- DDi vdi mot sDthuDc chi sau khi chuyen hoa mdi cotac dung hoac tang dQc tinh thi cam ling enzym lam tang tac dung hoac tang dQc tinh cua thuoc.
4. THAI TRU (elimination)
4.4.3. ThiJi gian ban thai (tllJ
Thai gian ban thai hay con goi la nua diJi (half life) cua thuoc la thai gian cdn thie't de' no'ng d¢ thuoc trong huye't tuang guim di con m¢t nlta. Thea
mo hinh dong h9C mot ngan co th~ tinh tl/2 nhu sau:
Ln2 0,693
t1l2= -- = -- (19)
x, x,
Ke: hang so toc de)thai tru cua co th~
Til (19) ta co: Ke= 0,693 (20)
t1/2
Vdi mo hinh de)ngh9Cmot ngan va thuOc thai trir tuan thea dong hoc bac I: K, = Kgan+Kth\lll (21) Kgan: hang so toc de)thai trir cua gan (chuyen hoa)
~h(\n: hang so toc dQthai tril cua than
Doi voi nhung thuoc bi chuy~n haa d gan Ia chu yeu thi t1l2phan anh muc de)va toc de)chuy~n haa cua chung. Doi voi nhung thuoc bai tiet qua th~n duoi d~,mgkhong bien d6i Ia chu yeu thi t1l2ph\l thuQCvao chuc nang th~n.
Cling nhu cac thong so dUQcdQng h9C (DDH) khac, t1l2thudng dUQccho s~n trang cac bang tra cuu, nhung do chi Ia nhung thong so tham khao ban d~u. Trong nhung trudng hQp th~t c~n thiet ngudi ta co th~ tinh toan C\lth~ cho phu hQp voi b~nh nhan.
Khi biet t1l2ngudi ta co th~ biet dUQcsau bao Iau thuoc se dlilt ncSngdQ d trlilng thai 6n dinh (Css) k~ til khi dua thuoc vao cd th~: sau It1l2co 50% IUQng thuoc dUQCthai tril; sau 2t1l2:75%; 3t1/2:87,50%; 4t1l2:93,75%; 5t1l2:96,87%; 6t1/2: 98,43%; 7t1l2:99,21%. Trang th\fC te ngudi ta thudng lay mau d~ xac dinh ncSng dQ thuoc d trlilng thai can bling (Cs.) sau 5t1l2bdi vi til giai dOlilnnay trd di qua
trinh bai xuat din bang vdi qua trinh phan b6 do do n6ng dQ thu6c giao dong it.
Ngoai ra khi khi biet tll2 nguoi ta con co th~ xac dinh duoc nong dQt6i da cua thu6c trong huyet tuong, n6ng dQcua thu6c dthoi di~m can biet va dieu chinh nong dQthu6c.
M6i lien quan giira cac thong s6DDH diineu dtren co su khac nhau. Co nhirng thong so' dQclap d6i vdi nhau nhung ciing co nhirng thong so'phu thuoc nhau. Vi du CI va Vdla nhimg thong so khong phu thuoc VaGnhau. Khi Vdthay d6i khong anh huong den gia tri cua CI va nguoc lai khi CI thay d6i cfing khong l1nh hudng den gia tri cua Yd.Trai lai t1l2phu thuoc vao ca CI va Yd.Khi CI hoac Vdthay d6i gia tri cua tl/2 cung thay d6i thea (t1l2giam khi CI tang ho(lcVdgiam; nguQc lai tl/2 tang khi CI giam ho(lc Vdtang). Cssph1,lthuQc VaGCI (Cssgiam khi CI tang va nguQcl~i Csstang khi CI giam).
Gia tr! clla cac thong s6DDH ph1,lthuQc VaGnhieu yeu to' nhu lUa tu6i, tr~ng thai b~nh ly (suy giam chuc nang gan, th~n), thai nghen, di truyen v.v...
Vi v~y khi dung thu6c cho cac d6i tUQngnay c:1nphai chu y d~ h~n che nhung tai bien ho(lc dQch~i do thu6c gay ra. D6i voi tre em, ngudi gia, ngudi suy giam chuc nang th~n trong nhung trudng hQp c:1n thiet phai tinh toan ho(lc dieu chinh lieu cho thich hQp.
Chuong 2
, ? ~
TAC DUNG CUA THUOC.
Tac dung cua thuoc la ket qua cua qua trinh tuong tac giira phan tiithuoc vdi cac thanh phan cua te bao co th~. Ket qua cua nhfmg tuong tac do lam thay d6i nhfmg tinh chat sinh ly, hoa sinh cua cac thanh ph An te bao, tao nen nhfing dap ling cua cac t6 chirc doi vdi thuoc. Thong thuong thudc co tac dung tang cuong hoac uc che' (dieu hoa) mQt chuc nang nao do cua co th~ chu khong tao ra chuc nang mdi.
1. MQT s6KHAI NI~M
1.1. Thai gian ti~m tang (latent time)
Thoi gian tiem tang (toc dQ xua:t hien tac dung) la thoi gian tit khi dua thuoc vao co th~ den khi xu at hien tac dung. Toc dQ nay phu thuoc vao duong dua thuoc, tinh chat ly hoa cua thuoc. Nghla la ph\l thuQc vao toc dQ hap thu, phan bo va chuy~n hoa cua thuoc.
1.2. Tac d'lolngchinh va tac d'lolngph'lol
Tac d\mg chinh la tac d\lng muon d~t du<;sctrong digu trj, con tac d\mg ph\lla tac d\lng khong mong muon co trong digu trj nhung v~n xuat hi~n khi dung thuoc. Vi d\l tac d\lng h~ sot, giam dau, chong viem la tac d\lng chinh cua cac thuoc nhom nay, con tac d\lng gay kich ling niem m~c dUdng tieu hoa la tac d\lng ph\l cua chung. Doi khi doi voi tac d\lng cua mQt thuoc trong trudng h<;sp nay la tac d\lng ph\l nhung trudng h<;spkhac l~i la tac d\lng chinh. Vi d\l tac d\lng gian d6ng tii cua atropin la tac d\lng ph\l khi dung atropin voi m\lc dich chong co thAt cd trdn (giam dau trong cac cdn dau do co thAt cd trdn dUdng tieu hoa, tiet ni~u ... ) nhung tac d\lng do l~i la tac d\lng chinh khi nho mAt d~ soi day mAt.
Trong dieu tri ngudi ta thuong tim cac bien phap d~ han che' nhimg tac dung phu cua thuoc bang each chon duong' dung thuoc thich hop, thoi di~m uong thuoc, dang bao che va c6 th~ ke't hop vdi thuoc khac mot each hop ly. Vi
du vdi cloral hydrat nguoi ta thuong uong dudi dang dich treo (suspension) hoac
dung dudng born true trang d~ tranh kich ung niem mac duong tieu h6a. D~ han che tai bien duong tieu h6a do sli dung keo dai cac thuoc chong viem nguoi ta
thuong dung kern vdi cac thuoc khang histamin H2(cimetidin).