Chemistry & Physics PowerPoint Chapter 9 Acid-Base 2023 Rev 7

Acids & Bases Dr. Joseph Curione 1 What is pH? } } } } pH = potential hydrogen pH is the concentration of hydrogen ions (H+) The pH of blood indicates the net result of normal acidbase regulation, any acid-base imbalance, and the body’s compensatory mechanisms Human blood must maintain a very narrow...

Acids & Bases Dr. Joseph Curione 1 What is pH? } } } } pH = potential hydrogen pH is the concentration of hydrogen ions (H+) The pH of blood indicates the net result of normal acidbase regulation, any acid-base imbalance, and the body’s compensatory mechanisms Human blood must maintain a very narrow pH range } } Blood = 7.35 – 7.45 < 6.8 or > 8.0 death occurs Hydrogen Ion 3 Hydrogen Atom Hydrogen Ion One Proton (+) One Electron (-) One Proton (+) electrically balanced Electrically charged (+) Definitions } An Acid is… } } A molecule that can donate a H+ ion Examples: } } } H Cl à H+ + ClH2CO3 à H+ + HCO3- An acid can be weak, moderate, or strong depending on its pH } } Weaker acids are closer to 7 Stronger acids are closer to 1 Definitions } A Base or alkali is… } } A molecule that can accept a H+ ion Examples: } } } H+ + OH- à H2O H+ + HCO3- à H2CO3 A base can be weak, moderate, or strong depending on its pH } } Weaker bases are closer to pH 7 Stronger bases are closer to pH 14 Brønsted Theory Defines an acid as a proton, or H+ ion donor. A base is a proton acceptor. When an acid donates its proton, what remains is called the conjugate base. Ex: HCl H+ + Cl(HCl to chloride ion) HNO3 H+ + NO3- (nitric acid to nitrate ion) protonated forms When a base accepts a proton, it is converted into its conjugate acid. protonated forms Ex: NH3 + H+ = NH4+ (ammonia to ammonium) HCO3- + H+ = H2CO3 (bicarb to carbonic acid) } } } } } } } } } 6 Henderson-Hasselbalch Equation } helps to predict the degree of ionization of a given drug in solution given the pH of that particular solution and the drugs pKa. } } pH = pKa + log [Base] [Acid] } conjugate base over acid, or base over conjugate acid 7 pKa } } } } The pKa is the pH at which 50% of a drug is ionized. When ambient pH = pKa, 50% is ionized. The more ionized a drug is, the less effective it is. For a drug that is a weak acid: } } } For a drug that is a weak base: } } } } If pH < pKa: Nonionized (HA) form predominates = more effective pH > pKa: Ionized (A-) form predominates = less effective pH < pKa: Ionized (BH+) form predominates = less effective pH > pKa: Nonionized (B) form predominates = more effective Summary: Acids work better in an acidic environment; Bases work better in a basic environment Ex. Lidocaine HCl (base drug) less effective injected into infx. ts. 9 Nearly linear across clinical range In blood, a normal [H+] of 40 nEq/L corresponds to a pH of 7.40. Because the pH is a negative logarithm of the [H+], changes in pH are inversely related to changes in [H+] (e.g., a decrease in pH is associated with an increase in [H+]). pH 7.7 7.5 7.4 7.3 7.1 7.0 6.8 [H+] 20 30 40 50 80 100 160 pH Review } } } pH Range is from 0 - 14 If [H+] is high, the solution is acidic; pH < 7 } ↑ H+ à ↓ pH (Acidic) If [H+] is low, the solution is basic or alkaline ; pH > 7 } ↓ H+ à ↑ pH (Alkaline) } Acids are H+ donors. } Bases are H+ acceptors, or give up OH- in solution. } Acids and bases can be: } Strong – dissociate completely in solution } HCl, NaOH } Weak – dissociate only partially in solution } Lactic acid, carbonic acid 11 pH Review } } } } } } } } pH scale = 1 to 14 The p-function operator means the negative logarithm of. pH = -log [H+] [H+] varies from 100 M to 10-14 M That is 14 orders of magnitude – a factor of a hundred million million. (100,000,000,000,000) A pH meter without the p-function logarithm would have a hundred million million divisions. The logarithm function is a way to map a large range of values onto a much smaller scale. In this case, a range of 100 to 10-14 onto a range of 1 to 14. 