Respiratory Mechanics

spirometry

measures the volume of air inspired and or expired by the lung and determines air flow

total lung capacity (TLC)

• volume of gas in the lungs at the end of maximal inspiration

• 6700 ml

• cannot be obtained by simple spirometry

functional residual capacity (FRC)

• volume of gas in the lungs at the end of passive expiration

• 2700 ml

• cannot be obtained by simple spirometry 

inspiratory reserve volume (IRV)

• additional volume of gas that can be inspired from end tidal respration

• 3500 ml

expiratory reserve volume (ERV)

• additional vlume of gas that can be expired from resting expiratory level

• 1500 ml

residual volume (RV)

• volume of gas in the lungs at the end of maximal expiration

• 1200 ml

• cannot be obtained by simple spirometry

inspiratory capacity (IC)

• maximal volume of gas that can be inspired from resting expiratory level 

• 4000 ml

tidal volume (Vt)

• volume of gas inspired during quiet breathing

• 500 ml

vital capacity (VC)

• maximal volume that can be expired after maximal inspiration

• 5500 ml

bulk flow equation

Q = P/R

pulmonary and systemic circuits exist in…

series

systemic gradient

93 mmHg

pulmonary graident 

10 mmHg

how much more is systemic resistance than pulmonary

10x

pulmonary artery

• gets blood from right ventricle

• supplies blood flow for gas exhange

bronchial artery 

• gets blood from aorta 

• nutritional and part of normla shunt 

lymphatics

remove fluid and defend lungs from airborne microbes

recruitment

addition of parallel vessels, lowers resistance

distension

dilation of uncollapsed vessels 

inspiration at large lung volume

• expansion of alveolus compresses capillaries

• reisistance increases

• distends small arteries and veins

expiration at low lung volume

• small arteries and veins compressed due to elevated intrathoracic pressure

• small alveoli allows expansion of capillary

where is there maximum resistance in the pulmonary circuit 

lung volumes near FRC 

blind sack air flow

• air enters and leaves by same pathway allowing air to intermix

• graident for flow is due to expansion of the thoracic cavity

what is the most important air volume

the air entering the alveolar spaces

conducting pathways

dead space, air flow with no gas exchnage

gas exchange pathway 

contain alveoli 

minute volume

• total ventilation/minute

• air movement into or out of the respiratory tract

• V = Vt(f)

respiratory frequency

number of breaths per minute

physiological dead space 

total measured dead space from both anatomical and alveolar 

dead space equation

Vd/Vt = PaCO2 - PcCO2 / PaCO2

alveolar ventilation

• volume of fresh air entering alveoli per breath

• V = (Vt - Vd)f

• largest determinant of gas exchnage

types of bulk flow 

turbulent, laminar, or transitional 

diffusion

due to brownian motion

neural air resistance

• parasympathetic causes bronchoconstricion

• sympathetic causes bronchodilation

mechanical air resistance 

• more negative IPP associated with larger lung volumes

• widens airways 

air movement into the lungs

• air flows from area of high pressure to low pressure

• contraction of the respiratory muscles during inspriation enlarges the thoracic cavity

• lungs in close apposition to respiratory muscles, sperated by the pleural space

• air movement into lungs result from creation of subatmospheric P in intraalveolar space during inspiration

• at end of inspiration, intraalveolar P remains to barometric P

• expiration is passive and depends on passive recoil of the lungs

pleural space

contains no gas and only a small volume of fluid

IPP during normal respiratory cycle

fluctuates solely in the negative range

how does IPP become positive during inspiration

• positive pressure respiration

• outward movement of the lungs compresses the intrapleural space and raises its pressure

how does IPP become positive during expiration

• active expiration

• respirtory muscles compress the intrapleual space

• IPP becomes positive so lungs can return to preinspiratory level more quickly 

assisted control mode ventilation

inspiratory cycle initiated by patient or automatically if no signal detected within a specified time window

positive end expiratory pressure

by not allowing IAP to return to 0 cm H2O at the end of expiration, the lung will be kept at a large volume

