血液ガスサンプルは患者が、処置や治療による影響を受けている最中に行うと結果解釈に誤りが生じます。処置や治療行為から、一定時間をおいて体内動態や呼吸動態が安定したタイミングで行なったときに初めて患者の血液ガス解釈が可能となります。また、採血時の緊張や興奮も測定値にバイアスが生じますので、できる限り静かな環境で安心させ、安定した状態で行う必要があります。このような安定した状態のことを、Steady Stateと呼んでいます。以下は、様々な論拠を示しながら、人においてSteady Stateの考え方を示したものです。分析前エラーを最小限にするため必要な知識です。
全身状態（Good, Poor, Very Poor)、状態（鎮静/麻酔/覚醒）、酸素投与法（room air , O2 flow-by, O2 face mask）、使用した分析機、採血部位（大腿動脈、足背動脈）、採血側（右、左）、使用したサンプラー（AVL microsampler、PICO70）、サンプラー使用本数（採血難度を示唆する）、使用した穿刺針（24G、25G、26G、27G）、使用した針の数（採血難度を示唆する）、サンプル量（0.15ml、0.20ml、0.30ml、0.50ml）、合併症（皮内出血、血腫など）、最終診断（後日記載）、検査の理由（初診、再診、入院中、周術期）、採血者
From ‘Malley WJ. Clinical Blood Gases: Assessment and Intervention. 2nd ed. St.Louis: Elsevier Saunders, 2005. pp10-12′
When oxygen therapy or mechanical ventilation is used, a period of time is required before the complete effect of the therapy is reflected in the arterial blood. Similarly, the same principle is true when therapy is changed or discontinued and following exercise. Because blood gases are often the major criteria on which therapeutic decisions are made regarding oxygenation and acid-base disturbances, it is crucial that the blood gas results provide us with an accurate current reflection of the patient’s status.
During this period of adjustment to a change in therapy, blood gas values are in a dynamic, changing state. In time, the entire cardiopulmonary system reaches a new equilibrium or steady state.Blood gas values remain relatively constant from this point on, and the complete impact of the therapy is reflected in the arterial blood.
Arterial blood samples must always be drawn only when the patient is in a steady state. The actual time required for the attainment of a steady state differs slightly with the patient’s pulmonary status. In patients free of overt pulmonary disease, a steady state is achieved probably in as few as 1 to 3minutes1,2and almost certainly within 10 minutes3,4.
In patients with chronic airway obstruction, up to 24 minutes after withdrawal of therapy may be necessary5. In clinical practice, a 20- to 30-minute waiting period is usually recommended before sampling arterial blood after a change in oxygen therapy or ventilation2,6,7. As shown earlier, however, only 3to 10minutes is necessary to achieve steady-state conditions in the absenceof pulmonarydisease.
Ideally, a patient who is breathing spontaneously should also be at rest for at least 5 minutes before sample acquisition 8. Likewise, temporary fluctuations in therapy also compromise steady-state conditions, which may occur if the patient removes his or her oxygen mask or must be suctioned for excessive pulmonary secretions. The clinician drawing the sample is responsible for ensuring that the patient is in a steady state before arterial puncture. When a sample is thought to represent non-steady-state conditions, a repeat puncture with related pain, risks, and cost is probably necessary. Worse yet, if the non-steady state goes unnoticed, incorrect or inappropriate therapy may be prescribed. Thus, before arterial puncture, the patient must be carefully assessed to ensure steady-state conditions.
Samples drawn to assess response to exercise require special considerations. They are best drawn at peak exercise, however, samples drawn within 15 seconds of termination of exercise are acceptable7. Outside this time range, samples may yield false-negative results for hypoxemia7.
Mild to moderate pain may accompany arterial puncture9. The clinician should be aware that pain and anxiety associating with arterial sampling may in itself cause changes in ventilation that, in turn, alter blood gas results. Thus, the patient should be approached calmly with a quiet voice and reassurance to promote physical and mental comfort8; and the sample should be obtained as quickly as possible. Some suggest the use of numbering agents before the actual puncture9 and this issue is discussed later in this chapter.