12 Calculate [H+] } } } } } } } } } } If given pH, we can calc. hydrogen ion conc. pH = - log [H+] [H+] = 10 –pH Example: Normal blood pH = 7.4 Calculate [H+] for pH of 7.4? [H+] = 10 -7.4 [H+] = 0.00000004 M = 0.00000004 Eq Recall from Ch. 8: Eq is analogous to a mole One Eq of a substance contains one mole of chemical reactivity. (to convert to mEq or nEq, move the decimal to the right) [H+] = 0.00004 mEq/L or 40 nEq/L pH Review } shortcut for nEq/L [H+] = 10 (9 - pH) Example: Calculate [H+] for pH of 7.4? } Example: Calculate [H+] for pH of 7.3? } } 14 pH Review } } } shortcut for nEq/L [H+] = 10 (9 - pH) Example: Calculate [H+] for pH of 7.6? 15 Calculate pH if given [H+] } } pH = - log [H+] Example: Calculate pH of a solution when the [H+] is 1.0 x 10^-3 M ABGs } Analysis indicates how well a patient is exchanging gases in the lungs and how well the body is maintaining normal pH. } 17 The Normal’s } pH } 7.35 – 7.45 } PaCO2 } 35 – 45 mmHg } HCO3- } 22 – 26 mEq/L } PaO2 } 80 – 100 mmHg 19 ABG Interpretation } Step 1: check PaO2 (80-100) } } Step 2: check pH (7.4) } } } Correlate with pH – if both pH & CO2 match, cause = respiratory Step 4: check HCO3 (22-26) acidotic or alkalotic range? } } 7.35 – 7.45 = compensated acidosis or alkalosis < 7.35 = uncompensated acidosis, > 7.45 = uncompensated alkalosis Step 3: check PaCO2 (35-45) alkalotic or acidotic range? } } < 80 = hypoxic, > 100 = hyperoxygenated (mask, ventilation, etc) Correlate with pH – if both pH & HCO3 match, cause = metabolic If all 3 (pH, CO2 & HCO3) match, cause = “combined” 21 22 Example } } } PaO2: 90 pH: 7.52 PaCO2: 43 HCO3: 30 PaO2 = normal pH = alkaline } } } } } Compensated or uncompensated? PaCO2 = normal HCO3 = high Interpretation: uncompensated metabolic alkalosis Example } } } } } } What if same values but with elevated CO2? PaO2: 90 pH: 7.52 PaCO2: 49 HCO3: 30 “Partially compensated” metabolic alkalosis Still metabolic alkalosis, but “partially compensated” due to a seen increase in PaCO2 (body’s attempt to correct) HCO3 in alkaline range CO2 in acidotic range, trying to compensate 24 } } } What if CO2 was in alkalotic range? PaO2: 90 pH: 7.52 PaCO2: 29 HCO3: 30 “Combined Alkalosis” because both CO2 and HCO3 are contributing to the pH alkalosis 25 Imbalances can be: } } } } } } } } Compensated – pH in range w/ both #s off Uncompensated – pH out of range w/ one off & one normal Partially Compensated - pH out of range w/ both #s off Combined – pH out of range w/ both #s contributing Acidosis Alkalosis Respiratory Metabolic 26 27 How does the body maintain pH? Buffer systems } } } Prevent major changes in pH by removing or releasing hydrogen (H+) ions Act chemically to change strong acids into weaker acids or to bind acids to neutralize their effects 29 How does the body maintain pH? Buffer Systems: } } } } } Carbonic acid (H2CO3) – Bicarbonate buffer system – most important ECF buffer against non-carbonic acid changes CO2 + H2O ↔ H2CO3 ↔ HCO3- + H+ Protein buffer system (includes Hgb) – largest buffer in the body; important ICF and ECF buffer Phosphate buffer system – important ICF and urinary buffer ¨ ¨ H+ + HPO42- ↔ H2PO4OH- + H2PO4- ↔ H2O + HPO42- Kassirer-Bleich Equation } } Kassirer-Bleich equation: Simplified formula that gives an approximation of [H+] (effective vs actual) based on PCO2 & HCO3 (ABG analyzers are used for more accurate calculations in clinical setting) } } } } [H+] = 24 x PCO2/HCO3 ¯ (allows calculation of [H+] if PCO2 and HCO3 are known) ((Can also rearrange to find the other variables – ex. [HCO3 -] = 24 x (PCO2 / [H+] ) )) To find [H+] in blood using Kassirer-Bleich Eq: [H+] = 24× PCO2/HCO3 ¯ x 10^(7.4-pH) (using ABG values) } Reflects how the acidity of blood is determined by the relative availability of acid and alkali (HCO3¯ & PaCO2) } Stresses how H+ ion concentration is determined by the ratio of PCO2/HCO3, rather than the absolute value of either value alone. Kassirer-Bleich equation } } } } } Your patient has the following ABG. Calculate the [H+] in the blood, in nEq/L? PaO2: 90 pH: 7.36 PaCO2: 29 HCO3: 18 32 Kassirer-Bleich equation } } } } } Your patient has the following ABG. Calculate the [H+] in the blood, in nEq/L? PaO2: 94 pH: 7.44 PaCO2: 27 HCO3: 16 33 PCO2/[HCO3- ] Ratio } When a primary acid-base disturbance alters one component of the PCO2/[HCO3- ] ratio, the compensatory response alters the other component in the same direction to keep the PCO2/[HCO3- ] ratio constant. PRIMARY AND SECONDARY ACID-BASE DERANGEMENTS End-Point: Constant PCO2/[HCO3- ] Ratio Acid-Base Disorder Primary Change Compensatory Change Respiratory acidosis Respiratory alkalosis Metabolic acidosis Metabolic alkalosis PCO2 up PCO2 down HCO3 down HCO3 up HCO3 up HCO3 down PCO2 down PCO2 up Summary of acid base abnormalities Scenario } } } } Very sick 56 year old woman being evaluated for a possible double lung transplant Dyspnea on minimal exertion On home oxygen therapy @ 2 lpm via NC Numerous pulmonary medications Scenario While she is being assessed an arterial blood gas sample is taken, revealing the following: pH PCO2 7.30 65 mm Hg Scenario What is the hydrogen ion concentration? (nEq/L) use: [H+] = 10 (9 - pH) What is the bicarbonate ion concentration? use: Kassirer-Bleich Eq. [H+] = 24 x (PCO2 / [HCO3 -] What is the acid-base disorder? Scenario What is the hydrogen ion concentration? [H+] = 10 (9-pH) Scenario What is the bicarbonate ion concentration? [H+] = 24 x (PCO2 / [HCO3 -] ) Kassirer-Bleich equation Scenario What is the acid-base disorder? Scenario What is the acid-base disorder? } For acute respiratory disturbances (where renal compensation does not have much time to occur) each arterial PCO2 shift of 10 mm Hg is accompanied by a pH shift of about 0.1, while for chronic respiratory disturbances (where renal compensation has time to occur) each PCO2 shift of 10 mm Hg is accompanied by a pH shift of about 0.03. Scenario What is the acid-base disorder? } } In our case an arterial PCO2 shift of 25 mm Hg (from 40 to 65 mm Hg) is accompanied by a pH shift of 0.10 units (from 7.40 to 7.30), or a 0.04 pH shift for each PCO2 shift of 10 mm. Since 0.04 is reasonably close to the expected value of 0.03 for an chronic respiratory disturbance, it is reasonable to say that clinically the patient has chronic respiratory acidosis. More accurately, assessing ABGs: } pH = 7.30 } PCO2 = 65 } HCO3 = 31.1 } Partially compensated respiratory acidosis