2017.11.22 Blog-Dr城下の呼吸器雑記帳

急性呼吸促迫時には100% 酸素をFlow-byがよいと思います。開口呼吸が消失すれば、ICU管理がよく、FIO2 25-30%, 室温 22-23℃に初期設定します。　Ⅱ型呼吸不全の代表疾患である慢性閉塞性肺疾患（COPD）では、長期経過に伴いある程度の低酸素血症には適応しており、ヘモグロビン-酸素解離曲線が急峻部から平坦部のカーブが変化するPao2 60mmHg程度に安定して維持されれば、ヘモグロビンと酸素との結合力が安定し、酸素運搬能としては十分です。逆に、低酸素血症に適応している環境で、急にFIO2 が上昇すると、体の反応としては、末梢化学受容体が即応し、negative feedback機構により呼吸抑制または停止します。その後、高炭酸ガス血症が生じ、体内の平衡と酸素療法の介入とのバランスをとることが困難な状況になり、呼吸状態が不安定となり、制御困難な状況になります。ヒトでは以下の文献のように、COPDの急性増悪時にはPao2は20-40mmHgまでに下がっていると考えられ、Pao2 60mmHgにまで回復することができれば、酸素化にも高炭酸ガス血症発症リスク回避にも適切と考えられています。したがって、急性呼吸促迫時には目標とするPao2は60mmHgです。安定期には、患者の呼吸状態によって調整しますが、FIO2＜30％での管理が適切と思います。人の文献ですが、COPD患者の酸素療法について記載されている部分の以下の資料を示します。

 

Malley WJ. Clinical Blood Gases: Application and Noninvasive Alternatives. Philadelphia: WB Saunders, 1990. pp184-185、本文中の文献リストは以下です。Malley90-ClinicalBloodGases-COPD患者の酸素療法, FIO２の設定と管理 -文献

Oxygen-Sensitive Patients¹

The approach to oxygen therapy in the patient with COPD or chronic hypercapnia is completely different. In these individuals, caution is the byword. Excessive oxygen therapy administered to these oxygen-sensi­tive patients may have fatal consequences. Neverthe­less, even in these patients, when hypoxia is suspected, oxygen therapy should never be withheld simply be­cause the patient may be sensitive to oxygen. Correc­tion of hypoxia is always the first priority.

Specifically, the first line of supportive treatment in acute exacerbation of COPD is low FIO₂ therapy. Typi­cally, the patient with COPD in acute respiratory⁷ failure has blood gases approximating those shown in Ex­ample 10-1.¹⁷⁹

Example 10-1. TYPICAL BLOOD GASES DURING ACUTE RESPIRATORY FAILURE IN PATIENTS WITH COPD

pH …………………………………….. 7.23-7.39

PaCO₂ ………………………………. 60-80 mm Hg

PaO₂ ………………………………… 20-40 mm Hg

Despite PaCO₂ levels in excess of 60 to 65 mm Hg, many patients with COPD can be managed without mechanical ventilation.¹⁸⁸ In general, mechanical ven­tilation should not be initiated unless pH falls below 7.20^187,188 and after all else fails.^187,190 The decision to institute mechanical ventilation in COPD is always dif­ficult and requires consideration of a host of variables.

A reasonable target PaO2 in acute exacerbation of COPD is 60 mm Hg.^177,191 This level guards against hypoxia and is unlikely to cause substantial hypercap­nia.

FIO₂ Selection. A useful guideline for FIO₂ selec­tion in acute exacerbation of COPD is the fact that PaO2 increases about 3 mm Hg for each 0.01 increase in FIO₂.^179,192 Thus, if a patient with COPD is seen in the emergency room during an acute exacerbation with a PaO2 of 39 mm Hg on FIO₂ of 0.21, the FIO₂ level indicated to achieve a PaO₂ of 60 mm Hg is 0.28. In other words, an FIO₂ increase of 0.07 should increase PaO₂ about 21 mm Hg (7 x 3 mm Hg).

