Does How You’re Positioned During Kidney Stone Surgery Actually Matter? What the Research Says About PCNL and Arterial Blood Gases
Kidney stones affect roughly one in ten people at some point in their lives — and for the largest, most complex stones, surgery is often the only solution. Percutaneous nephrolithotomy (PCNL) has become the gold-standard minimally invasive procedure for removing stones too big or stubborn for other methods. A surgeon makes a small incision in the patient’s back, guides a scope directly into the kidney, and breaks the stone apart for removal.
Simple enough in concept. But here’s something that rarely comes up in patient conversations: the position you’re placed in during that surgery — face down (prone), face up (supine), or on your side (flank) — can meaningfully affect how your body handles the procedure at a physiological level. In particular, it can influence your arterial blood gases: the balance of oxygen, carbon dioxide, and acid in your bloodstream. A clinical trial published in Urology Journal (Karami et al., 2012) put all three positions to the test, and the findings offer important insight for both patients and surgical teams.
What Is Percutaneous Nephrolithotomy, and Why Does Position Matter?
PCNL is recommended when kidney stones are 2 cm or larger, or when stones are resistant to less invasive treatments like shock wave lithotripsy. The procedure is performed under general or regional anesthesia, with the patient lying in a fixed position for the duration of surgery — which can last anywhere from 45 minutes to several hours.
The Three Main Positions
- Prone position: The patient lies face down. This is the traditional approach and offers excellent access to the kidney’s posterior structures.
- Supine position: The patient lies face up. This became more popular after the “Valdivia” technique was developed in the 1990s, and it allows for simultaneous retrograde access.
- Flank (lateral) position: The patient lies on their side. This position can be guided by ultrasound rather than fluoroscopy (X-ray), reducing radiation exposure.
Each position affects the chest wall, diaphragm, lung volumes, and circulation differently — which is precisely why researchers wanted to know whether those differences show up measurably in arterial blood gases.
Understanding Arterial Blood Gases: A Quick Primer
Arterial blood gas (ABG) analysis is one of the most informative tests in critical care. A small sample of blood drawn from an artery reveals:
- pH — how acidic or alkaline the blood is (normal: 7.35–7.45)
- PaO₂ — partial pressure of oxygen (normal: 80–100 mmHg)
- PaCO₂ — partial pressure of carbon dioxide (normal: 35–45 mmHg)
- HCO₃⁻ — bicarbonate, a marker of metabolic acid-base status (normal: 22–26 mEq/L)
- Base excess — a summary measure of metabolic acid-base balance
These values give anesthesiologists a real-time window into how well the lungs are ventilating, how well oxygen is reaching the tissues, and whether the body is compensating for metabolic changes. During PCNL, large volumes of irrigation fluid flow through the kidney, the patient is sedated, and the body is held in an unnatural position — all of which can shift these numbers.
What the Study Found: Flank vs. Prone vs. Supine
The clinical trial enrolled 90 patients scheduled for PCNL with no upper urinary tract abnormalities. They were divided into three equal groups of 30:
- Group 1: Ultrasound-guided PCNL in the flank position
- Group 2: Fluoroscopy-guided PCNL in the prone position
- Group 3: Fluoroscopy-guided PCNL in the supine position
Arterial blood gases were collected at two time points: just before repositioning, and 20 minutes after repositioning into the surgical position. This design allowed researchers to isolate the effect of position itself on blood gas parameters.
Key Findings
The results showed measurable differences across positions when blood gas values were compared before and after repositioning. Notably:
- In the prone position, patients experienced more pronounced changes in PaCO₂ and oxygenation parameters. The face-down posture compresses the abdomen and anterior chest wall, which can restrict the movement of the diaphragm and alter ventilation-perfusion matching in the lungs.
- In the supine position, circulatory dynamics were generally better maintained, though abdominal compression from the prone-related effects was absent.
- In the flank position, blood gas parameters remained comparatively more stable, possibly because the lateral orientation avoids both the abdominal compression of prone and the reduced lung compliance associated with some supine variants.
These differences were statistically significant, suggesting that position is not a neutral variable in PCNL — it actively shapes the body’s physiological response.
Comparing PCNL Positions: A Summary Table
| Feature | Prone | Supine | Flank |
| Traditional use | Yes (classic standard) | Growing since 1990s | Increasingly popular |
| Guidance method | Fluoroscopy (X-ray) | Fluoroscopy | Ultrasound (less radiation) |
| Kidney access | Posterior calyces | Anterior + posterior | Posterior |
| Respiratory impact | Greatest restriction | Moderate | Least restriction |
| ABG changes | Most pronounced | Moderate | Most stable |
| Simultaneous ureteroscopy | Difficult | Easy | Moderate |
| Ideal for obese patients | Less preferred | Often preferred | Studied with good outcomes |
| Anesthesia complexity | Higher | Lower | Moderate |
Note: Individual patient anatomy, surgeon expertise, and stone characteristics also influence position choice.
