เรียน หมอ ไอศวรรย์ ครับ
ขอบคุณที่ถามมา ว่าจะเขียนมาหลายวันแล้วแต่ไม่มีจังหวะว่าง อาจตอบยากถ้าให้โจทย์คำถามมาแค่นี้ ว่าจะไปซ้ายหรือไปขวา และผมอาจตอบไม่ตรงกับโจทย์ผู้ป่วยก็ได้ การพิจารณา urine Na ต้องเข้าใจว่า เรากำลังพิจารณาเรื่อง concentration of Na, not the total amount of Na that appears in the urine. คงต้องเข้าใจสรีรวิทยาที่ไตก่อน
The " concentration" means a ration between solute ( Na) and solvent ( urine water). The amount of urine Na is up to tubular ability to reabsorb Na, and the amount of urine water depends on the state of ADH secretion. If your are considering a normal person receiving NSS, it is true that any person is supposed to have a constant release of certain amount of
ADH , making our urine slightly hyperosmotic with respect to plasma osmolarity. If you administer iv
NSS , you will expand more ECF, increasing renal hemodynamics as well as plasma Atrial natriuretic peptide, causing renal vasodilation and natriuresis. We had observed that patient sometimes produced urine with urine Na as high as 120-140 mEg/L of urine after rapid andprolonged NSS infusiton. In patients with low ECF volume, you may expect higher ADH but also higher RAS activation. urine Na may be low because of both compromised GFR ( not eminent at an earlier stage of the mild illness) , enhanced prox,. tubule reabsorption of Na, and alsosterone effect on distal tubular Na reabsorption. At the early phase of NSS replacement, though urine flow is about to increase, you may observe a urine with lower-than-expected Na concentration. But it wil not last long . Perhaps, this is caused by ongoing action of circulating aldosterone . Please remember that the half life of ADH is quite short whereas that of aldosterone ( a steroid compound) is longer. At later stage of NSS replacement in a previously healthy person, you usually observe an increase in urine flow as well as u. Na concentration. In contrast , in patients with edematous state namely CHF or cirrhosis with marked ascites, high activation of RAAS is present. Moreover, high ADH is common due to low " effective " circulatory volume. Then, you expect that infusion of NSS will not have much contribution to change in UNa. Presence of high angiotensin, low GFR and high aldo. will make UNa very low ( eg. severe cirrhosis , hepatorenal syndrome or severe CHF) and high ADH will make urine flow at minimal.
IN conclusion, U. Na after NSS infusion may be unchanged or increased, depending on clinical circumstances. Infrequently, urine Na will be decreased.
Apology for replying in English. it is faster to type in one language only.
Kriang Tungsanga , MD