Spontaneous Variability of Pao2
The clinician should also appreciate that some studies have shown considerable spontaneous variability in Pao2 in apparently stable patients10. This variability may be as much as 10% and may be due to patient or machine issues11. The important point here is that changes in Pao2 of as much as ±8 mmHg should be viewed with skepticism because they are commonly a result of spontaneous variability.
Documenting Current Status
Many times, the individual who interprets and acts upon the blood gas report is not the same individual who drew the sample. Therefore, it is important that sufficient information regarding the patient’s status at the time of the sample be documented. Sound decisions can be made only in the proper context of circumstances at the time of sampling.
Specific information regarding identification of the sample and the date and time of acquisition is essential on the requisition slip. This information mustinclude the patient’s full name and hospital or emergency room number. The blood vessel source should also be noted (i.e., arterial, venous, or mixed venous). Potential technical issues that may impact the quality of the sample should be noted as well. These might include issues such as improper storage of the sample or transportation delays. Other desirable information includes the location of the patient, working diagnosis, clinical indication, name of the physician requesting the sample, the initials of the individual who obtained the sample, and the sample site8.
The patient’s temperature and respiratory rate should likewise be recorded. The position of the patient (e.g., supine, sitting) at the time of sampling and the activity of the patient (e.g., comatose, convulsing) may also provide valuable information when the data are interpreted. Hemoglobin concentration may be useful in assessing oxygenation status or calculating [BE].Notations of fluid infusions and location may likewise be useful in some cases. The type and flow rate of O2therapy should be checked and recorded. When positive airway pressure (e.g., Continuous Positive Airway Pressure [CPAP] or Biphasic Positive Airway Pressure [BIPAP]) is being applied, the inspiratory and expiratory pressures being delivered should be observed and recorded.
In the case of the patient receiving mechanical ventilation, a host of other variables should be documented. The type of ventilator and mode of ventilation should be recorded. The respiratory rate setting on the machine as well as the actual respiratory rate of the patient should be determined and included on the report. When applicable, the positive end-expiratory pressure (PEEP) level or BIPAP levels should be observed on the pressure manometer of the machine and recorded. Finally, the fraction of inspired oxygen (FIo2) and exhaled tidal volume (VT)should be measured and recorded. All of this information may be important for interpreting blood gas results. In plotting the future course of treatment, it is essential to know clearly what has transpired.
1. Mathews PJ. The validity of Pao2 values 3,6, and 9 minutes after an FIO2 change in machanically vetilated heart-surgery patients. Respir Care1987;32:1029-1034.
2. NCCLS. Blood Gas and ph Analysis and Related Measurements; Approved Guideline.NCCLS document C46-AA[ISBN 1-56238-444-9]. 940 West Valley Road, Suite 1400, Wayne, PA, 2001;198007-191898.
3. Howe JP, 3rd, Alpert JS, Rickman FD, et al. Return of arterial PO2 values to baseline after supplemental oxygen in patients with cardiac disease. Chest1975;67:256-258.
4. Hess D, Good C, R. D, et al. The validity of assessing arterial blood gases 10 minutes after an FIO2 Change in mechanically ventilalated patients without chronic pulmonary disease. Respir Care1985;30:1037-1041.
5. Sherter CB, Jabbour SM, Kovnat DM, et al. Prolonged rate of decay of arterial PO2 following oxygen breathing in chronic airways obstruction. Chest1975;67:259-261.
6. Woolf CR. Letter: Arterial blood gas levels after oxygen therapy. Chest1976;69:808-809.
7. AARC. Sampling for Arterial Blood Gas Analysis. AARC Clinical Practice Guideline. Respir Care1992;37:913-917.
8. NCCLS. Percutaneous Ccollection of Arterial Blood for Laboratory Analysis-second Edition:Approved Standard. NCCLS document H11-A2 (ISBN1-56238-130-X). 771 E. Lancaster Avenue, Villanova, PA 19085, 1992.
9. Giner J, Casan P, Belda J, et al. Pain during arterial puncture. Chest1996;110:1443-1445.
10. Hess D, Agarwal NN. Variability of blood gases, pulse oximeter saturation, and end-tidal carbon dioxide pressure in stable, mechanically ventilated trauma patients. J Clin Monit1992;8:111-115.
11. Hess D. Detection and monitoring of hypoxemia and oxygen therapy. Respir Care2000;45:65-80; discussion 80-63.