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Intra-anesthesia hypoxia may be due to a problem with the anesthesia delivery system or a problem within the patient. The following are possible causes of hypoxia. A low supply of oxygen may be due to an inadequate FiO 2 mixture. If the proportioning system fails to allow a proper ratio of N 2 O to O 2 (3: 1) then a hypoxic mixture will be delivered.
A bobbin in the flowmeter that is stuck can fool the anesthetist into believing that more O 2 is being delivered than in actuality. A depleted O 2 supply will also cause hypoxia, as a decreased volume of O 2 will be administered to the patient. If the pipeline valves or cylinder valves are closed then no O 2 reaches the breathing circuit. The hospital should have an O 2 backup supply on facility grounds for emergencies. It is important to note that cylinders are full and that hoses for pipelines are hooked up correctly and function prior to anesthetizing a patient. Pipeline pressures should read a minimum of 50 psig and O 2 cylinders a minimum of 600 psig to assure adequate supply.
Leaks can occur inside the machine, the CO 2 absorber, at hose connections, in the bellows (especially hanging bellows), in the ventilator, flowmeter, and proportioned. (4) Broken valves allow leakage within the circuit. Tests for high and low pressure leaks should be performed prior to use of the machine. Do not use hoses that are damaged or frayed and have a potential for leakage. Incompetent unidirectional valves may cause re breathing of gases and a hypoxic mixture. (3) Leaks can be detected by low expired volume, increase in ETCO 2, or inadequate bellow expansion after expiration. Ventilator alarms for low airway pressures indicate a leak in the ventilator.
Applying soapy water on connectors will result in bubbling if a leak is present. (1) Obstruction of flow into the circuit will prevent O 2 from reaching the patient. This can occur as a result of sticking flow control valves, kinking of tubes, or foreign bodies in the system. Be sure to inspect all tubes and valves prior to use. When a cylinder or pipeline is hooked up incorrectly the DISS and PISS systems have been violated. Make sure that O 2 connections are green, air is yellow, and N 2 O blue.
Check to be sure that each cylinder has only 1 gasket at the connection. Closed circuits (Maple sons) give rise to the risk of re breathing when inadequate gas flow is supplied. Be sure to identify the amount of FGF needed for each Mapleson system in order to prevent hypoxic re breathing. A scavenger that is hooked up to high suction will pull FGF in and bypass the patient if the negative pressure relief valve is dysfunctional and cannot entrain air. A negative pressure alarm will alert the anesthetist to this problem. The collapsing reservoir bag will be the first indication that negative pressure is in the system.
Any catheter that is inadvertently passed into the trachea with suction will steal O 2 supply from the lungs. (1) Be sure to check NG placement after insertion by listening to air being injected into the stomach via the NG. An improperly placed ETT results in right mainstem bronchus or esophageal intubation. Verify ETT placement by chest movement, ETCO 2 response, and bilateral breath sounds auscultated post intubation. It is a good standard of practice to pre oxygenate the patient with O 2 prior to intubation to store O 2 in the FRC for physiologic backup. Reposition the ETT as indicated if improper placement is suspected. Inadequate O 2 supply can be prevented by performing a machine checkout of the anesthesia equipment each day.
Problems occurring intra operatively should be detected by troubleshooting the equipment for disconnects or leaks. Temporary increase of FiO 2 can be of assistance until the problem is found. O 2 analyzers on the inspiratory limb should ensure adequate O 2 delivery to the patient. The analyzer should alert the anesthetist when O 2 delivery is less than 21 % but may be as high as 100 %.
Anesthetic drugs and diseases that depress medullary function can cause deep sedation with low respiratory volume exchanges. Inadequate MV or apnea occurs frequently with use of anesthesia drugs. (4) MV for each patient should be calculated according to IBW prior to putting the patient to sleep to ensure adequate ventilation and perfusion. Extended intubation time sometimes requires intermittent ventilation and O 2 via the ambu bag to prevent hypoxia between attempts. Patients who have been over sedated can be reversed with anti-narcotic drugs such as Norman. Be sure to set proper TV and RR to meet each patients required MV. Watch the bellows to see that the volume being delivered is the volume you have set.
