Physiology Flashcards
Factors affecting systolic pressure
Stroke volume (The main factor)
Ventricular contractility
In chronic conditions, a decrease in the compliance of the systemic arteries (age-related arteriosclerosis)
Factors affecting diastolic pressure
Total peripheral resistance Heart rate (directly related) Stroke volume (not a major factor)
Factors affecting Pulse Pressure
Increase in stroke volume
Decrease in vessel compliance (systolic increases and diastolic decreases)
Mean Pressure Formula
Diastolic + 1/3 Pulse Pressure
Or
2/3 Diastolic + 1/3 Systolic
CO Formula
CO = MAP/TPR
Or
MAP = CO x TPR
But if venous or RAP is severely increased, it must be taken into account when estimating TPR
TPR = (MAP - RAP)/CO
O2 consumption (Fick Principle)
VO2 = CO x (CaO2 - CvO2)
O2 Delivery
O2 Delivery = Q x CaO2
Q (Flow) Formula
Q = Oxygen consumption/[O2]pv - [O2]pa
What does a fall in PvO2 or SvO2 mean?
Indicates the patient’s O2 consumption increased and/or there was a fall in Q (flow) or CaO2 or both
Intrinsic or auto regulation of blood flow
Metabolic mechanism
Tissue vasodilatory metabolite products such as Adenosine, CO2, H+ and K+
Myogenic mechanism
Increased perfusing pressure causes stretch of the arteriolar wall and the surrounding smooth muscle, causing contraction, arteriole radius decreases and Q doesn’t increase
Alveolar ventilation Formula
Va = (Vt - Vd)f
Where Vt (Tidal volume)
Vd (dead space)
f (Respiration rate)
Units of pressure (Lung mechanics)
1 cm H2O = 0.74 mmHg
1 mm Hg = 1.36 cm H2O
Lung Compliance Formula
Compliance = Change in lung volume (tidal volume) / Change in surrounding Pressure
Examen
Vt = 0.6 L
IPP before inspiration -5 cm H2O
IPP after inspiration -8 cm H2O
Compliance = 0.6/3 = 0.200 L/cm H2O
Partial Pressure of a Gas in ambient air
Pgas = Fgas x Patm
Pgas = partial Pressure of a gas Fgas = concentration of a gas Patm = atmospheric pressure (760 mmHg)
Partial Pressure of a gas in inspired air
PIgas = Fgas (Patm - PH2O)
PIgas = partial pressure of inspired gas Fgas = concentration of the gas Patm = atmospheric pressure PH2O = partial pressure of H2O vapor (47 mmHg)
Pulmonary capillary gases
PAO2 = 100 mmHg PACO2 = 40 mmHg PaO2 = 95 mmHg PaCO2 = 40 mmHg PvO2 = 40 mmHg PvCO2 = 47 mmHg
Factor affecting alveolar PCO2
Alveolar ventilation (Inverse relationship) = If VA increases, PACO2 decreases and viceversa
Metabolic rate = Direct relationship
Factors affecting Alveolar PO2
Patm = An increase, increases PAO2 and a decrease (high altitude) decreases PAO2
FiO2 = an increase increases PAO2 (normally 0.21)
PACO2 = An increase, decreases PAO2 and a decrease, increases PAO2
RQ (Respiratory exchange ratio) = normally 0.8
Formula
PAO2 = (Patm - 47)FiO2 - PACO2/RQ
Rate of Gas difusión
Vgas = A/T x D x (P1-P2)
A = surface area for exchange (decrease in emphysema, increase in exercise) T = Thickness of the membranes (increase in fibrosis and restrictive diseases) D = Diffusion constant = Solubility P1-P2 = gradient across the membranes (e.g gradient for O2 = 100-40 = 60 mmHg)
Hemoglobin O2 Content
Each gram of Hb combine with 1.34 mL of O2
For example, if the Hb is 15 g/100 mL (15g%), then the maximal amount of O2 per 100 mL (100% saturation) in combination with Hb is
1.34(Hb) = 1.34(15) = 20 mL O2/100 mL blood = 20 vol%
Oxygen - Hb dissociation curves
Shifting the curve to the right
Increased CO2
Increased hydrogen ion (Acidosis)
Increased temperature
Increased 2-3 biphosphoglycerate
Reduced affinity of the Hb molecule for oxygen
Easier for tissues to extract oxygen
Strep part of curve, O2 content decreases
P50 increased (PO2 required for 50% saturation)
Oxygen - Hb dissociation curves
Shift the curve to the left
Decrease temperature
Decrease PCO2
Decrease 2-3 biphosphoglycerate
Decrease hydrogen ion (Alkalosis)
More difficult for tissues to extract oxygen
Steep part of curve, O2 content increased
P50 decreases
Carbon monoxide and Hemoglobin
CO ha a greater affinity for Hb than does oxygen (240 times greater)
O2-Hb dissociation curve is shifted to the left
HbO2 content is reduced
Carbamino compounds
When Carbon Dioxide reacts with terminal amine groups of proteins (hemoglobin). About 5% of the total CO2 is carried as carbamino compounds
Causes of Hypoxemia - Hypoventilation
No increase in the A-a oxygen gradient
Supplemental oxygen can relieve the hypoxemia
End tidal air still reflects the systemic arterial compartment
The problem is not within the lung itself
Equal decrease in PO2 in all 3 compartments (PAO2, pulmonary end capillary PO2 and PaO2)
Causes of hypoxemia - Diffusion impairment
Increase in A-a oxygen gradient
Supplemental oxygen can relieve the hypoxemia
End tidal air does not reflect the arterial values
Decrease in DLCO
PAO2 is OK
Pulmonary end capillary Pressure < PAO2
PaO2< PAO2
Causes of hypoxemia - Low VA/Q units
Increased A-a oxygen gradient because PAO2 is normal in areas that don’t have low VA/Q
Supplemental oxygen corrects the hypoxemia because regions still have some ventilation, just much lower than normal
End tidal air does not reflect the arterial values
Low VA/Q creates alveolar and end pulmonary capillary blood gases that are approaching venous gases
Examples (status astmaticus, cystic fibrosis, anaphylaxis, partial occlusion of an airway)
Causes of hypoxemia - Intrapulmonary shunt (right to left)
Increase in A-a gradient
Supplemental oxygen ineffective at returning PaO2 to normal
End tidal air does not reflect the arterial values
Examples (at electric lung regions in pneumothorax and ARDS, complete occlusion of an airway, right to left shunts created by heart defects)