Why Blood Gas Changes Matter During Surgery
A patient under general anesthesia cannot regulate their own breathing the way they would while awake. The anesthesiologist controls ventilation through a machine — but the machine’s settings must account for what the body is actually doing. If the surgical position is causing a patient’s PaCO₂ to rise (hypoventilation pattern) or their pH to drift toward acidosis, the anesthesiologist must detect and correct this in real time.
Clinical implications include:
- Respiratory acidosis can develop if CO₂ is not adequately cleared in the prone position
- Impaired oxygenation can emerge when lung volumes are reduced by abdominal or chest wall compression
- Metabolic shifts from irrigation fluid absorption can compound position-related changes
- Prolonged procedures increase cumulative physiological stress regardless of position
Monitoring arterial blood gases during PCNL — rather than relying solely on pulse oximetry — allows the surgical team to catch these changes before they become complications.
What This Means for High-Risk Patients
Not all patients undergoing PCNL are equally resilient. Some groups face heightened risk from position-related physiological changes:
- Obese patients: Adipose tissue around the chest and abdomen can amplify the restrictive effects of prone positioning on respiratory mechanics
- Elderly patients: Reduced lung compliance and cardiovascular reserve mean less tolerance for blood gas fluctuations
- Patients with pre-existing lung disease: Even modest changes in ventilation-perfusion matching can cause meaningful drops in oxygenation
- Patients with cardiovascular conditions: Position-related shifts in preload and cardiac output can destabilize hemodynamics
- Long-stone cases: Extended operative time compounds position-related physiological burden
For these patients, choosing the flank or supine position — and using ultrasound guidance where possible — may offer a meaningful safety advantage, independent of stone clearance rates.
Stone-Free Rates: Does Safety Come at a Cost to Effectiveness?
A reasonable concern is whether choosing a “safer” position compromises the ability to fully remove the stone. The evidence here is reassuring. Multiple studies, including a comparative outcomes trial by the same research group (Karami et al., World Journal of Urology, 2013), found that stone-free rates across prone, supine, and flank positions were statistically similar — ranging from approximately 86% to 92% across groups, with no significant difference.
In other words, the physiological advantages of the flank position in terms of blood gas stability do not appear to come at the expense of surgical effectiveness. The trade-offs between positions are largely about access logistics, surgeon familiarity, and patient anatomy — not efficacy.
This is particularly significant because it suggests surgical teams can prioritize patient safety considerations — including respiratory and metabolic stability — without necessarily sacrificing outcomes.
The Role of Ultrasound Guidance
One underappreciated dimension of the flank position is that it enables ultrasound-guided access to the kidney, as opposed to the fluoroscopic guidance required for prone and supine PCNL. Ultrasound guidance:
- Eliminates radiation exposure for both patient and surgical team
- Provides real-time visualization of surrounding structures, potentially reducing inadvertent injury
- Has been shown to achieve comparable stone-free rates to fluoroscopic approaches in experienced hands
The combination of flank positioning and ultrasound guidance therefore represents a two-pronged approach to reducing procedural risk — less physiological disruption and less radiation.
What Should Patients Know Before PCNL?
If you or someone you care for is scheduled for PCNL, here are practical points to discuss with your urologist:
- Ask about positioning options. Not all centers offer all three positions; surgeon training and equipment matter. But it’s worth asking which position is planned and why.
- Disclose respiratory or cardiac conditions. Your anesthesiologist needs a complete picture of your health to anticipate how you’ll tolerate position-related physiological changes.
- Understand that monitoring will be in place. Arterial blood gas monitoring during PCNL is standard in well-resourced centers; it’s one of the key safety checks of the procedure.
- Don’t assume bigger stones mean a harder recovery. Stone size affects operative time, but modern techniques — including modified supine and flank variants — have significantly narrowed the differences between approaches.
- Ask about your surgeon’s experience. Outcomes in PCNL are strongly influenced by the operator’s familiarity with a given technique. A skilled team using a “less traditional” position often outperforms a less experienced team using the classic one.
Conclusion: Positioning Is More Than a Logistical Detail
The choice of surgical position in percutaneous nephrolithotomy is not merely an ergonomic convenience for the surgeon — it is a physiological variable with real consequences for how patients breathe, oxygenate their blood, and maintain acid-base balance under anesthesia. Research published in Urology Journal and supported by subsequent studies confirms that the flank position is associated with the most stable arterial blood gas profiles, while prone positioning carries the greatest respiratory burden.
As PCNL techniques continue to evolve and as minimally invasive surgery becomes more accessible globally, these physiological considerations will become increasingly important in tailoring procedures to individual patients. For anyone facing kidney stone surgery, the key takeaway is this: the details matter. Talk to your surgical team, understand your options, and trust that in experienced hands, your safety and your stone-free rate can both be priorities.
For more peer-reviewed research on urological surgery and kidney stone management, visit Urology Journal.