If the ventilator is not delivering the set MV, ambu the patient until the problem with the machine has been fixed. Watch the patients chest rise and fall appropriately. Note any pathologic changes in patients history that may affect MV status. Airway resistance prevents delivery of O 2 to the lungs. First be sure that the ETT is correctly placed. Monitor for laryngospasm for broncho spasm by auscultation.
If present, administer positive airway pressure by ambu or PEEP. (4) Assess for pneumothorax or CHF. Notify the surgeon for possibility of CXR/chest tube if indicated. Administer diuretics as needed for pulmonary edema in CHF. Increasing amounts of secretions in the lungs and airway also increase resistance.
Suction as needed to clear the airway. A lightly anesthetized patient may breathe against the ventilator. Administer more anesthetic if indicated. Check the inspiratory valve for condensation and stickiness. Is the ceramic valve moving to allow inward flow? Clean and dry the valve or replace a broken ceramic valve as needed. (3) Be sure that the APL valve is not fully open during ventilator mode.
Any delivery of gases to the patient including O 2 will escape the open valve. The knob should be partially closed and adjusted to control ventilatory pressure. (3) If expired gases are inadvertently rebreather then hypoxia can occur as well as hypercapnia. Incorrect placement of the PEEP valve on the inspiratory limb will not only result in failure of end expiratory pressure, but inspiration may be obstructed or diminished by the valve. Be sure that the PEEP valve is only placed on the inspiratory limb for proper inspiratory air flow. (3) Pathologic changes in patients respiratory systems can contribute to the prevention of O 2 delivery at the alveolar membrane. Some of these causes are sepsis, liver failure, AV malformation, pulmonary emboli, and right to left shunting.
Increasing the amount of FiO 2 delivery does not generally help the problem. The anesthetist must treat the underlying pathology in order to improve oxygenation. (4) When ventilation and perfusion ratios are not in balance, O 2 can not be delivered to the tissues at an adequate rate. Some causes of V/Q mismatch are atelectasis, positioning (especially lateral de cubitus), bronchial intubation, broncho spasm, pneumonia, plugs, ARDS, airway obstruction, and decreased FRC. One lung ventilation can cause a mismatch if the DLT is malposition. Most V/Q mismatches will improve with increase in mean airway pressure or PEEP. Treatment of underlying problems is also very important. (4) Anemia's including carboxyhemoglobinemia and methemoglobinemia prevent O 2 delivery to the tissues.
Low Hub impedes carrying of O 2 while Met and Hgb carry O 2 but do not release it to the tissues. CO poisoning increases the amount of carboxy hemoglobin in the blood therefore any re breathing potentiates the problem. Some anemia's can be detected by pulse oximetry. Severe anemia's from blood loss should be treated by blood replacement. (4) A patient with severe vasoconstriction as is shock cannot get adequate perfusion of vital organs. If perfusion is decreased then the O 2 is not being delivered to the organ.
An algorithm for severe vasoconstriction in shock should be followed. (4) 8) Left shift of oxy hemoglobin curve. When the oxy hemoglobin curve shifts to the left there is a decreased affinity of O 2 for the Hub and less O 2 being carried to the tissues. Some causes of the shift are hypothermia, alkalosis, and hypocarbia. Prevention by adequate warming intra operatively, adequate MV's and control of blood pH can deter the shift.
Shivering from hypothermia will increase O 2 consumption which can be monitored by comparing O 2 analyzer and ETO 2 readings. (4) When N 2 O is suddenly discontinued there is a rapid diffusion of N back into the alveoli displacing O 2. Pre-treatment with 100 % O 2 during emergence can help curve the detrimental effects of diffusion hypoxia. A decrease in SaO 2 should be minimal and short-lived to prevent hypoxic events. (3) Always check the surgical field for events contributing to hypoxia. Severe blood loss, retractors, and positioning can cause rapid and negative effects on O 2 delivery. Keep the surgical team informed of patients O 2 status as needed. (3) The O 2 analyzer and pulse odometer are the main monitors to assess O 2 delivery to the patient.
The pulse odometer reflects arrival of O 2 in the patients tissue and the O 2 analyzer ensures delivery of O 2 from the machine to the patient. Both monitors have audiovisual alarms for low oxygen levels that need immediate attention by the anesthetist. Bibliography:
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