2019B Question 11
Describe the respiratory and cardiovascular changes associated with a carbon dioxide pneumoperitoneum for laparoscopic surgery.
Examiner Report
73.2% of candidates achieved a pass in this question.
The domains assessed on this question were the effects of increased intra-abdominal pressure on the respiratory system, and the effects of increased intra-abdominal pressure on the cardiovascular system. The effects of CO2 absorption on both the respiratory and cardiovascular systems were also assessed. Other relevant facts pertaining to pneumoperitoneum gained extra marks.
Common failures included lack of addressing all domains, especially those relating to intra-abdominal pressure effects commonly seen in practice. Simple lists of effects with no explanation gained lower marks.
This topic is covered briefly in Perrin, M & Fletcher, A Continuing Education in Anaesthesia, Critical Care and Pain, Vol 4., No. 4, 200, as well as in more detail in Nunn’s applied respiratory physiology 8th ed. Ch. 20, or Miller’s Anaesthesia 7th ed. Ch. 21.
Model Answer
Structure:
- Introduction
- Effects of intra-abdominal hypertension
- Effects of hypercapnoea
- Effects of position change
- Miscellaneous complications
Introduction
| Factor | Details |
|---|---|
| Capnoperitoneum | - Steady state insufflation rate 200-400mL.min-1 - Pressure 10-20mmHg |
| Cause of derangements | - Raised intra-abdominal pressure (↑ IAP) - Hypercapnoea - Positioning |
| Summary of effects | - Respiratory: - Dose-dependent embarrassment - Worse if obese or Trendelenburg) - Can be offset by recruitment manoeuvres and PEEP, but at expense of CVS system - Cardiovascular: - Variable - Significantly worse if hypovolaemic |
Intra-abdominal Hypertension
| Factor | Details |
|---|---|
| Respiratory | - Cephalad diaphragm displacement → ↓ Lung volumes - ↓ FRC ± closing capacity - Small airway closure - Atelectasis - Shunt - Hypoxaemia - ↓ Lung compliance - Restrictive deficit - ↑ Peak airway pressure - ↑ Risk of barotrauma - Hypoventilation - Changes worse if obese, pregnant, Trendelenburg |
| Cardiovascular | - Biphasic effects on venous return - At first: Autotransfusion of splanchnic blood → ↑ VR → ↑ CO (± initial ↑ MAP) - Then: Compression of IVC → ↓ VR → ↓ CO (more pronounced if hypovolaemic) - ↑ SVR due to - ↑ IAP - ↑ PaCO2 → ↑ Catecholamine release - Dose-dependent effects on steady-state blood pressure - IAP <~20mmhg →="" ↑="" map="" (↑="" svr="" outweighs="" ↓="" venous="" return="" cardiac="" output)<="" p=""> - IAP >~20mmHg → ↓ MAP (↓ venous return/cardiac output outweighs ↑ SVR) - Effect on myocardial oxygen supply and demand - ↑ Demand: ↑ HR, ↑ contractility, ↑ stroke work - ↑ Supply: Metabolic autoregulation (but impaired if coronary artery disease) - Oliguria - ↑ IAP →?Renal venous obstruction → ↓ GFR |
Hypercapnoea
| Factor | Details |
|---|---|
| Mechanism | - CO2 is rapidly absorbed into systemic circulation (30-50mL.min-1) - Diffuses into rapidly and slowly equilibrating compartments down partial pressure gradient (note large body stores ~120L) - CO2 + H2O ↔ H2CO3 ↔ HCO3- + H+ - Hence ↑ PaCO2 → ↓ PH (whole body including CNS) - Prolonged laparoscopy with ↑ PaCO2 → ↑ Accumulation → ↑ Minute ventilation requirement |
| Respiratory effects | - Respiratory acidosis - Potentiation of HPV - Distribution of blood away from poorly-ventilated dependent lung - ↑ V/Q matching - ↑ PaO2 - Hence offsets V/Q inequality caused by small airway closure |
| Cardiovascular | - If pH >7.2: ↑ SNS output → ↑ Inotropy - If pH ≤7.2: ↓ pH → ↓ Inotropy - ↓ PH → Sensitisation of myocardium to arrhythmogenic effects of catecholamines - Pulmonary vasoconstriction - Systemic vasodilatation (offsets ↑ IAP → ↑ SVR) - ↑ Myocardial O2 demand (↑ HR) - ↑ Myocardial O2 supply (metabolic autoregulation – but impaired if coronary artery disease) |
Position
| Factor | Details |
|---|---|
| Trendelenburg | - Respiratory - ± Endobronchial intubation → Massive shunt → ↓ PaO2 - ↑ Diaphragm compression - Cardiovascular - ↑ Venous return: Offsets venous compression |
| Reverse Trend. | - Respiratory - ↓ Diaphragm compression - Cardiovascular - ↓ Venous return: Compounds venous compression |
Miscellaneous Complications
| Factor | Details |
|---|---|
| Gas in wrong place | - Pneumomediastinum - Pneumothorax - Subcutaneous emphysema - Venous gas embolism → Cardiovascular collapse: - Either intravascular insufflation directly into vessel - Or Venturi effect |
| CVS collapse at insufflation | - Peritoneal stretch → Vagal response - Sinus bradycardia, nodal rhythm, asystole) - Empty ventricle → Bezold-Jarisch reflex - Contraction of underfilled LV - Stimulation of unmyelinated PSNS C fibres - ↑↑ PSNS output → ↓ HR, ↓ BP, coronary vasodilatation |
| Surgical misadventure | - Damage to vessel by trochar - Difficult to control bleeding - Difficult to estimate blood loss |