The formula shown in Equation 1 may be used to determine the appropriate percentage of oxygen to be applied in acute exacerbation of COPD, assuming a target PaO₂ of 60 mm Hg. Of course, this is just a guideline, and individual cases may vary considerably. It must also be remembered that this guideline applies only to the patient with COPD in acute exacerbation.

（60 mm Hg – room air PaO2）/3

= required % FIO₂ increase (1)

It is noteworthy that SaO₂ increases on average 3 to 4% per 0.01 increase in FIO₂ in acute COPD.¹⁷⁹ This substantial increase shows the tremendous value of oxygen therapy in acute COPD. Because these patients are often on the steep portion of the oxyhemoglobin curve, the amount of oxygen actually present in the blood rises sharply with only a small increase in PaO2.

CO2 Narcosis. One must always keep in mind that increases in PaO2 in patients with COPD may also be accompanied by increases in PaCO2. Although a slight increase in PaCO2 is inconsequential, a large increase must be avoided. The patient is more likely to have an increase in PaCO2 when initial PaCO2 is greater than 70 mm Hg,¹⁹² or when the initial PaO2 is very low.¹⁹¹ Also, the clinician should be aware that worsening hypercarbia is usually observed several hours after the onset of oxygen therapy.¹⁸⁸

The increase in PaCO2 also tends to be proportional to the level of FIO₂ delivered. In one study, PaCO2 increased on average 5 mm Fig with administration of FIO₂ 0.24 and 8 mm Hg on FIO₂ 0.28.¹⁹⁰  In another study, modest FIO₂ levels of 0.35 to 0.4 substantially aggravated hypercapnia.¹⁹³ When excessive oxygen therapy appears to be responsible for progressive hy­percapnia, FIO₂ should be reduced gradually, because abrupt cessation of oxygen may result in further de­terioration.

The buildup of CO2 to high levels in the blood gives rise to the syndrome known as CO2 narcosis. This syn­drome is characterized by increasing PaCO2 levels, aci­demia, stupor, and coma.¹⁹¹ Additional clinical signs suggestive of mild-to-moderate hypercardia include decreased cerebral function, headache, drowsiness, lethargy, and asterixis.¹⁹⁴ A more complete list of the signs and symptoms associated with hypercapnia is provided in Table 9-4, Chapter 9（Malley90-ClinicalBloodGases-Table9-4, Chapter9）.

Sometimes, the hypercarbic patient may be rela­tively asymptomatic. Nevertheless, the clinician must be continuously on guard for any signs or symptoms of hypercardia while administering oxygen to the pa­tient with COPD.

Some authors suggest that high doses of oxygen therapy may be safely administered to patients with COPD if hypercapnia is not present initially.^177,189  In my personal, anecdotal experience, I have observed increasing PaCO2 levels after oxygen therapy despite the absence of pre-existing hypercarbia.

The phenomenon of normal PaCO2 levels in a pa­tient who actually has chronic hypercapnia could be explained by the observation that PaCO2 levels in some patients with COPD seem to decrease during acute exacerbation of the disease (see Chapter 14).⁴  Thus, when blood gases are first sampled in the hospital, PaCO2 levels could appear lower than their normal baseline. In other words, chronic CO2 retention may- go unrecognized during acute exacerbation because PaCO2 is within the normal range. In any event, it ap­pears prudent to approach oxygen therapy cautiously in all patients with COPD, regardless of PaCO2 levels.

The device selected for the administration of oxygen is somewhat a matter of personal preference. High- flow systems such as air-entrainment masks with low FIO₂ levels have the advantage of accurate FIO₂ rates regardless of breathing pattern. However, the patient’s comfort and compliance with these devices are not good. In addition, these masks are often unsightly, awkward, and noisy.

Low-flow systems (e.g., nasal cannula) are more often used. They are less obtrusive, quieter, and better tolerated. For more precise FIO₂ control however, flowmeters marked in flow increments less than 1 L/min may be required.¹⁹⁵  Furthermore, one must always remember that a nasal cannula is a low-flow- system and, as such, may allow considerable FIO₂ varia­tion with changes in ventilatory pattern.