsingborg, Sweden) was then inserted in the medial, caudal, and dorsal directions, perpendicular to the skin to a depth of 2 to 3 cm. Parasthesias were elicited with a Myotest nerve detector (DBS. Biometer International, Odense, Denmark) in the ND group. One of the two electrodes of the nerve detector was placed on the corresponding shoulder and the other connected to the venflon needle through the hub and a stimulus given. Twitches of the neck muscles and parasthesias over the shoulder and upper arm were then elicited. Ten to 1.5 mL of 0.5% bupivicaine was then injected and the cannula withdrawn. In the non-ND group, parasthesias were elicited by the needle itself. A superficial cervical plexus block was performed with 10 mL of the same solution between the skin and muscle along the posterior border of the SCM near its midpoint in both groups. Effect of the block was then confirmed by testing with a needle. Quality of analgesia was assessed by questioning the surgeon, anesthesiologist, and the patient. Local anesthetic was neither injected in the carotid sheath nor in the line of incision in either group. Doses of morphine sulfate and diazepam were used as required. Surgery time averaged 1.5 to 2 hours in both groups. Excellent analgesia resulted in all patients of the ND group. Top-up doses of sedatives and analgesics were not required in any of the ND group. In the non-ND group, however, they were used in 7 of the 9 cases, and nalorphine had to be used in 1 case in the postoperative period. A Javid shunt was used in 1 patient of the ND group following development of slurring of speech during the period of clamping. This recovered on restoration of blood flow. No neurologic complications were observed except for hoarseness in one case in the ND group, which recovered within 72 hours. Hypertension developed in 1 patient in the ND group and in 2 cases of the non-ND group and had to be treated with vasodilators. Local hematoma developed in 1 of the ND group patients who had been on aspirin until the day of surgery.
