Mechanical Ventilation

SaO2,weil

Sauerstoffangebot=Cardiac output x Sauerstoffgehalt x 10

Sauerstoffgehalt = Sauerstoffsättigung x Hb x 1.37 + 0.003 x PaO2 (sehr kleiner Anteil am O2-Gehalt!)

Durch wenig Sauerstoffgabe kann SpO2 schnell gesteigert werden. Bei Hypoventilation fehlt aber der Gasaustausch und nicht das Angebot (meist PaCO2-Anstieg)

1. Versagen der Atemmechanik->führt zu Lungenversagen
   1. "intrinsic dysfunction"
2. Lungenversagen->führt zu Versagen der Atemmechanik
   1. Atemwege
   2. Alveolär
   3. Perfusion (shunting)

Compliance, resistance (druckunterschied/Gasfluss) und PEEPi

The patient is maintained at a fairly high CPAP level that is intermittently released to a much lower CPAP level for a short period of time.

The intermittent release and resumption of the upper CPAP level generates alveolar ventilation that augments the ventilation provided by spontaneous breathing. It aims at improving oxygenation without allowing excess hypercarbia.

Proportional assist Ventilation

In this mode, the ventilator generates pressure in proportion to patient-generated flow and volume. The operator sets the percentage of flow-assisted and volume-assisted ventilation. This ventilation mode requires the measurement of respiratory system compliance and resistance.

This mode uses the diaphragmatic electrical activity during spontaneous inspirations to trigger and control the delivery of ventilator assistance. The diaphragm electrical activity is recorded by a multiple-array oesophageal electrode positioned in the lower oesophagus. The pressure applied to the airway by the ventilator is moment-by-moment proportional to the diaphragm electrical activity according to a proportionality factor set by the operator

• Synchronised Intermittent Mandatory Ventilation (SIMV). This mode alternates assist-control volumetric inflations (delivered according to a user-set mandatory frequency) and breaths that can be either assisted-spontaneous (when pressure support is set above zero) or fully spontaneous (when pressure support is set to zero).
• Pressure-Control SIMV (PC-SIMV). This mode is the same as SIMV, except that the mandatory inflations are developed with pressure-control rather than of volume-control.
•  

Der Anteil des Tidalvolumens, der für Gasautausch sorgt (Tidalvolumen - Totraumvolumen)

Alveoläre Ventilation= Atemfrequ. x Alveoläres Volumen

Elimination ist proportional zur alveolärer Ventilation und hängt von der CO2-Produktion ab

Wir kennen zwei Determinaten nicht: 

PaCO2=k x met. CO2-Produktion (V'CO2) /alv. Ventilation V'A

k ist unklar: wie schiwerig CO2 eliminiert wird

V'CO2 ist unklar

Wir kontrollieren V'A nur indirekt

Mechanical safety criteria include:
- Limited tidal volume at 6 ml/Kg ideal body weight (IBW),
- Limited static end-inspiratory pressure (max plateau pressure at 28-30 cmH₂O),
- Limited peak airway pressure,
- Avoiding intrinsic PEEP.

approximately 2.2 ml per kg of IBW

mindestens doppelt soviel wie Totraum =4.4 ml

über transpulmonalen Druck: Atemwegsdruck - Pleuradruck 

The static end-inspiratory pressure =plateau-druck depends on a number of factors besides the Vt, namely PEEP, intrinsic PEEP and compliance

->high Vt of 12–15 ml/kg is within pressure safety limits when compliance is normal–high and total PEEP is low. On the contrary a Vt as low as 6 ml/kg can produce excessive plateau pressures when the compliance is extremely low and a high PEEP level is applied.

= die Frequenz bei der es zu vollständiger Expiration kommt.

1. PEEP

2. RCe: respiratory system expiratory time constant: corresponds to the product of resistance and compliance, and quantifies the speed of exhalation

With a Te of at least three times the RCe, the equilibrium is at least nearly reached. A Te shorter than twice the RCe generates significant dynamic hyperinflation, and intrinsic PEEP accumulates above the externally applied PEEP

thanks to the low RCe with high exhalation speed, typical of these patients.

However, inversed I:E increases the mean intrathoracic pressure and may compromise the circulation.

• Low FiO₂, due for example to altitude
• Hypoventilation, especially when breathing low FiO₂
• Impaired pulmonary diffusion capacity (rarely a cause of hypoxaemia
• Ventilation-perfusion (V/Q) mismatch
• Shunt, due to perfusion of non-ventilated lung regions
• Desaturation of mixed venous blood (if combined with shunt or V/Q imbalance).

Steigerung des FiO2

Hypoxic pulmonary vasoconstriction (HPV) increases pulmonary vascular resistance in poorly aerated regions of the lung, thus redirecting pulmonary blood flow to better ventilated regions.

HPV can be inhibited if the patient is ventilated with high FiO₂ or if alveolar hypoventilated units are recruited (local increase in P_AO₂).

HPV can be inhibited if the patient is ventilated with high FiO₂ or if alveolar hypoventilated units are recruited (local increase in P_AO₂).

Verbesserung des CO nur bei vorbestehend schlechter Herzfunktion. Bei Herzgesunden eher geringeres CO

Extr. wird von Ventilator erzeugt und ist kontrollierbar

Intr.PEEP depends on the dynamic balance between ventilator and patient. Intrinsic PEEP can be easily measured in passively ventilated patients, but continuous monitoring is difficult. Therefore, improving recruitment by artificially generating intrinsic PEEP cannot be considered a safe practice.

• May improve ventilation distribution and recruitment in the dependent and basal lung regions, thanks to the tone and pump action of the diaphragm, thus improving the matching of ventilation with perfusion
• Reduces the positive intrathoracic pressure associated with mechanical ventilation, thus decreasing the adverse effects of positive pressure on haemodynamics and extrathoracic organs.

P_0.1 = occlusion pressure at 0.1 sec negative pressure generated by the inspiratory muscles in the first 100 msec of an inspiratory attempt with airway occlusion i.e. in the absence of flow and intrathoracic volume changes.
P_0.1 of 1.5 cmH₂O are expected in healthy subject at rest while it may increase to 5 cmH₂O during exercise. Values of P_0.1 are affected by the level of sedation, presence of PEEPi and respiratory muscles atrophy.

Changes in lung compliance result in changes in tidal volume

Changes in lung resistence result in changes in tidal volume

Requires back-up ventilation if there is a risk that patient will stop initiating breathing

Requires compensation for airway leak if present

Simple to set

Avoids high inspiratory pressures

Better patient–ventilator synchrony

Time-cycling means that the ventilator switches to exhalation as soon as the set inspiratory time (Ti) has elapsed

Patient-cycling is based on an expiratory trigger that allows the automatic adaptation of the ventilator Ti to the patient's own Ti