Anesth Analg 2002;94:1674-1675
© 2002 International Anesthesia Research Society
LETTERS TO THE EDITOR
Subambient Gas Delivery
Stephen Stayer, MD,
Jill Gouvion, RRT,
Lee Evey, BS RRT, and
Dean Andropoulos, MD
Texas Children’s Hospital, Houston, TX
To the Editor:
When providing positive pressure ventilation to children with unrepaired hypoplastic left heart syndrome (HLHS) or other single-ventricle lesions, it is sometimes desirable to deliver a subambient oxygen concentration (FIO2 0.18). Children with HLHS have mitral atresia, aortic atresia, and a hypoplastic left ventricle. Systemic venous blood returns to the right atrium, and pulmonary venous blood returns to the left atrium. Because of mitral atresia, blood in the left atrium will cross to the right atrium, mixing with the systemic venous blood and passing through the tricuspid valve to be ejected by the right ventricle into the pulmonary artery. This mixture of oxygenated and deoxygenated blood will flow into the pulmonary arteries to the lungs or to the systemic circulation via a patent ductus arteriosus. The ratio of blood passing to the lungs versus the systemic circulation (Qp:Qs) is proportionate to the pulmonary and systemic vascular resistances. At birth the pulmonary vascular resistance is elevated and rapidly decreases over the first days of life. As the pulmonary vascular resistance decreases, children with HLHS have increasing pulmonary blood flow. Even though oxygen saturation increases, systemic perfusion becomes compromised because most of the cardiac output is ejected into the lungs. Alterations in ventilation are used as first-line therapy to maintain pulmonary vascular resistance. These infants are usually sedated, paralyzed, and mechanically ventilated to induce hypercarbia and maintain an elevated pulmonary vascular resistance (1). If this is ineffective, a subambient gas mixture is used, also with the effort to reduce pulmonary blood flow (2,3). Methods to deliver a subambient gas mixture have not been previously described.
We find that infants who are managed with a subambient gas mixture before their cardiac surgical repair frequently experience compromised systemic perfusion when the FIO2 is increased to 0.21, despite maintaining hypercarbia. We developed a method of hypoxic gas delivery that allows us to deliver FIO2 between 0.18 and 1.0. This method of blending extra nitrogen into the breathing circuit can be accomplished by two methods, depending on the type of anesthesia machine utilized. The first method is used for an anesthesia machine that uses a blender to adjust the FIO2, such as the Siemens 900C (Siemens-Elema AB, Sweden) (Fig. 1). Blender 1 is connected to oxygen and nitrogen from the wall source. The oxygen hose is connected in the routine fashion, and the nitrogen hose has a modified connection to allow it to be connected where the air hose would routinely be used. The output from blender 1 is connected to the oxygen inlet of blender 2 (on the anesthesia machine). The air hose is connected in routine fashion to blender 2. With this setup blender 1 functions as a switch; when set to deliver FIO2 of 1.0, the output is an FIO2 of 1.0. However, when set to deliver FIO2 of 0.21, the output is 100% nitrogen. When a subambient gas delivery is desired, blender 1 is set to deliver 100% nitrogen. Blender 2 will progressively deliver lower oxygen concentrations, as the knob is turned to a higher FIO2. For example, when set to deliver 32% oxygen, the ventilator output is approximately 18% oxygen.
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Figure 1. First blending method. 1) Blender 1 is used as a "switch" set at either 21 or 100 percent oxygen. 2) When Blender 1 is set to 21%, the output is actually 100% nitrogen. 3) When Blender 2 receives 100% nitrogen from Blender 1, as the FIO2 knob is rotated toward a higher oxygen concentration, the ventilator output will deliver a lower FIO2. To deliver 18 oxygen, the knob should be set at 32%. 4) The auxiliary FIO2 monitor will accurately display the FIO2 of delivered gas. 5) When Blender 1 is set to deliver 100%, Blender 2 will function as it does normally.
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To prevent an error in gas delivery the staff are educated to the setup, signs are placed above blender 1 and on the anesthesia machine indicating that a hypoxic gas mixture is in use, a cover is placed over the FIO2 knob on blender 2 indicating that the FIO2 progressively goes from 0.21 to 0 as the knob is turned, and a second oxygen sensor is placed in the anesthesia circuit with the limit set to alarm at FIO2 below 0.18.
The second method is designed to deliver subambient oxygen concentrations via an anesthesia machine with flow meters (Figure 2). An additional blender is connected to air and nitrogen gas sources, and the output of this blender is connected to the air inlet on the anesthesia machine. When this additional blender is set to deliver FIO2 of 1.0, the actual delivery of gas is 100% nitrogen. Therefore, the air flowmeter will deliver added nitrogen to the circuit so the anesthesiologist may adjust oxygen and air flows to deliver the desired oxygen concentrations. Again staff are educated about this setup, warning signs are placed on the anesthesia machine, and the oxygen sensor is set to alarm if the FIO2 is below 0.18.
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Figure 2. Second blending method. 1) The blender is used as a "switch" set at either 21% oxygen or 100% nitrogen. 2) When the blender is set to 100%, the output is actually 100% nitrogen. 3) When air flowmeter (on the anesthesia machine) receives 100% nitrogen from the blender, the anesthesiologist may blend additional nitrogen to decrease the FIO2 to subambient concentrations.
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We have found that proportioning through a blender or delivery through flowmeter designed to deliver air will deliver a gas concentration exactly to what would be calculated. For example, with a total flow of 5 L, settings of 4.1 L/min nitrogen and 0.9 L/min oxygen will deliver an FIO2 of 0.18; setting "Blender 2" on the Servo 900C to 32% oxygen will actually deliver an FIO2 of 0.18. These modifications of gas source violate the standards of the Compressed Gas Association, American Society Testing Materials, and the International Standards Organization; and should only be used when a subambient oxygen concentration is necessary for hemodynamic stability. We have successfully utilized this setup in 19 patients with HLHS before cardiopulmonary bypass and have consistently been able to deliver 18% oxygen without adverse effect.
References
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Jobes DR, Nicolson SC, Steven JM, et al. CO2 prevents pulmonary overcirculation in hypoplastic left heart syndrome. Ann Thorac Surg 1992; 54: 150–1.[Abstract]
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Day RW, Barton AJ, Pysher TJ, Shaddy RE. Pulmonary vascular resistance of children treated with nitrogen during early infancy. Ann Thorac Surg 1998; 65: 1400–4.[Abstract/Free Full Text]
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Ramamoorthy C, Tabbutt S, Kurth CD, et al. Effects of inspired 17% O2 or 3% CO2 on cerebral O2 saturations in neonates with hypoplastic left heart syndrome. Anesth Analg 2000; 90: SCA26.