PEEP increases 

FRC 

positive pressure ventilation

• minimizes development of ventilator induced lung injury

• transalveolar pressure less than 28-30 cm H2O

continous positive aiway pressure

• not true support mode of ventilation

• breathing is spontaneous but via a circuit that is pressurized

• maintains airway size and prevents respiratory miscle atrophy

sleep apnea primary event 

critical negative pressure during inspiration

sleep apnea contributing factors

• sleep reduces the tone of the muscles of the oropharynx

• obesity

• alcohol

• increased masopharyngeal resistance

clinical consequences of sleep apnea

• cognitive and behavioral deficits

• excessive daytime sleepiness

• modd swings

• poor deciion making

• major risks of cardiovascular disease

assessing pulmonary mechanical function

• normally uses a forced expiration

• basis of spirometry 

pressure outside the lungs

intrapleural pressure

pressure inside the lungs

intraalveolar pressure

trasmural pressure 

responsible for lung movement is the difference between IAP and IPP 

fluid in intrapleural space

subjected to distending force and its pressure is negative

V/Q ratio ____ as one move sup the lung toward the apex

increases

pneumothorax 

• perforation of the chest wall or the lung causes air to move into the intrapelural space because IPP is negative 

• presence of air in the IPS breaks the liquid seal that attaches the lungs to the chest wall and that region of the lung collapses 

• chest wall expands at the same time 

traumatic pneumothorax

• wound to the best causes air to move from the envirnment into the IPS

• ruputre of the alveolus by barotrauma causes air to move from the intralveolar space into the IPS

spontaneous pneumothorax

• spontaneous rupture of the alveolus causes air to move from the intralaveolar space into the IPS

• most common at the apex

tension pneumothorax 

air in the lungs continues to accumulate and lungs completely collapse and seriously compromose both gas exchange and cardaic mechanics 

elasticity

• property of matter that causes it to resist distortion '

• elastic tissue returns to its original shape after having been deformed

work of breathing =

PV

pressure volume relationship for the normal lung 

describes the fall in IPP required to obtain a chnage in lung volume 

compliance

inverse of elasticity, slope of VP curve

surfactant

• reduces the surface tension and increases lung compliance

• greatly reduces work of respiration

absence of surfactant 

surfae tension of the film lining the inside of the alveolus is costant 

P =

2ST/r

affect of air water interface on compliance

• it takes more work to inflate a lung with air than with saline

• air forms a surface tension when in contact with water that lessens presence of surfactant

RDS of newborn 

• lungs very elastic but difficult to inflate 

• premature birth and maternal diabetes are risk factors 

interdependence

alveoli share speta and do not exist as independent units

what determines the FRC

elastic properties of the lungs and chest wall

vital capacity 

maximum volume of ai that an individual can move in a single breath as quickly and forcibly as possible 

forced expiratory volume

the volume of air exhaled in the first second of the FVC maneuver

normal ratio of FEV to FVC

• normally a value of 80%

• during forced expiration, IPP becomes positive and he airways are compressed

• maximum expiratory flow rates are effort independent

peak expiratory flow rate 

maximum flow rate achieved during a FVC

forced expiratory volume 

the volume of air exhaled in the mid portion of a FVC maneuver between 25 and 75% of the VC exhaled 

values not measured with spirometry

• residual volume

• total lung capacity

• diffusing capacity

obstructive pulmonary disease

characterized by an increase in airway resistance and is measured as a decrease in expiratory flow rates

chronic bronchitis 

hypertrophied smooth muscle and mucus glands, increased mucus secretion, and narrow airways 

asthma

hyperreative airways, inflammation, and narrow airways

emphysema

loss of tissue elasticity, loss of support for small airways, ad easily distorted airways

restrictive lung disease 

characterized by an increase in elasticity that is measured as a decrease in all lung volumes 

types of restrictive lung disease

• RDS of newborn

• fibrotic lung disease

• pulmonary vascular congestion

• pulmonary edema