anesthesia monitoring
Terms
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- Normal VT
- 10-20 mL/kg in most domestic species
- Alveolar Ventilation ( VA) =
- RR x (VT– VD)
- Hypercapnia: PaCO2 > 45 mmHg
- § From 45 - ˜100 mmHg it is a respiratory stimulant, § CNS stimulant and vasodilator § In excess of 100mmHg it is an anesthetic & CNS depressant
- PAO2 =
- [(Patm-PH20) x FiO2] - PaCO2 /0.8
- “Cyanosis†requires >
- 5gm/dL of deoxygenated Hb
- CaO2=
- (Hg x %Sat x 1.34)+ (0.003 x PaO2)
- Apnea Monitor
- detects air flow, rate only, alarms w/apnea §continuous, auditory signal
- Ventilometry
- •Measures the volume of a breath (tidal volume) §simple turbine-propelled vWright’s respirometer §computerized, flow-generated
- Pulse oximetry
- •Detects infrared light absorption at two wavelengths, and calculates saturation of hemoglobin with O2 •Contains algorithm to subtract non-pulsatile reflectance •PaO2 = SpO2 – 30 (estimate)
-
MAP needed for
1.brain
2.renal
3.muscles - Cerebral and coronary (50-60 mmHg) §Renal and mesenteric (60-70 mmHg) §Muscle compartments (>70 mmHg)
- pulse Not palpable lower than MAP of about
- 40
- sphyngomanometry:
- uses a cuff to occlude arterial supply to a known pressure, and define the return of flow (systolic)
- oscilometric:
- uses cuff, but computer detects oscillations of artery. (S, D and M)
- calculated MAP=
- D+1/3(S-D)
- Pulse Oximetry⬢Accuracy
- ⬢Fair SpO2 ,<> 75-90% ⬢Overestimation SpO2< 70% ⬢ Underestimation SpO2 > 90%
- Heart Rate limits on CO
- ⬢Heart rate directly contributes to CO up to a point, then it decreases CO: §Decreasing preload (most of filling occurs in first half of diastole) §Decreasing SV §Decreasing myocardial oxygenation potential (coronary arteries fill in diastole)
- ⬢Type of murmur changes treatment during anesthesia:
- §Restrictive conditions require increased filling pressures §Regurgitant conditions are aided by increasing forward flow §Failing myocardium requires contractile assistance
-
Central Venous Pressure
Monitoring - ⬢Zero reference = RA ⬢Measurement in cmH2O ⬢ with a water column ⬢Or in mmHg with ⬢ a pressure transducer ⬢1 mmHg = 1.36 cmH2O ⬢CVP = 0-5 cmH2O in SA ⬢CVP> 12 cmH2O, High ⬢CVP < 0 cm H20, Low
- BP cuff should be what % of limb circumference
- 30-40%
- Results of cooling
- Increased recovery time: mortality in some studies nDecreased metabolic rate and MAC (5% for each C°) nDecreased oxygen demand of tissues
- Capillary Filtration Pressure
- •Net Driving Pressure into Capillary •[(Pc-Pi) – (pp- pi)] §Pc = capillary hydrostatic pressure vArteriole approx. 35 mmHg vVenuole approx. 15 mmHg §Pi = interstitial hydrostatic pressure (0) §pp = plasma oncotic pressure (20-28 mmHg) §pi = interstitial oncotic pressure (3mmHg) •Balance » 10 mmHg fluid OUT of capillary • » 10 mmHg fluid IN to venule
- Disadvantages of Colloids
- •Circulatory overload •Anaphylactic reactions (incidence 0.033%) •Coagulation disorders: §‘Coating of platelets’ §Volume dilution of clotting factors vDextrans (worst) vStarches (hetastarch) vGelatins (not in use)
- Assessment of Hydration
- ⬢Hydration is the presence of fluid in the interstitial space (intravascular and intracellular are far more difficult parameters to assess) §Dehydration for <3 days 80% ECF, 20% ICF §Dehydration for >3 days 60% ECF, 40% ICF ⬢Koch and Graber
- Emergency Fluid Resuscitation
- ⬢Aim: restore tissue perfusion & oxygenation ⬢Shock fluid rates (high end value- constant re-evaluation will most likely limit these numbers): ⬢Think ¼ blood volume to start §Blood volume: vDog 80-90 ml/kg (isotonic) vCat 60-70 ml/kg (isotonic) vColloid (dextran or hetastarch) 20ml/kg §7.5% Hypertonic saline in colloid 4ml/kg over 10min
- Acute Blood Loss
- ⬢Goal: restore circulating blood volume (CBV) ⬢Initially any fluid type will restore volume and maintain cardiac output ⬢Volume of crystalloid must be 3 x volume of colloid for the same CBV ⬢Replace red cells or use whole blood once bleeding is under control, or when loss exceeds 20% of blood volume ⬢What comes out must go in